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J.P. Magué & B. Ménard

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[1]  arXiv:1502.07343 [pdf, other]
Title: Long term variability of Cygnus X-1: VII. Orbital variability of the focussed wind in Cyg X-1 / HDE 226868 system
Comments: proposed for acceptance in A&A, 11 pages, 11 figures (two in appendix)
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR)

Binary systems with an accreting compact object are a unique chance to investigate the strong, clumpy, line-driven winds of early type supergiants by using the compact object's X-rays to probe the wind structure. We analyze the two-component wind of HDE 226868, the O9.7Iab giant companion of the black hole Cyg X-1 using 4.77 Ms of RXTE observations of the system taken over the course of 16 years. Absorption changes strongly over the 5.6 d binary orbit, but also shows a large scatter at a given orbital phase, especially at superior conjunction. The orbital variability is most prominent when the black hole is in the hard X-ray state. Our data are poorer for the intermediate and soft state, but show signs for orbital variability of the absorption column in the intermediate state. We quantitatively compare the data in the hard state to a toy model of a focussed Castor-Abbott-Klein-wind: as it does not incorporate clumping, the model does not describe the observations well. A qualitative comparison to a simplified simulation of clumpy winds with spherical clumps shows good agreement in the distribution of the equivalent hydrogen column density for models with a porosity length on the order of the stellar radius at inferior conjunction; we conjecture that the deviations between data and model at superior conjunction could be either due to lack of a focussed wind component in the model or a more complicated clump structure.

[2]  arXiv:1502.07344 [pdf, ps, other]
Title: Galaxy formation with radiative and chemical feedback
Comments: This version has coloured figures not present in the printed version. Submitted to MNRAS, minor revisions
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Here we introduce GAMESH, a novel pipeline which implements self-consistent radiative and chemical feedback in a computational model of galaxy formation. By combining the cosmological chemical-evolution model GAMETE with the radiative transfer code CRASH, GAMESH can post process realistic outputs of a N-body simulation describing the redshift evolution of the forming galaxy. After introducing the GAMESH implementation and its features, we apply the code to a low-resolution N-body simulation of the Milky Way formation and we investigate the combined effects of self-consistent radiative and chemical feedback. Many physical properties, which can be directly compared with observations in the Galaxy and its surrounding satellites, are predicted by the code along the merger-tree assembly. The resulting redshift evolution of the Local Group star formation rates, reionisation and metal enrichment along with the predicted Metallicity Distribution Function of halo stars are critically compared with observations. We discuss the merits and limitations of the first release of GAMESH, also opening new directions to a full implementation of feedback processes in galaxy formation models by combining semi-analytic and numerical methods.

[3]  arXiv:1502.07345 [pdf, ps, other]
Title: A Census of Variability in Globular Cluster M68 (NGC 4590)
Comments: 25 pages, 18 figures, A&A in press
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We analyse 20 nights of CCD observations in the V and I bands of the globular cluster M68 (NGC 4590), using these to detect variable objects. We also obtained electron-multiplying CCD (EMCCD) observations for this cluster in order to explore its core with unprecedented spatial resolution from the ground.
We reduced our data using difference image analysis, in order to achieve the best possible photometry in the crowded field of the cluster. In doing so, we showed that when dealing with identical networked telescopes, a reference image from any telescope may be used to reduce data from any other telescope, which facilitates the analysis significantly. We then used our light curves to estimate the properties of the RR Lyrae (RRL) stars in M68 through Fourier decomposition and empirical relations. The variable star properties then allowed us to derive the cluster's metallicity and distance.
We determine new periods for the variable stars, and search for new variables, especially in the core of the cluster where our method performs particularly well. We detect an additional 4 SX Phe stars, and confirm the variability of another star, bringing the total number of confirmed variable stars in this cluster to 50. We also used archival data stretching back to 1951 in order to derive period changes for some of the single-mode RRL stars, and analyse the significant number of double-mode RRL stars in M68. Furthermore, we find evidence for double-mode pulsation in one of the SX Phe stars in this cluster. Using the different types of variables, we derived an estimate of the metallicity, [Fe/H]=$-2.07 \pm 0.06$ on the ZW scale, and 4 independent estimates of the distance modulus ($\mu_0 \sim$ 15.00 mag) for this cluster. Thanks to the first use of difference image analysis on time-series observations of M68, we are now confident that we have a complete census of the RRL stars in this cluster.

[4]  arXiv:1502.07346 [pdf, ps, other]
Title: A primordial origin for the composition similarity between the Earth and the Moon
Comments: 34 pages, 7 Figures, 4 Tables. Accepted for publication in Nature
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Most of the properties of the Earth-Moon system can be explained by a collision between a planetary embryo and the growing Earth late in the accretion process. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor. However, analysis of the terrestrial and lunar isotopic composition show them to be highly similar. In contrast, the compositions of other solar system bodies are significantly different than the Earth and Moon. This poses a major challenge to the giant impact scenario since the Moon-forming impactor is then thought to also have differed in composition from the proto-Earth. Here we track the feeding zones of growing planets in a suite of simulations of planetary accretion, in order to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are systematically more similar to the planets they impact. A significant fraction of planet-impactor pairs have virtually identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.

[5]  arXiv:1502.07347 [pdf, other]
Title: Galaxy Cluster Mass Reconstruction Project: II. Quantifying scatter and bias using contrasting mock catalogues
Comments: 25 pages, 19 figures, 7 tables. Accepted for publication in MNRAS
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA)

This article is the second in a series in which we perform an extensive comparison of various galaxy-based cluster mass estimation techniques that utilise the positions, velocities and colours of galaxies. Our aim is to quantify the scatter, systematic bias and completeness of cluster masses derived from a diverse set of 25 galaxy-based methods using two contrasting mock galaxy catalogues based on a sophisticated halo occupation model and a semi-analytic model. Analysing 968 clusters, we find a wide range in the RMS errors in log M200c delivered by the different methods (0.18 to 1.08 dex, i.e., a factor of ~1.5 to 12), with abundance matching and richness methods providing the best results, irrespective of the input model assumptions. In addition, certain methods produce a significant number of catastrophic cases where the mass is under- or over-estimated by a factor greater than 10. Given the steeply falling high-mass end of the cluster mass function, we recommend that richness or abundance matching-based methods are used in conjunction with these methods as a sanity check for studies selecting high mass clusters. We see a stronger correlation of the recovered to input number of galaxies for both catalogues in comparison with the group/cluster mass, however, this does not guarantee that the correct member galaxies are being selected. We do not observe significantly higher scatter for either mock galaxy catalogues. Our results have implications for cosmological analyses that utilise the masses, richnesses, or abundances of clusters, which have different uncertainties when different methods are used.

[6]  arXiv:1502.07353 [pdf, other]
Title: Determining the covering factor of Compton-thick active galactic nuclei with NuSTAR
Comments: Accepted for publication in ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The covering factor of Compton-thick obscuring material associated with the torus in active galactic nuclei (AGN) is at present best understood through the fraction of sources exhibiting Compton-thick absorption along the line of sight ($N_{H}>1.5\times10^{24}$ cm$^{-2}$) in the X-ray band, which reveals the average covering factor. Determining this Compton-thick fraction is difficult however, due to the extreme obscuration. With its spectral coverage at hard X-rays ($>$10 keV), NuSTAR is sensitive to the AGN covering factor since Compton scattering of X-rays off optically thick material dominates at these energies. We present a spectral analysis of 10 AGN observed with NuSTAR where the obscuring medium is optically thick to Compton scattering, so called Compton-thick (CT) AGN. We use the torus models of Brightman & Nandra which predict the X-ray spectrum from reprocessing in a torus and include the torus opening angle as a free parameter and aim to determine the covering factor of the Compton-thick gas in these sources individually. Across the sample we find mild to heavy Compton-thick columns, with $N_{H}$ measured from $10^{24}-10^{26}$ cm$^{-2}$, and a wide range of covering factors, where individual measurements range from 0.2-0.9. We find that the covering factor, $f_{c}$, is a strongly decreasing function of the intrinsic 2-10 keV luminosity, $L_{X}$, where $f_{c}=(-0.41\pm0.13)$log$_{10}$($L_{X}$/erg s$^{-1}$)$+18.31\pm5.33$, across more than two orders of magnitude in $L_{X}$ (10$^{41.5}-10^{44}$ erg s$^{-1}$). The covering factors measured here agree well with the obscured fraction as a function of $L_{X}$ as determined by studies of local AGN with $L_{X}>10^{42.5}$ erg s$^{-1}$.

[7]  arXiv:1502.07354 [pdf, other]
Title: Asymmetric condensed dark matter
Comments: 35 pages, 1 figure
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

We explore the viability of a boson dark matter candidate with an asymmetry between the number densities of particles and antiparticles. A simple thermal field theory analysis confirms that, under certain general conditions, this component would develop a Bose-Einstein condensate in the early universe that, for appropriate model parameters, could survive the ensuing cosmological evolution until now. The condensation of a dark matter component in equilibrium with the thermal plasma is a relativistic process, hence the amount of matter dictated by the charge asymmetry is complemented by a hot relic density frozen out at the time of decoupling. Contrary to the case of ordinary WIMPs, dark matter particles in a condensate can be very light, $10^{-22}\,{\rm eV} \lesssim m \lesssim 10^2\,{\rm eV}$; the lower limit arises from constraints on small-scale structure formation, while the upper bound ensures that the density from thermal relics is not too large. Big-Bang nucleosynthesis constrains the temperature of decoupling to the scale of the QCD phase transition or above. This requires large dark matter-to-photon ratios and very weak interactions with standard model particles. Finally, we argue that a given boson particle that was in thermal equilibrium in the early universe may be in a condensate, or in the form of thermal relics, but we cannot have a combination of both contributing significantly to the mass density today.

[8]  arXiv:1502.07355 [pdf, other]
Title: Identification of the brightest Lyα emitters at z=6.6: implications for the evolution of the luminosity function in the re-ionisation era
Comments: 20 pages, main results shown in Fig. 6 and Fig. 7, submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Using wide field narrow-band surveys, we provide a new measurement of the $z=6.6$ Lyman-$\alpha$ Emitter (LAE) luminosity function (LF), which constraints the bright end for the first time. We use a combination of archival narrow-band NB921 data in UDS and new NB921 measurements in SA22 and COSMOS/UltraVISTA, all observed with the Subaru telescope, with a total area of $\sim 5$ deg$^2$. We exclude lower redshift interlopers by using broad-band optical and near-infrared photometry and also exclude three supernovae with data split over multiple epochs. We spectroscopically confirm the two most luminous Ly$\alpha$ emitters ever found at $z=6.604$ and $6.541$ in the COSMOS field using Keck/DEIMOS and VLT/FORS2. Combining the UDS and COSMOS samples we find no evolution of the bright end of the Ly$\alpha$ LF between $z=5.7$ and $6.6$, which is supported by spectroscopic follow-up, and conclude that \emph{Himiko}-like sources are not as rare as previously thought, with number densities of $\sim 1.5\times10^{-5}$ Mpc$^{-3}$. Combined with our wide-field SA22 measurements, our results indicate a non-Schechter-like bright end of the LF at $z=6.6$ and a different evolution of \emph{observed} faint and bright LAEs. This differential evolution was not addressed in previous studies, or discarded as cosmic variance, but we argue instead that it may be an effect of re-ionisation. Using a toy-model, we show that such differential evolution of the LF is expected, since brighter sources are able to ionise their surroundings earlier, such that Ly$\alpha$ photons are able to escape. Our targets are excellent candidates for detailed follow-up studies and provide the possibility to give a unique view on the earliest stages in the formation of galaxies and re-ionisation process.

[9]  arXiv:1502.07356 [pdf, other]
Title: Milking the spherical cow: on aspherical dynamics in spherical coordinates
Comments: Submitted to MNRAS. Comments welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Galaxies and the dark matter halos that host them are not spherically symmetric, yet spherical symmetry is a helpful simplifying approximation for idealised calculations and analysis of observational data. The assumption leads to an exact conservation of angular momentum for every particle, making the dynamics unrealistic. But how much does that inaccuracy matter in practice for analyses of stellar distribution functions, collisionless relaxation, or dark matter core-creation?
We provide a general answer to this question for a wide class of aspherical systems; specifically, we consider distribution functions that are "maximally stable", i.e. that do not evolve at first order when external potentials (which arise from baryons, large scale tidal fields or infalling substructure) are applied. We show that a spherically-symmetric analysis of such systems gives rise to the false conclusion that the density of particles in phase space is ergodic (a function of energy alone).
Using this idea we are able to demonstrate that: (a) observational analyses that falsely assume spherical symmetry are made more accurate by imposing a strong prior preference for near-isotropic velocity dispersions in the centre of spheroids; (b) numerical simulations that use an idealised spherically-symmetric setup can yield misleading results and should be avoided where possible; and (c) triaxial dark matter halos (formed in collisionless cosmological simulations) nearly attain our maximally-stable limit, but their evolution freezes out before reaching it.

[10]  arXiv:1502.07357 [pdf, other]
Title: Baryon impact on the halo mass function: Fitting formulae and implications for cluster cosmology
Comments: 12 pages, 5 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We calibrate the halo mass function accounting for halo baryons and present fitting formulae for spherical overdensity masses $M_{500\textrm c}$, $M_{200\textrm c}$, and $M_{200\textrm m}$. We use the hydrodynamical Magneticum simulations, which are well suited because of their high resolution and large cosmological volumes of up to $\sim2$ Gpc$^3$. Baryonic effects globally decrease the masses of galaxy clusters, which, at given mass, results in a decrease of their number density. This effect vanishes at high redshift $z\sim2$ and for high masses $\gtrsim 5\times10^{14}M_\odot$. We perform cosmological analyses of three idealized approximations to the cluster surveys by the South Pole Telescope (SPT), Planck, and eROSITA. For the SPT-like and the Planck-like samples, we find that the impact of baryons on the cosmological results is negligible. In the eROSITA-like case, we find that neglecting the baryonic impact leads to an underestimate of $\Omega_\textrm m$ by about 0.01, which is comparable to the expected uncertainty from eROSITA. We compare our mass function fits with the literature. In particular, in the analysis of our Planck-like sample, results obtained using our mass function are shifted by $\Delta(\sigma_8)\simeq0.05$ with respect to results obtained using the Tinker et al. (2008) fit. This shift represents a large fraction of the observed difference between the latest results from Planck clusters and CMB anisotropies, and the tension is essentially removed. We discuss biases that can be introduced through inadequate mass function parametrizations that introduce false cosmological sensitivity. Additional work to calibrate the halo mass function is therefore crucial for progress in cluster cosmology.

[11]  arXiv:1502.07358 [pdf, ps, other]
Title: An improved SPH scheme for cosmological simulations
Comments: 22 figures, 2 tables, submitted to MNRAS
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA)

We present an implementation of smoothed particle hydrodynamics (SPH) with improved accuracy for simulations of galaxies and the large-scale structure. In particular, we combine, implement, modify and test a vast majority of SPH improvement techniques in the latest instalment of the GADGET code. We use the Wendland kernel functions, a particle wake-up time-step limiting mechanism and a time-dependent scheme for artificial viscosity, which includes a high-order gradient computation and shear flow limiter. Additionally, we include a novel prescription for time-dependent artificial conduction, which corrects for gravitationally induced pressure gradients and largely improves the SPH performance in capturing the development of gas-dynamical instabilities. We extensively test our new implementation in a wide range of hydrodynamical standard tests including weak and strong shocks as well as shear flows, turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas clouds. We jointly employ all modifications; however, when necessary we study the performance of individual code modules. We approximate hydrodynamical states more accurately and with significantly less noise than standard SPH. Furthermore, the new implementation promotes the mixing of entropy between different fluid phases, also within cosmological simulations. Finally, we study the performance of the hydrodynamical solver in the context of radiative galaxy formation and non-radiative galaxy cluster formation. We find galactic disks to be colder, thinner and more extended and our results on galaxy clusters show entropy cores instead of steadily declining entropy profiles. In summary, we demonstrate that our improved SPH implementation overcomes most of the undesirable limitations of standard SPH, thus becoming the core of an efficient code for large cosmological simulations.

[12]  arXiv:1502.07359 [pdf, ps, other]
Title: Spectroscopic Confusion: Its Impact on Current and Future Extragalactic HI Surveys
Comments: Accepted to MNRAS, 14 pages, 9 figures, 2 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present a comprehensive model to predict the rate of spectroscopic confusion in HI surveys, and demonstrate good agreement with the observable confusion in existing surveys. Generically the action of confusion on the HI mass function was found to be a suppression of the number count of sources below the `knee', and an enhancement above it. This results in a bias, whereby the `knee' mass is increased and the faint end slope is steepened. For ALFALFA and HIPASS we find that the maximum impact this bias can have on the Schechter fit parameters is similar in magnitude to the published random errors. On the other hand, the impact of confusion on the HI mass functions of upcoming medium depth interferometric surveys, will be below the level of the random errors. In addition, we find that previous estimates of the number of detections for upcoming surveys with SKA-precursor telescopes may have been too optimistic, as the framework implemented here results in number counts between 60% and 75% of those previously predicted, while accurately reproducing the counts of existing surveys. Finally, we argue that any future single dish, wide area surveys of HI galaxies would be best suited to focus on deep observations of the local Universe (z < 0.05), as confusion may prevent them from being competitive with interferometric surveys at higher redshift, while their lower angular resolution allows their completeness to be more easily calibrated for nearby extended sources.

[13]  arXiv:1502.07360 [pdf, ps, other]
Title: Herschel Hi-GAL imaging of massive young stellar objects
Comments: 11 pages, 12 figures, 3 tables, accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We used Herschel Hi-GAL survey data to determine whether massive young stellar objects (MYSOs) are resolved at 70$\mu$m and to study their envelope density distribution. Our analysis of three relatively isolated sources in the l=30{\deg} and l=59{\deg} Galactic fields show that the objects are partially resolved at 70$\mu$m. The Herschel Hi-GAL survey data have a high scan velocity which makes unresolved and partially resolved sources appear elongated in the 70$\mu$m images. We analysed the two scan directions separately and examine the intensity profile perpendicular to the scan direction. Spherically symmetric radiative transfer models with a power law density distribution were used to study the circumstellar matter distribution. Single dish sub-mm data were also included to study how different spatial information affects the fitted density distribution. The density distribution which best fits both the 70$\mu$m intensity profile and SED has an average index of ~0.5. This index is shallower than expected and is probably due to the dust emission from bipolar outflow cavity walls not accounted for in the spherical models. We conclude that 2D axisymmetric models and Herschel images at low scan speeds are needed to better constrain the matter distribution around MYSOs.

[14]  arXiv:1502.07361 [pdf, ps, other]
Title: WS1: one more new Galactic bona fide luminous blue variable
Comments: 5 pages, 3 figures, accepted for publication in MNRAS Letters
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

In this Letter, we report the results of spectroscopic and photometric monitoring of the candidate luminous blue variable (LBV) WS1, which was discovered in 2011 through the detection of a mid-infrared circular shell and follow-up optical spectroscopy of its central star. Our monitoring showed that WS1 brightened in the B, V and I bands by more than 1 mag during the last three years, while its spectrum revealed dramatic changes during the same time period, indicating that the star became much cooler. The light curve of WS1 demonstrates that the brightness of this star has reached maximum in 2013 December and then starts to decline. These findings unambiguously proved the LBV nature of WS1 and added one more member to the class of Galactic bona fide LBVs, bringing their number to sixteen (an updated census of these objects is provided).

[15]  arXiv:1502.07365 [pdf, other]
Title: Probing the Spacetime Around Supermassive Black Holes with Ejected Plasma Blobs
Comments: 4 pages, 6 figures
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

Millimeter-wavelength VLBI observations of the supermassive black holes in Sgr A* and M87 by the Event Horizon Telescope could potentially trace the dynamics of ejected plasma blobs in real time. We demonstrate that the trajectory and tidal stretching of these blobs can be used to test general relativity and set new constraints on the mass and spin of these black holes.

[16]  arXiv:1502.07369 [pdf, ps, other]
Title: The Impact of Dust Evolution and Photoevaporation on Disk Dispersal
Comments: The Astrophysical Journal, accepted for publication
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Protoplanetary disks are dispersed by viscous evolution and photoevaporation in a few million years; in the interim small, sub-micron sized dust grains must grow and form planets. The time-varying abundance of small grains in an evolving disk directly affects gas heating by far-ultraviolet photons, while dust evolution affects photoevaporation by changing the disk opacity and resulting penetration of FUV photons in the disk. Photoevaporative flows, in turn, selectively carry small dust grains leaving the larger particles---which decouple from the gas---behind in the disk. We study these effects by investigating the evolution of a disk subject to viscosity, photoevaporation by EUV, FUV and X-rays, dust evolution, and radial drift using a 1-D multi-fluid approach (gas + different dust grain sizes) to solve for the evolving surface density distributions. The 1-D evolution is augmented by 1+1D models constructed at each epoch to obtain the instantaneous disk structure and determine photoevaporation rates. The implementation of a dust coagulation/fragmentation model results in a marginal decrease in disk lifetimes when compared to models with no dust evolution; the disk lifetime is thus found to be relatively insensitive to the evolving dust opacity. We find that photoevaporation can cause significant reductions in the gas/dust mass ratio in the planet-forming regions of the disk as it evolves, and may result in a corresponding increase in heavy element abundances relative to hydrogen. We discuss implications for theories of planetesimal formation and giant planet formation, including the formation of gas-poor giants. After gas disk dispersal, $\sim 3\times 10^{-4}$ \ms\ of mass in solids typically remain, comparable to the solids inventory of our solar system.

[17]  arXiv:1502.07385 [pdf, ps, other]
Title: An updated gamma-ray analysis of the Be-BH binary HD~215227
Comments: Accepted for publication in MNRAS, 10 pages, 5 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); High Energy Astrophysical Phenomena (astro-ph.HE)

We report an updated analysis of the gamma-ray source AGL J2241+4454 that was detected as a brief two-day flare in 2010 by the AGILE satellite. The high-energy emission of AGL J2241+4454 has been attributed to the binary system HD 215227, which consists of a Be star being orbited by a black hole making it the first known Be-black hole binary system. We have analyzed the AGILE data and find a gamma-ray flux of $(1.8\pm0.7)\times10^{-6}$ ph cm$^{-2}$ s$^{-1}$, in agreement with the initial report. Additionally, we examined data from the Fermi LAT over several time intervals including the two day flare, the folded orbital phase, and the entire mission ($\sim$6-years). We do not detect AGL J2241+4454 over any of these time periods with Fermi and find upper limits of $1.1\times10^{-7}$ ph cm$^{-2}$ s$^{-1}$ and $5.2\times10^{-10}$ ph cm$^{-2}$ s$^{-1}$ for the flare and the full mission, respectively. We conclude that the HD 215227 Be-black hole binary is not a true gamma-ray binary as previous speculated. While analyzing the Fermi data of the AGL J2241+4454 region, we discovered a previously unknown gamma-ray source with average flux of $(13.56\pm0.02)\times10^{-8}$ ph cm$^{-2}$ s$^{-1}$ that is highly variable on monthly timescales. We associate this emission with the known quasar 87GB 215950.2+503417.

[18]  arXiv:1502.07389 [pdf, other]
Title: Cosmological perturbation theory in 1+1 dimensions
Comments: 31 pages + appendices; 10 figures; submitted to JCAP
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Many recent studies have highlighted certain failures of the standard Eulerian-space cosmological perturbation theory (SPT). Its problems include (1) not capturing large-scale bulk flows [leading to an O(1) error in the 1-loop SPT prediction for the baryon acoustic peak in the correlation function], (2) assuming that the Universe behaves as a pressureless, inviscid fluid, and (3) treating fluctuations on scales that are non-perturbative as if they were. Recent studies have highlighted the successes of perturbation theory in Lagrangian space or theories that solve equations for the effective dynamics of smoothed fields. Both approaches mitigate some or all of the aforementioned issues with SPT. We discuss these physical developments by specializing to the simplified 1D case of gravitationally interacting sheets, which allows us to substantially reduces the analytic overhead and still (as we show) maintain many of the same behaviors as in 3D. In 1D, linear-order Lagrangian perturbation theory ("the Zeldovich approximation") is exact up to shell crossing, and we prove that n^{th}-order Eulerian perturbation theory converges to the Zeldovich approximation as n goes to infinity. In no 1D cosmology that we consider (including a CDM-like case and power-law models) do these theories describe accurately the matter power spectrum on any mildly nonlinear scale. We find that theories based on effective equations are much more successful at describing the dynamics, and we test some of the assumptions that underlie prior applications of these theories. Finally, we discuss many topics that have recently appeared in the perturbation theory literature such as beat coupling, the shift and smearing of the baryon acoustic oscillation feature, and the advantages of Fourier versus configuration space. Our simplified 1D case serves as an intuitive review of these perturbation theory results.

[19]  arXiv:1502.07401 [pdf, other]
Title: Stellar Activity and Coronal Heating: an overview of recent results
Authors: Paola Testa (1), Steve Saar (1), Jeremy Drake (1) ((1) Harvard-Smithsonian Center for Astrophysics)
Comments: Accepted for publication on Philosophical Transactions A. 29 pages, 5 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Observations of the coronae of the Sun and of solar-like stars provide complementary information to advance our understanding of stellar magnetic activity, and of the processes leading to the heating of their outer atmospheres. While solar observations allow us to study the corona at high spatial and temporal resolution, the study of stellar coronae allows us to probe stellar activity over a wide range of ages and stellar parameters. Stellar studies therefore provide us with additional tools for understanding coronal heating processes, as well as the long-term evolution of solar X-ray activity. We discuss how recent studies of stellar magnetic fields and coronae contribute to our understanding of the phenomenon of activity and coronal heating in late-type stars.

[20]  arXiv:1502.07412 [pdf, ps, other]
Title: Volatile Delivery to Planets from Water-rich Planetesimals around Low Mass Stars
Comments: Accepted to ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Most models of volatile delivery to accreting terrestrial planets assume that the carriers for water are similar in water content to the carbonaceous chondrites in our Solar System. Here we suggest that the water content of primitive bodies in many planetary systems may actually be much higher, as carbonaceous chondrites have lost some of their original water due to heating from short-lived radioisotopes that drove parent body alteration. Using N-body simulations, we explore how planetary accretion would be different if bodies beyond the water line contained a water mass fraction consistent with chemical equilibrium calculations, and more similar to comets, as opposed to the more traditional water-depleted values. We apply this model to consider planet formation around stars of different masses and identify trends in the properties of Habitable Zone planets and planetary system architecture which could be tested by ongoing exoplanet census data collection. Comparison of such data with the model predicted trends will serve to evaluate how well the N-body simulations and the initial conditions used in studies of planetary accretion can be used to understand this stage of planet formation.

[21]  arXiv:1502.07418 [pdf, ps, other]
Title: An ultra-luminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30
Comments: 24 pages, 4 figures plus 4 extended data figures, published in Nature on 26 February 2015
Journal-ref: Nature, Vol. 518, 512-515 (2015)
Subjects: Astrophysics of Galaxies (astro-ph.GA)

So far, roughly 40 quasars with redshifts greater than z=6 have been discovered. Each quasar contains a black hole with a mass of one billion solar masses ( $10^9 M_\odot$). The existence of such black holes when the Universe was less than 1 billion years old presents substantial challenges to theories of the formation and growth of black holes and the coevolution of black holes and galaxies. Here we report the discovery of an ultra-luminous quasar, SDSS J010013.02+280225.8, at redshift z=6.30. It has an optical and near-infrared luminosity a few times greater than those of previously known z>6 quasars. On the basis of the deep absorption trough on the blue side of the Ly $\alpha$ emission line in the spectrum, we estimate the proper size of the ionized proximity zone associated with the quasar to be 26 million light years, larger than found with other z>6.1 quasars with lower luminosities. We estimate (on the basis of a near-infrared spectrum) that the black hole has a mass of $\sim 1.2 \times 10^{10} M_\odot$, which is consistent with the $1.3 \times 10^{10} M_\odot$ derived by assuming an Eddington-limited accretion rate.

[22]  arXiv:1502.07426 [pdf, other]
Title: James Webb Space Telescope can Detect Kilonovae in Gravitational Wave Follow-up Search
Comments: 8 pages, 2 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Kilonovae represent an important electromagnetic counterpart for compact binary mergers, which could become the most commonly detected gravitational wave (GW) source. Follow-up observations, triggered by GW sources, of kilonovae are nevertheless difficult due to poor localization by GW detectors and due to their faint near-infrared peak emission that has limited observational capability. We show that the Near-Infrared Camera (NIRCam) on the James Webb Space Telescope (JWST) will be able to detect kilonovae within the relevant GW-detection range of $\sim300$ Mpc, in less than NIRCam's minimum exposure time, for over a week following the merger. Despite this sensitivity, a kilonova search in all galaxies within 300 Mpc in the GW-localized sky area will not be viable with NIRCam because of JWST slew rates. However, targeted surveys may be developed to optimize the likelihood of discovering kilonovae efficiently within limited observing time. We estimate that a targeted survey focused on galaxies within 200 Mpc in a fiducial localized area of $10 \mbox{deg}^2$ would require $\sim$ 6.0 hours, dominated by overhead times; a targeted survey further focused on galaxies exhibiting high star-formation rates would require $\sim$ 2.8 hours. Required times may be reduced by as much as 40%, without compromising the likelihood of detecting kilonovae, in a targeted survey of localized areas associated with 50%, rather than 90%, confidence regions. On detection and identification of a kilonova, a limited number of NIRCam follow-up observations could constrain the properties of matter ejected by the binary and the equation of state of dense nuclear matter.

[23]  arXiv:1502.07438 [pdf, ps, other]
Title: Iron and s-elements abundance variations in NGC5286: comparison with anomalous globular clusters and Milky Way satellites
Comments: 28 pages, 21 figures, accepted for publication in MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We present a high resolution spectroscopic analysis of 62 red giants in the Milky Way globular cluster NGC5286. We have determined abundances of representative light proton-capture, alpha, Fe-peak and neutron-capture element groups, and combined them with photometry of multiple sequences observed along the colour-magnitude diagram. Our principal results are: (i) a broad, bimodal distribution in s-process element abundance ratios, with two main groups, the s-poor and s-rich groups; (ii) substantial star-to-star Fe variations, with the s-rich stars having higher Fe, e.g. <[Fe/H]>_s-rich - <[Fe/H]>_s-poor ~ 0.2~dex; and (iii) the presence of O-Na-Al (anti-)correlations in both stellar groups. We have defined a new photometric index, c_{BVI}=(B-V)-(V-I), to maximise the separation in the colour-magnitude diagram between the two stellar groups with different Fe and s-element content, and this index is not significantly affected by variations in light elements (such as the O-Na anticorrelation). The variations in the overall metallicity present in NGC5286 add this object to the class of "anomalous" GCs. Furthermore, the chemical abundance pattern of NGC5286 resembles that observed in some of the anomalous GCs, e.g. M22, NGC1851, M2, and the more extreme Omega Centauri, that also show internal variations in s-elements, and in light elements within stars with different Fe and s-elements content. In view of the common variations in s-elements, we propose the term s-Fe-anomalous GCs to describe this sub-class of objects. The similarities in chemical abundance ratios between these objects strongly suggest similar formation and evolution histories, possibly associated with an origin in tidally disrupted dwarf satellites.

[24]  arXiv:1502.07442 [pdf, ps, other]
Title: ngravs: Distinct gravitational interactions in GADGET-2
Authors: K. A. S. Croker
Comments: 6 pages, 5 figures
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We discuss an extension of the massively parallel cosmological simulation code GADGET-2, which enables investigation of distinct gravitational force laws between particle species. In addition to simplifying investigations of a universally modified force law, the ngravs extension allows state-of-the-art collisionless cosmological simulations of quite exotic gravitational scenarios. We briefly review the algorithms used by GADGET-2, and present our extension to multiple gravities, highlighting additional features that facilitate consideration of exotic force laws. We discuss the accuracy and performance of the ngravs extension, both internally and with an unaltered GADGET-2, under all relevant operational modes. The ngravs extension is publicly released to the research community.

[25]  arXiv:1502.07460 [pdf, other]
Title: Optical - Near Infrared Photometric Calibration of M-dwarf Metallicity and Its Application
Comments: Accepted for Publication in the Astronomical Journal
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Based on a carefully constructed sample of dwarf stars, a new optical-near infrared photometric calibration to estimate the metallicity of late-type K and early-to-mid-type M dwarfs is presented. The calibration sample has two parts; the first part includes 18 M dwarfs with metallicities determined by high-resolution spectroscopy and the second part contains 49 dwarfs with metallicities obtained through moderate-resolution spectra. By applying this calibration to a large sample of around 1.3 million M dwarfs from the Sloan Digital Sky Survey and the Two-Micron All Sky Survey, the metallicity distribution of this sample is determined and compared with those of previous studies. Using photometric parallaxes, the Galactic heights of M dwarfs in the large sample are also estimated. Our results show that stars farther from the Galactic plane, on average, have lower metallicity, which can be attributed to the age-metallicity relation. A scarcity of metal-poor dwarf stars in the metallicity distribution relative to the Simple Closed Box Model indicates the existence of the "M dwarf problem," similar to the previously known G and K dwarf problems. Several more complicated Galactic chemical evolution models which have been proposed to resolve the G and K dwarf problems are tested and it is shown that these models could, to some extent, mitigate the M dwarf problem as well.

[26]  arXiv:1502.07477 [pdf, ps, other]
Title: The Concept of Few-Parameter Modelling of Eclipsing Binary and Exoplanet Transit Light Curves
Comments: 5 pages, 4 figures, Astronomical Society of the Pacific Conference Series - Litomysl 2014
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We present a new few-parameter phenomenological model of light curves of eclipsing binaries and stars with transiting planets that is able to fit the observed light curves with the accuracy better than 1\% of their amplitudes. The model can be used namely for appropriate descriptions of light curve shapes, classification, mid-eclipse time determination, and fine period analyses.

[27]  arXiv:1502.07489 [pdf, other]
Title: Hubble Space Telescope observations of the NUV transit of WASP-12b
Comments: Accepted into the Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

We present new observations of four closely-spaced NUV transits of the hot Jupiter-like exoplanet WASP-12b using HST/COS, significantly increasing the phase resolution of the observed NUV light curve relative to previous observations, while minimising the temporal variation of the system. We observe significant excess NUV absorption during the transit, with mean normalised in-transit fluxes of $F_\mathrm{norm}\simeq0.97$, i.e. $\simeq$2-5 $\sigma$ deeper than the optical transit level of $\simeq0.986$ for a uniform stellar disk (the exact confidence level depending on the normalisation method used). We further observe an asymmetric transit shape, such that the post-conjunction fluxes are overall $\simeq$2-3 $\sigma$ higher than pre-conjunction values, and characterised by rapid variations in count rate between the pre-conjunction and out of transit levels. We do not find evidence for an early ingress to the NUV transit as suggested by earlier HST observations. However, we show that the NUV count rate observed prior to the optical transit is highly variable, but overall $\simeq$2.2-3.0 $\sigma$ below the post-transit values and comparable in depth to the optical transit, possibly forming a variable region of NUV absorption from at least phase $\phi\simeq$0.83, limited by the data coverage.

[28]  arXiv:1502.07496 [pdf, ps, other]
Title: A minimal width of the arrival direction distribution of ultra-high energy cosmic rays detected with the Yakutsk array
Authors: A.A. Ivanov
Comments: Submitted to ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

This paper presents the results of searches for anisotropy in arrival directions of ultra-high energy cosmic rays detected with the Yakutsk Array during the 1974--2008 observational period together with available data from other giant extensive air shower arrays working at present. A method of analysis based on a comparison of the minimal width of distributions in equatorial coordinates is applied. As a result, a hypothesis of isotropy in arrival directions is rejected at the $99.5\%$ significance level. The observed decrease in the minimal width of distribution can be explained by the presence of cosmic ray sources in energy intervals and sky regions according to the recent indications inferred from data of the Yakutsk Array and Telescope Array experiments.

[29]  arXiv:1502.07501 [pdf, ps, other]
Title: Long-Term X-ray Spectral Variability of Seyfert Galaxies with Swift
Comments: 6 pages, 5 figues, to appear in "Swift: 10 years of discovery", Proceedings of Science
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We present analysis of the long-term X-ray spectral variability of Seyfert galaxies as observed by Swift, which provides well-sampled observations over a much larger flux range and a much longer timescale than any other X-ray observatory. We examine long-term variability of three AGN: NGC 1365 (see Connolly et al. 2014), Mkn 335 and NGC 5548. At high fluxes, the 0.5-10 keV spectra soften with increasing flux, as seen previously within the 2-10 keV band. However, at very low fluxes the sources also become very soft. We have fitted a number of models to the data and find that both intrinsic luminosity variability and variable absorption are required to explain the observations. In some systems, e.g. NGC 1365, the best explanation is a two-component wind model in which one component represents direct emission absorbed by a disc wind wind, with the absorbing column inversely proportional to the intrinsic luminosity, and the second component represents unabsorbed emission reflected from the wind. In other AGN the situation is more complex.

[30]  arXiv:1502.07502 [pdf, ps, other]
Title: The largest Swift AGN monitoring campaign: UV/optical variability in NGC 5548
Comments: 8 pages, 5 figures, to appear in "Swift: 10 years of discovery", Proceedings of Science
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We report on the largest Swift AGN monitoring program, concerning UV/optical variability in Seyferts. From 554 observations, over a 750d period, of the Seyfert galaxy NGC 5548, we see (McHardy et al. 2014) a good overall correlation between the X-ray and UV/optical bands,particularly on short timescales (tens of days). The UVOT bands are found to lag behind X-rays with a lag scaling as wavelength to the power 1.23 +/- 0.31, in excellent agreement with that expected (1.33) if UV/optical variability arises from reprocessing of X-rays by the accretion disc. However, the observed lags are ~3 times longer than expected from a standard Shakura-Sunyaev disc, raising real concerns about the detailed validity of this model. The results can be explained with a slightly larger mass and accretion rate, and a hotter disc, or if the disc is clumpy, thereby enhancing the emission from the outer regions.

[31]  arXiv:1502.07513 [pdf, ps, other]
Title: Planetary systems and real planetary nebulae from planets destruction near white dwarfs
Authors: Ealeal Bear, Noam Soker (Technion, Israel)
Comments: submitted
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

We suggest that tidal destruction of Earth-like and icy planets near a white dwarf (WD) might lead to the formation of one or more low-mass planets in tight orbits around the WD. More massive planets contain hydrogen which will start burning on the surface of the WD and inflate an envelope, part of which be ejected to form a nebula. This nebula will be ionized and be observed as a planetary nebulae. The formation of the WD planetary system starts with a tidal break-up of icy or lower mass planets to planetesimals near their tidal radius of about 1Rsun. Internal stress forces keep the planetesimal from tidal break-up when their radius is less than about 100km. We suggest that the planetesimals then bind together to form new sub-Earth-like planets around the WD at a few solar radii. More massive planets that contain hydrogen will supply the WD with fresh nuclear fuel to reincarnate its stellar-giant phase. Some of the hydrogen will be inflated in a large envelope that will cause the planetesimal formed from the core of the giant planets to be evaporated. In the post-giant phase the hot WD ionizes the gas that was blown in the wind of the stellar-giant envelope, and form a nebula that originated from a planet--a real planetary nebula (RPN).

[32]  arXiv:1502.07543 [pdf, other]
Title: Transient sources at the highest angular resolution
Authors: Maria Massi
Comments: 8 pages, 5 figures. Proceeding of the 12th European VLBI Network Symposium and Users Meeting: 7-10 October 2014, Cagliari, Italy. Invited talk. PoS(EVN 2014)062
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

By definition transients are sudden events, some, like supernovae, are catastrophic, while others might be due to recurrent phenomena. The aim of studying transients is to reveal the physical conditions causing them, in this sense ideal targets for monitoring are transients in binary systems. In these systems the physical process responsible for the transient depends directly or indirectly on the interaction of the two components of the system. Here I report on transients in stellar binary systems at two extremes of stellar evolution: a T~Tauri system formed by two young low mass stellar objects, and X-ray binary systems formed by a star and a neutron star or a black hole, i.e., end points in the life of massive stars. VLBI observations of the young binary system V773 Tau A resolve the binary separation and can be overlapped with the optical frame. Consecutive VLBI observations showing the evolution of the radio emission with respect to the two stellar objects are an unvaluable tool for a better understanding of the magnetic field topology of T Tauri stars. The characteristics of radio jets in X-ray binaries are summarised and compared with those of the radio emission of the gamma-ray binary LS I +61303. Timing analysis of radio and Fermi-LAT observations provide constraints for theoretical models that can be tested by VLBI observations.

[33]  arXiv:1502.07552 [pdf, ps, other]
Title: Investigating the Global Collapse of Filaments Using Smoothed Particle Hydrodynamics
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We use Smoothed Particle Hydrodynamic simulations of cold, uniform density, self-gravitating filaments, to investigate their longitudinal collapse timescales; these timescales are important because they determine the time available for a filament to fragment into cores. A filament is initially characterised by its line-mass, $\mu$, its radius, $R$ (or equivalently its density $\rho\!=\!\mu/\pi R^2$), and its aspect ratio, $A\;\,(\equiv Z/R$, where $Z$ is its half-length). The gas is only allowed to contract longitudinally, i.e. parallel to the symmetry axis of the filament (the $z$-axis). Pon et al. (2012) have considered the global dynamics of such filaments analytically. They conclude that short filaments ($A\! < \!5$) collapse along the $z$-axis more-or-less homologously, on a time-scale $t_{_{\rm HOM}} \sim 0.44\,A\,(G\rho)^{-1/2}$; in contrast, longer filaments ($A\! > \!5$) undergo end-dominated collapse, i.e. two dense clumps form at the ends of the filament and converge on the centre sweeping up mass as they go, on a time-scale $t_{_{\rm END}} \sim 0.98\,A^{1/2}\,(G\rho)^{-1/2}$. Our simulations do not corroborate these predictions. First, for all $A\! > \!2$, the collapse time satisfies a single equation \[t_{_{\rm COL}}\;\sim\;(0.49+0.26A)(G\rho)^{-1/2}\,,\] which for large $A$ is much longer than the Pon et al. prediction. Second, for all $A\! > \!2$, the collapse is end-dominated. Third, before being swept up, the gas immediately ahead of an end-clump is actually accelerated outwards by the gravitational attraction of the approaching clump, resulting in a significant ram pressure. For high aspect ratio filaments the end-clumps approach an asymptotic inward speed, due to the fact that they are doing work both accelerating and compressing the gas they sweep up. Pon et al. appear to have neglected the outward acceleration and its consequences.

[34]  arXiv:1502.07579 [pdf, ps, other]
Title: The unusual photometric variability of the PMS star GM Cep
Comments: 9 pages, 4 figures, accepted for publication in PASA. arXiv admin note: text overlap with arXiv:1111.2166
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Results from UBVRI photometric observations of the pre-main sequence star GM Cep obtained in the period April 2011 - August 2014 are reported in the paper. Presented data are a continuation of our photometric monitoring of the star started in 2008. GM Cep is located in the field of the young open cluster Trumpler 37 and over the past years it has been an object of intense photometric and spectral studies. The star shows a strong photometric variability interpreted as a possible outburst from EXor type in previous studies. Our photometric data for a period of over six years show a large amplitude variability (Delta V ~ 2.3 mag.) and several deep minimums in brightness are observed. The analysis of the collected multicolor photometric data shows the typical of UX Ori variables a color reversal during the minimums in brightness. The observed decreases in brightness have a different shape, and evidences of periodicity are not detected. At the same time, high amplitude rapid variations in brightness typical for the classical T Tauri stars also present on the light curve of GM Cep. The spectrum of GM Cep shows the typical of classical T Tauri stars wide H/alpha emission line and absorption lines of some metals. We calculate the outer radius of the H/alpha emitting region as 10.4 +/-0.5 Rsun and the accretion rate as 1.8 x 10 E-7 Msun/yr.

[35]  arXiv:1502.07580 [pdf, ps, other]
Title: CARMENES input catalogue of M dwarfs. I. Low-resolution spectroscopy with CAFOS
Comments: A&A, in press
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Context. CARMENES is a stabilised, high-resolution, double-channel spectrograph at the 3.5 m Calar Alto telescope. It is optimally designed for radial-velocity surveys of M dwarfs with potentially habitable Earth-mass planets. Aims. We prepare a list of the brightest, single M dwarfs in each spectral subtype observable from the northern hemisphere, from which we will select the best planet-hunting targets for CARMENES. Methods. In this first paper on the preparation of our input catalogue, we compiled a large amount of public data and collected low-resolution optical spectroscopy with CAFOS at the 2.2 m Calar Alto telescope for 753 stars. We derived accurate spectral types using a dense grid of standard stars, a double least-squares minimisation technique, and 31 spectral indices previously defined by other authors. Additionally, we quantified surface gravity, metallicity, and chromospheric activity for all the stars in our sample. Results. We calculated spectral types for all 753 stars, of which 305 are new and 448 are revised. We measured pseudo-equivalent widths of Halpha for all the stars in our sample, concluded that chromospheric activity does not affect spectral typing from our indices, and tabulated 49 stars that had been reported to be young stars in open clusters, moving groups, and stellar associations. Of the 753 stars, two are new subdwarf candidates, three are T Tauri stars, 25 are giants, 44 are K dwarfs, and 679 are M dwarfs. Many of the 261 investigated dwarfs in the range M4.0-8.0 V are among the brightest stars known in their spectral subtype. Conclusions. This collection of low-resolution spectroscopic data serves as a candidate target list for the CARMENES survey and can be highly valuable for other radial-velocity surveys of M dwarfs and for studies of cool dwarfs in the solar neighbourhood.

[36]  arXiv:1502.07583 [pdf, ps, other]
Title: Constraints on the dark matter sound speed from galactic scales: the cases of the Modified and Extended Chaplygin Gas
Comments: 4 pages
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph)

We show that the observed rotation curves of spiral galaxies constrain the sound speed of the dark matter to be $c_s < 10^{-4} c$, where $c$ is the speed of light in vacuum. Using the Modified Chaplygin Gas as a representative example of a class of unified dark energy models incorporating an effective dark matter component with a non-zero sound speed, we determine the most stringent constraint to date on the value of the constant contribution to the equation of state parameter in this class of models. Finally, we explain the reason why previous constraints using the Cosmic Microwave Background and Baryonic Acoustic Oscillations were not as competitive as the one presented in this paper and discuss the limitations of the recently proposed Extended Chaplygin Gas.

[37]  arXiv:1502.07585 [pdf, ps, other]
Title: Tidal Downsizing model. II. Planet-metallicity correlations
Authors: Sergei Nayakshin (University of Leicester)
Comments: 13 pages. Submitted to MNRAS 2 months ago, no report yet (!)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Core Accretion (CA), the de-facto accepted theory of planet formation, requires formation of massive solid cores as a prerequisite for assembly of gas giant planets. The observed metallicity correlations of exoplanets are puzzling in the context of CA. While gas giant planets are found preferentially around metal-rich host stars, planets smaller than Neptune orbit hosts with a wide range of metallicities. We propose an alternative interpretation of these observations in the framework of a recently developed planet formation hypothesis called Tidal Downsizing (TD). We perform population synthesis calculations based on TD, and find that the connection between the populations of the gas giant and the smaller solid-core dominated planets is non linear and not even monotonic. While gas giant planets formed in the simulations in the inner few AU region follow a strong positive correlation with the host star metallicity, the smaller planets do not. The simulated population of these smaller planets shows a shallow peak in their formation efficiency at around the Solar metallicity. This result is driven by the fact that at low metallicities the solid core's growth is damped by the scarcity of metals, whereas at high metallicities the fragments within which the cores grow contract too quickly, cutting the core's growth time window short. Finally, simulated giant gas planets do not show a strong host star metallicity preference at large separations, which may explain why one of the best known directly imaged gas giant planet systems, HR 8799, is metal poor.

[38]  arXiv:1502.07596 [pdf, ps, other]
Title: Foregrounds in Wide-Field Redshifted 21 cm Power Spectra
Comments: Accepted for publication in ApJ
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Detection of 21 cm emission of HI from the epoch of reionization, at redshifts z>6, is limited primarily by foreground emission. We investigate the signatures of wide-field measurements and an all-sky foreground model using the delay spectrum technique that maps the measurements to foreground object locations through signal delays between antenna pairs. We demonstrate interferometric measurements are inherently sensitive to all scales, including the largest angular scales, owing to the nature of wide-field measurements. These wide-field effects are generic to all observations but antenna shapes impact their amplitudes substantially. A dish-shaped antenna yields the most desirable features from a foreground contamination viewpoint, relative to a dipole or a phased array. Comparing data from recent Murchison Widefield Array observations, we demonstrate that the foreground signatures that have the largest impact on the HI signal arise from power received far away from the primary field of view. We identify diffuse emission near the horizon as a significant contributing factor, even on wide antenna spacings that usually represent structures on small scales. For signals entering through the primary field of view, compact emission dominates the foreground contamination. These two mechanisms imprint a characteristic pitchfork signature on the "foreground wedge" in Fourier delay space. Based on these results, we propose that selective down-weighting of data based on antenna spacing and time can mitigate foreground contamination substantially by a factor ~100 with negligible loss of sensitivity.

[39]  arXiv:1502.07607 [pdf, ps, other]
Title: Seismic sensitivity to sub-surface solar activity from 18 years of GOLF/SoHO observations
Comments: Accepted for publication in A&A
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Solar activity has significantly changed over the last two Schwabe cycles. After a long and deep minimum at the end of Cycle 23, the weaker activity of Cycle 24 contrasts with the previous cycles. In this work, the response of the solar acoustic oscillations to solar activity is used in order to provide insights on the structural and magnetic changes in the sub-surface layers of the Sun during this on-going unusual period of low activity. We analyze 18 years of continuous observations of the solar acoustic oscillations collected by the Sun-as-a-star GOLF instrument onboard the SoHO spacecraft. From the fitted mode frequencies, the temporal variability of the frequency shifts of the radial, dipolar, and quadrupolar modes are studied for different frequency ranges which are sensitive to different layers in the solar sub-surface interior. The low-frequency modes show nearly unchanged frequency shifts between Cycles 23 and 24, with a time evolving signature of the quasi-biennial oscillation, which is particularly visible for the quadrupole component revealing the presence of a complex magnetic structure. The modes at higher frequencies show frequency shifts 30% smaller during Cycle~24, which is in agreement with the decrease observed in the surface activity between Cycles 23 and 24. The analysis of 18 years of GOLF oscillations indicates that the structural and magnetic changes responsible for the frequency shifts remained comparable between Cycle 23 and Cycle 24 in the deeper sub-surface layers below 1400 km as revealed by the low-frequency modes. The frequency shifts of the higher-frequency modes, sensitive to shallower regions, show that Cycle 24 is magnetically weaker in the upper layers of Sun.

[40]  arXiv:1502.07620 [pdf, ps, other]
Title: VUV-absorption cross section of CO2 at high temperatures and impact on exoplanet atmospheres
Comments: 8 pages, 3 figures, BIO Web of Conferences, Vol. 2, EPOV 2012 : From Planets to Life - Colloquium of the CNRS Interdisciplinary Initiative Planetary Environments and Origins of Life (2014)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Ultraviolet (UV) absorption cross sections are an essential ingredient of photochemical atmosphere models. Exoplanet searches have unveiled a large population of short-period objects with hot atmospheres, very different from what we find in our solar system. Transiting exoplanets whose atmospheres can now be studied by transit spectroscopy receive extremely strong UV fluxes and have typical temperatures ranging from 400 to 2500 K. At these temperatures, UV photolysis cross section data are severely lacking. Our goal is to provide high-temperature absorption cross sections and their temperature dependency for important atmospheric compounds. This study is dedicated to CO2, which is observed and photodissociated in exoplanet atmospheres. We performed these measurements for the 115 - 200 nm range at 300, 410, 480, and 550 K. In the 195 - 230 nm range, we worked at seven temperatures between 465 and 800 K. We found that the absorption cross section of CO2 is very sensitive to temperature, especially above 160 nm. Within the studied range of temperature, the CO2 cross section can vary by more than two orders of magnitude. This, in particular, makes the absorption of CO2 significant up to wavelengths as high as 230 nm, while it is negligible above 200 nm at 300 K. To investigate the influence of these new data on the photochemistry of exoplanets, we implemented the measured cross section into a 1D photochemical model. The model predicts that accounting for this temperature dependency of CO2 cross section can affect the computed abundances of NH3, CO2, and CO by one order of magnitude in the atmospheres of hot Jupiter and hot Neptune.

[41]  arXiv:1502.07624 [pdf, ps, other]
Title: Spectral Studies of Flaring FSRQs at GeV Energies Using Pass 8 Fermi-LAT Data
Comments: 7 pages, 4 figures, 4 tables, 2014 Fermi Symposium proceedings - eConf C14102.1
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Flat spectrum radio quasars (FSRQs) are bright active galactic nuclei surrounded by gas clouds within a UV-visible intense radiation field that form the so-called broad line region (BLR). These objects emit relativistic jets from a region close to the central supermassive black hole and through the BLR. The Fermi-Large Area Telescope (Fermi-LAT) is sensitive to gamma-ray photons from $\sim$30 MeV to more than 300 GeV. We have performed spectral analysis of bright FSRQs in a 5.5 year (2008-2014) data sample collected by Fermi-LAT, using the new Pass 8 event selection and instrument response function. Also, our study of flaring episodes in a limited time range brings interesting results while compared to the full 5.5 year data samples.

[42]  arXiv:1502.07669 [pdf, ps, other]
Title: Suzaku broad-band spectrum of 4U 1705-44: Probing the Reflection component in the hard state
Comments: 21 pages, 5 figures, 4 tables. To appear in Monthly Notices of the Royal Astronomical Society
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Iron emission lines at 6.4-6.97 keV, identified with Kalpha radiative transitions, are among the strongest discrete features in the X-ray band. These are one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disk around a compact object. In this paper we present a recent Suzaku observation, 100-ks effective exposure, of the atoll source and X-ray burster 4U 1705-44, where we clearly detect signatures of a reflection component which is distorted by the high-velocity motion in the accretion disk. The reflection component consists of a broad iron line at about 6.4 keV and a Compton bump at high X-ray energies, around 20 keV. All these features are consistently fitted with a reflection model, and we find that in the hard state the smearing parameters are remarkably similar to those found in a previous XMM-Newton observation performed in the soft state. In particular, we find that the inner disk radius is Rin = 17 +/- 5 Rg (where Rg is the Gravitational radius, GM/c^2), the emissivity dependence from the disk radius is -2.5 +/- 0.5, the inclination angle with respect to the line of sight is i = 43 +/- 5 degrees, and the outer radius of the emitting region in the disk is Rout > 200 Rg. We note that the accretion disk does not appear to be truncated at large radii, although the source is in a hard state at about 3 % of the Eddington luminosity for a neutron star. We also find evidence of a broad emission line at low energies, at 3.03 +/- 0.03 keV, compatible with emission from mildly ionized Argon (Ar XVI-XVII). Argon transitions are not included in the self-consistent reflection models that we used and we therefore added an extra component to our model to fit this feature. The low energy line appears compatible with being smeared by the same inner disk parameters found for the reflection component.

[43]  arXiv:1502.07675 [pdf, ps, other]
Title: Long-term Observations of Three Nulling Pulsars
Comments: 10 pages, 8 figures, accepted for publication in MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We present an analysis of approximately 200 hours of observations of the pulsars J1634$-$5107, J1717$-$4054 and J1853$+$0505, taken over the course of 14.7 yr. We show that all of these objects exhibit long term nulls and radio-emitting phases (i.e. minutes to many hours), as well as considerable nulling fractions (NFs) in the range $\sim67\,\% - 90\,\%$. PSR J1717$-$4054 is also found to exhibit short timescale nulls ($1 - 40~P$) and burst phases ($\lesssim 200~P$) during its radio-emitting phases. This behaviour acts to modulate the NF, and therefore the detection rate of the source, over timescales of minutes. Furthermore, PSR J1853$+$0505 is shown to exhibit a weak emission state, in addition to its strong and null states, after sufficient pulse integration. This further indicates that nulls may often only represent transitions to weaker emission states which are below the sensitivity thresholds of particular observing systems. In addition, we detected a peak-to-peak variation of $33\pm1\,\%$ in the spin-down rate of PSR J1717$-$4054, over timescales of hundreds of days. However, no long-term correlation with emission variation was found.

[44]  arXiv:1502.07692 [pdf, other]
Title: CSI 2264: Probing the inner disks of AA Tau-like systems in NGC 2264
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The classical T Tauri star AA Tau presented photometric variability attributed to an inner disk warp, caused by the interaction between the inner disk and an inclined magnetosphere. Previous studies of NGC 2264 have shown that similar photometric behavior is common among CTTS.
The goal of this work is to investigate the main causes of the observed photometric variability of CTTS in NGC 2264 that present AA Tau-like light curves, and verify if an inner disk warp could be responsible for their variability. We investigate veiling variability in their spectra and u-r color variations and estimate parameters of the inner disk warp using an occultation model proposed for AA Tau. We compare infrared and optical light curves to analyze the dust responsible for the occultations. AA Tau-like variability is transient on a timescale of a few years. We ascribe it to stable accretion regimes and aperiodic variability to unstable accretion regimes and show that a transition, and even coexistence, between the two is common. We find evidence of hot spots associated with occultations, indicating that the occulting structures could be located at the base of accretion columns. We find average values of warp maximum height of 0.23 times its radial location, consistent with AA Tau, with variations of on average 11% between rotation cycles. We show that extinction laws in the inner disk indicate the presence of grains larger than interstellar grains.
The inner disk warp scenario is consistent with observations for all but one periodic star in our sample. AA Tau-like systems comprise 14% of CTTS observed in NGC 2264, though this increases to 35% among systems of mass 0.7M_sun<M<2.0M_sun. Assuming random inclinations, we estimate that nearly all systems in this mass range likely possess an inner disk warp, possibly because of a change in magnetic field configurations among stars of lower mass.

[45]  arXiv:1502.07696 [pdf, other]
Title: Why are dense planetary rings only found between 8 AU and 20 AU?
Authors: M.M. Hedman
Comments: 4 Pages, 1 Figure, Accepted for publication in APJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

The recent discovery of dense rings around the Centaur Chariklo (and possibly Chiron) reveals that complete dense planetary rings are not only found around Saturn and Uranus, but also around small bodies orbiting in the vicinity of those giant planets. This report examines whether there could be a physical process that would make rings more likely to form or persist in this particular part of the outer Solar System. Specifically, the ring material orbiting Saturn and Uranus appears to be much weaker than the material forming the innermost moons of Jupiter and Neptune. Also, the mean surface temperatures of Saturn's, Uranus' and Chariklo's rings are all close to 70 K. Thus the restricted distribution of dense rings in our Solar System may arise because icy materials are particularly weak around that temperature.

[46]  arXiv:1502.07705 [pdf, other]
Title: On the Abundance of Extreme Voids
Comments: 10 pages, 4 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Cosmic voids have been shown to be an effective probe of cosmology, complementary to galaxy clusters. In this work, we present a simple theoretical framework for predicting of the size of the largest voids expected within a given redshift and volume. Our model is based on the exact extreme-value statistics which has previously been successfully applied to massive galaxy clusters. We implement our formalism using the void-abundance model of Sheth and Van de Weygaert and find results that are consistent with dark-matter simulation. We discuss several ways in which our framework can be adapted to other void models.

[47]  arXiv:1502.07733 [pdf, other]
Title: Planck, LHC, and $α$-attractors
Comments: 7 pages, 4 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

We describe a simple class of cosmological models called $\alpha$ attractors, which provide an excellent fit to the latest Planck data. These theories are most naturally formulated in the context of supergravity with logarithmic Kahler potentials. We develop generalized versions of these models which can describe not only inflation but also dark energy and supersymmetry breaking.

[48]  arXiv:1502.07740 [pdf, ps, other]
Title: The Contribution of Faint Galaxy Wings to Source-subtracted Near-infrared Background Fluctuations
Comments: Accepted for publication in MNRAS. 8 pages, 9 figures, 2 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

The source-subtracted, 1.1 and 1.6 {\mu}m NICMOS images used in earlier analyses of the near-infrared Hubble Ultra Deep Field contained residual flux in extended wings of identified sources that contributed an unknown amount to fluctuation power. When compared to the original results, a reanalysis after subtracting this residual flux shows that mean-square and rms fluctuations decrease a maximum of 52 and 31 per cent at 1.6 {\mu}m and 50 and 30 per cent at 1.1 {\mu}m. However, total mean-square fluctuations above 0.5 arcsec only decrease 6.5 and 1.4 per cent at 1.6 and 1.1 {\mu}m, respectively. These changes would not affect any published conclusions based on the prior analyses. These results exclude previous suggestions that extended wings of detected galaxies may be a major contributor to the source-subtracted near-infrared background and confirm that most fluctuation power in these images must be explained by other means.