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Diaz Trigo M.,ESO | Sidoli L.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Boirin L.,11 Rue Of Luniversite | Parmar A.N.,ESAC
Astronomy and Astrophysics | Year: 2012

We analysed data from five XMM-Newton observations of GX 13 + 1 to investigate the variability of the photo-ionised absorber in this source. We fitted EPIC and RGS spectra obtained from the "least-variable" intervals with a model consisting of disc-blackbody and blackbody components together with a Gaussian emission feature at ∼6.55-6.7 keV modified by absorption due to cold and photo-ionised material. We found a significant correlation between the hard, ∼6-10 keV, flux, the ionisation and column density of the absorber and the equivalent width of the broad iron line. We interpret the correlation in a scenario in which a disc wind is thermally driven at large, ∼1010 cm, radii and the broad line results from reprocessed emission in the wind and/or hot atmosphere. The breadth of the emission line is naturally explained by a combination of scattering, recombination and fluorescence processes. We attribute the variations in the absorption and emission along the orbital period to the view of different parts of the wind, possibly located at slightly different inclination angles. We constrain the inclination of GX 13 + 1 to be between 60 and 80° from the strong absorption in the line of sight, which obscures up to 80% of the total emission in one observation, and the absence of eclipses. We conclude that either a disc wind and/or a hot atmosphere can explain the current observations of narrow absorption and broad iron emission features in neutron star low-mass X-ray binaries as a class. © 2012 ESO.

Dovciak M.,Czech Republic Astronomical Institute | Muleri F.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Goosmann R.W.,11 Rue Of Luniversite | Karas V.,Czech Republic Astronomical Institute | Matt G.,Third University of Rome
Astrophysical Journal | Year: 2011

We discuss a model of an X-ray illuminating source above an accretion disk of a rotating black hole. Within the so-called lamp-post scheme we compute the expected (observed) polarization properties of the radiation reaching an observer. We explore the dependences on model parameters, employing Monte Carlo radiation transfer computations of the X-ray reflection on the accretion disk and taking general relativity effects into account. In particular, we discuss the role of the black hole spin, of the observer viewing angle, and of the primary X-ray source distance from the black hole. We give several examples of the resulting polarization degree for two types of exemplary objects - active galactic nuclei and Galactic black holes. In order to assess potential observability of the polarization features, we assume the sensitivity of the proposed New Hard X-ray Mission (NHXM). We examine the energy range from several keV to ∼50 keV, so the iron line complex and the Compton hump are included in our model spectra. We find the resultant polarization degree to increase at the higher end of the studied energy band, i.e., at ≳20 keV. Thus, the best results for polarimetry of reflection spectra should be achieved at the Compton hump energy region. We also obtain a higher polarization degree for large spin values of the black hole, small heights of the primary source, and low inclination angles of the observer. © 2011. The American Astronomical Society. All rights reserved.

Baty H.,11 Rue Of Luniversite | Petri J.,11 Rue Of Luniversite | Zenitani S.,Japan National Astronomical Observatory
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2013

Magnetic reconnection associated to the double tearing mode is investigated by means of resistive relativistic magnetohydrodynamic simulations. A linearly unstable double current sheet system in two-dimensional Cartesian geometry is considered. For initial perturbations of large enough longitudinal wavelengths, a fast reconnection event is triggered by a secondary instability that is structurally driven by the non-linear evolution of the magnetic islands. The latter reconnection phase and time-scale appear to weakly depend on the plasma resistivity and magnetization parameter. We discuss the possible role of such explosive reconnection dynamics to explain the MeV flares observed in the Crab Pulsar nebula. Indeed, the time-scale and the critical minimum wavelength give constraints on the Lorentz factor of the striped wind and on the location of the emission region, respectively. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Halbwachs J.-L.,11 Rue Of Luniversite | Mayor M.,Observatoire de Geneva | Udry S.,Observatoire de Geneva
Monthly Notices of the Royal Astronomical Society | Year: 2012

Wide binaries are tracers of the gravity field of the Galaxy, but their study requires some caution. A large list of common proper motion stars selected from the third Astronomischen Gesellschaft Katalog (AGK3) was monitored with the CORAVEL (for COrrelation RAdial VELocities) spectrovelocimeter, in order to prepare a sample of physical binaries with very wide separations. 66 stars received special attention, since their radial velocities (RV) seemed to be variable. These stars were monitored over several years in order to derive the elements of their spectroscopic orbits. In addition, 10 of them received accurate RV measurements from the SOPHIE spectrograph of the T193 telescope at the Observatory of Haute-Provence. For deriving the orbital elements of double-lined spectroscopic binaries (SB2), a new method was applied, which assumed that the RV of blended measurements are linear combinations of the RV of the components. 13 SB2 orbits were thus calculated. The orbital elements were eventually obtained for 52 spectroscopic binaries (SB), two of them making a triple system. 40 SB received their first orbit and the orbital elements were improved for 10 others. In addition, 11 SB were discovered with very long periods for which the orbital parameters were not found. The median period of the 40 first orbits is 1yr, and several SB should be resolved or should receive an astrometric orbit in future, providing the masses of the components. In addition, it appeared that HD153252 has a close companion, which is a candidate brown dwarf with a minimum mass of 50 Jupiter masses. The final selection of wide binaries and the derivation of their statistical properties will be presented in a second paper. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Grise F.,University of Iowa | Kaaret P.,University of Iowa | Pakull M.W.,11 Rue Of Luniversite | Motch C.,11 Rue Of Luniversite
Astrophysical Journal | Year: 2011

Holmberg IX X-1 is an archetypal ultraluminous X-ray source (ULX). Here we study the properties of the optical counterpart and of its stellar environment using optical data from SUBARU/Faint Object Camera and Spectrograph, GEMINI/GMOS-N and Hubble Space Telescope (HST)/Advanced Camera for Surveys, as well as simultaneous Chandra X-ray data. The V ∼ 22.6 spectroscopically identified optical counterpart is part of a loose cluster with an age ≲ 20 Myr. Consequently, the mass upper limit on individual stars in the association is about 20 M⊙. The counterpart is more luminous than the other stars of the association, suggesting a non-negligible optical contribution from the accretion disk. An observed UV excess also points to non-stellar light similar to X-ray active low-mass X-ray binaries. A broad He II λ4686 emission line identified in the optical spectrum of the ULX further suggests optical light from X-ray reprocessing in the accretion disk. Using stellar evolutionary tracks, we have constrained the mass of the counterpart to be ≳ 10 M⊙, even if the accretion disk contributes significantly to the optical luminosity. Comparison of the photometric properties of the counterpart with binary models show that the donor may be more massive, ≳ 25 M⊙, with the ULX system likely undergoing case AB mass transfer. Finally, the counterpart exhibits photometric variability of 0.14mag between two HST observations separated by 50 days which could be due to ellipsoidal variations and/or disk reprocessing of variable X-ray emission. © 2011. The American Astronomical Society. All rights reserved.

Trigo M.D.,ESO | Boirin L.,11 Rue Of Luniversite
Acta Polytechnica | Year: 2013

We review the current status of studies of disc atmospheres and winds in low mass X-ray binaries. We discuss the possible wind launching mechanisms and compare the predictions of the models with the existent observations. We conclude that a combination of thermal and radiative pressure (the latter being relevant at high luminosities) can explain the current observations of atmospheres and winds in both neutron star and black hole binaries. Moreover, these winds and atmospheres could contribute significantly to the broad iron emission line observed in these systems. © Czech Technical University in Prague, 2013.

Ocvirk P.,11 Rue Of Luniversite | Aubert D.,11 Rue Of Luniversite
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

We present a new semi-analytical model of the population of satellite galaxies of the Milky Way, aimed at estimating the effect of the geometry of reionization at galaxy scale on the properties of the satellites. In this model, reionization can be either (A) externally driven and uniform, or (B) internally driven by the most massive progenitor of the Milky Way. In the latter scenario, the propagation of the ionization front and photon dilution introduce a delay in the photoevaporation of the outer satellites' gas with respect to the inner satellites. As a consequence, outer satellites experience a longer period of star formation than those in the inner halo. We use simple models to account for star formation, the propagation of the ionization front, photoevaporation and observational biases. Both scenarios yield a model satellite population that matches the observed luminosity function and mass-to-light ratios. However, the predicted population for scenario (B) is significantly more extended spatially than for scenario (A), by about 0.3 dex in distance, resulting in a much better match to the observations. The survival of the signature left by the local ultraviolet field during reionization on the radial distribution of satellites makes it a promising tool for studying the reionization epoch at galaxy scale in the Milky Way and nearby galaxies resolved in stars with forthcoming large surveys. However, more work is needed to determine how the effect reported here can be disentangled from that of cosmic variance between different realizations of Milky Way haloes. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Petri J.,11 Rue Of Luniversite
Astrophysics and Space Science | Year: 2010

Neutron stars are the densest objects known in the Universe. Being the final product of stellar evolution, their internal composition and structure is rather poorly constrained by measurements. It is the purpose of this paper to put some constrains on the mass and moment of inertia of neutron stars based on the interpretation of kHz quasi-periodic oscillations observed in low mass X-ray binaries. We use observations of high-frequency quasi-periodic observations (HF-QPOs) in low mass X-ray binaries (LMXBs) to look for the average mass and moment of inertia of neutron stars. This is done by applying our parametric resonance model to discriminate between slow and fast rotators. We fit our model to data from ten LMXBs for which HF-QPOs have been seen and the spin of the enclosed accreting neutron star is known. For a simplified analysis we assume that all neutron stars possess the same properties (same mass M* and same moment of inertia I*). We find an average mass M* ≈ 2.0-2.2M⊙. The corresponding average moment of inertia is then I* ≈ 1-3 × 1038 kgm2 ≈ 0.5-1.5 (10 km)2 M⊙ which equals to dimensionless spin parameter ã ≈ 0.05-0.15 for slow rotators (neutron stars with a spin frequency roughly about 300 Hz) respectively ã ≈ 0.1-0.3 for fast rotators (neutron stars with the spin frequency roughly about 600 Hz). © Springer Science+Business Media B.V. 2010.

Goosmann R.W.,11 Rue Of Luniversite | Matt G.,Third University of Rome
Monthly Notices of the Royal Astronomical Society | Year: 2011

We model the expected X-ray polarization induced by complex reprocessing in the active nucleus of the Seyfert 2 galaxy NGC 1068. Recent analysis of infrared interferometry observations suggests that the ionized outflows ejected by the central engine are not aligned with the symmetry axis of the obscuring torus. This conclusion was obtained by extrapolating the apparent orientation of the narrow-line region to the inner parts of the ionization cones. We show that future measurements of the soft X-ray polarization vector unambiguously determine the orientation of the ionization cones. Furthermore, X-ray polarimetry across a broad photon energy range may independently verify the misalignment between the ionization cones and the axis of the torus. To model the expected polarization percentage and position angle, we apply the radiative transfer code stokes. Reprocessing of the primary X-ray radiation takes place in the accretion disc, the surrounding equatorial torus and the inclined, ionized outflows. We also examine additional equatorial scattering occurring in between the accretion disc and the inner surfaces of the torus. Radiative coupling between the different reprocessing components is computed coherently. The resulting polarization properties depend on the optical depth of the reprocessing regions and on the viewing angle of the observer. We show that even under unfavourable conditions the misalignment of the outflows with respect to the torus axis can be determined from a rotation of the polarization position angle between softer and harder X-rays. We argue that the misalignment of the outflows with respect to the torus axis in NGC 1068 may be constrained by a future X-ray mission if equipped with a broad-band polarimeter. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Mouhcine M.,Liverpool John Moores University | Ibata R.,11 Rue Of Luniversite | Rejkuba M.,European Southern Observatory
Astrophysical Journal Letters | Year: 2010

Recent panoramic observations of the dominant spiral galaxies of the Local Group have revolutionized our view of how these galaxies assemble their mass. However, it remains completely unclear whether the properties of the outer regions of the Local Group large spirals are typical. Here, we present the first panoramic view of a spiral galaxy beyond the Local Group, based on the largest, contiguous, ground-based imaging survey to date resolving the stellar halo of the nearest prime analog of the Milky Way, NGC 891 (D ≈ 10 Mpc). The low surface brightness outskirts of this galaxy are populated by multiple, coherent, and vast substructures over the ∼ 90 kpc × ∼ 90 kpc extent of the survey. These include a giant stream, the first to be resolved into stars beyond the Local Group using ground-based facilities, that loops around the parent galaxy up to distances of ∼ 50 kpc. The bulge and the disk of the galaxy are found to be surrounded by a previously undetected large, flat, and thick cocoon-like stellar structure at vertical and radial distances of up to ∼ 15 kpc and ∼ 40 kpc, respectively. © 2010. The American Astronomical Society.

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