Spectrum formation in X-ray pulsars at very low mass accretion rate: Monte Carlo approach

Large scale structure and cosmolog

Mean opacities of a strongly magnetized high-temperature plasma

Galaxie

Constraints on the magnetic field structure in accreting compact objects from aperiodic variability

Galaxie

Statistical features of multiple Compton scattering in a strong magnetic field

Galaxie

The remarkable outburst of the highly-evolved post period-minimum dwarf nova SSS J122221.7−311525

We report extensive 3-yr multiwavelength observations of the WZ Sge-type dwarf nova SSS J122221.7−311525 during its unusual double superoutburst, the following decline and in quiescence. The second segment of the superoutburst had a long duration of 33 d and a very gentle decline with a rate of 0.02 mag d−1, and it displayed an extended post-outburst decline lasting at least 500 d. Simultaneously with the start of the rapid fading from the superoutburst plateau, the system showed the appearance of a strong near-infrared excess resulting in very red colours, which reached extreme values (B − I ≃ 1.4) about 20 d later. The colours then became bluer again, but it took at least 250 d to acquire a stable level. Superhumps were clearly visible in the light curve from our very first time-resolved observations until at least 420 d after the rapid fading from the superoutburst. The spectroscopic and photometric data revealed an orbital period of 109.80 min and a fractional superhump period excess ≲0.8 per cent, indicating a very low mass ratio q ≲ 0.045. With such a small mass ratio the donor mass should be below the hydrogen-burning minimum mass limit. The observed infrared flux in quiescence is indeed much lower than is expected from a cataclysmic variable with a near-main-sequence donor star. This strongly suggests a brown-dwarf-like nature for the donor and that SSS J122221.7−311525 has already evolved away from the period minimum towards longer periods, with the donor now extremely dim

The x-ray polarimetry view of the accreting pulsar Cen X-3

Galaxie

Testing the equation of state of neutron stars with electromagnetic observations

Neutron stars are the densest, directly observable stellar objects in the universe and serve as unique astrophysical laboratories to study the behavior of matter under extreme physical conditions. This book chapter is devoted to describing how electromagnetic observations, particularly at X-ray, optical and radio wavelengths, can be used to measure the mass and radius of neutron stars and how this leads to constraints on the equation of state of ultra-dense matter. Having accurate theoretical models to describe the astrophysical data is essential in this effort. We will review different methods to constrain neutron star masses and radii, discuss the main observational results and theoretical developments achieved over the past decade, and provide an outlook of how further progress can be made with new and upcoming ground-based and space-based observatories.Comment: To appear as a chapter in the White Book of "NewCompStar" European COST Action MP1304. Accepted by the editorial board in February 2018; 61 pages, 17 figures, 1 tabl

Statistics of white dwarf properties in intermediate polars

© The Author(s), 2020. Many intermediate polars are hard X-ray sources. The theory of their hard X-ray radiation is well developed and allows us to determine white dwarf masse in this kind of cataclysmic variables. Here we present the results of determination of the masses of 35 white dwarfs in the intermediate polars observed by observatories NuSTAR (10 sources) and Swift/BAT (25 sources). The corresponding mass accrerion rates and the luminosity function were also derived due to accurate distances to the sources well known now after Gaia DR2

X-ray polarimetry of the accreting pulsar GX 301−2

The phase- and energy-resolved polarization measurements of accreting X-ray pulsars (XRPs) allow us to test different theoretical models of their emission, and they also provide an avenue to determine the emission region geometry. We present the results of the observations of the XRP GX 301−2 performed with the Imaging X-ray Polarimetry Explorer (IXPE). A persistent XRP, GX 301−2 has one of the longest spin periods known: ∼680 s. A massive hyper-giant companion star Wray 977 supplies mass to the neutron star via powerful stellar winds. We did not detect significant polarization in the phase-averaged data when using spectro-polarimetric analysis, with the upper limit on the polarization degree (PD) of 2.399. Using the phase-resolved spectro-polarimetric analysis, we obtained a significant detection of polarization (above 99 in two out of nine phase bins and a marginal detection in three bins, with a PD ranging between ∼3100° to ∼160°. Using the rotating vector model, we obtained constraints on the pulsar geometry using both phase-binned and unbinned analyses, finding excellent agreement. Finally, we discuss possible reasons for a low observed polarization in GX 301−2

Observational appearance of rapidly rotating neutron stars: X-ray bursts, cooling tail method, and radius determination

© ESO 2020. Neutron stars (NSs) in low-mass X-ray binaries rotate at frequencies high enough to significantly deviate from sphericity (ν∗ ∼200-600 Hz). First, we investigate the effects of rapid rotation on the observational appearance of a NS. We propose analytical formulae relating gravitational mass and equatorial radius of the rapidly rotating NS to the mass M and radius R of a non-rotating NS of the same baryonic mass using accurate fully relativistic computations. We assume that the NS surface emission is described by the Planck function with two different emission patterns: the isotropic intensity and that corresponding to the electron-scattering dominated atmosphere. For these two cases we compute spectra from an oblate rotating NS observed at different inclination angles using the modified oblate Schwarzschild approximation, where light bending is computed in Schwarzschild metric, but frame dragging and quadrupole moment of a NS are approximately accounted for in the photon redshift calculations. In particular, we determine the solid angle at which a rotating NS is seen by a distant observer, the observed colour temperature and the blackbody normalization. Then, we investigate how rapid rotation affects the results of NS radius determination using the cooling tail method applied to the X-ray burst spectral evolution. We approximate the local spectra from the NS surface by a diluted blackbody with the luminosity-dependent dilution factor using previously computed NS atmosphere models. We then generalize the cooling tail method to the case of a rapidly rotating NS to obtain the most probable values of M and R of the corresponding non-rotating NS with the same baryonic mass. We show that the NS radius could be overestimated by 3-3.5 km for face-on stars of R ≈ 11 km rotating at ν∗= 700 Hz if the version of the cooling tail method for a non-rotating NS is used. We apply the method to an X-ray burst observed from the NS rotating at ν∗ ≈ 532 Hz in SAX J1810.8-2609. The resulting radius of the non-rotating NS (assuming M = 1.5 M) becomes 11.8 ± 0.5 km if it is viewed at inclination i = 60° and R = 11.2 ± 0.5 km for a face-on view, which are smaller by 0.6 and 1.2 km than the radius obtained using standard cooling tail method ignoring rotation. The corresponding equatorial radii of these rapidly rotating NSs are 12.3 ± 0.6 km (for i = 60°) and 11.6 ± 0.6 km (for i = 0°)