Polarized emission from strongly magnetized sources

Anomalous X-ray pulsars (AXPs) and Soft gamma repeaters (SGRs) form together a single class of astrophysical sources, commonly associated to magnetars. New-generation X-ray polarimeters will play a key role in assessing the nature of these sources by directly probing the star magnetic field. In the highly magnetized environment radiation is expected to be strongly polarized and such a measure will be easily within reach of IXPE and eXTP. Polarization measurements will eventually confirm the presence of ultra-strong magnetic fields, probing the magnetar scenario. In this work we will discuss theoretical expectations for the polarization signature of AXPs and SGRs and present numerical simulations for the detector response of the polarimeters currently under construction. We will also show how these sources can be used to test vacuum birefringence, a QED effect predicted by Heisemberg and Euler in the Thirties and not experimentally verified as yet

Vertical Structure of the Outer Accretion Disk in Persistent Low-Mass X-Ray Binaries

We have investigated the influence of X-ray irradiation on the vertical structure of the outer accretion disk in low-mass X-ray binaries by performing a self-consistent calculation of the vertical structure and X-ray radiation transfer in the disk. Penetrating deep into the disk, the field of scattered X-ray photons with energy $E\gtrsim10$\,keV exerts a significant influence on the vertical structure of the accretion disk at a distance $R\gtrsim10^{10}$\,cm from the neutron star. At a distance $R\sim10^{11}$\,cm, where the total surface density in the disk reaches $\Sigma_0\sim20$\,g\,cm$^{-2}$, X-ray heating affects all layers of an optically thick disk. The X-ray heating effect is enhanced significantly in the presence of an extended atmospheric layer with a temperature $T_{atm}\sim(2\div3)\times10^6$\,K above the accretion disk. We have derived simple analytic formulas for the disk heating by scattered X-ray photons using an approximate solution of the transfer equation by the Sobolev method. This approximation has a $\gtrsim10$\,% accuracy in the range of X-ray photon energies $E<20$\,keV.Comment: 19 pages, 8 figures, published in Astronomy Letter

Broadband X-ray spectrum of the intermediate polar V2400 Oph

We present the results of our analysis of the observations of the intermediate polar V2400 Oph by the INTEGRAL and RXTE observatories. We reconstructed the spectrum of the source over a wide (3-100 keV) energy range. The spectrum obtained can be fitted by a computed theoretical model of the post-shock emitting region with Tmax ∼ 22 keV. As a result, we estimated the mass (0.59M⊙) and radius (8.8 × 10 8 cm) of the white dwarf in the system V2400 Oph. © 2004 MAIK "Nauka/Interperiodica"

BeppoSAX observations of GRO J1744-28: Cyclotron line detection and the softening of the burst spectra

© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. We present an analysis of BeppoSAX observations of the unique transient bursting X-ray pulsar GRO J1744-28. The observations took place in 1997 March during the decay phase of the outburst. We find that the persistent broad-band X-ray continuum of the source is consistent with a cutoff power law typical for the accreting pulsars. We also detect the fluorescence iron line at 6.7 keV and an absorption feature at ~4.5 keV, which we interpret as a cyclotron line. The corresponding magnetic field strength in the line-forming region is ~3.7(1 + z) × 1011 G. Neither line is detected in the spectra of the bursts. However, an additional soft thermal component with kT ~ 2 keV was required to describe the burst spectrum. We briefly discuss the nature of this component and argue that among other possibilities it might be connected with thermonuclear flashes at the neutron star surface which accompany the accretion-powered bursts in the source

Asymptotics for a special solution to the second member of the Painleve I hierarchy

We study the asymptotic behavior of a special smooth solution y(x,t) to the second member of the Painleve I hierarchy. This solution arises in random matrix theory and in the study of Hamiltonian perturbations of hyperbolic equations. The asymptotic behavior of y(x,t) if x\to \pm\infty (for fixed t) is known and relatively simple, but it turns out to be more subtle when x and t tend to infinity simultaneously. We distinguish a region of algebraic asymptotic behavior and a region of elliptic asymptotic behavior, and we obtain rigorous asymptotics in both regions. We also discuss two critical transitional asymptotic regimes.Comment: 19 page

Optical Identification of Four Hard X-ray Sources from the Swift All-Sky Survey

We present the results of our optical identifications of four hard X-ray sources from the Swift all-sky survey. We obtained optical spectra for each of the program objects with the 6-m BTA telescope (Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz), which allowed their nature to be established. Two sources (SWIFT J2237.2+6324} and SWIFT J2341.0+7645) are shown to belong to the class of cataclysmic variables (suspected polars or intermediate polars). The measured emission line width turns out to be fairly large (FWHM ~ 15-25 A), suggesting the presence of extended, rapidly rotating (v~400-600 km/s) accretion disks in the systems. Apart from line broadening, we have detected a change in the positions of the line centroids for SWIFT J2341.0+7645, which is most likely attributable to the orbital motion of the white dwarf in the binary system. The other two program objects (SWIFT J0003.3+2737 and SWIFT J0113.8+2515) are extragalactic in origin: the first is a Seyfert 2 galaxy and the second is a blazar at redshift z=1.594. Apart from the optical spectra, we provide the X-ray spectra for all sources in the 0.6-10 keV energy band obtained from XRT/Swift data.Comment: 9 pages, 6 figures, will be published in Astronomy Letters, 38, No.5, pp.281-289 (2012

Investigation of the new cataclysmic variable 1RXS J180834.7+101041

We present the results of our photometric and spectroscopic studies of the new eclipsing cataclysmic variable star 1RXS J180834.7+101041. Its spectrum exhibits double-peaked hydrogen and helium emission lines. The Doppler maps constructed from hydrogen lines show a nonuniform distribution of emission in the disk similar to that observed in IP Peg. This suggests that the object can be a cataclysmic variable with tidal density waves in the disk. We have determined the component masses (M_WD =0.8 \pm 0.22 M_sun and M_RD =0.14 \pm 0.02 M_sun) and the binary inclination (i =78 \pm 1.5 deg) based on well-known relations between parameters for cataclysmic variable stars. We have modeled the binary light curves and showed that the model of a disk with two spots is capable of explaining the main observed features of the light curves.Comment: 22 pages, 9 figures, 2 tables, published in Astronomy Letters, 2011, 37, 845-85