Using RXTE/ASM archival data, we investigate the behaviour of the spectral hardness ratio as a function of X-ray luminosity in a sample of six transient X-ray pulsars (EXO 2030+375, GX 304−1, 4U ...0115+63, V 0332+63, A 0535+26 and MXB 0656−072). In all sources we find that the spectral hardness ratio defined as F
5−12 keV/F
1.33−3 keV increases with the ASM flux (1.33–12 keV) at low luminosities and then saturates or even slightly decreases above some critical X-ray luminosity falling into the range ∼(3–7) × 1037 erg s−1. Two-dimensional structure of accretion columns in the radiation–diffusion limit is calculated for two possible geometries (filled and hollow cylinder) for mass accretion rates
$\dot{M}$
ranging from 1017 to 1.2 × 1018 g s−1. The observed spectral behaviour in the transient X-ray pulsars with increasing
$\dot{M}$
can be reproduced by a Compton-saturated sidewall emission from optically thick magnetized accretion columns with taking into account the emission reflected from the neutron star atmosphere. At
$\dot{M}$
above some critical value
$\dot{M}_{\rm cr}\sim (6{\rm -}8)\times 10^{17}$
g s−1, the height of the column becomes such that the contribution of the reflected component to the total emission starts decreasing, which leads to the saturation and even slight decrease of the spectral hardness. Hollow-cylinder columns have a smaller height than the filled-cylinder ones, and the contribution of the reflected component in the total emission does not virtually change with
$\dot{M}$
(and hence the hardness of the continuum monotonically increases) up to higher mass accretion rates than
$\dot{M}_{\rm cr}$
for the filled columns.
Aims. We investigate the long-term evolution of the cyclotron resonance scattering feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and present evidence of a true long-term decrease ...in the centroid energy Ecyc of the cyclotron line in the pulse phase averaged spectra from 1996 to 2012. Methods. Our results are based on repeated observations of Her X-1 by those X-ray observatories capable of measuring clearly beyond the cyclotron line energy of ~40 keV; these are RXTE, INTEGRAL, Suzaku, and NuSTAR. We consider results based on our own successful observing proposals as well as results from the literature. Results. The historical evolution of the pulse phase averaged CRSF centroid energy Ecyc since its discovery in 1976 is characterized by an initial value around 35 keV, an abrupt jump upwards to beyond ~40 keV between 1990 and 1994, and an apparent decay thereafter. Much of this decay, however, was found to be due to an artifact, namely a correlation between Ecyc and the X-ray luminosity Lx discovered in 2007. In observations after 2006, however, we now find a statistically significant true decrease in the cyclotron line energy. At the same time, the dependence of Ecyc on X-ray luminosity is still valid with an increase of ~5% in energy for a factor of two increase in luminosity. We also report on the first evidence of a weak dependence of Ecyc on phase of the 35 d precessional period, which manifests itself not only in the modulation of the X-ray flux, but also in the systematic variation in the shape of the 1.24 s pulse profile. One of our motivations for repeatedly observing Her X-1, namely the suspicion that the cyclotron line energy may be gradually decreasing after its strong upward jump in the early 1990s, is finally confirmed. A decrease in Ecyc by 4.2 keV over the 16 years from 1996 to 2012 can either be modeled by a linear decay, or by a slow decay until 2006 followed by a more abrupt decrease thereafter. Conclusions. The observed timescale for the decrease in Ecyc of a few decades is too short for a decay of the global magnetic field. We speculate that the physical reason could be connected to a geometric displacement of the cyclotron resonant scattering region in the polar field or to a true physical change in the magnetic field configuration at the polar cap by the continued accretion. In the second scenario, the upward jump in Ecyc observed around 1991 may have been due to a relatively fast event in which the polar magnetic field rearranged itself after releasing part of the accumulated material to larger areas of the neutron star surface.
The flute instability at the inner edge of a thin diamagnetic accretion disk is analyzed. The magnetic field configuration model from Aly (1980) is used. We have analyzed a modified dispersion ...relation for the flute instability that takes into account the Keplerian disk rotation. We have derived the inner radius of the accretion disk within our analysis of the flute instability. We show that the inner radius does not differ from the Alfvén radius for spherical accretion to within a dimensionless coefficient, with the proportionality coefficient depending only on the turbulence alpha parameter and the relative disk thickness (
).
eROSITA (extended ROentgen Survey with an Imaging Telescope Array) instrument onboard the Russian-German ‘Spectrum-Roentgen-Gamma’ (SRG) mission observed the Her X-1/HZ Her binary system in multiple ...scans over the source during the first and second SRG all-sky surveys. Both observations occurred during a low state of the X-ray source when the outer parts of the accretion disk blocked the neutron star from view. The orbital modulation of the X-ray flux was detected during the low states. We argue that the detected X-ray radiation results from scattering of the emission of the central source by three distinct regions: (a) an optically thin hot corona with temperature ~(2−4) × 10
6
K above the irradiated hemisphere of the optical star; (b) an optically thin hot halo above the accretion disk; and (c) the optically thick cold atmosphere of the optical star. The latter region effectively scatters photons with energies above 5–6 keV.
We analyse spin-up/spin-down of the neutron star in Be-X-ray binary system GX 304-1 observed by Swift/X-ray telescope (XRT) and Fermi/gamma-ray burst monitor (GBM) instruments in the period of the ...source activity from 2010 April to 2013 January and discuss possible mechanisms of angular momentum transfer to/from the neutron star. We argue that the neutron star spin-down at quiescent states of the source with an X-ray luminosity of L
x ∼ 1034 erg s−1 between a series of Type I outbursts and spin-up during the outbursts can be explained by quasi-spherical settling accretion on to the neutron star. The outbursts occur near the neutron star periastron passages, where the density is enhanced due to the presence of an equatorial Be-disc tilted to the orbital plane. We also propose an explanation to the counterintuitive smaller spin-up rate observed at higher luminosity in a double-peak Type I outburst due to lower value of the specific angular momentum of matter captured from the quasi-spherical wind from the Be-star by the neutron star moving in an elliptical orbit with eccentricity e ≳ 0.5.
Abstract
Microlensing by the stellar population of lensing galaxies provides an important opportunity to resolve the accretion disc structure spatially in strongly lensed quasars. Disc sizes ...estimated in this way are on average larger than the predictions of the standard Shakura-Sunyaev accretion disc model. An analysis of the observational data on microlensing variability suggests that some fraction of lensed quasars (primarily smaller-mass objects) are accreting in the super-Eddington regime. Super-Eddington accretion leads to the formation of an optically thick envelope scattering the radiation formed in the disc. This makes the apparent disc size larger and practically independent of wavelength. In the framework of our model, it is possible to make self-consistent estimates of mass accretion rates and black hole masses for the cases when both amplification-corrected fluxes and radii are available.
Although the bulk of the observed optical flux from the discs of intermediate-redshift lensed quasars is formed well outside the region of strong relativistic boosting and light bending, relativistic ...effects have an important influence on microlensing curves. The reason lies in the divergent nature of amplification factors near fold caustics, which are increasingly sensitive to small spatial size details. Higher-order disc images produced by strong light bending around the black hole may affect the amplification curves, making a contribution of up to several per cent near maximum amplification. In accordance with theoretical predictions, some of the observed high-amplification events possess fine structure. Here we consider three putative caustic-crossing events, one by SBS J1520+530 and two events for individual images of Einstein's cross (QSO J2237+0305). Using relativistic disc models allows us to improve the fits but the required inclinations are high,
. Such high inclinations apparently contradict the absence of any strong absorption that is likely to arise if a disc is observed edge-on through a dust torus. Still, high inclinations are required only for the central parts of the disc, which allows the disc itself initially to be tilted by 60-90° with respect to the black hole and aligned toward the black hole equatorial plane near the last stable orbit radius. For SBS J1520+530, an alternative explanation for the observed amplification curve is a superposition of two subsequent fold-caustic crossings. While relativistic disc models favour black hole masses ∼1010 M⊙ (several times higher than the virial estimates) or small Eddington ratios, this model is consistent with the observed distribution of galaxies over peculiar velocities only if the black hole mass is
.
Aims. We present the results of ten years of repeated measurements of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and report the discovery of ...a positive correlation of the centroid energy of this absorption feature in pulse phase averaged spectra with source luminosity. Methods. Our results are based on a uniform analysis of observations by the RXTE satellite from 1996 to 2005, using sufficiently long observations of 12 individual 35-day Main-On states of the source. Results. The mean centroid energy E_{\rm c} of the CRSF in pulse phase averaged spectra of Her X-1 during this time is around 40 keV, with significant variations from one Main-On state to the next. We find that the centroid energy of the CRSF in Her X-1 changes by similar to 5% in energy for a factor of 2 in luminosity. The correlation is positive, contrary to what is observed in some high luminosity transient pulsars. Conclusions. Our finding is the first significant measurement of a positive correlation between E_{\rm c} and luminosity in any X-ray pulsar. We suggest that this behaviour is expected in the case of sub-Eddington accretion and present a calculation of a quantitative estimate, which is very consistent with the effect observed in Her X-1. We urge that Her X-1 is regularly monitored further and that other X-ray pulsars are investigated for a similar behaviour.
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