V. Doroshenko

A reflection model for the cyclotron lines in the spectra of X-ray pulsars

Cyclotron resonance scattering features observed in the spectra of some X-ray pulsars show significant changes of the line energy with the pulsar luminosity. At high luminosities, these variations are often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and of the cyclotron lines. However, this scenario inevitably implies a large gradient of the magnetic field strength within the line-forming region, which makes the formation of the observed line-like features problematic. Moreover, the observed variation of the cyclotron line energy is much smaller than could be anticipated for the corresponding luminosity changes. We argue here that a more physically realistic situation is that the cyclotron line forms when the radiation emitted by the accretion column is reflected from the neutron star surface, where the gradient of the magnetic field strength is significantly smaller. Here we develop a reflection model and apply it to explain the observed variations of the cyclotron line energy in a bright X-ray pulsar V 0332+53 over a wide range of luminosities.

Outburst of GX 304-1 monitored with INTEGRAL: positive correlation between the cyclotron line energy and flux

Context. X-ray spectra of many accreting pulsars exhibit significant variations as a function of flux and thus of mass accretion rate. In some of these pulsars, the centroid energy of the cyclotron line(s), which characterizes the magnetic field strength at the site of the X-ray emission, has been found to vary systematically with flux. Aims. GX 304−1 is a recently established cyclotron line source with a line energy around 50 keV. Since 2009, the pulsar shows regular outbursts with the peak flux exceeding one Crab. We analyze the INTEGRAL observations of the source during its outburst in January−February 2012. Methods. The observations covered almost the entire outburst, allowing us to measure the source’s broad-band X-ray spectrum at different flux levels. We report on the variations in the spectral parameters with luminosity and focus on the variations in the cyclotron line. Results. The centroid energy of the line is found to be positively correlated with the luminosity. We interpret this result as a manifestation of the local sub-Eddington (sub-critical) accretion regime operating in the source.

Suzaku Observations of the HMXB 1A 1118-61

We present broadband analysis of the Be/X-ray transient 1A 1118-61 by Suzaku at the peak of its third observed outburst in 2009 January and two weeks later when the source flux had decayed by an order of magnitude. The continuum was modeled with a cutoffpl model as well as a compTT model, with both cases requiring an additional blackbody component at lower energies. We confirm the detection of a cyclotron line at {approx}55 keV and discuss the possibility of a first harmonic at {approx}110 keV. Pulse profile comparisons show a change in the profile structure at lower energies, an indication for possible changes in the accretion geometry. Phase-resolved spectroscopy in the outburst data shows a change in the continuum throughout the pulse period. The decrease in the cyclotron resonance scattering feature centroid energy also indicates that the viewing angle on the accretion column is changing throughout the pulse period.

Witnessing the magnetospheric boundary at work in Vela X−1

We present an analysis of the Vela X−1 “off-states” based on Suzaku observations taken in June 2008. Defined as states in which th e flux suddenly decreases below the instrumental sensitivity , these “off-states” have been interpreted by several authors as the onset of the “propeller regime”. For the first time ever, however, we fi nd that the source does not turn off and, although the flux drops by a factor of 20 during the three recorded “off-states”, pulsations are still observed. The spectrum and t he pulse profiles of the “o ff-states” are also presented. We discuss our findings in the framework o f the “gated accretion” scenario and conclude that most likely the residual flux is due to the accretion of matter leaking throug h the magnetosphere by means of Kelvin-Helmholz instabilities (KHI).

Is there a highly magnetized neutron star in GX 301–2?

We present the results of an in-depth study of the long-period X-ray pulsar GX 301−2. Using archival data of INTEGRAL, RXTE ASM ,a ndCGRO BATSE, we study the spectral and timing properties of the source. Comparison of our timing results with previously published work reveals a secular decay of the orbital period at a rate of �− 3.25 × 10 −5 dy r −1 , which is an order of magnitude faster than for other known systems. We argue that this is probably result either of the apsidal motion or of gravitational coupling of the matter lost by the optical companion with the neutron star, although current observations do not allow us to distinguish between those possibilities. We also propose a model to explain the observed long pulse period. We find that a very strong magnetic field B ∼ 10 14 G can explain the observed pulse period in the framework of existing models for torques affecting the neutron star. We show that the apparent contradiction with the magnetic field strength BCRSF ∼ 4 × 10 12 G derived from the observed cyclotron line position may be resolved if the line formation region resides in a tall accretion column of height ∼2.5–3 RNS. The color temperature measured from the spectrum suggests that such a column may indeed be present, and our estimates show that its height is sufficient to explain the observed cyclotron line position.

Propeller effect in two brightest transient X-ray pulsars: 4U 0115+63 and V 0332+53

We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well known X-ray pulsars 4U 0115+63 and V 0332+53 during their giant outbursts in 2015. Such transitions were detected at the threshold luminosities of

RXTE observations of the 1A 1118–61 in an outburst, and the discovery of a cyclotron line

(1.4pm0.4)times10^{36}

Supergiant, fast, but not so transient 4U 1907+09

erg s

Transient X-ray pulsar V 0332+53: pulse-phase-resolved spectroscopy and the reflection model

^{-1}

Luminosity dependence of the cyclotron line and evidence for the accretion regime transition in V 0332+53

and