We investigate the structure of the wind in the neutron star X-ray binary system Vela X-1 by analyzing its flaring behavior. Vela X-1 shows constant flaring, with some flares reaching fluxes of more ...than 3.0 Crab between 20–60 keV for several 100 s, while the average flux is around 250 mCrab. We analyzed all archival INTEGRAL data, calculating the brightness distribution in the 20–60 keV band, which, as we show, closely follows a log-normal distribution. Orbital resolved analysis shows that the structure is strongly variable, explainable by shocks and a fluctuating accretion wake. Analysis of RXTE ASM data suggests a strong orbital change of NH. Accreted clump masses derived from the INTEGRAL data are on the order of 5 × 1019 – 1021 g. We show that the lightcurve can be described with a model of multiplicative random numbers. In the course of the simulation we calculate the power spectral density of the system in the 20–100 keV energy band and show that it follows a red-noise power law. We suggest that a mixture of a clumpy wind, shocks, and turbulence can explain the measured mass distribution. As the recently discovered class of supergiant fast X-ray transients (SFXT) seems to show the same parameters for the wind, the link between persistent HMXB like Vela X-1 and SFXT is further strengthened.
We have investigated the dipping activity observed in the high-mass X-ray binary 4U 1907+09 and shown that the source continues to pulsate in the “off” state, noting that the transition between the ...“on” and “off” states may be either dip-like or flare-like. This behavior may be explained in the framework of the “gated accretion” scenario proposed to explain the flares in supergiant fast X-ray transients (SFXTs). We conclude that 4U 1907 + 09 might prove to be a missing link between the SFXTs and ordinary accreting pulsars.
Context. In addition to coherent pulsation, many accreting neutron stars exhibit flaring activity and strong aperiodic variability on time scales comparable to or shorter than their pulsation period. ...This behavior shows that the accretion flow in the vicinity of the accretor must be highly non-stationary. Observational study of this phenomenon is often problematic because it requires very high statistics of X-ray data and a specific analysis technique. Aims. In our research we used high-resolution data taken with RXTE and INTEGRAL on a sample of bright transient and persistent pulsars to perform an in-depth study of their variability on time scales comparable to the pulsation period – ”pulse-to-pulse variability”. Methods. The high-quality data allowed us to collect individual pulses of different amplitude and explore their X-ray spectrum as a function of pulse amplitude. The described approach allowed us for the first time to study the luminosity dependence of pulsars’ X-ray spectra in observations where the averaged (over many pulse cycles) luminosity of the source remains constant. Results. In all studied pulsars we reveal significant spectral changes as a function of the pulse height both in the continuum and in the cyclotron absorption features. The sources appear to form two groups that show different dependencies of the spectrum on pulse height. We interpret this division as a manifestation of two distinct accretion regimes that are at work in different pulsars.
Context. High-mass X-ray binaries are among the brightest objects of our Galaxy in the high-energy domain (0.1-100 keV). Despite our relatively good knowledge of their basic emission mechanisms, the ...complex problem of understanding their time- and energy- dependent X-ray emission has not been completely solved. Aims. In this paper, we study the energy-dependent pulse profiles of the high-mass X-ray binary pulsar 4U 0115 + 63 to investigate how they are affected by cyclotron resonant scattering. Methods. We analyzed archival BeppoSAX and RXTE observations performed during the giant outburst of the source that occurred in 1999. We exploited a cross correlation technique to compare the pulse profiles in different energy ranges and developed a relativistic ray-tracing model to interpret our findings. We also studied the phase dependency of the cyclotron absorption features by performing phase-resolved spectroscopy. Results. The pulse profiles of 4U 0115 + 63 displayed clear “phase-lags” at energies close to those of the cyclotron absorption features that characterize the X-ray emission of the source. We reproduce this phenomenon qualitatively by assuming an energy-dependent beaming of the emission from the column surface and verify that our model is also compatible with the results of phase-resolved spectral analysis. Conclusions. We show that cyclotron resonant scattering affects the pulse profile formation mechanisms in a complex way, which necessitates both improvements in the modeling and the study of other sources to be better understood.
Context. Accretion-powered X-ray pulsars show highly energy-dependent and complex pulse-profile morphologies. Significant deviations from the average pulse profile can appear, in particular close to ...the cyclotron line energies. These deviations can be described as energy-dependent phase lags, that is, as energy-dependent shifts of main features in the pulse profile. Aims. Using a numerical study we explore the effect of cyclotron resonant scattering on observable, energy-resolved pulse profiles. Methods. We generated the observable emission as a function of spin phase, using Monte Carlo simulations for cyclotron resonant scattering and a numerical ray-tracing routine accounting for general relativistic light-bending effects on the intrinsic emission from the accretion columns. Results. We find strong changes in the pulse profile coincident with the cyclotron line energies. Features in the pulse profile vary strongly with respect to the average pulse profile with the observing geometry and shift and smear out in energy additionally when assuming a non-static plasma. Conclusions. We demonstrate how phase lags at the cyclotron energies arise as a consequence of the effects of angular redistribution of X-rays by cyclotron resonance scattering in a strong magnetic field combined with relativistic effects. We also show that phase lags are strongly dependent on the accretion geometry. These intrinsic effects will in principle allow us to constrain a system’s accretion geometry.
Aims. We performed a detailed pulse phase resolved spectroscopy of the accreting binary X-ray pulsar Her X-1 in the energy range 3.5–75 keV and have established pulse phase profiles for all spectral ...parameters. For three parameters, the centroid energy of the cyclotron line, the photon index and the flux of the 6.4 keV iron line, we have studied the variation as a function of 35 d phase. Methods. We analyzed RXTE observations of the Main-On of November 2002 using data from the PCA and the HEXTE instruments. Four different time intervals of about ~1 d duration were selected to provide a good coverage of a complete Main-On. The intervals are centered at 35 d phases 0.03, 0.10, 0.15, and 0.20, respectively. Results. All spectral parameters show a strong modulation with pulse phase. While the centroid energy of the cyclotron line follows roughly the shape of the pulse profile, showing higher values close to the peak of the X-ray pulse, both the photon index and the iron line intensity exhibit distinct minima around the peak of the X-ray pulse. With respect to variations of the observed profiles with 35 d phase, we find that there is a clear evolution of the shape of the pulse profiles (flux versus pulse phase), a moderate increase of the maximum cyclotron line energy (found around pulse phase 0.7), but no significant evolution of the shape of the pulse phase profiles of the cyclotron line energy, the spectral power law index or the iron line intensity. Conclusions. The variation of spectral parameters as a function of the pulse phase provides important information about the system: i. the disappearance of the Fe line flux near the highest continuum flux may be an indication of a hollow cone geometry of the accretion structure; ii. the apparent non-dependence of the cyclotron line energy profiles on 35 d phase provides a new possibility to test the model of free precession of the neutron star, proposed to be responsible for the systematic variations in the pulse profiles.
Context. Be/X-ray binaries (BeXRBs) are the most populous class of high-mass X-ray binaries. Their X-ray duty cycle is tightly related to the optical companion wind activity, which in turn can be ...studied through dedicated optical spectroscopic observations. Aims. We study optical spectral features of the Be circumstellar disk to test their long-term variability and their relation with the X-ray activity. Special attention has been given to the Hα emission line, one of the best tracers of the disk conditions. Methods. We obtained optical broadband medium resolution spectra from a dedicated campaign with the Anglo-Australian Telescope and the Southern African Large Telescope in 2014−2015. Data span over one entire binary orbit, and cover both X-ray quiescent and moderately active periods. We used Balmer emission lines to follow the evolution of the circumstellar disk. Results. We observe prominent spectral features, like double-peaked Hα and Hβ emission lines. The HαV/R ratio significantly changes over a timescale of about one year. Our observations are consistent with a system observed at a large inclination angle (i ≳ 60°). The derived circumstellar disk size shows that the disk evolves from a configuration that prevents accretion onto the neutron star, to one that allows only moderate accretion. This is in agreement with the contemporary observed X-ray activity. Our results are interpreted within the context of inefficient tidal truncation of the circumstellar disk, as expected for this source’s binary configuration. We derived the Hβ-emitting region size, which is equal to about half of the corresponding Hα-emitting disk, and constrain the luminosity class of V850 Cen as III–V, consistent with the previously proposed class.
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, and CGRO 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 $\simeq$ - 3.25 × 10-5 d yr-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 ~ 1014 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 × 1012 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.
We present an update of the orbital ephemeris of the binary X-ray pulsar Her X-1 and determine an improved value for the rate of orbital decay. In addition, we report the first measurement of the ...orbital eccentricity. We analyzed pulse timing data of Her X-1 from X-ray observations by RXTE (Rossi X-Ray Timing Explorer) and INTEGRAL over the period 1996–2007. Accurate pulse arrival times were determined from solar system bary-centered photon arrival times by generating pulse profiles averaged over appropriately short integration times. Applying pulse phase connection techniques, it was possible to determine sufficiently accurate local ephemeris data for seven observation periods distributed over 12 years. Combining the new local $T_{\pi/2}$ values with historical values from the literature we update the orbital ephemeris of Her X-1 to $T_{\pi/2}$ = MJD 46 359.871940(6) and Porb = 1.700167590(2) d and measure a continuous change of the orbital period of ${\rm d}P_{\rm orb}/{\rm d}t = -(4.85 \pm 0.13) \times 10^{-11}\,{\rm s\,s^{-1}}$. For the first time, a value for the eccentricity of the orbit of Her X-1 has been measured: $\epsilon = (4.2 \pm 0.8) \times 10^{-4}$.
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.
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