D. Klochkov

Spectral formation in accreting X-ray pulsars: bimodal variation of the cyclotron energy with luminosity

Context. Accretion-powered X-ray pulsars exhibit significant variability of the cyclotron resonance scattering feature (CRSF) centroid energy on pulse-to-pulse timescales, and also on much longer timescales. Two types of spectral variability are observed. For sources in group 1, the CRSF energy is negatively correlated with the variable source luminosity, and for sources in group 2, the opposite behavior is observed. The physical basis for this bimodal behavior is currently not well understood. Aims. We explore the hypothesis that the accretion dynamics in the group 1 sources is dominated by radiation pressure near the stellar surface, and that Coulomb interactions decelerate the gas to rest in the group 2 sources. Methods. We derive a new expression for the critical luminosity, Lcrit, such that radiation pressure decelerates the matter to rest in sources with X-ray luminosity LX > Lcrit. The formula for Lcrit is based on a simple physical model for the structure of the accretion column in luminous X-ray pulsars that takes into account radiative deceleration, the energy dependence of the cyclotron cross section, the thermodynamics of the accreting gas, the dipole structure of the pulsar magnetosphere, and the diffusive escape of radiation through the column walls. We show that for typical neutron star parameters, Lcrit = 1.5 × 10 37 B 16/15 12 erg s −1 ,w hereB12 is the surface magnetic field strength in units of 10 12 G. Results. The formula for the critical luminosity is evaluated for five sources, using the maximum value of the CRSF centroid energy to estimate the surface magnetic field strength B12. The results confirm that the group 1 sources are supercritical (LX > Lcrit )a nd the group 2 sources are subcritical (LX < Lcrit), although the situation is less clear for those highly variable sources that cross over the line LX = Lcrit. We also explain the variation of the CRSF energy with luminosity as a consequence of the variation of the characteristic emission height. The sign of this dependence is opposite in the supercritical and subcritical cases, hence creating the observed bimodal behavior. Conclusions. We have developed a new model for the critical luminosity in accretion-powered X-ray pulsars that explains the bimodal dependence of the CRSF centroid energy on the X-ray luminosity LX. Our model provides a physical basis for the observed variation of the CRSF energy as a function of LX for both the group 1 (supercritical) and the group 2 (subcritical) sources as a result of the variation of the emission height in the column.

Discovery of a flux-related change of the cyclotron line energy in Hercules X-1

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 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 ~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 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.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.Our results are based on a uniform analysis of observations bythe RXTE satellite from 1996 to 2005, using sufficiently long observations of 12 individual 35-day Main-On states of the source. The mean centroid energy E_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 ~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. Our finding is the first significant measurement of a positive correlation between E_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.

X-Ray Variation Statistics and Wind Clumping in Vela X-1

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 seconds, 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 10 19 ‐10 21 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.

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.

Pulse-amplitude-resolved spectroscopy of bright accreting pulsars: indication of two accretion regimes

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.

The neutron star in HESS J1731−347: Central compact objects as laboratories to study the equation of state of superdense matter

Context: Central Compact Objects (CCOs) in supernova remnants are isolated thermally emitting neutron stars (NSs). They are most probably characterized by a magnetic field strength that is roughly two orders of magnitude lower than that of most of the radio and accreting pulsars. The thermal emission of CCOs can be modeled to obtain constraints on the physical parameters of the star such as its mass, radius, effective temperature, and chemical composition. Aims: The CCO in HESS, J1731-347 is one of the brightest objects of this class. We present our analysis of two new XMM-Newton observations of the source which increase the total exposure time of the by a factor of five compared to the analyses presented before. Methods: We use our numerical spectral models for carbon and hydrogen atmospheres to fit the spectrum of the CCO. From our fits, we derive constraints on the physical parameters of the emitting star. We also use the new data to derive new upper limits on pulsations and to confirm the absence of a long-term variability. Results: The analysis shows that atmosphere spectral models are clearly preferred by the fit over the blackbody function. Under the assumption that the X-ray emission is uniformly produced by the entire star surface, hydrogen atmosphere models lead to uncomfortably large distances of the CCO, above 7-8 kpc. On the other hand, the carbon atmosphere model formally excludes distances above 5-6 kpc and is compatible with the source located in the Scutum-Crux (3 kpc) or Norma-Cygnus (4.5 kpc) Galactic spiral arm. We provide and discuss the corresponding confidence contours in the NS mass-radius plane. The measured effective temperature indicates that the NS is exceptionally hot for the estimated age of 30 kyr. We discuss possible cooling scenarios to explain this property, as well as possible additional constraints on the star mass and radius from cooling theory.

INTEGRAL observations of Hercules X-1

Aims. We investigate the X-ray spectral and timing properties of the accreting X-ray pulsar Her X-1 observed with the INTEGRAL satellite in July–August 2005. Methods. The data analyzed in this work cover a substantial part of one main-on state of the source. The short time scale pulse period development is measured. X-ray pulse profiles for different energy ranges and time intervals are constructed. Pulse-averaged and pulse-phase resolved broad band X-ray spectra are studied. Spectral changes during X-ray dips are explored. Results. The X-ray pulse profiles are found to change significantly during the period of observations. For the first time a strong spinup is measured within one 35 d cycle. Spectral characteristics observed during the X-ray dips are consistent with their interpretation as being due to partial covering as has been reported by several authors. The fundamental cyclotron absorption line is firmly observed in both pulse-averaged and pulse-phase resolved X-ray spectra. The energy, width, and the depth of the line are found to vary significantly with pulse phase.

THE SMOOTH CYCLOTRON LINE IN HER X-1 AS SEEN WITH NUCLEAR SPECTROSCOPIC TELESCOPE ARRAY

Her X-1, one of the brightest and best studied X-ray binaries, shows a cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an ideal target for a detailed study with the Nuclear Spectroscopic Telescope Array (NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We observed Her X-1 three times, coordinated with Suzaku, during one of the high flux intervals of its 35 day superorbital period. This paper focuses on the shape and evolution of the hard X-ray spectrum. The broadband spectra can be fitted with a power law with a high-energy cutoff, an iron line, and a CRSF. We find that the CRSF has a very smooth and symmetric shape in all observations and at all pulse phases. We compare the residuals of a line with a Gaussian optical-depth profile to a Lorentzian optical-depth profile and find no significant differences, strongly constraining the very smooth shape of the line. Even though the line energy changes dramatically with pulse phase, we find that its smooth shape does not. Additionally, our data show that the continuum only changes marginally between the three observations. These changes can be explained with varying amounts of Thomson scattering in the hot corona of the accretion disk. The average, luminosity-corrected CRSF energy is lower than in past observations and follows a secular decline. The excellent data quality of NuSTAR provides the best constraint on the CRSF energy to date.

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.

The pre-outburst flare of the A 0535+26 August/September 2005 outburst

Aims. We study the spectral and temporal behavior of the High Mass X -ray Binary A 0535+26 during a ‘pre-outburst flare’ which took place∼ 5 d before the peak of a normal (type I) outburst in August /September 2005. We compare the studied behavior with that observed during the outburst. Methods. We analyseRXTE observations that monitored A 0535 +26 during the outburst. We complete spectral and timing anal yses of the data. We study the evolution of the pulse period, prese nt energy-dependent pulse profiles both at the initial pre-o utburst flare and close to outburst maximum, and measure how the cyclotron resonance-scattering feature (hereafter CRSF) evolves. Results. We present three main results: a constant period P=103.3960(5) s is measured until periastron passage, followed by a spin-up with a decreasing period derivative of Ṗ=(−1.69±0.04)×10−8s s−1 at MJD 53618, andP remains constant again at the end of the main outburst. The spin-up provides evidence for the existence o f an accretion disk during the normal outburst. We measure a C RSF energy of Ecyc∼50 keV during the pre-outburst flare, and Ecyc∼46 keV during the main outburst. The pulse shape, which varie s significantly during both pre-outburst flare and main outburst, evolves st rongly with photon energy.Aims. We study the spectral and temporal behavior of the High Mass X-ray Binary A 0535+26 during a “pre-outburst flare” which took place ∼5 d before the peak of a normal (type I) outburst in August/September 2005. We compare the studied behavior with that observed during the outburst. Methods. We analyse RXTE observations that monitored A 0535+26 during the outburst. We complete spectral and timing analyses of the data. We study the evolution of the pulse period, present energy-dependent pulse profiles both at the initial pre-outburst flare and close to outburst maximum, and measure how the cyclotron resonance-scattering feature (hereafter CRSF) evolves. Results. We present three main results: a constant period P = 103.3960(5) s is measured until periastron passage, followed by a spin-up with a decreasing period derivative of u P = (−1.69 ± 0.04) × 10 −8 ss −1 at MJD 53 618, and P remains constant again at the end of the main outburst. The spin-up provides evidence for the existence of an accretion disk during the normal outburst. We measure a CRSF energy of Ecyc ∼ 50 keV during the pre-outburst flare, and Ecyc ∼ 46 keV during the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.