X-ray variation statistics and wind clumping in Vela X-1

Abstract

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 <i>INTEGRAL<i/> 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 <i>RXTE<i/> ASM data suggests a strong orbital change of <i>N<i/><sub>H<sub/>. Accreted clump masses derived from the <i>INTEGRAL<i/> data are on the order of 5 × 10<sup>19<sup/> – 10<sup>21<sup/> 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.