The clumpy absorber in the high-mass X-ray binary Vela X-1

Abstract

Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase ∼0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium and neon whose strengths and presence depend on the overall level of absorption. They imply a co-existence of cool and hot gas phases in the system that we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.

Spectral evidence of an accretion disc in wind-fed X-ray pulsar Vela X-1 during an unusual spin-up period

“…As shown in previous studies (e.g. Schulz et al 2002;Watanabe et al 2006;Grinberg et al 2017), the low-energy spectrum of Vela X-1 is composed of many photoionized lines and neutral fluorescence lines, indicating the coexistence of the cool and hot gas around the neutron star. We limit the fitting energy range to 3-10 keV to avoid the complex low-energy lines.…”

A Comprehensive Chandra Study of the Disk Wind in the Black Hole Candidate 4U 1630-472

“…Physically, the increased dynamical broadening of the re-emission, lower ionization, and lower absorbing column would result filling factors of f = 0.024 +0.029 . This high degree of clumpiness would likely result in variability that is not observed in the lightcurves of any of our observations, as in the case of the highly clumpy stellar winds in high-mass X-ray binaries such as Cygnus X-1 (Hanke et al 2009;Grinberg et al 2015;Miškovičová et al 2016) and Vela X-1 (Grinberg et al 2017). Although we cannot ultimately rule out the possibility of very clumpy yet homogeneous structure, as could be the case in the cold and partially neutral gas in the BLR and/or tori of AGN, it is unlikely for such a small filling factor to occur in either a hot thermal wind (T > 1 keV) or highly ion- ized magnetic wind without some additional instability to drive this highly specific type of clumpiness.…”

Chandra reveals a possible ultrafast outflow in the super-Eddington Be/X-ray binary Swift J0243.6+6124

“…Similarly, the Ne X and Mg XII Lyα emission lines at respectively 12.15Å and 8.4Å, often observed in neutron star HMXBs (e.g. Grinberg et al 2017;Koliopanos & Vasilopoulos 2018;La Palombara et al 2016), are visible in the spectra even by eye (c.f. the top panel of Figure 3).…”

“…However, no hints for absorbtion features or an outflow are present. Even further down in the sub-Eddington regime, Grinberg et al (2017) report a plethora of rest emission lines in Vela X-1, including the Fe, Si, Mg, and Ne species identified in Sw J0243. But again, no outflow is detected, despite the high-quality observations which would likely reveal an outflow similar to the one we possibly detect in Sw J0243.…”

“…High-resolution X-ray observations of both super-Eddington X-ray binaries (Pinto et al 2016;Koliopanos & Vasilopoulos 2018) and BeXRBs at lower accretion rates (La Palombara et al 2016;Grinberg et al 2017) typically reveal emission lines at rest . Indeed, six out of seven emission lines detected by our line-search algorithm can be straightforwardly identified as such.…”