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

¹ ESA European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
e-mail: vgrinberg@cosmos.esa.int
² Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
³ Centre for Mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
⁴ Villanova University, Department of Physics, Villanova, PA 19085, USA
⁵ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
⁶ CRESST, Department of Physics, and Center for Space Science and Technology, UMBC, Baltimore, MD 21250, USA
⁷ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ ESA European Space Astronomy Centre (ESAC), Science Operations Departement, 28692 Villanueva de la Cañada, Madrid, Spain
⁹ Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, Cambridge, MA 02139, USA
¹⁰ Dr. Karl Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics (ECAP), Universität Erlangen-Nürnberg, Sternwartstrasse 7, 96049 Bamberg, Germany
¹¹ Instituto de Física de Cantabria (CSIC-Universidad de Cantabria), 39005 Santander, Spain
¹² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China

Received: 28 August 2017
Accepted: 17 November 2017

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. These features imply a co-existence of cool and hot gas phases in the system, which 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.