Footprints in the wind of Vela X–1 traced with MAXI^⋆

¹ Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany
e-mail: doroshv@astro.uni-tuebingen.de
² MAXI team, RIKEN, 2-1 Hirosawa, Wako, 351-0198 Saitama, Japan
³ School of Dentistry at Matsudo, Nihon University, 2-870-1, Sakaecho-nishi, Matsudo, 271-8587 Chiba, Japan
⁴ ISS Science Project Office, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba, 305-8505 Ibaraki, Japan
⁵ Department of Physics, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan

Received: 16 February 2013
Accepted: 21 April 2013

Abstract

The stellar wind around a compact object in luminous wind-accreting high-mass X-ray binaries is expected to be strongly ionized with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionization significantly reduces the radiative force within the so-called Strömgren region, leading to wind stagnation around the compact object. In close binaries like Vela X−1 this effect might alter the wind structure throughout the system. Using the spectral data from the Monitor of All-sky X-ray Image (MAXI), we studied the observed dependence of the photoelectric absorption as a function of the orbital phase in Vela X−1 and found that it is inconsistent with expectations for a spherically symmetric smooth wind. Taking into account previous investigations, we developed a simple model for wind structure with a stream-like photoionization wake region of slower and denser wind trailing the neutron star, which is responsible for the observed absorption curve.