Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit

¹ DSM/DAPNIA/Service d'Astrophysique, CEA Saclay, Bât. 709, L'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
² AIM – Unité Mixte de Recherche – CEA – CNRS – Université Paris VII – UMR 7158, France e-mail: mribo@discovery.saclay.cea.fr
³ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Escuela Politécnica Superior, Universitat d'Alacant, Ap. 99, 03080 Alicante, Spain e-mail: [ignacio;jmt]@dfists.ua.es
⁴ Institut de Ciència dels Materials, Universitat de València, PO Box 22085, 46071 Valencia, Spain e-mail: pere.blay@uv.es
⁵ IESL, Foundation for Research and Technology, 71110 Heraklion, Crete, Greece
⁶ University of Crete, Physics Department, PO Box 2208, 710 03 Heraklion, Crete, Greece e-mail: pau@physics.uoc.gr

Received: 14 September 2005
Accepted: 14 November 2005

Abstract

Massive X-ray binaries are usually classified by the properties of the donor star in classical, supergiant and Be X-ray binaries, the main difference being the mass transfer mechanism between the two components. The massive X-ray binary 4U 2206+54 does not fit in any of these groups, and deserves a detailed study to understand how the transfer of matter and the accretion on to the compact object take place. To this end we study an IUE spectrum of the donor and obtain a wind terminal velocity () of ~350 km s^-1, which is abnormally slow for its spectral type. We also analyse here more than 9 years of available RXTE/ASM data. We study the long-term X-ray variability of the source and find it to be similar to that observed in the wind-fed supergiant system Vela X-1, reinforcing the idea that 4U 2206+54 is also a wind-fed system. We find a quasi-period decreasing from ~270 to ~130 d, noticed in previous works but never studied in detail. We discuss possible scenarios for its origin and conclude that long-term quasi-periodic variations in the mass-loss rate of the primary are probably driving such variability in the measured X-ray flux. We obtain an improved orbital period of d with maximum X-ray flux at MJD . Our study of the orbital X-ray variability in the context of wind accretion suggests a moderate eccentricity around 0.15 for this binary system. Moreover, the low value of solves the long-standing problem of the relatively high X-ray luminosity for the unevolved nature of the donor, BD +53°2790, which is probably an O9.5 V star. We note that changes in and/or the mass-loss rate of the primary alone cannot explain the different patterns displayed by the orbital X-ray variability. We finally emphasize that 4U 2206+54, together with LS 5039, could be part of a new population of wind-fed HMXBs with main sequence donors, the natural progenitors of supergiant X-ray binaries.