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

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 (v ∞ ) 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 P orb = 9.5591 ± 0.0007 d with maximum X-ray flux at MJD 51856.6 ± 0.1. 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 v ∞ 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 v ∞ 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.

X-ray irradiation of the winds in binaries with massive components

“…Within the classical Bondi-Hoyle-Lyttleton picture (Hoyle & Lyttleton 1941;Bondi & Hoyle 1944) this affects the accretion rate and the X-ray luminosity. This effect can, for example, explain, why the wind velocity required to explain the X-ray luminosity of 4U 2206+54 is significantly lower than the terminal velocity expected for this type of star (Ribó et al 2006).…”

Orbital parameters of the high‐mass X‐ray binary 4U 2206+54

“…The system also appeared in the Ariel V catalogue (Warwick et al 1981). The mass donor is classified as an O9.5Vp star with a higher than normal helium abundance, underfilling its Roche lobe and losing mass via a slow but dense stellar wind, υ ∼ 350 km s −1 (Ribó et al 2006). However, there are some metallic lines typical for a later-type spectrum (Negueruela & Reig 2001).…”

Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54

“…The distance to 4U 2206+54 is estimated to be 2.6 kpc . Ribó et al (2006) found that the wind terminal velocity of 4U 2206+54 is abnormally slow (∼350 km s −1 ). With such a low velocity, an eccentric orbit and using the Bondi-Hoyle formalism it is possible to reproduce the average X-ray luminosity of the system as well as the orbital variability seen in the RXTE/ASM light curve between 1996 and 2005.…”