Bibcode
Ribó, M.; Negueruela, I.; Blay, P.; Torrejón, J. M.; Reig, P.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 449, Issue 2, April II 2006, pp.687-698
Fecha de publicación:
4
2006
Revista
Número de citas
46
Número de citas referidas
40
Descripción
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.