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

The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries.
In spite of intensive searches, X-ray pulsations have not been detected in
the time range 0.001-1000 s. A cyclotron line at ~30 keV has been suggested
by various authors but never detected with significance. The stellar wind
of the optical companion is abnormally slow. The orbital period, initially
reported to be 9.6 days, disappeared and a new periodicity of 19.25 days
emerged. Our new long and uninterrupted RXTE observations allow us to search
for long (~1 hr) pulsations for the first time. We have discovered 5560-s
pulsations in the light curve of 4U 2206+54. Initially detected in RXTE data,
these pulsations are also present in INTEGRAL and EXOSAT observations. The
average X-ray luminosity in the energy range 2-10 keV is
1.5 x 10^{35} erg s^{-1} with a ratio Fmax/Fmin ~ 5. This ratio implies
an eccentricity of ~0.4, somewhat higher than previously suggested. The
source also shows a soft excess at low energies. If the soft excess is
modelled with a blackbody component, then the size and temperature of
the emitting region agrees with its interpretation in terms of a hot
spot on the neutron star surface. The source displays variability on time
scales of days, presumably due to changes in the mass accretion rate as
 the neutron star moves around the optical companion in a moderately
 eccentric orbit.

0812_2365v1.pdf548.34 KB
P. Reig (FORTH/U. of Crete), J.M Torrejon (U. of Alicante), I. Negueruela (U. of Alicante), P. Blay (U. of Valencia), M. Ribo (U. of Barcelona), J. Wilms (U. of Erlangen-Nuremberg)
Thursday, 1 Jan 2009