Open AccessSimultaneous X‐ray/optical observations of GX 9+9 (4U 1728−16)
Author(s) -
Kong A. K. H.,
Charles P. A.,
Homer L.,
Kuulkers E.,
O'Donoghue D.
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10157.x
Subject(s) - physics , light curve , astrophysics , photometry (optics) , orbital period , black body radiation , x ray binary , low mass , accretion disc , astronomy , x ray astronomy , observatory , neutron star , x ray , optics , stars , radiation
We report on the results of the first simultaneous X‐ray ( RXTE ) and optical [South African Astronomical Observatory (SAAO)] observations of the luminous low‐mass X‐ray binary (LMXB) GX 9+9 in 1999 August. The high‐speed optical photometry revealed an orbital period of 4.1958 h and confirmed previous observations, but with greater precision. No X‐ray modulation was found at the orbital period. On shorter time‐scales, a possible 1.4‐h variability was found in the optical light curves which might be related to the MHz quasi‐periodic oscillations seen in other LMXBs. We do not find any significant X‐ray/optical correlation in the light curves. In X‐rays, the colour–colour and hardness‐intensity diagrams indicate that the source shows characteristics of an atoll source in the upper banana state, with a correlation between intensity and spectral hardness. Time‐resolved X‐ray spectroscopy suggests that two‐component spectral models give a reasonable fit to the X‐ray emission. Such models consist of a blackbody component which can be interpreted as the emission from an optically thick accretion disc or an optically thick boundary layer, and a hard Comptonized component for an extended corona.