A CLOSER LOOK AT THE SOFT X-RAY TRANSIENT X1608-52: LONG-TERM OPTICAL AND X-RAY OBSERVATIONS

摘要

We report long-term simultaneous optical and (RXTE) X-ray observations of the soft X-ray transient and low-mass X-ray binary X1608- 52 spanning from 1999 to 2001. In addition to the usual X-ray outburst and quiescent states, X1608- 52 also exhibits an extended low-intensity state, during which the optical counterpart, QX Nor, is found to be about 2 mag brighter than during quiescence. We detect optical photometric variability on a possible period of 0.5370 days with a semiamplitude of ～0.27 mag in the Ⅰ band. The modulation could be orbital but is also consistent with a scenario invoking a superhump with decreasing period. We discuss the possibilities of distinguishing between the orbital- and superhump-period cases. Observations of QX Nor during quiescence indicate an F- to G-type main-sequence secondary, while theoretical considerations argue for an evolved mass donor. Only an evolved mass donor would satisfy the condition for the occurrence of superhumps.