3. X-ray and optical monitoring
In order to obtain tight limits for the temporal development of the radius and the effective temperature of the photosphere, we have monitored RX J0513 with the ROSAT HRI detector at intervals of about two days covering one complete X-ray outburst cycle (Fig. 1, upper part). The source remained undetectable during the X-ray off state and showed a sudden increase of the soft X-ray flux by a factor of at the end of August 1997, reaching maximum flux after 5 days. The X-ray outburst lasted 40 days and ended with a steep flux decline by again a factor of to non-detectability within 2 days. The slow flux variation during the X-ray outburst can be approximated by an exponential decline with a time constant of days.
Detailed light curves of several optical low states are available from regular monitoring of RX J0513 as a serendipitous source on CCD images taken for the MACHO project (Southwell et al. 1996). In Fig. 1 (lower part) the light curves of different observed optical minima are shifted in time such that the onset of the optical decline occurs at . MACHO data obtained quasi-simultaneously with our HRI monitoring indicate that the steep onset of the X-ray outburst occurred within 1-2 days after the beginning of the optical decline (W. Sutherland, private communication).
The shape of the optical low states repeats fairly well with an uncertainty of some 5 days in the duration of the low state. Within this uncertainty, its length coincides with the duration of the X-ray outburst. Both, the X-ray and the optical light curves show a similar small flux gradient before the final fast transition to the X-ray off/optical high state occurs.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000