X-ray and optical variability in the low-mass burst source X 1735-444.

Abstract

The low-mass X-ray burst source X1735 - 444 was observed with Exosat for 9 hr on August 23, 1984. The source exhibited a high level of persistent X-ray emission throughout the observation, with variations of about 20 percent and one burst. In contrast to previous observations of this source, the softness of the X-ray emission was found to be correlated with the source intensity. This softness/intensity relationship suggests that the emission is caused by inverse Compton scattering of soft photons in a corona or in the inner regions of the accretion disc; the relationship between softness and intensity can thus be understood as due to Compton cooling of the hot plasma. An iron line at 6.8 + or 0.1 keV with equivalent width about 90 eV was required to adequately fit the high-resolution spectral data. No quasi-periodic oscillations were detected in the frequency range less than or equal to 32 Hz, with an upper limit of less than 1 percent. Five hours of optical B-band photometry with 700 s time resolution were obtained simultaneously at the South African Astronomical Observatory; the optical flux is found to be anticorrelated with the X-ray flux at the time of this observation witha high level of significance.