We examine a variety of X-ray and optical data for evidence of evolution in the accretion structure of the dipping, eclipsing, bursting low-mass X-ray binary (LMXB) EXO 0748-676. Our results are divided into three main sections.
New ASCA and archival EXOSAT data reveal behavior related to the source brightness state. An anomalous dip (Φ [congruent with] 0.45) can be associated with the source low state. This dip does not appear to precess and is the only dipping feature showing significant changes between brightness states. The quiescent (non-dipping, non-bursting, non-eclipsing) spectral behavior evolves between high and low states. The hardness of quiescent spectrum is uncorrelated with intensity in the high state, but becomes anti-correlated with intensity in the low state. Origins of the anti-correlated behavior are not clear, but could be related to an absorption component not present in the high state.
We proceed to examine the moderate X-ray resolution spectra of EXO 0748-676 in two different brightness states. Spectra were obtained from both BBXRT on 1990 December 6 (high state) and ASCA on 1993 May 7 (low state). The greatest difference between the spectra of the source in either state is that a previously un-noted soft (