Eclipse Timings of the Low‐Mass X‐Ray Binary EXO 0748−676. II. Detection of an Apparent Orbital Period Change and of Orbital Period Noise

The rare eclipsing low-mass X-ray binary EXO 0748-676 has now been monitored accurately for over a decade in the hope of detecting orbital period evolution. We report observations of 10 complete eclipses at resolution less than 0.1 s with the Rossi X - ray Timing Explorer during 1996 May and 1996 August. We have timed the eclipses with an accuracy limited by photon statistics to 0.2-0.5 s. When combined with historical measurements, the eclipse timings imply an unexpectedly large orbital period derivative, orb =3.3 × 10 −11 , and a rapid timescale for orbital evolution, τ orb = 1.3 × 10 7 yr. The observations also require considerable intrinsic jitter in the mideclipse timings; a maximum likelihood estimate is 0.15 s per orbital cycle. We consider various models and scenarios that might account for these unexpected results.