Intermittent dipping in a low-mass X-ray binary

Periodic dips observed in ≈20 per cent of low-mass X-ray binaries (LMXBs) are thought to arise from obscuration of the neutron star by the outer edge of the accretion disc. We report the detection with the Rossi X-ray Timing Explorer of two dipping episodes in Aql X-1, not previously a known dipper. The X-ray spectrum during the dips exhibited an elevated neutral column density, by a factor between one and almost two orders of magnitude. Dips were not observed in every cycle of the 18.95-h orbit, so that the estimated frequency for these events is 0.10^+0.07 _-0.05 cycle^-1. This is the first confirmed example of intermittent dipping in such a system. Assuming that the dips in Aql X-1 occur because the system inclination is intermediate between the non-dipping and dipping sources, implies a range of 72°-79° for the source. This result lends support for the presence of a massive (>2M⊙) neutron star in Aql X-1, and further implies that ≈30 additional LMXBs may have inclinations within this range, raising the possibility of intermittent dips in those systems also. Thus, we searched for dips from 24 other bursting systems, without success. For the system with the largest number of dip phases covered, 4U 1820-303, the non-detection implies a 95 per cent upper limit to the dip frequency of 1.4 × 10^-3 cycle^-1.