  Periodic dips observed in approx. 20% of low-mass X-ray binaries are thought
to arise from obscuration of the neutron star by the outer edge of the
accretion disk. 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 1 and almost two orders of magnitude. Dips were not observed in
every cycle of the 18.95-hr orbit, so that the estimated frequency for these
events is 0.10 (+0.07,-0.05)/cycle. 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 deg. for the source. This result
lends support for the presence of a massive (> 2 M_sun) neutron star in Aql
X-1, and further implies that approx. 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 nondetection implies a 95% upper limit to the dip frequency of
1.4E-3/cycle.

High Energy Astrophysical Phenomena