Intermittent dipping in a low-mass X-ray Binary

TL;DR

This paper reports the first confirmed case of intermittent dipping in the low-mass X-ray binary Aql X-1, providing insights into system inclination and neutron star mass, and explores the prevalence of dips in similar systems.

Contribution

It presents the discovery of intermittent dips in Aql X-1 and discusses implications for system inclination and neutron star mass, expanding understanding of dipping behavior in LMXBs.

Findings

Detected two dipping episodes in Aql X-1 with RXTE.

Estimated dipping frequency of 0.10 per orbit cycle.

Suggested inclination range of 72-79 degrees for Aql X-1.

Abstract

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.