Finding a 24-day orbital period for the X-ray binary 1A 1118-616

TL;DR

This paper determines the orbital period and ephemeris of the Be X-ray binary 1A 1118-616, revealing a 24-day orbit through analysis of RXTE observations during an outburst, with results indicating a circular orbit.

Contribution

First measurement of the binary period and orbital parameters of 1A 1118-616 using phase connection of pulse arrival times.

Findings

Orbital period is 24.0 +/- 0.4 days.

Orbit is consistent with being circular.

Orbital phase T90 coincides with X-ray peak.

Abstract

We report the first determination of the binary period and orbital ephemeris of the Be X-ray binary containing the pulsar 1A 1118-616 (35 years after the discovery of the source). The orbital period is found to be Porb = 24.0 +/- 0.4 days. The source was observed by RXTE during its last large X-ray outburst in January 2009, which peaked at MJD 54845.4, by taking short observations every few days, covering an elapsed time comparable to the orbital period. Using the phase connection technique, pulse arrival time delays could be measured and an orbital solution determined. The data are consistent with a circular orbit, and the time of 90 degrees longitude was found to be T90 = MJD 54845.37(10), which is coincident with that of the peak X-ray flux.