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

¹ Institut für Astronomie und Astrophysik, Abteilung Astronomie, Universität Tübingen (IAAT), Sand 1, 72076 Tübingen, Germany
e-mail: staubert@astro.uni-tuebingen.de
² NASA-Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
³ Center for Space Science and Technology (CRESST), University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ Dr. Karl Remeis-Sternwarte and Erlangen Center for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁵ Center for Astrophysics and Space Sciences (CASS), University of California San Diego, La Jolla, CA 92093-0424, USA

Received: 10 September 2010
Accepted: 2 November 2010

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 P_orb = 24.0 ± 0.4days. 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 T_π/2 = MJD 54845.37(10), which is coincident with that of the peak X-ray flux.