Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany
e-mail: pmaggi@mpe.mpg.de
² Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warzawa, Poland
³ Institut für Astronomie und Astrophysik Tübingen, Universität Tübingen, Sand 1, 72076 Tübingen, Germany

Received: 5 February 2013
Accepted: 5 April 2013

Abstract

Aims. We report the discovery of LXP 169, a new high-mass X-ray binary in the Large Magellanic Cloud. The optical counterpart has been identified and appears to exhibit an eclipsing light curve. We performed follow-up observations to clarify the eclipsing nature of the system.

Methods. Energy spectra and time series were extracted from two XMM-Newton observations to search for pulsations, characterise the spectrum, and measure spectral and timing changes. Long-term X-ray variability was studied using archival ROSAT data. The XMM-Newton positions were used to identify the optical counterpart. We obtained ultraviolet to near-infrared photometry to characterise the companion, along with its 4000 d I-band light curve and colour–magnitude variability. We observed LXP 169 with Swift at two predicted eclipse times.

Results. We found a spin period of 168.8 s that did not change between two XMM-Newton observations. The X-ray spectrum, well characterised by a power law, was harder when the source was brighter. The X-ray flux of LXP 169 is found to be variable by a factor of at least 10. The counterpart is highly variable on short and long timescales, and its photometry is that of an early-type star with an ouflowing circumstellar disc producing a near-infrared excess. This classifies the source as a Be/X-ray binary pulsar. We observe a transit in the ultraviolet, thereby confirming that the companion star itself is eclipsed. We give an ephemeris for the transit of MJD 56 203.877_-0.197^+0.934 + N × (24.329±0.008). We propose and discuss the scenario where the matter captured from the companion’s equatorial disc creates an extended region of high density around the neutron star, which partially eclipses the companion as the neutron star transits in front of it.

Conclusions. This is most likely the first time the compact object in an X-ray binary is observed to eclipse its companion star. LXP 169 would be the first eclipsing Be/X-ray binary, and a wealth of important information might be gained from additional observations, such as a measure of the possible Be disc/orbital plane misalignment, or the mass of the neutron star.