1302.4665 (P. Maggi et al.)

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

P. Maggi, F. Haberl, R. Sturm, W. Pietsch, A. Rau, J. Greiner, A. Udalski, M. Sasaki

Aims: We report the discovery of LXP169, a new high-mass X-ray binary in the Large Magellanic Cloud. The optical counterpart is identified and exhibits 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 long I-band light curve. We observed LXP169 with Swift at two predicted eclipse times. Results: We found a spin period of 168.8 s which 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 LXP169 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 a near-infrared excess. This classifies the source as a Be/X-ray binary pulsar. We observed a transit in the ultraviolet, thereby confirming that the companion star itself is eclipsed. We give an ephemeris for the transit of MJD56203.877(-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: LXP169 is the first confirmed eclipsing Be/X-ray binary. For the first time we observe the compact object in an X-ray binary eclipsing its companion star, and much can be learned by studying this important system.

View original: http://arxiv.org/abs/1302.4665