Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud. (arXiv:1302.4665v1 [astro-ph.HE])

Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud. (arXiv:1302.4665v1 [astro-ph.HE]):
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.