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

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 +0.934−0.197 + 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.

SXP 5.05 = IGR J00569-7226: using X-rays to explore the structure of a Be star's circumstellar disc

“…In addition, there is evidence for a complete eclipse of the neutron star by the Be star -if so, this will be the first eclipsing Be/X-ray system in the SMC. The only previously known "eclipsing" Be/X-ray binary system is the recently discovered system, LXP 169, in the LMC (Maggi et al, 2013). But that system only exhibits transits by the neutron star across the face of the Be star and no actual eclipses by the Be star of thew neutron star -presumably possible if the orbital eccentricity is large enough.…”

Spectral and temporal properties of RX J0520.5-6932 (LXP 8.04) during a type-I outburst

“…Approximately 60 confirmed pulsars have been found in the SMC so far (Haberl & Pietsch 2004;Coe et al 2005Coe et al , 2010, and 45 candidates have been identified during the recent XMM-Newton survey of the SMC Haberl et al 2012). On the other hand the LMC, which is 10 times more massive than the SMC, has only about 20 confirmed HMXB (see Liu et al 2005;Maggi et al 2013;Vasilopoulos et al 2013a, and references within), of which 14 are HMXB pulsars. Still, since the X-ray coverage of the LMC is not nearly as complete as that of the SMC, early interpretations of the connection between star-formation rate and its BeXRB population would be dubious.…”

SXP 7.92: A Recently Rediscovered Be/X-ray Binary in the Small Magellanic Cloud, Viewed Edge On

“…1 by the vertical dashed lines. We note that this is not the first report of "dips" in the optical light curve of a BeXRB: similar sized dips were seen in the OGLE light curve of LXP 169 (Maggi et al 2013) Figure 5. Left hand panel shows the SALT spectrum of SXP 7.92 in the wavelength range 3900-4900Å.…”

“…The value of R derived above is not compatible with the radius of a NS alone (generally assumed to be 10-15 km). Maggi et al (2013) invoke a similarly large sized obscuring object, ≥ 3.5 × 10 9 m, to explain the dips in the light curve of LXP 169. They claim that these dips are due to the transit of the NS system across the Be star, rather than an obscuration of part of the circumstellar disc, and that the size can be understood in terms of the inflow of material onto the NS from the stellar wind which would create a total accreting system of the requisite size.…”