在大麦云星云中发现一颗168.8秒的X射线脉冲星凌日
Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud
摘要：
报道了在大麦哲伦星系中发现的一颗新的大质量X射线双星LXP 169。光学对应物已被确定，似乎表现出一个日食光曲线。我们进行了后续观测，以澄清该系统的日食性质。能量谱和时间序列是从两个XMM-牛顿观测中提取的，用于搜索脉动，分析光谱，测量光谱和时间变化。长期X射线变异性研究使用档案ROSAT数据。XMM位置用于识别光学对应物。我们获得了紫外到近红外光度测量来确定伴星，以及它的4000 d长的I波段光变曲线。我们用Swift在两个预测的日食时间观测了LXP 169。我们发现了一个168.8秒的自旋周期，在两个XMM观测之间没有变化。X射线光谱，很好地表征了幂律，当源更亮时更难。发现LXP 169的X射线通量至少可变化10倍。它的对应物在短时间和长时间尺度上都是高度可变的，它的测光是一颗具有近红外过量的早期型恒星。这将其归类为BeXRB脉冲星。我们观察到了紫外线的凌日，从而证实了伴星本身被遮蔽了。给出了MJD 56203.877 + N*24.329凌日的星历表。我们提出并讨论了这样一种情况，即从伴星赤道盘捕获的物质在中子星（NS）周围产生了一个扩展的高密度区域，当NS在它前面过境时，它部分地遮蔽了伴星。这很可能是第一次观察到XRB中的紧凑物体遮蔽了伴星。LXP 169将是第一颗与BeXRB交食的星，并且可能会从额外的观测中获得大量的重要信息，例如测量可能的Be盘/轨道平面不对准，或NS的质量。
英文摘要：
We report the discovery of LXP169, a new high-mass X-ray binary (XRB) in the LMC. 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. 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 positions were used to identify the optical counterpart. We obtained UV to NIR 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. We found a spin period of 168.8 s that did not change between two XMM 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 NIR excess. This classifies the source as a BeXRB pulsar. We observed a transit in the UV, thereby confirming that the companion star itself is eclipsed. We give an ephemeris for the transit of MJD 56203.877 + N*24.329. 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 (NS), which partially eclipses the companion as the NS transits in front of it. This is most likely the first time the compact object in an XRB is observed to eclipse its companion star. LXP169 would be the first eclipsing BeXRB, 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 NS.
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