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arXiv 1106.0744astro-ph.HEastro-ph.GA
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在2XMM星表中发现超软X射线瞬变源:潮汐扰动事件的候选者

Discovery of an Ultrasoft X-ray Transient Source in the 2XMM Catalog: a Tidal Disruption Event Candidate

Dacheng Lin, Eleazar R. Carrasco, Dirk Grupe, Natalie A. Webb, Didier Barret, Sean A. Farrell

摘要:

我们发现了一个超软X射线瞬变源2XMMi J184725.1-631724,它是在两次XMM-牛顿观测中偶然发现的,方向是星系IC 4765-f01-1504的中心,红移为0.0353。这两次观测相隔211天,0.2-10千电子伏的吸收通量增加了约9倍。它们的光谱最好由热盘或单温黑体分量(贡献>80%的通量)加上弱幂律分量主导的模型来描述。热发射的温度为几十eV,弱幂律分量的光子指数约为3.5。类似于热态的黑洞X射线双星,我们的源表现出吸积盘,其光度似乎遵循$L\propto T^4$关系。这将表明,使用最佳拟合的内盘半径,黑洞的质量约为10^5-10^6 M_sun。两个XMM-牛顿观测都显示了大约21%的小时时间尺度的变化,这可以解释为由于质量吸积速率的快速变化。ROSAT在1992年的一次观测中没有探测到该辐射源,这表明在较长时间尺度上的变异系数大于64。在2011年2月的一次雨燕观测中,该源在X射线中没有再次被探测到,这意味着自上次XMM-牛顿观测以来,通量减少了12倍以上。除了X射线光谱的极端柔软性和缺乏强光发射线所暗示的星系的不活动性之外,瞬态性质使其成为候选的潮汐破裂事件。如果是这种情况,第一个XMM-牛顿观测将处于上升阶段,第二个处于衰减阶段。

英文摘要:

We have discovered an ultrasoft X-ray transient source, 2XMMi J184725.1-631724, which was detected serendipitously in two XMM-Newton observations in the direction of the center of the galaxy IC 4765-f01-1504 at a redshift of 0.0353. These two observations were separated by 211 days, with the 0.2-10 keV absorbed flux increasing by a factor of about 9. Their spectra are best described by a model dominated by a thermal disk or a single-temperature blackbody component (contributing >80% of the flux) plus a weak power-law component. The thermal emission has a temperature of a few tens of eV, and the weak power-law component has a photon index of ~3.5. Similar to the black hole X-ray binaries in the thermal state, our source exhibits an accretion disk whose luminosity appears to follow the $L\propto T^4$ relation. This would indicate that the black hole mass is about 10^5-10^6 M_sun using the best-fitting inner disk radius. Both XMM-Newton observations show variability of about 21% on timescales of hours, which can be explained as due to fast variations in the mass accretion rate. The source was not detected by ROSAT in an observation in 1992, indicating a variability factor of >64 over longer timescales. The source was not detected again in X-rays in a Swift observation in 2011 February, implying a flux decrease by a factor of >12 since the last XMM-Newton observation. The transient nature, in addition to the extreme softness of the X-ray spectra and the inactivity of the galaxy implied by the lack of strong optical emission lines, makes it a candidate tidal disruption event. If this is the case, the first XMM-Newton observation would have been in the rising phase, and the second one in the decay phase.

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