论文标题
NGC 55中的瞬态超X射线源
A transient ultraluminous X-ray source in NGC 55
论文作者
论文摘要
超X射线源（ULX）是一类积聚的紧凑对象，X射线照明超过10 $^{39} $ erg s $^{ - 1} $。每个星系的平均ULX数量仍然没有很好地约束，尤其是考虑到ULX瞬变分数的不确定性。在这里，由于最近的XMM-Newton和Neil Gehrels Swift天文台观测值，我们报告了Galaxy NGC 55（我们将其标记为ULX-2）中新型瞬态ULX的鉴定。此对象先前被归类为瞬态X射线源，其亮度左右在2010 XMM-Newton观察中的几个10 $^{38} $ erg s $^{ - 1} $。得益于新的和更深入的观察（$ \ sim $ 130 ks），我们表明该来源达到了亮度峰值$> 1.6 \ times 10^{39} $ erg s $ s $^{ - 1} $。 ULX-2的X射线频谱比以前的观测值和适合软ulx类的X射线光谱要柔软得多。它可以使用具有两个热分子的模型来很好地描述，就像在ULX中经常发现一样。 X射线变异性的时间尺度为一个月的顺序，可能是由于积聚率的小变化或由于超级轨道调制而造成的，这是由于积聚盘的进攻，这与其他ULX相似。
Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 10$^{39}$ erg s$^{-1}$. The average number of ULXs per galaxy is still not well constrained, especially given the uncertainty on the fraction of ULX transients. Here, we report the identification of a new transient ULX in the galaxy NGC 55 (which we label as ULX-2), thanks to recent XMM-Newton and the Neil Gehrels Swift Observatory observations. This object was previously classified as a transient X-ray source with a luminosity around a few 10$^{38}$ erg s$^{-1}$ in a 2010 XMM-Newton observation. Thanks to new and deeper observations ($\sim$ 130 ks each), we show that the source reaches a luminosity peak $>1.6 \times 10^{39}$ erg s$^{-1}$. The X-ray spectrum of ULX-2 is much softer than in previous observations and fits in the class of soft ULXs. It can be well described using a model with two thermal components, as often found in ULXs. The time scales of the X-ray variability are of the order of a month and are likely driven by small changes in the accretion rate or due to super-orbital modulations, attributed to precession of the accretion disc, which is similar to other ULXs.
