标题： M51a中一个可能的可变ULX和IMBH候选体 显示英文标题

标题： A variable ULX and possible IMBH candidate in M51a

摘要： ULX-7位于M51星系的北部螺旋臂中，表现出超亮X射线源（ULX）中不寻常的行为特征，具有硬X射线谱但短期变化非常显著。 这表明它并不处于典型的超亮状态。 我们利用XMM-Newton、Chandra和NuSTAR的存档数据以及哈勃太空望远镜（HST）和甚大阵列（VLA）的光学和无线电数据对该源进行了分析。 我们的X射线光谱分析显示该源具有硬幂律谱形状，光子指数Gamma~1.5，尽管其X射线亮度变化超过一个量级，这种形状仍然保持不变。 源的功率谱在6.5^{+0.5}_{-1.1}x10^-3 Hz处出现一个转折点，从低频谱指数alpha_1=-0.1^{+0.5}_{-0.2}转变为高频谱指数alpha_2=0.65^{+0.05}_{-0.14}，使其类似于低频/硬态黑洞（BH）功率谱中发现的低频转折点。 我们可以取一个较低的频率下限来计算对应的高频转折点，并由此得出一个质量上限为1.6x10^3太阳质量的黑洞。 利用X射线/无线电基本平面，我们还计算了另一种情况下的质量上限，即低频/硬态黑洞的质量上限为3.5x10^4太阳质量。 硬谱、高均方根变异性以及质量限制与ULX-7是一个中等质量黑洞一致；然而，我们不能排除该源有趣行为的其他解释，最值得注意的是具有极端吸积率的中子星。 显示英文摘要

摘要： ULX-7, in the northern spiral arm of M51, demonstrates unusual behaviour for an ultraluminous X-ray source, with a hard X-ray spectrum but very high short-term variability. This suggests that it is not in a typical ultraluminous state. We analyse the source using archival data from XMM-Newton, Chandra and NuSTAR, and by examining optical and radio data from HST and VLA. Our X-ray spectral analysis shows that the source has a hard power-law spectral shape with a photon index Gamma~1.5, which persists despite the source's X-ray luminosity varying by over an order of magnitude. The power spectrum of the source features a break at 6.5^{+0.5}_{-1.1}x10^-3 Hz, from a low-frequency spectral index of alpha_1=-0.1^{+0.5}_{-0.2} to a high-frequency spectral index of alpha_2=0.65^{+0.05}_{-0.14}, making it analogous to the low-frequency break found in the power spectra of low/hard state black holes (BHs). We can take a lower frequency limit for a corresponding high-frequency break to calculate a BH mass upper limit of 1.6x10^3 solar masses. Using the X-ray/radio fundamental plane we calculate another upper limit to the BH mass of 3.5x10^4 solar masses for a BH in the low/hard state. The hard spectrum, high rms variability and mass limits are consistent with ULX-7 being an intermediate-mass BH; however we cannot exclude other interpretations of this source's interesting behaviour, most notably a neutron star with an extreme accretion rate.