标题： NGC5474和NGC3627（M66）中两个ULX的光学对应体 显示英文标题

标题： Optical counterparts of two ULXs in NGC5474 and NGC3627 (M66)

摘要： 我们发现了两个最亮的超亮X射线源（ULXs）在星系NGC5474和NGC3627（M66）中的光学对应体。 在哈勃太空望远镜图像中的对应体非常微弱，它们的V星等分别为24.7 ($M_V \approx -4.5$) 和25.9 ($M_V \approx -4.2$)。 NGC5474 X-1 的X射线流量变化超过两个数量级，在其明亮状态下它有$L_X \approx 1.6 \times 10^{40}$erg/s，其光谱由吸收幂律模型最佳拟合，光子指数为$\Gamma \approx 0.94$。 M66 X-1 在X射线中变化约2.5倍，其最大亮度为$2.0 \times 10^{40}$erg/s，伴随有$\Gamma \approx 1.7$。 对NGC5474 X-1 的光学光谱显示为蓝光谱，但被一颗23星等附近的恒星污染，而对应体的光谱更红。 在狭缝捕获的其他天体中，包括一个背景发射线星系（z=0.359）和NGC5474的一个新年轻星团。 我们发现这两个ULXs的X射线到光学的比例分别为 NGC5474 X-1 (在明亮状态) 的$L_X/L_{opt}$~ 7000 和 M66 X-1 的8000，这是迄今为止测量到的最暗的光学对应体。 它们的光学发射可能起源于供体星。 如果它们具有恒星级质量黑洞的超爱丁顿吸积盘，那么它们可能是所有ULXs中最低的吸积率，例如在M81 X-6和NGC1313 X-1中。 显示英文摘要

摘要： We identified two optical counterparts of brightest ultraluminous X-ray Sources (ULXs) in galaxies NGC5474 and NGC3627 (M66). The counterparts in Hubble Space Telescope images are very faint, their V magnitudes are 24.7 ($M_V \approx -4.5$) and 25.9 ($M_V \approx -4.2$), respectively. NGC5474 X-1 changes the X-ray flux more than two orders of magnitude, in its bright state it has $L_X \approx 1.6 \times 10^{40}$ erg/s, the spectrum is best fitted by an absorbed power-law model with a photon index $\Gamma \approx 0.94$. M66 X-1 varies in X-rays with a factor of ~2.5, its maximal luminosity being $2.0 \times 10^{40}$ erg/s with $\Gamma \approx 1.7$. Optical spectroscopy of the NGC5474 X-1 has shown a blue spectrum, which however was contaminated by a nearby star of 23 mag, but the counterpart has a redder spectrum. Among other objects captured by the slit are a background emission-line galaxy (z=0.359) and a new young cluster of NGC5474. We find that these two ULXs have largest X-ray-to-optical ratios of $L_X/L_{opt}$ ~ 7000 for NGC5474 X-1 (in its bright state) and 8000 for M66 X-1 both with the faintest optical counterparts ever measured. Probably their optical emission originates from the donor star. If they have super-Eddington accretion discs with stellar-mass black holes, they may also have the lowest mass accretion rates among ULXs such as in M81 X-6 and NGC1313 X-1.