A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source

摘要

"超亮X-射线源"(ULXs)是非核点源，它们被普遍认为要么含有中等质量的黑洞，要么含有较小的、从一个双星伴星吸积而成的恒星质量的黑洞。对ULXs进行研究，能提供关于黑洞形成和/或高"Eddington速度"吸积模式的信息。本期Nature上的两篇论文描述了具有异常性质的脉动ULXs。 Christian Motch等人发现，星系NGC 7793中的P13源在一个周期约为64天的双星体系中。通过模拟来自B9la供体星的X-射线加热作用的强可见光和紫外线调制，他们将该黑洞质量约束为小于15个太阳质量。Matteo Bachetti等人对星系M82中的一个源进行了观测，脉动数据表明，这个源含一个中子星而不是一个黑洞，从而向认为黑洞为大多数明亮X-射线双星提供动力的假设提出了质疑。%Most ultraluminous X-ray sources have a typical set of properties not seen in Galactic stellar-mass black holes. They have luminosities of more than 3 × 10~(39) ergs per second, unusually soft X-ray components (with a typical temperature of less than about 0.3 kiloelectronvolts) and a characteristic downturn in their spectra above about 5 kilo­electronvolts. Such puzzling properties have been interpreted either as evidence of intermediate-mass black holes or as emission from stellar-mass black holes accreting above their Eddington limit, analogous to some Galactic black holes at peak luminosity. Recently, a very soft X-ray spectrum was observed in a rare and transient stellar-mass black hole. Here we report that the X-ray source P13 in the galaxy NGC 7793 is in a binary system with a period of about 64 days and exhibits all three canonical properties of ultraluminous sources. By modelling the strong optical and ultraviolet modulations arising from X-ray heating of the B9Ia donor star, we constrain the black hole mass to be less than 15 solar masses. Our results demonstrate that in P13, soft thermal emission and spectral curvature are indeed signatures of supercritical accretion. By analogy, ultraluminous X-ray sources with similar X-ray spectra and luminosities of up to a few times 10~(40) ergs per second can be explained by supercritical accre­tion onto massive stellar-mass black holes.