利用eROSITA研究银河系高光度X射线双星（HMXBs）的低光度端：确立吸积型BeXRBs的光度下限
The low luminosity end of Galactic HMXBs with eROSITA: Establishing a luminosity floor for accreting BeXRBs
浏览论文内容
中文总结 AI 辅助
该研究利用eROSITA首次观测到HMXBs低至10^32 erg/s的光度，确立BeXRBs的光度下限，发现多数BeXRBs爆发外吸积为常态，暗示部分孤立Be星存在致密天体伴星。
中文摘要 AI 辅助
我们首次展示了SRG/eROSITA在其前四次巡天中观测到的银河系已知HMXBs总体情况。eROSITA的灵敏度约为$10^{-13}\\,\mathrm{erg}\\,\mathrm{s}^{-1}\\,\mathrm{cm}^{-2}$，对应银河系大多数已知HMXBs的光度为$10^{32}$-$10^{34}\\,\mathrm{erg}\\,\mathrm{s}^{-1}$，这一灵敏度是此前任何宽视场巡天仪器都未达到过的。我们利用eROSITA得到了已知HMXBs延伸的log N-log L分布，其光度下限可达$10^{32}\\,\mathrm{erg}\\,\mathrm{s}^{-1}$，并展示了源本征变异性可能引发的大范围离散度。我们还给出了按亚类型划分的光度分布，显示超巨X射线双星（SgXBs）和Be X射线双星（BeXRBs）占据总体分布的不同区域，同时重新分析了RXTE/ASM和MAXI的数据以与eROSITA的结果进行对比。eROSITA所覆盖的光度范围使得能够对“暂现”BeXRBs进行系统研究，这类源在爆发外通常低于其他监测仪器的探测阈值，其低光度行为长期以来是一个悬而未决的问题。目前仅通过定点仪器在部分样本中观测到了低光度下稳定吸积的迹象，基于eROSITA的结果，我们认为爆发外的吸积很可能是常态，因为绝大多数（>80%）BeXRBs的探测光度至少比最亮Be星预期的光度高一个数量级。我们结合冷盘吸积和“螺旋桨”机制讨论了观测到的光度，并强调了一小部分“孤立”Be星，其光度与光度最低的BeXRBs相当，暗示存在致密天体伴星。
英文摘要
We present a first look at the Galactic population of heretofore known HMXBs as observed by SRG/eROSITA during its first four surveys. eROSITA's sensitivity of $\sim10^{-13}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$, translating to $10^{32}$-$10^{34},\mathrm{erg}\,\mathrm{s}^{-1}$ in luminosity for most known HMXBs in the Milky Way, has thus far never been reached by any wide-area survey instrument. We present the extended log N-log L distribution of known HMXBs reaching down to $10^{32}\,\mathrm{erg}\,\mathrm{s}^{-1}$ using eROSITA, and show the large scatter that can be induced by source intrinsic variability. We present sub-type resolved luminosity distributions, showing that the Supergiant X-ray binaries (SgXBs) and Be X-ray binaries (BeXRBs) occupy different parts of the overall distribution, and reanalyse RXTE/ASM data and MAXI for comparison to eROSITA. The luminosity regime uncovered by eROSITA allows a systematic study of the "transient" BeXRBs, which are typically below the detection threshold of monitors outside of outburst, and whose low luminosity behavior has been a longstanding question. Signatures of stable accretion at low luminosities have been observed with pointed instruments for a fraction of the overall sample, so far. With the eROSITA results, we posit that accretion outside of outburst is likely the norm, since a vast majority (> 80%) of BeXRBs are detected at luminosities at least an order of magnitude higher than expected for the most X-ray luminous Be stars. We discuss the observed luminosity in the context of cold disk accretion and the "propeller" mechanism. We highlight a small subpopulation of "isolated" Be-stars that reach luminosities comparable to the least luminous BeXRBs, hinting at the presence of compact object companions.
