用甚大阵分辨M32的核射电辐射
Resolving the Nuclear Radio Emission from M32 with Very Large Array
摘要：
本星系群矮椭圆星系M32拥有已知的最近和最不亮的超大质量黑洞（SMBH）之一，这为研究最静止状态下吸积SMBH的物理学提供了难得的机会。最近的甚大天线阵（VLA）观测在M32的核心发现了一个射电源，它被认为是SMBH的射电对应物。为了进一步研究这个核源的射电性质，我们在2015- 2017年之间的四个时期进行了后续的高分辨率VLA观测，每个时期都有双频。在6 GHz时，核源在$\sim$ 0 \farcs 4的角分辨率下被分辨，表现出无芯的、略微不平衡的形态，具有$\sim$ 2.5 \arcsec（$\sim$ 10秒差距）的可检测范围。在四个时期之间没有发现显著的变化。在15 GHz时，在同一区域内没有检测到显著的发射，指向陡峭的固有无线电频谱（频谱指数的3\，$σ$上限为-1.46）。我们讨论了这种核源的性质的可能方案，并得出结论，恒星的起源，特别是行星状星云，X射线双星，超新星遗迹或弥漫的电离气体供电的大质量恒星，可以排除。相反，所观察到的无线电特性可以解释为同步辐射从一个假设的风驱动的弱吸积SMBH。
英文摘要：
The Local Group dwarf elliptical galaxy M32 hosts one of the nearest and most under-luminous super-massive black holes (SMBHs) ever known, offering a rare opportunity to study the physics of accreting SMBHs at the most quiescent state. Recent Very Large Array (VLA) observations have detected a radio source at the nucleus of M32, which is suggested to be the radio counterpart of the SMBH. To further investigate the radio properties of this nuclear source, we have conducted follow-up, high-resolution VLA observations in four epochs between 2015--2017, each with dual frequencies. At 6 GHz, the nuclear source is resolved under an angular resolution of $\sim$0\farcs4, exhibiting a coreless, slightly lopsided morphology with a detectable extent of $\sim$2.5 \arcsec ($\sim$10 parsec). No significant variability can be found among the four epochs. At 15 GHz, no significant emission can be detected within the same region, pointing to a steep intrinsic radio spectrum (with a 3\,$σ$ upper limit of -1.46 for the spectral index). We discuss possible scenarios for the nature of this nuclear source and conclude that a stellar origin, in particular planetary nebulae, X-ray binaries, supernova remnants or diffuse ionized gas powered by massive stars, can be ruled out.Instead, the observed radio properties can be explained by synchrotron radiation from a hypothetical wind driven by the weakly accreting SMBH.
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