47 Tuc的深射电成像将特殊的X射线源X9确定为新的黑洞候选者
Deep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate
摘要：
本文报道在球状星团47 Tuc中探测到来自已知X射线源X9的稳定射电辐射。在其紫外光谱中有一个双峰的C IV发射线，提供了一个清晰的吸积信号，这个来源以前被归类为激变变星。在2010年和2013年的深ATCA成像中，我们确定了5.5和9.0 GHz的稳定射电源，无线电频谱指数（定义为$S_ν\proptoν^α$）为$α=-0.4\pm0.4$。我们在5.5 GHz处测得的$42\pm4$微焦/束的通量密度意味着射电光度（$νL_ν$）为5.8 e27 erg/s，明显高于以前任何一个吸积白矮星的射电探测。过渡毫秒双星在吸积中子星中具有最高的射电-X射线通量比（在相同的X射线光度下仍比吸积黑洞低几倍），显示出与X9明显不同的X射线和射电变率模式。当与档案X射线测量相结合时，我们的无线电探测将47 Tuc X9放置在非常接近吸积黑洞的无线电/X射线相关性的位置，并且我们探索了这种来源是静止的恒星质量黑洞X射线二进制的可能性。施主星的性质尚不确定;虽然光学对应物的光度与低质量主序星的施主星一致，但产生1 e33 erg/s的高静态X射线光度所需的质量转移速率表明该系统可能是超紧凑的，轨道周期为25分钟。这是迄今为止在银河系球状星团中发现的第四个静止黑洞候选者，也是唯一一个从其光学/紫外光谱中确认吸积特征的黑洞。
英文摘要：
We report the detection of steady radio emission from the known X-ray source X9 in the globular cluster 47 Tuc. With a double-peaked C IV emission line in its ultraviolet spectrum providing a clear signature of accretion, this source had been previously classified as a cataclysmic variable. In deep ATCA imaging from 2010 and 2013, we identified a steady radio source at both 5.5 and 9.0 GHz, with a radio spectral index (defined as $S_ν\proptoν^α$) of $α=-0.4\pm0.4$. Our measured flux density of $42\pm4$ microJy/beam at 5.5 GHz implies a radio luminosity ($νL_ν$) of 5.8e27 erg/s, significantly higher than any previous radio detection of an accreting white dwarf. Transitional millisecond pulsars, which have the highest radio-to-X-ray flux ratios among accreting neutron stars (still a factor of a few below accreting black holes at the same X-ray luminosity), show distinctly different patterns of X-ray and radio variability than X9. When combined with archival X-ray measurements, our radio detection places 47 Tuc X9 very close to the radio/X-ray correlation for accreting black holes, and we explore the possibility that this source is instead a quiescent stellar-mass black hole X-ray binary. The nature of the donor star is uncertain; although the luminosity of the optical counterpart is consistent with a low-mass main sequence donor star, the mass transfer rate required to produce the high quiescent X-ray luminosity of 1e33 erg/s suggests the system may instead be ultracompact, with an orbital period of order 25 minutes. This is the fourth quiescent black hole candidate discovered to date in a Galactic globular cluster, and the only one with a confirmed accretion signature from its optical/ultraviolet spectrum.
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