47 Tucanae的超深ATCA成像揭示一个中央致密射电源
Ultra-deep ATCA imaging of 47 Tucanae reveals a central compact radio source
AI总结：
该研究利用约480小时ATCA超深观测在球状星团47 Tucanae中心发现暗弱平谱致密射电源，并通过多波段分析认为其最可能是毫秒脉冲星或弱吸积黑洞。
摘要：
我们展示了使用Australia Telescope Compact Array对银河系球状星团47 Tucanae进行的一项超深射电连续谱巡天结果，该巡天包含约480小时的观测。对该星团的这一全面覆盖使我们能够在5.5 GHz达到1.19 μJy beam⁻¹的RMS噪声水平，在9 GHz达到940 nJy beam⁻¹，并在叠加的7.25 GHz图像中达到790 nJy beam⁻¹。这是球状星团的最深射电图像，也是使用Australia Telescope Compact Array迄今获得的最深图像。我们识别出ATCA J002405.702-720452.361，这是一个暗弱（5.5 GHz下为6.3±1.2 μJy，9 GHz下为5.4±0.9 μJy）、平谱（α=-0.31±0.54）的射电源，其位置与星团中心重合，并可能与一个暗弱X射线源相关。未发现令人信服的光学对应体。我们利用射电、X射线、光学和紫外数据表明，涉及背景活动星系核、色球活动双星或包含白矮星的双星的解释均不太可能。最合理的解释是该源为一颗尚未发现的毫秒脉冲星或一个弱吸积黑洞。如果该X射线源与该射电源相关，黑洞活动基本面给出的黑洞质量约为54–6000 M⊙，表明其为一个中等质量黑洞或一个重恒星质量黑洞。
英文摘要：
We present the results of an ultra-deep radio continuum survey, containing $\sim480$ hours of observations, of the Galactic globular cluster 47 Tucanae with the Australia Telescope Compact Array. This comprehensive coverage of the cluster allows us to reach RMS noise levels of 1.19 $μJy~\textrm{beam}^{-1}$ at 5.5 GHz, 940 $nJy~\textrm{beam}^{-1}$ at 9 GHz, and 790 $nJy~\textrm{beam}^{-1}$ in a stacked 7.25 GHz image. This is the deepest radio image of a globular cluster, and the deepest image ever made with the Australia Telescope Compact Array. We identify ATCA J002405.702-720452.361, a faint ($6.3\pm1.2$ $μJy$ at 5.5 GHz, $5.4\pm0.9$ $μJy$ at 9 GHz), flat-spectrum ($α=-0.31\pm0.54$) radio source that is positionally coincident with the cluster centre and potentially associated with a faint X-ray source. No convincing optical counterpart was identified. We use radio, X-ray, optical, and UV data to show that explanations involving a background active galactic nucleus, a chromospherically active binary, or a binary involving a white dwarf are unlikely. The most plausible explanations are that the source is an undiscovered millisecond pulsar or a weakly accreting black hole. If the X-ray source is associated with the radio source, the fundamental plane of black hole activity suggests a black hole mass of $\sim54-6000$ M$_{\odot}$, indicating an intermediate-mass black hole or a heavy stellar-mass black hole.
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