NuSTAR Observations of Candidate Subparsec Binary Supermassive Black Holes

We present analysis of NuSTAR X-ray observations of three AGN that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on apparent periodicity in their optical light curves. Simulations predict that close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs. We previously observed these AGN with Chandra and found no differences between their low energy X-ray properties and the larger AGN population. However some models predict differences to be more prominent at energies higher than probed by Chandra. We find that even at the higher energies probed by NuSTAR, the spectra of these AGN are indistinguishable from the larger AGN population. This could rule out models predicting large differences in the X-ray spectra in the NuSTAR bands. Alternatively, it might mean that these three AGN are not binary SMBHs.

我们对三个活动星系核（AGN）的NuSTAR X射线观测结果展开分析。这些AGN基于其光学光变曲线中的明显周期性特征，被卡特琳娜实时暂现源巡天（Catalina Real-Time Transient Survey）识别为候选亚秒差距双超大质量黑洞（SMBH）系统。模拟结果预测，近距吸积的双SMBH系统将具有与单个吸积SMBH不同的X射线光谱。我们此前利用钱德拉（Chandra）望远镜观测这些AGN，发现其低能X射线特性与更广泛的AGN群体无显著差异。然而，部分模型预测，在钱德拉望远镜探测范围之外的更高能量段，这种差异会更为显著。我们发现，即便在NuSTAR探测的更高能量段，这些AGN的光谱仍与更广泛的AGN群体无法区分。这一结果或可排除那些预测NuSTAR波段X射线光谱存在显著差异的模型，亦或表明这三个AGN并非双SMBH系统。