A supermassive black hole under the radar: repeating X-ray variability in a Seyfert galaxy

Abstract: In the last few years, a few supermassive black holes (SMBHs) have shown short-term (of the order of hours) X-ray variability. Given the limited size of the sample, every new addition to this class of SMBHs can bring invaluable information. Within the context of an automated search for X-ray sources showing flux variability in the \textit{Chandra} archive, we identified peculiar variability patterns in 2MASX J12571076+2724177 (J1257), a SMBH in the Coma cluster, during observations performed in 2020. We investigated the long-term evolution of the flux, together with the evolution of the spectral parameters throughout the \textit{Chandra} and \textit{XMM-Newton} observations, which cover a time span of approximately 20 years. We found that J1257 has repeatedly shown peculiar variability over the last 20 years, on typical timescales of $\simeq20-25$ ks. From our spectral analysis, we found hints of a softer-when-brighter behaviour and of two well-separated flux states. We suggest that J1257 might represent a new addition to the ever-growing size of relatively low mass SMBHs ($M\simeq10<sup>6-10<sup>7\mathrm{M}_\odot$) showing extreme, possibly quasi-periodic X-ray variability on short time scales. The available dataset does not allow for a definitive classification of the nature of the variability. However, given the observed properties, it could either represent a quasi-periodic oscillation at particularly low frequency or be associated with quasi-periodic eruptions in an AGN with peculiar spectral properties.