Letter to the Editor

Possible ∼0.4 h X-ray quasi-periodicity from an ultrasoft active galactic nucleus

¹ Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, PR China
e-mail: xwshu@ahnu.edu.cn
² CAS Key Laboratory for Researches in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, Anhui 230026, PR China
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁴ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China
⁵ Department of Astronomy, Guangzhou University, Guangzhou 510006, PR China

Received: 12 September 2020
Accepted: 15 November 2020

Abstract

RX J1301.9+2747 is an ultrasoft active galactic nucleus (AGN) with unusual X-ray variability that is characterized by a long quiescent state and a short-lived flare state. The X-ray flares are found to recur quasi-periodically on a timescale of 13−20 ks. Here, we report the analysis of the light curve in the quiescent state from two XMM-Newton observations spanning 18.5 years, along with the discovery of a possible quasi-periodic X-ray oscillation (QPO) with a period of ∼1500 s. The QPO is detected at the same frequency in the two independent observations, with a combined significance of > 99.89%. The QPO is in agreement with the relation between frequency and black hole mass (M_BH) that has been reported in previous works for AGNs and Galactic black hole X-ray binaries (XRBs). The QPO frequency is stable over almost two decades, suggesting that it may correspond to the high-frequency type found in XRBs and originates, perhaps, from a certain disk resonance mode. In the 3:2 twin-frequency resonance model, our best estimate on the M_BH range implies that a maximal black hole spin can be ruled out. We find that all ultrasoft AGNs reported so far display quasi-periodicities in the X-ray emission, suggesting a possible link on the part of the extreme variability phenomenon to the ultrasoft X-ray component. This indicates that ultrasoft AGNs could be the most promising candidates in future searches for X-ray periodicities.