X-ray quasi-periodic eruptions from the galactic nucleus of RX J1301.9+2747

Following the recent discovery of X-ray quasi-periodic eruptions (QPEs) from the nucleus of the galaxy GSN 069, we report here on the detection of QPEs in the active galaxy RX J1301.9+2747. QPEs are rapid and recurrent increases of the X-ray count-rate by more than one order of magnitude with respect to a stable quiescent level. During a 48 ks long XMM-Newton observation performed on 30 and 31 May 2019, three strong QPEs lasting about half an hour each were detected in the light curves of RX J1301.9+2747. The first two QPEs are separated by a much longer recurrence time (about 20 ks) compared to the second and third (about 13 ks); this pattern is consistent with the alternating long-short recurrence times of the GSN 069 QPEs. Longer X-ray observations will better clarify the temporal pattern of the QPEs in RX J1301.9+2747 and will allow to perform a detailed comparison with GSN 069. The X-ray spectral properties of QPEs in the two sources are remarkably similar, with QPEs representing fast transitions from a relatively cold and likely disc-dominated state to a state characterized by warmer emission similar to the so-called soft X-ray excess, a component that is almost ubiquitously seen in the X-ray spectra of unobscured, radiatively efficient active galaxies. Previous X-ray observations of RX J1301.9+2747 in 2000 and 2009 strongly suggest that QPEs have been present for at least the past 18.5 years. The detection of QPEs from a second galactic nucleus after GSN 069 rules out contamination by a Galactic source in both cases, so that QPEs have to be considered as a novel extragalactic phenomenon associated with accreting supermassive black holes.