Possible ~0.4 hour X-ray quasi-periodicity from an ultrasoft active galactic nucleus

TL;DR

This study reports a stable ~1500s X-ray quasi-periodic oscillation in the ultrasoft AGN RX J1301.9+2747 over 18.5 years, linking it to disk resonance modes and suggesting ultrasoft AGNs as prime candidates for X-ray periodicity searches.

Contribution

First detection of a long-term stable QPO in an ultrasoft AGN, connecting it to black hole mass and disk resonance models, and highlighting ultrasoft AGNs as promising for future periodicity studies.

Findings

QPO detected at ~1500s in two observations with >99.89% significance.

QPO frequency consistent with black hole mass and disk resonance models.

Ultrasoft AGNs commonly show quasi-periodicities, indicating a potential link to their extreme variability.

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 observations spanning 18.5 years, along with the discovery of a possible quasi-periodic X-ray oscillation (QPO) with a period of ~1500s. 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.