Variability in a low-mass active galactic nucleus: oscillation or eruption?

Abstract

ABSTRACT Following the discovery of a new class of X-ray variability seen in four galaxies, dubbed quasi-periodic eruptions (QPEs), we reconsider the variability seen in the low-mass active galactic nucleus (AGN) 2XMM J123103.2+110648 to ascertain whether it should be considered a QPE host galaxy. We apply the autocorrelation function to two archival XMM–Newton observations to determine characteristic time-scales for variability of ∼13.52 and ∼14.35 ks. The modelling of light curves, both folded at these time-scales and unfolded, indicates that a Gaussian model is preferable over a sinusoidal model, with average durations for the bright phases of 6.17 and 7.69 ks. In a broad 0.2–1.0 keV band the average amplitude of the bright phases was found to be 2.86 and 8.56 times the quiescent count rate. The pattern of variability seen in 2XMM J123103.2+110648 cannot be definitively declared as a series of QPEs. Instead, this suggests there may be a continuum of quasi-periodic variability ranging from eruptions to oscillations being caused by a single mechanism. This offers the possibility of finding further sources that continue to bridge the gap between QPEs and quasi-periodic oscillations (QPOs). A targeted analysis of 47 observations of 11 other low-mass AGN (log (MBH) ≲ 6) found no evidence of QPE- or QPO-like behaviour in a sample of other similar mass objects.