The intermediate-mass black hole 2XMM J123103.2+110648: a varying disc accretion rate during possible X-ray quasi-periodic eruptions?

TL;DR

This study analyzes the X-ray spectral evolution of the IMBH candidate 2XMM J123103.2+110648, suggesting that its variability, including possible QPEs, could be driven by changes in accretion rate or disc inclination.

Contribution

The paper provides the first spin measurement for this source and models its spectral evolution with a slim disc, linking short-term QPOs to accretion rate variations.

Findings

Black hole mass estimated at (~6±3)×10^4 M_sun

Spectral evolution explained by varying accretion rate or inclination

QPO spectral differences consistent with changing accretion rate

Abstract

We fit the evolving X-ray spectra of the variable and fading source 2XMM J123103.2+110648 (J1231), which is an intermediate-mass black hole (IMBH) candidate. Recent X-ray timing studies have proposed that J1231's quasi-periodic oscillation (QPO) observed at the peak of its X-ray lightcurve is a variant of the quasi-periodic eruptions (QPEs) observed in other sources. Here, we fit X-ray spectra from XMM-Newton, Swift, and Chandra using a slim disc model for the black hole's accretion disc, obtaining a best-fit black hole mass of ($6\pm3)\times10^{4}$ $M_\odot$ and spin of $>0.6$ at 2$\sigma$ confidence. This mass is consistent with past estimates, supporting the IMBH interpretation, and the spin measurement is new. Yet the nature of J1231 remains uncertain: its long-term variability (decade-long continuum evolution) could signal a tidal disruption event or active galactic nuclear variability. We find that the spectral evolution within the first three years after the source's detection can be well explained by either a varying disc accretion rate $\dot m$ or a varying disc inclination $\theta$. Meanwhile, we find that during the short-term variability (the QPO with a ~3.8hr period), each oscillation does not show the "hard-rise-soft-decay" typical of QPEs. We fit the average spectrum at the QPO lightcurve maxima and the average spectrum at its minima, finding that the spectral difference is well explained by $\dot m$ decreasing from peaks to valleys if $\theta<30^{\circ}$ and constant between all data epochs. This result suggests that the short-term QPO behaviour might also be driven by a varying disc $\dot m$.