# A ~3.8 hour Periodicity from an Ultrasoft Active Galactic Nucleus Candidate

## TL;DR

This study reports a significant ~3.8-hour X-ray quasi-periodic oscillation in an ultrasoft AGN candidate, suggesting a low-mass black hole and providing insights into AGN variability and accretion processes.

## Contribution

First detection of a ~3.8-hour QPO in an ultrasoft AGN candidate, linking it to low-frequency QPOs in galactic black hole binaries and analyzing its spectral properties.

## Key findings

- QPO detected at ~5 sigma significance in 2005 observations
- QPO rms increases from ~25% to ~50% across energy bands
- Spectral fits consistent with thermal disk or low-temperature Comptonization

## Abstract

Very few galactic nuclei are found to show significant X-ray quasi-periodic oscillations (QPOs). After carefully modeling the noise continuum, we find that the ~3.8 hr QPO in the ultrasoft active galactic nucleus (AGN) candidate 2XMM J123103.2+110648 was significantly detected (~5sigma) in two XMM-Newton observations in 2005, but not in the one in 2003. The QPO rms is very high and increases from ~25% in 0.2-0.5 keV to ~50% in 1-2 keV. The QPO probably corresponds to the low-frequency type in Galactic black hole X-ray binaries, considering its large rms and the probably low mass (~10^5 msun) of the black hole in the nucleus. We also fit the soft X-ray spectra from the three XMM-Newton observations and find that they can be described with either pure thermal disk emission or optically thick low-temperature Comptonization. We see no clear X-ray emission from the two Swift observations in 2013, indicating lower source fluxes than those in XMM-Newton observations.