An X-ray View of the Ambiguous Nuclear Transient AT2019pev

TL;DR

This study presents detailed X-ray observations of the ambiguous nuclear transient AT2019pev, revealing complex spectral and luminosity evolution that suggests it is more akin to an active galactic nucleus (AGN) transient than a tidal disruption event.

Contribution

It provides the first comprehensive X-ray analysis of AT2019pev, combining multi-wavelength data to clarify its nature as an AGN-like transient rather than a TDE.

Findings

X-ray luminosity increased fivefold within five days

X-ray spectra showed a 'harder-when-brighter' trend before peak

Lightcurves indicate a transition in accretion states

Abstract

AT2019pev is a nuclear transient in a narrow-line Seyfert 1 galaxy at $z=0.096$. The archival ultraviolet, optical and infrared data showed features of both tidal disruption events (TDEs) and active galactic nuclei (AGNs), and its nature is not fully understood. We present detailed X-ray observations of AT2019pev taken with Swift, Chandra and NICER over 173 days of its evolution since the first Swift XRT epoch. The X-ray luminosity increases by a factor of five in five days from the first Swift XRT epoch to the lightcurve peak. The lightcurve decays by a factor of ten over $\sim$75 days and then flattens with a weak re-brightening trend at late times. The X-ray spectra show a "harder-when-brighter" trend before peak and a "harder-when-fainter" trend after peak, which may indicate a transition of accretion states. The archival ground-based optical observations show similar time evolution as the X-ray lightcurves. Beyond the seasonal limit of the ground-based observations, the Gaia lightcurve is rising toward an equally bright or brighter peak 223 days after the optical discovery. Combining our X-ray analysis and archival multi-wavelength data, AT2019pev more closely resembles an AGN transient.