Insights from the "Red devil" AT 2022fpx: A Dust-reddened Family of Tidal Disruption Events Excluded by Their Apparent Red Color?

TL;DR

This paper investigates the peculiar, dust-reddened tidal disruption event AT 2022fpx, challenging the typical 'blue color' criterion for optical TDEs and exploring implications for their detection and energy estimates.

Contribution

It reveals that red-colored TDEs like AT 2022fpx exist and may be overlooked by traditional blue-based selection methods, suggesting a need to revise TDE identification criteria.

Findings

Red TDEs challenge the 'blue color' selection criterion.

Blackbody fits underestimate the energy of optical-UV bright, X-ray faint TDEs.

SEDs peak at shorter wavelengths than blackbody models suggest.

Abstract

We report unnoticed but intriguing features in the peculiar nuclear transient AT 2022fpx, and investigate its type. These features include the constantly red optical color of $g-r>0$, a stable soft X-ray flare ($kT\sim100$ eV) in the past $\sim$550 days, a prominent mid-infrared echo peaked at $\sim$$10^{43.3}$ erg s$^{-1}$ and the confirmation of a weak active galactic nucleus by weak flares in pre-event Wide-field Infrared Survey Explorer mid-infrared light curves with no contemporary optical, radio or X-ray counterparts. The combination of the optical red color and possible origin of a tidal disruption event (TDE) of AT 2022fpx is particularly attractive, as it challenges the most widely accepted and adopted "blue color" criterion for optical TDE selection. Although we still cannot confirm whether the red color is intrinsic, we do find that the "blue color" criterion can filter out normal TDEs whose optical-UV spectral energy distributions (SEDs) are either severely contaminated by prominent emission lines (especially H$\alpha$) or heavily dust-reddened. Hence, its potential selection effect may have been imprinted on the whole optical TDE family. Blackbody fitting on the optical (rest-frame $\sim$$4000-7000$ \AA) and optical-UV ($\sim$$2000-7000$ \AA) SEDs of four TDEs with high-cadence UV observations shows that $T_\mathrm{bb}$ rise by $\sim$40$-$110 \% when the UV bands are included. The power-law models ($f_{\lambda}\propto\lambda^{-\alpha}$ with $\alpha=2-3$) can fit the rest-frame $\sim$$2000-7000$ \AA SEDs more consistently, indicating that SEDs should peak at shorter wavelengths, but not simple blackbodies. Hence, the estimated released energy for the optical-UV bright but X-ray faint TDEs based on blackbody SED fitting should be significantly lower than the intrinsic energy.