An Off-Nucleus Nonstellar Black Hole in the Seyfert Galaxy NGC 5252

TL;DR

This paper reports the discovery of an off-nucleus ultraluminous X-ray source in NGC 5252, likely an accreting black hole with a mass over 10,000 solar masses, showing multiwavelength counterparts and associated emission lines.

Contribution

It presents the first detailed multiwavelength characterization of a ULX in NGC 5252, suggesting it is a massive black hole remnant rather than a background AGN.

Findings

ULX has X-ray luminosity of 1.5×10^40 erg/s

ULX exhibits radio, optical, UV counterparts

Optical emission lines indicate gravitational binding to NGC 5252

Abstract

We report the discovery of a ultraluminous X-ray source (ULX; CXO J133815.6+043255) in NGC 5252. This ULX is an off-nuclear point-source, which is 22$^{\prime\prime}$ away from the center of NGC 5252, and has an X-ray luminosity of 1.5 $\times$ $10^{40}$erg s$^{-1}$. It is one of the rare examples of ULX, which exhibits clear counterparts in radio, optical, UV bands. Follow-up optical spectrum of the ULX shows strong emission lines. The redshift of [O III] emission line coincides with the systematic velocity of NGC 5252, suggesting the ULX is gravitationally bound to NGC 5252. The flux of [O III] appears to be correlated with both X-ray and radio luminosity in the same manner as ordinary AGNs, indicating that the [O III] emission is intrinsically associated with the ULX. Based on the multiwavelength data, we argue that the ULX is unlikely to be a background AGN. A more likely option is an accreting BH with a black hole mass of $\geq 10^4M_\odot$, which might be a stripped remnant of a merging dwarf galaxy.