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

Abstract

We report the discovery of an ultraluminous X-ray source (ULX; CXO J133815.6+043255) in NGC 5252. This ULX is an off-nuclear point source, which is 22″ away from the center of NGC 5252, and has an X-ray luminosity of 1.5 × 10⁴⁰ erg s^-1. It is one of the rare examples of a ULX, which exhibits clear counterparts in radio, optical, and UV bands. A follow-up optical spectrum of the ULX shows strong emission lines. The redshift of the [O iii] emission line coincides with the systematic velocity of NGC 5252, suggesting that 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 active galactic nuclei (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 black hole with a mass of ≥slant {10}⁴ {M}_⊙ , which might be a stripped remnant of a merging dwarf galaxy.