A New Ultraluminous X-Ray Source in the nearby Edge-on Spiral NGC 891

Abstract

We report the discovery of a new candidate ultraluminous X-ray source in the nearby edge-on spiral galaxy NGC 891. The source, which has an absorbed flux of F_x ~ 1 × 10^-12 erg s cm^-2 (corresponding to an L_x >~ 10⁴⁰ erg s^-1 at 9 Mpc), must have begun its outburst in the past five years as it is not detected in prior X-ray observations between 1986 and 2006. We try empirical fits to the XMM-Newton spectrum, finding that the spectrum is fit very well as emission from a hot disk, a cool irradiated disk, or blurred reflection from the innermost region of the disk. The simplest physically motivated model with an excellent fit is a hot disk around a stellar-mass black hole (a super-Eddington outburst), but equally good fits are found for each model. We suggest several follow-up experiments that could falsify these models.