Luminous X-Ray Flares from Low-Mass X-Ray Binary Candidates in the Early-Type Galaxy NGC 4697

Abstract

We report results of the first search specifically targeting short-timescale X-ray flares from low-mass X-ray binaries in an early-type galaxy. A new method for flare detection is presented. In NGC 4697, the nearest, optically luminous, X-ray-faint elliptical galaxy, three out of 157 sources are found to display flares at >99.95% probability, and all show more than one flare. Two sources are coincident with globular clusters and show flare durations and luminosities similar to (but larger than) type I X-ray superbursts found in Galactic neutron star (NS) X-ray binaries (XRBs). The third source shows more extreme flares. Its flare luminosity (~6×10³⁹ ergs s^-1) is very super-Eddington for an NS and is similar to the peak luminosities of the brightest Galactic black hole (BH) XRBs. However, the flare duration (~70 s) is much shorter than are typically seen for outbursts reaching those luminosities in Galactic BH sources. Alternative models for the flares are considered.