Luminous X-ray Flares from Low Mass X-ray Binary Candidates in the Early-type Galaxy Ngc

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, 3 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 erg 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. Subject headings: binaries: close — galaxies: elliptical and lenticular — galaxies: individual (NGC 4697) — X-rays: binaries — X-rays: bursts — X-rays: galaxies