Ultraluminous X-ray bursts in two ultracompact companions to nearby elliptical galaxies

Abstract

An X-ray flaring source was found near the galaxy NGC 4697. Two flares were seen, separated by four years. The flux increased by a factor of 90 on a timescale of about one minute. Both flares were very brief. There is no optical counterpart at the position of the flares, but if the source was at the distance of NGC 4697, the luminosities were 10^39 erg/s. Here we report the results of a search of archival X-ray data for 70 nearby galaxies looking for similar such flares. We found two flaring sources in globular clusters or ultra-compact dwarf companions of parent elliptical galaxies. One source flared once to a peak luminosity of 9 x 10^40 erg/s, while the other flared five times to 10^40 erg/s. All of the flare rise times were <1 minute, and they then decayed over about an hour. When not flaring, the sources appear to be normal accreting neutron star or black hole X-ray binaries, but they are located in old stellar populations, unlike the magnetars, anomalous X-ray pulsars or soft gamma repeaters that have repetitive flares of similar luminosities.