title: Ultraluminous X-ray bursts in two ultracompact companions to nearby elliptical galaxies.
abstract: A flaring X-ray source was found near the galaxy NGC 4697 (ref. 1). Two brief flares were seen, separated by four years. During each flare, the flux increased by a factor of 90 on a timescale of about one minute. There is no associated optical source at the position of the flares, but if the source was at the distance of NGC 4697, then the luminosities of the flares were greater than 1039erg per second. Here we report the results of a search of archival X-ray data for 70 nearby galaxies looking for similar flares. We found two ultraluminous flaring sources in globular clusters or ultracompact dwarf companions of parent elliptical galaxies. One source flared once to a peak luminosity of 9 × 1040erg per second; the other flared five times to 1040erg per second. The rise times of all of the flares were less than one minute, and the flares 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 γ repeaters that have repetitive flares of similar luminosities.
authors.affiliation: Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, Alabama 35487, USA.
authors.forename: Jimmy A
authors.initials: JA
authors.lastname: Irwin
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
authors.forename: W Peter
authors.initials: WP
authors.lastname: Maksym
authors.affiliation: Department of Physics, University of Alberta, CCIS 4-181, Edmonton, Alberta T6G 2E1, Canada.
authors.forename: Gregory R
authors.initials: GR
authors.lastname: Sivakoff
authors.affiliation: Department of Physics and Astronomy, San José State University, One Washington Square, San Jose, California 95192, USA.|University of California Observatories, 1156 High Street, Santa Cruz, California 95064, USA.
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Romanowsky
authors.affiliation: Space Science Center, University of New Hampshire, Durham, New Hampshire 03824, USA.
authors.forename: Dacheng
authors.initials: D
authors.lastname: Lin
authors.affiliation: Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, Alabama 35487, USA.
authors.forename: Tyler
authors.initials: T
authors.lastname: Speegle
authors.affiliation: Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, Alabama 35487, USA.
authors.forename: Ian
authors.initials: I
authors.lastname: Prado
authors.affiliation: Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, Alabama 35487, USA.
authors.forename: David
authors.initials: D
authors.lastname: Mildebrath
authors.affiliation: Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA.
authors.forename: Jay
authors.initials: J
authors.lastname: Strader
authors.affiliation: Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China.|College of Astronomy and Space Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China.
authors.forename: Jifeng
authors.initials: J
authors.lastname: Liu
authors.affiliation: Department of Astronomy, The University of Michigan, 1085 South University Avenue, Ann Arbor, Michigan 48103, USA.
authors.forename: Jon M
authors.initials: JM
authors.lastname: Miller