The Ultraluminous X-ray Sources near the Center of M82

TL;DR

This paper reports the discovery and analysis of a recurrent ultraluminous X-ray source in M82, likely an intermediate-mass black hole, along with other X-ray sources, based on multi-epoch Chandra and HST observations.

Contribution

It identifies a new recurrent ULX near M82's center, characterizes its spectral and temporal properties, and suggests it is an intermediate-mass black hole, also reporting related X-ray sources.

Findings

ULX first appeared in 1999, reappeared, and has been active since.

ULX luminosity varies from below detection to ~1.3e40 erg/s.

ULX is located within a young star cluster and associated with an H II region.

Abstract

We report the identification of a recurrent ultraluminous X-ray source (ULX), a highly absorbed X-ray source (possibly a background AGN), and a young supernova remnant near the center of the starburst galaxy M82. From a series of Chandra observations taken from 1999 to 2005, we found that the transient ULX first appeared in 1999 October. The source turned off in 2000 January, but later reappeared and has been active since then. The X-ray luminosity of this source varies from below the detection level (~2.5e38 erg/s) to its active state in between ~7e39 erg/s and 1.3e40 erg/s (in the 0.5-10 keV energy band) and shows unusual spectral changes. The X-ray spectra of some Chandra observations are best fitted with an absorbed power-law model with photon index ranging from 1.3 to 1.7. These spectra are similar to those of Galactic black hole binary candidates seen in the low/hard state except that a very hard spectrum was seen in one of the observations. By comparing with near infrared images taken with the Hubble Space Telescope, the ULX is found to be located within a young star cluster. Radio imaging indicates that it is associated with a H II region. We suggest that the ULX is likely to be a > 100 solar mass intermediate-mass black hole in the low/hard state. In addition to the transient ULX, we also found a highly absorbed hard X-ray source which is likely to be an AGN and an ultraluminous X-ray emitting young supernova remnant which may be related to a 100-year old gamma-ray burst event, within 2 arcsec of the transient ULX.