天文学：天の川銀河系の外側で見つかった謎のX線フレア

Two new, enigmatic sources of X-ray flares - explosive bursts of X-rays - in galaxies near the Milky Way are described in a study published in Nature this week. Although the nature of these sources is uncertain, the authors note that they are unlike any known object in the Milky Way and seem to be located in old stellar populations.

Prior to this study, two very brief X-ray flares with high luminosity were detected near the galaxy NGC 4697, in 2003 and 2007. Jimmy Irwin and colleagues examined archival data from Chandra X-ray observations for 70 galaxies near the Milky Way to find similar flares. They investigated several thousand X-ray point sources and identified just two sources of similar flares. One of the sources flared once, the other flared five times. The authors find that the time it took the flares to rise was less than one minute in all cases, and the flares decayed over the course of about one hour. They note that, unlike other astronomical objects that produce flares, such as those associated with gamma ray bursts or supernovae, these sources do not self-destruct in the process of flaring.

The authors' analysis of the flares suggests that the sources, when not flaring, appear to be accreting in globular clusters or ultracompact dwarf galaxies near elliptical galaxies.