The physical nature of intensely bright X-ray sources found in external galaxies, but offset from the galactic nucleus, is currently a hot topic in high-energy astrophysics. This paper exploited some of the first observations from the Chandra X-ray observatory to provide a sharp look at the most extreme case known of this class of source.

The key to the paper is the use of a new instrument: Chandra. Using Chandra, we were able to isolate the intensely bright source from surrounding sources, to demonstrate that it is significantly offset from the galaxy nucleus, and to show that it is highly variable.

The first very bright X-ray sources were found in external galaxies by Pepi Fabbiano and collaborators with the Einstein observatory about 20 years ago. Over the past decade, Ed Colbert, Andy Ptak, Tim Roberts, and several others extended those results and showed that some sources are too bright to be powered by stellar-mass black holes if the X-rays are emitted isotropically. In this case, the objects would be "intermediate-mass" black holes with masses higher than those of normal stars, but well below those of supermassive black holes found in galactic nuclei. If the emission is beamed towards us, rather than isotropic, then the objects could be stellar-mass black holes or neutron stars and likely represent an unusual stage in the evolution of binary systems containing these objects. Much of the current work focuses on whether the X-rays are beamed or isotropic, and on how "intermediate-mass" black holes could be formed.

The brightest X-ray sources in our own Milky Way are black holes accreting matter from a companion star. The brightness of an accreting black hole is limited because the outward pressure exerted by radiation from near the black hole will overwhelm the graviational attraction of the black hole if the radiation is too strong and stop the inflow of matter which powers the radiation (the Eddington limit). More massive black holes have stronger gravitational fields and can, therefore, be brighter X-ray sources. The X-ray source described in the paper appears to be more than a factor of 10 times brighter than any X-ray source in our Galaxy. This suggests that it may be an unusually massive black hole. The lower limit on the black hole mass—if the X-rays are radiated isotropically—is 500 times the mass of our sun. This is above the mass of any known star and would require a new mechanism for its production. Alternatively, the X-rays may be beamed towards us. In this case a "standard" black hole could suffice, but the binary system would need to be in an unusual and previously unknown evolutionary state to produce the beaming. In either case, the X-ray source represents a novel object and further study will help extend our understanding of accreting black holes beyond those found in our own Galaxy.