An ultra-luminous X-ray source (ULX) is an object (excluding a galactic nucleus) that is more luminous in its X-ray emission alone than one million suns are at all wavelengths. Most galaxies, including our own Milky Way, have no ULXs; galaxies that do host a ULX usually have only one. ULXs are mysterious because if they were stars (or anything else) with about the mass of the sun, their huge luminosities should tear them apart. Because ULXs stretch our current understanding they are not just mysterious -- they are also interesting and important.
Several possible explanations for ULXs have been advanced. If the X-ray emission were somehow beamed directly towards us, for example, then our estimates of their total luminosities would be very much smaller, and so the radiation would be less destructive. Unfortunately this solution cannot realistically address the most extreme cases. A more likely suggestion is that ULXs are black holes accreting material from a surrounding disk -- but even they must be more massive than the sun...and no one understands how such "intermediate mass" black holes can form.
Writing in the last issue of Monthly Notices of the Royal Astronomical Society, SAO astronomers Roberto Soria, Alessandro Baldi, Guido Risaliti, Giuseppina Fabbiano, and Andreas Zezas, together with two colleagues, report seeing the flaring of a ULX in a large, relatively nearby spiral galaxy. Using the Chandra X-ray Observatory, the team saw the source vary by a factor of three over one week. Combining this result with other information about the source, the astronomers argue that their serendipitous discovery has enabled them to constrain the most-likely scenario for this ULX: a black hole with a mass of between about 50 and 150 solar masses, and with a powerful jet of ejected material. Although they cannot determine how such an object was produced in the first place, the result does strengthen the case that some ULXs are much more massive than a single, sun-like star.