Some black holes are too big. Some black holes are too small. A letter appearing in this week's edition of Nature describes how astronomers may have found one that is just right.

The letter, written by a team of British and French astronomers, does not state that they have found an intermediate mass black hole—one that could be termed just right—but that they have found an object where most other explanations fail to explain its behavior.

The object, 2XMM J011028.1-460421 or (more conveniently) HLX-1, is a source of ultraluminous X-rays near the spiral galaxy ESO 243-49. These X-rays have been postulated to be the product of an intermediate mass black hole, one between 100 and 10,000 solar masses, but to date no candidate object has been widely accepted.

Initially observed in November 2004, HLX-1 was re-imaged in November 2008. In order to understand the nature of the ultraluminous X-rays emanating from HLX-1, the researchers needed to understand its relationship with the host galaxy. They first attempted to see if the X-rays at HLX-1 could be due to a random alignment of some background galaxies within the field of the ESO 243-49 host galaxy, but a series of 1,000,000 Monte Carlo simulations found this outcome to have a probability of only nine percent. The authors are able to rule out any foreground interference due to the shape of the X-ray spectrum.

Given that that HLX-1 appears to be part of ESO 243-49, the authors went on to examine other possible causes. According to the paper, a Galactic white dwarf accreting from a low-mass neighbor would show up in the X-ray spectrum, but not the visible spectrum. A low mass binary neutron star system would not exhibit the same type of spectrum observed—one that is consistent with an accreting black hole.

Some blazars could have a similar spectrum, but the low adsorption seen here rules that possibility out. The fact that there is a difference in the observations between 2004 and 2008 suggest that this is not a case of multiple low-intensity sources adding together. Finally, the derived luminosity and spectrum shape aren't consistent with the idea that this is due to either relativistic or geometric beaming.

With most other causes exhausted, we're left with the conclusion that HLX-1 is the "strongest case thus far for the existence of intermediate-mass black holes." Calculations based around the luminosity of this object suggest that it has a lower mass limit of approximately 500 solar masses, putting it squarely in the middle range for this class of object. However, the authors admit that this is a conservative estimate.

Nature, 2009. DOI: 10.1038/nature08083

Listing image by Heidi Sagerud