How do you bolster the case for bagging a middleweight black hole?

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Astronomers are intrigued with this week's announcement that a team of Europeans has found the best evidence yet for a missing link along the evolutionary line of black holes -- a so-called intermediate-mass black hole.

The ultimate evidence may await a new generation of space telescopes to observe such objects.

Harvard University astrophysicist Avi Loeb points out that one way to confirm a very large black hole's mass is through its effect on the orbits of stars nearby. Those closer to the intermediate black hole will have far faster orbital velocities than stars farther away. That's what astronomers have done to estimate the mass of supermassive black holes at the hearts of galaxies, including the Milky Way.

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Another approach involves observing the object in the ultraviolet portion of the electromagnetic spectrum and comparing the results with modeling estimates of what those emissions should look like for an intermediate-mass black hole. Sean Farrell, who led the research team, says that's the next step the team is taking.

Dr. Farrell's colleague Oliver Godet adds that another way to confirm HLX-1's mass is by looking at the signature of iron atoms in the object's spectrum. Iron's spectral lines "are thought to originate from ionized iron gas in the accretion disc close to the black hole," he writes in an email. That line should be highly distorted by the effects of being so close to the black hole.

But it will take a new generation of more sensitive x-ray telescopes to tease out that signature, he adds.

Help may be on the way. NASA, ESA, and Japan's space agency are looking at a joint International X-ray Observatory project. Simulations of that orbiting telescope's capabilities suggest it would be able to make the kind of observation Dr. Godet describes.

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