Preposterous Universe

Wednesday, May 11, 2005
Black hole being born

It looks like we may have seen the first moments of a black hole being born. This image (from Joshua Bloom's web page) shows the optical counterpart of a new gamma-ray burst discovered by the Swift satellite. It seems likely that this event has resulted from the coalescence of two neutron stars. As the neutron stars spiral together, the smaller one is ripped apart by tidal forces, spreading into a disk of material. Some of the material accretes on the remaining neutron star, which reaches the point of no return and explodes, creating a black hole. Part of the matter outside the black hole is violently ejected, crashing into the surrounding interstellar medium, producing the flash of gamma-rays detected by Swift.

I won't go into too many details about the story, since Steinn SigurĂ°sson knows this stuff much better than I do, and has explained what is happening in a series of great posts (one, two, three), from which I've stolen everything I'm writing here (and apologies in advance for what I mess up). Gamma-ray bursts are ultra-high energy events at cosmological distances (i.e., well outside our galaxy) that have fascinated astrophysicists for years now. The crucial point is that there are two types of bursts, long-duration (a few seconds) and short-duration (hundredths of seconds). The long-duration bursts are thought to arise from especially violent supernova explosions, and are typically found in galaxies undergoing copious star formation. The short-duration ones are likely to come from the coalescence of compact objects like neutron stars or black holes. This leads to a couple of expectations: close binary systems of compact objects can take a very long time to coalesce, so there's no reason to find them near star-forming regions (all the stars having formed long ago). Also, the closeness of the binary often arises through close gravitational interactions with other stars, which can serve to give the binary a serious kick, so that it zooms right out of the host galaxy.

However, until now astronomers had never been able to pinpoint an optical counterpart to a short-period burst. (It's hard to determine the direction from which gamma rays are coming. Swift uses a multi-stage technique, in which it first detects the gammas and then zooms in with X-ray and optical telescopes.) The image above shows the optical counterpart of this particular burst, GRB 050509b. It is short-duration, and appears to be lurking on the outskirts of a giant elliptical galaxy. Such galaxies are no longer forming stars, and the fact that the object is outside the galaxy proper (by perhaps 35000 parsecs) makes it fit nicely with the coalescing-neutron-star model. So those few glowing pixels are perhaps the baby photos of a new black hole. Amazing what stories astronomers can spin from such sparse data.

Update: In the comments, Matt points out that the purported optical counterpart doesn't seem to be varying, which a real gamma-ray burst afterglow certainly would be. But as Steinn says, the X-ray counterpart is definitely there.

Ideas on culture, science, politics.
Sean Carroll

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