Double whammy. Minutes after the initial fireworks, some gamma-ray bursts flare up again in x-rays, as depicted in this artist's rendering.

Do Black Holes Play With Their Food?

Like the aftershocks of an earthquake, violent x-ray flares have been discovered in the aftermath of gamma-ray bursts, the most powerful explosions in the universe. Although the source of the flares is unclear, the finding may help astronomers better understand the first few minutes in the lives of black holes.

Most gamma-ray bursts are believed to signal the violent death of rapidly spinning, massive stars. The core of the star collapses into a black hole, while powerful jets of matter and energy are blown into space almost at the speed of light. Shocks in the jets produce the initial burst of gamma rays, which may last from just a few seconds to a couple of minutes. Subsequent interactions between the jets and the surrounding interstellar gas create a slowly fading afterglow, lasting for days, weeks, or even months.

Now, the X-ray Telescope onboard NASA's Swift satellite has found energetic flares of x-rays happening minutes after the initial gamma-ray burst. Swift was launched in November 2004 (ScienceNOW, 24 January) and is the first satellite able to quickly point its telescopes at a newly spotted gamma-ray burst. On 6 April and 2 May 2005, Swift detected x-ray flares minutes after the onset of two gamma-ray bursts in the constellations Eridanus the River and Leo the Lion. The 2 May x-ray flare, occurring 12 minutes after the burst, carried almost as much energy as the original gamma-ray explosion--"something never before seen and quite unexpected," according to a paper by David Burrows of Pennsylvania State University in University Park and colleagues published online today by Science. The 6 April flare was less powerful but still very conspicuous.

Burrows's team argues that the x-ray flares can best be explained as postnatal burps and hiccups of the newly-born black hole. Perhaps the hungry baby is feasting on ejected material that falls back into its gravitational muzzle, they report.

The finding is "quite surprising," says gamma-ray burst researcher Ralph Wijers of the University of Amsterdam, the Netherlands. But it's not self-evident that the black hole is messing around with its food minutes after its birth, he says. Instead, the x-ray flares could be due to delayed shocks in the jets that were produced at the time of the burst, or they could be part of the early burst afterglow. More observations may reveal whether the x-ray aftershocks are telling astronomers something about the black hole itself or about its environment. Right now, says Wijers, "it's an interesting riddle."

Related Web sites
NASA's Swift satellite
Gamma-ray burst primer