Strange gamma ray flash caused by black hole eating star

Posted by Kate Taylor

A mysterious gamma ray flash observed by the Swift satellite in late March was probably caused by a star being swallowed by a massive black hole, astronomers have concluded.

The gamma-ray flare, called Sw 1644+57, was highly unusual because of its long duration and the fact that it appeared to come from the center of a galaxy nearly four billion light years away.

Analysis of the Swift data and subsequent observations by the Hubble Space Telescope and the Chandra X-ray Observatory have now given support to the theory that the busrst was caused by the relatively slow tidal disruption of an infalling star.

"This burst produced a tremendous amount of energy over a fairly long period of time, and the event is still going on more than two and a half months later," says Joshua Bloom, an associate professor of astronomy at UC Berkeley.

"That's because as the black hole rips the star apart, the mass swirls around like water going down a drain, and this swirling process releases a lot of energy. This is truly different from any explosive event we have seen before," Bloom said.

Bloom reckons around 10 percent of the infalling star's mass has been turned into energy and blasted out as X-rays from the swirling accretion disk, or as X-rays and higher energy gamma rays from a relativistic jet that punches out along the rotation axis. Earth just happened to be in exactly the right direction to reecive the full force of the gamma-ray beam.

"We argue that this must be jetted material and we're looking down the barrel," he said. "Jetting is a common phenomenon when you have accretion disks, and black holes actually prefer to make jets."

Looking back at previous observations of this region of the cosmos, Bloom and his team found no evidence of X-ray or gamma-ray emissions, supporting the view that this was a one-off event.

"Here, you have a black hole sitting quiescently, not gobbling up matter, and all of a sudden something sets it off," Bloom said. "This could happen in our own galaxy, where a black hole sits at the center living in quiescence, and occasionally burbles or hiccups as it swallows a little bit of gas. From a distance, it would appear dormant, until a star randomly wanders too close and is shredded."

Probable tidal disruptions of a star by a massive black hole have previously been seen at X-ray, ultraviolet and optical wavelengths, but never before at gamma-ray energies. Such random events, especially looking down the barrel of a jet, are incredibly rare - "probably once in 100 million years in any given galaxy," says Bloom.

"I would be surprised if we saw another one of these anywhere in the sky in the next decade," he says.

The astronomers suspect that the gamma-ray emissions began March 24 or 25 in the uncatalogued galaxy at a redshift of 0.3534, putting it at a distance of about 3.8 billion light years. Bloom and his colleagues expect the emissions to fade over the next year.

"We think this event was detected around the time it was as bright as it will ever be, and if it's really a star being ripped apart by a massive black hole, we predict that it will never happen again in this galaxy," he says.