Supermassive Black Hole Swallows Star, Flashes Bright Gamma-Ray Burst

A dazzling burst of gamma rays observed in March from a supermassive black hole could have been due to a star falling into the massive black hole and being ripped apart, according to a team of astronomers.

The burst of gamma rays was first observed by NASA's Swift spacecraft on March 28, Swift's Burst Alert Telescope discovered the source in the constellation Draco when it erupted with the first in a series of powerful X-ray blasts. The satellite determined a position for the explosion, now cataloged as gamma-ray burst (GRB) 110328A, and informed astronomers worldwide.

At first, the astronomers thought it was an emission of gamma rays from a dying star - a normal phenomenon. Astronomers say they have never seen anything this bright, long-lasting and variable before. Usually, gamma-ray bursts mark the destruction of a massive star, but flaring emission from these events never lasts more than a few hours.

But the when the flash sustained for weeks although it faded in intensity, they realized that it wasn't a typical gamma-ray burst at all, but a high-energy jet produced as a star about the size of our sun was shredded by a black hole a million times more massive.

This is truly different from any explosive event we have seen before, Joshua Bloom, an associate professor of astronomy at UC Berkeley, said in a statement.

As dozens of telescopes turned to study the spot, astronomers quickly noticed that a small, distant galaxy appeared very near the Swift position. A deep image taken by Hubble on April 4 pinpoints the source of the explosion at the center of this galaxy, which lies 3.8 billion light-years away.

Astronomers say that the unusual blast likely arose when a star wandered too close to its galaxy's central black hole. Intense tidal forces tore the star apart, and the infalling gas continues to stream toward the hole. According to this model, the spinning black hole formed an outflowing jet along its rotational axis.

Scientists think that the X-rays may be coming from matter moving near the speed of light in a particle jet that forms as the star's gas falls toward the black hole.

What made this gamma-ray flare, called Sw 1644+57, stand out from a typical burst were its long duration and the fact that it appeared to come from the center of a galaxy nearly 4 billion light years away, Bloom added.

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, said Bloom. 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.

Since most, if not all, galaxies are thought to contain a massive black hole at the center, a long-duration burst could conceivably come from the relatively slow tidal disruption of an infalling star, the astronomers said.

We know of objects in our own galaxy that can produce repeated bursts, but they are thousands to millions of times less powerful than the bursts we are seeing now. This is truly extraordinary, said Andrew Fruchter at the Space Telescope Science Institute in Baltimore, said in a statement in April.

The astronomers suspect that the gamma-ray emissions began March 24 or 25 in the uncatalogued galaxy at a redshift of 0.3534. Bloom and his colleagues estimate that the emissions will fade over the next year.

Most galaxies, including our own, contain central black holes with millions of times the sun's mass; those in the largest galaxies can be a thousand times larger. The disrupted star probably succumbed to a black hole less massive than the Milky Way's, which has a mass four million times that of our sun.

Astronomers previously have detected stars disrupted by supermassive black holes, but none have shown the X-ray brightness and variability seen in GRB 110328A. The source has repeatedly flared. Since April 3, for example, it has brightened by more than five times.

Scientists think that the X-rays may be coming from matter moving near the speed of light in a particle jet that forms as the star's gas falls toward the black hole.

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 said.

Bloom and his colleagues propose in their Science Express paper that some 10 percent of the infalling star's mass is turned into energy and irradiated 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 the eye of the gamma-ray beam.

In addition, Bloom draws an analogy with a quasar, a distant galaxy that emits bright, high-energy light because of the massive black hole at its center gobbling up stars and sending out a jet of X-rays along its rotation axis. Observed from an angle, these bright emissions are called active galactic nuclei, but when observed down the axis of the jet, they're referred to as blazars.

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 could find no evidence of X-ray or gamma-ray emissions, leading them to conclude that this is a one-off event, Bloom said.

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.

A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.

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. I would be surprised if we saw another one of these anywhere in the sky in the next decade, Bloom said.