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Gamma rays burst from M87's black hole

Posted: JULY 3, 2009

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High energy gamma rays bursting from the galaxy M87 are found to be emanating from a region very close to the supermassive black hole at its heart.

Last month we reported on new computer modelling that upped the mass of M87's black hole by three times over, making it a monstrous 6.4 billion solar masses (read the story here). Like all black holes, it sucks material in from its surroundings, forming a tightly rotating accretion disc around the jaws of the monster. Immense gravitational energy propels material outwards as jets.

Streaming out from the centre of the galaxy M87 is a black-hole powered jet of electrons and other sub-atomic particles traveling at nearly the speed of light. Image: NASA and The Hubble Heritage Team (STScI/AURA).

It has been known since 1998 that M87 was emitting impressively high energy flares of gamma rays, but their exact point of origin could not be discerned. Now, thanks to an international collaboration of nearly 400 scientists working on three different telescope arrays, the gamma rays and accompanying radio flux are found to originate from the immediate vicinity of the black hole.

The astronomers used the National Science Foundation's continent-wide Very Long Baseline Array (VLBA), the Very Energetic Radiation Imaging Telescope Array System (VERITAS), the High Energy Stereoscopic System (HESS) and the Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC).

"Combining the gamma-ray observations with the supersharp radio 'vision' of the VLBA allowed us to see that the gamma rays are coming from a region very near the black hole itself," says Craig Walker of the National Radio Astronomy Observatory (NRAO).

"We had scheduled gamma-ray observations of M87 in a close cooperative effort with the three major gamma-ray observatories VERITAS, HESS and MAGIC, and we were lucky that an extraordinary gamma-ray flare happened just when the source was observed with the VLBA and its impressive spatial resolving power," adds Matthias Beilicke of Washington University.

A very high energy gamma ray flare was accompanied by a radio flare from the black hole vicinity.

The team also found that M87's most energetic gamma-ray flares were accompanied by the largest flare of radio waves seen from that galaxy. The radio flare began at about the time of the gamma-ray flares, but continued to increase in brightness for at least two months. "This tells us that energetic material burst out very close to the black hole, causing the gamma rays to be emitted and the radio flare to begin," says Walker. "As that material traveled down the jet, expanding and losing energy, the gamma-ray emission ceased, but the radio continued to increase in brightness. The VLBA showed us with great precision where the radio emission came from, so we know the gamma rays came from closer in toward the black hole."

Pinning down the location of these emissions provides important clues for understanding how such highly energetic emissions are produced in the jets of active galaxies. The new measurements indicate that the gamma rays are coming from an area no larger than 50 times the size of the event horizon - the boundary around the black hole from which matter cannot escape. In the case of M87, the black hole's event horizon equates to a distance of two times our Solar System.

The scientists reported their findings in the July 2 online edition of the journal Science.

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