Messier 87 Shows Off for Hundreds of Earth-bound Astronomers

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When the giant radio galaxy Messier 87 (M 87) unleashed a torrent of gamma radiation and radio flux, an international collaboration of 390 scientists happened to be watching. They’re reporting the discovery in this week’s issue of Science Express.

The results give first experimental evidence that particles are accelerated to extremely high energies in the immediate vicinity of a supermassive black hole and then emit the observed gamma rays. The gamma rays have energies a trillion times higher than the energy of visible light.

Matthias Beilicke and Henric Krawczynski, both physicists at Washington University in St. Louis, coordinated the project using the Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaboration. The effort involved three arrays of 12-meter (39-foot) to 17-meter (56-foot) telescopes, which detect very high-energy gamma rays, and the Very Long Baseline Array (VLBA) that detects radio waves with high spatial precision.

“We had scheduled gamma-ray observations of M 87 in a close cooperative effort with the three major gamma-ray observatories VERITAS, H.E.S.S. 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,” Beilicke said.

“Only combining the high-resolution radio observations with the VHE gamma-ray observations allowed us to locate the site of the gamma-ray production,” added R. Craig Walker, a staff scientist at the National Radio Astronomy Observatory in Socorro, New Mexico.

M 87 is located at a distance of 50 million light years from Earth in the Virgo cluster of galaxies. The black hole in the center of M 87 is six billion times more massive than the Sun.

The size of a non-rotating black hole is given by the Schwarzschild radius. Everything — matter or radiation — that comes within one Schwarzschild radius of the center of the black hole will be swallowed by it. The Schwarzschild radius of the supermassive black hole in M 87 is comparable to the radius of our Solar System.

In the case of some supermassive black holes — as in M 87 — matter orbiting and approaching the black hole powers highly relativistic outflows, called jets. The matter in the jets travels away from the black hole, escaping its deadly gravitational force. The jets are some of the largest objects in the Universe, and they can reach out many thousands of light years from the vicinity of the black hole into the intergalactic medium.

Very high-energy gamma-ray emission from M 87 was first discovered in 1998 with the HEGRA Cherenkov telescopes. “But even today, M 87 is one of only about 25 sources outside our galaxy known to emit [very high energy] gamma rays,” says Beilicke.

The new observations now show that the particle acceleration, and the subsequent emission of gamma rays, can happen in the very “inner jet,” less than about 100 Schwarzschild radii away from the black hole, which is an extremely narrow space as compared with the total extent of the jet or the galaxy.

In addition to VERITAS and the VLBA, the High Energy Stereoscopic System (H.E.S.S.) and the Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC) gamma-ray observatories were involved in these observations.

Lead image caption: Artists’s Conception of M87’s inner core: Black hole, accretion disk, and inner jets. Credit: Bill Saxton, NRAO/AUI/NSF

Second image: Large-scale VLA image of M87: White circle indicates the area within which the gamma-ray telescopes could tell the very energetic gamma rays were being emitted. To narrow down the location further required the VLBA. CREDIT: NRAO/AUI/NSF

Collage: At top left, a VLA image of the galaxy shows the radio-emitting jets at a scale of about 200,000 light-years. Subsequent zooms progress closer into the galaxy’s core, where the supermassive black hole resides. In the artist’s conception (background). the black hole illustrated at the center is about twice the size of our Solar System, a tiny fraction of the size of the galaxy, but holding some six billion times the mass of the Sun. Credit: Bill Saxton, NRAO/AUI/NSF

Sources: Science and the National Radio Astronomy Observatory, via Eurekalert.