Galactic rays

Big things you can see—like a beam of light half a million light years long — need big explanations. Ditto for big things you can’t see — like a black hole holding the mass of a billion suns.

In a study published today, a group of scientists has linked the two, forging a clearer picture of how a gargantuan black hole in galaxy M87 is spewing a jet of light far longer than the galaxy is wide.

Black holes exist at the center of most galaxies. Now, using four radio telescopes spaced up to 4,500 kilometers apart, Sheperd Doeleman of MIT and colleagues have tracked the source of M87’s jet right down to edge of the blackness we call the black hole.

Perhaps because nobody else would understand the term, astronomers call the edge of darkness the “event horizon.”

At the edge of blackness

Albert Einstein taught that intense gravity bends space-time, and thus light. Nowhere is this more evident than around a black hole, whose stupendous gravity prevents light from escaping.

The event horizon is a tempestuous dividing line. Inside, gravitation gobbles all matter and light. Outside, astrophysical wonders release gobs of energy. “A naked black hole would be invisible,” says Doeleman, a radio astronomer at MIT’s Haystack Observatory, “but its intense gravitational pull drags everything — gas, dust and stars — into a cosmic traffic jam that heats up through friction as all this matter is trying to cram into a space where it can’t fit. We see a black hole because it’s a messy eater; it can’t eat fast enough to consume all the matter that is dragged into it.”

The event horizon, with its chaotic collisions of hot matter, creates massive amounts of X-rays and radio waves, which is what Doeleman is measuring with the “Event Horizon Telescope,” that merger of radio telescopes in Arizona, California and Hawaii.

By reading short wavelength radio waves, and using a good estimation of the distance to M87, Doeleman and colleagues were able to get a precise fix on the source of the jet. “Until now, we did not have a telescope with magnification powerful enough to resolve this region around the event horizon, so this has been the purview of computer simulations,” Doeleman says.

Primo ‘scope!

The jumbo combine of radio telescopes adds signals from the individual telescopes to give 2,000 times as much resolution as the Hubble Space Telescope. The system could, theoretically, see a grapefruit on the moon.

If the grapefruit held a strong radio transmitter.

The event horizon around the 7-billion solar mass black hole in M87 is about as big as our solar system. But seeing details from a distance of 50 light years requires the phenomenal resolution of the group of radio telescopes. Says Doeleman, “This is the first time we have been able to look so closely at the birth of one of these relativistic [near speed of light] jets, with an angular resolution that matches the scale on which the energy is extracted from the black hole.”

In other words, the enormous telescope, reading short radio waves, can see enough detail to see the jet being born just above the event horizon.

A jet with juice!

The astonishing energy in the jet — equivalent to about 10 billion suns — starts when electrons are accelerated along magnetic fields, Doeleman says. “The magnetic field lines are like a bunch of wires that have one end fixed to a turntable while the other ends whip around. The hot electrons love to corkscrew around the field lines, and are centrifugally accelerated away from the accretion disk, creating this visible beam that is moving as fast as matter can go.”

The photons — particles of light — are created through synchrotron emission: when a charged particle is accelerated, it makes a photon.

A region of ultra-strong gravity, where matter is moving close to the speed of light, is a place ruled by Einstein’s theories of relativity, Doeleman says. “We are using the strangeness of matter and light near a black hole to draw the conclusion that it has to be spinning, and also that the accretion disk has to be orbiting in the same direction. Most theoretical models show we can get jets from that configuration, but these are the first observations to confirm that.”

Now that signals from a radio telescope in Chile’s Atacama Desert are starting to feed into the system, stay tuned for more details on black holes. “Einstein tells you that if the black hole is spinning as fast as it can, the minimum orbit would be three days,” Doeleman says. “That means if we look consistently for a few days, we might be able to see evidence of features on the accretion disk. That would be a significant addition to our understanding of the dynamics of black holes.”