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Attack of the Blobs

Published: 07-09-2009

A Hubble Space Telescope image of a blazar galaxy. Credit: NASA, HST (WFPC2), J. Bahcall

Supermassive black holes inside blazar galaxies emit powerful jets of particles traveling in opposite directions near the speed of light. Some are aimed toward the Earth. These jets emit radio waves, which makes them visible to radio telescopes as they streak across the sky. By studying these radio waves, scientists have determined that the jets are traveling at about 99.5% the speed of light and thus exhibit the effects of relativity. The blazars themselves are unusually variable, and many emit ultrahigh-energy γ-rays.

Two blazars occasionally spit out blobs emitting ultrahigh-energy γ-rays about every 3–5 minutes, a veritable attack of the blobs! The luminosity of the γ-ray flares emitted by the blobs is equivalent to all the stars in a galaxy simultaneously lighting up.

The γ-ray-emitting blobs travel at extremely high speeds toward the Earth. An array of four telescopes on Earth recently detected their ultrahigh-energy γ-rays as the γ-rays interacted with our upper atmosphere, producing flashes of blue light.

Several research groups have used data gathered by the telescopes to learn more about the origin of the γ-ray flares. The researchers realized that their source must be very small and compact. The source must also have been created inside the primary jets produced by the two blazars. One of the main problems researchers had to solve was why the source of the γ-ray flares was traveling a lot faster than the primary jets, at 99.99% the speed of light, as compared to 99.5% the speed of light for the primary jets.

Fellow Mitch Begelman and colleagues from Princeton University and Germany’s Max Planck Institute for Astrophysics recently explained all these observations. The attack of the blobs is due to a "jet in a jet." The researchers determined that small jets containing γ-ray-emitting blobs are created during explosions inside the primary jets. These explosions release internal tension created by the energy of very high magnetic fields. It's as if the main jets have high-energy battery packs that must be discharged every few minutes. The energy of the explosions propels the little jets to nearly the speed of light. The little jets then spit out the blobs that radiate ultrahigh-energy γ-rays.

"We've come up with an explanation that reconciles the observation of two different measurements of jet speeds at different wavelengths," said Begelman. "There are actually two jets. Under the right conditions, little jets just start popping off inside the big jets every three to five minutes until things settle down again."

Begelman says it’s likely that there are huge numbers of such little jets going off inside the big jets produced by supermassive black holes inside galaxies. However, most of them aren’t pointed toward the Earth where we can observe them.   - Julie Phillips


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