Gamma rays from distant blazar originated from within relativistic jet

December 21, 2015

Very Energetic Radiation Imaging Telescope Array System (VERITAS) detected gamma rays from a distant blazar with supermassive black hole in its middle. This offered a chance to pinpoint the source of the gamma rays. Surprisingly, the source is most likely 5 light-years from the supermassive black hole, within the relativistic jet that the supermassive black hole emitted.

Artist's impression of a blazar. Credit: M. Weiss/CfA

First, let’s see what quasar, blazar and relativistic jet are. Then we’ll talk about where the source of gamma rays is and finally how they have to travel through a fog-like extragalactic background light before they reach us.

Quasars and blazars emit gamma rays

A quasar is a region of space that emits strong electromagnetic radiation. It is basically a supermassive black hole surrounded by heated gas and dust. The gas and dust rotate around and feed the supermassive black hole, but some of the material gets ejected in the form of two jets at the speeds near to the speed of light. A more compact version of a quasar is called blazar.

Quasars, blazars and their jets are a strong source of electromagnetic radiation over the entire spectrum, including powerful gamma rays. When one of the jets, called relativistic jets, point in our direction, we can detect its gamma radiation and gain insight about its source.

Where is the source of the observed gamma rays?

One such stream of gamma rays came from a galaxy called PKS 1441+25 and was picked up by Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona, USA. 

"We're looking down the barrel of this relativistic jet," explains one of the study’s team members, Wystan Benbow of the Harvard-Smithsonian Center for Astrophysics, Cambridge, USA. "That's why we're able to see the gamma rays at all."

Now, we know that relativistic jets come from the supermassive black hole, but where is the exact source of this gamma burst in a blazar? Using data from VERITAS, astronomers found it to be within one the relativistic jets, at least 0.1 light-year from the supermassive black hole, but most likely 5 light-years away. The source is also very huge - about one third of a light-year in diameter, which is larger than what we normally see in quasars.

"These jets tend to have clumps in them. It's possible that two of those clumps may have collided and that's what generated the burst of energy," says co-author Matteo Cerruti, also of Harvard-Smithsonian Center for Astrophysics.

The paper with results has been accepted for publication in The Astrophysical Journal Letters.

Gamma rays passing through the extragalactic background light

Although they are powerful, gamma rays are not that easy detect when they come from a source almost 13 billion light-years away. They have to be strong enough to pass through something called an extragalactic background light (EBL). Think of it as a light fog that saturates the Universe.

The extragalactic background light comes from all of the stars and other light sources in all the galaxies that ever existed, making other sources harder to spot. This includes gamma rays from blazars. However, when we do detect gamma rays from such a distant source, it can help determine the upper limit of density of the extragalactic background light.

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