A Side of Science: What is a blazar?

In my latest cosmic game connection, I discuss a recent discovery of a neutrino that came from near a black hole in a galaxy more than 4 billion light years away.  That black hole was at the center of a type of galaxy called a blazar, which are cool galaxies shooting a jet of material directly toward us.

Credit: M. Weiss/CfA
  • Blazars are galaxies with supermassive black holes at their centers – just like most (all?) large galaxies including our Milky Way.
  • In some galaxies, that central black hole has a disk of material, called an accretion disk, that is full of material actively being pulled in to the black hole. The matter forms a disk to get rid of some of its angular momentum, and spends time spiraling around. As the matter falls inward, it accelerates and some of its gravitational energy gets converted into light. These galaxies are called “active galaxies” or “active galactic nuclei” (since it’s that central region that’s active).
  • Even cooler, though, in some of these active galaxies, there is a jet of material that gets launched out from near the black hole.
  • What we see from active galaxies can depend on how that black hole is oriented relative to our line of sight. In blazars, we are looking right down the jet!
Credit: ESA/Hubble & NASA
  • The first blazar was identified in 1929, but it was listed as a variable star – a star that changes over time. However, after the development of radio astronomy, some of these variable stars were showing significant radio emission, which is unusual for stars. There were other unusual things about these objects – they didn’t vary the way stars typically do, for example. Then, researchers found that one of these objects, called 3C 273, was several billion light years away – way further than any individual star. (For comparison, the Milky Way is 100,000 light years in diameter.) And, evidence for a host galaxy was found for another blazar in 1974 – clearly knocking stars out of contention for explaining these objects.

    This visualization shows gamma rays detected during 3C 279’s big flare by the LAT instrument on NASA’s Fermi satellite. Credit: NASA/DOE/Fermi LAT Collaboration
  • Blazars can be very bright in gamma-ray light. In fact, one of the instruments on NASA’s Fermi Gamma-ray Space Telescope has seen nearly 1500 blazars, which constitutes more than half of the sources detected by that instrument.

And, in case you missed my blazar neutrino video, be sure to check it out!

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