Fermi Spies Energetic Blazar Flare

by Nicholos Wethington on December 11, 2009

A comparison of the Fermi images from November 2nd and December 3rd of this year, showing the brightening of 3C 454.3. Image Credit: NASA

A comparison of the Fermi images from November 3rd and December 2nd of this year, showing the brightening of 3C 454.3. Image Credit: NASA/DOE/Fermi LAT Collaboration

The blazar 3C 454.3, a bright source of gamma rays from a galaxy 7 billion light-years away just got a whole lot brighter. Observations from the Fermi gamma-ray telescope confirm that since September 15th the blazar has flared up considerably, increasing in gamma-ray brightness by about ten times in the from earlier this past summer, making it currently the brightest gamma-ray source in the sky.

3C 454.3 is a blazar, a jet of energetic particles that is caused by the supermassive black hole at the center of a galaxy. Most galaxies are thought to house a supermassive black hole at their center, and as it chomps down matter from the accretion disk that surrounds it, the supermassive black hole can form large jets that stream out light and energy in fantastic proportions. In the case of 3C 454.3, one of these jets is aimed at the Earth, which allows for us to see and study it.

This blazar has started to outshine the Vela pulsar, which because it is only 1,000 light-years away from the Earth is generally the brightest gamma-ray source in the sky. 3C 454.3 is almost twice as bright as Vela in the gamma-ray part of the spectrum, even though it lies 7 million times further away from the Earth. 3c 454.3 has also brightened significantly in the infrared, X-ray, radio and visible light.

This is not the first time the blazar has shown an increase in brightness. Over the course of observations of the blazar, it flared-up in brightness in May 2005, and again in July and August of 2007.

Dr. Erin Wells Bonning, Postdoctoral Associate at the Yale Center for Astronomy and Astrophysics, said of the recent flare in comparison with previous brightening events:

“In 2005, it reached a R-band magnitude of 12. Our peak observed R-band magnitude was 13.83, so we’re still not at the brightness of the 2005 outburst (about a factor of 5 below). On July 19, 2007, it reached a R-band magnitude of 13, not as bright as the 2005 event, but still brighter than we see it now. In 2005, there were no gamma-ray instruments to observe 3C 454.3, but the 2007 flare was observed by AGILE with a flux above 100 MeV of 3 +- 1 * 10^-6 cts/s/cm^s. The Fermi and AGILE count rates for Dec 2-3, 2009  are 6-9 times as high. So, interestingly, although it is not currently as bright optically as it was in 2007, it is a good deal brighter in gamma-rays.”

The Fermi gamma-ray space telescope (formerly GLAST) keeps tabs on the gamma-ray emissions from many sources in the sky. 3C 454.3 is just one of the top ten brightest sources of gamma-rays visible to the satellite, a list of which can be found in an article Nancy wrote in March, The Top Ten Gamma-Ray Sources from the Fermi Telescope.

Of course, the blazar 3C 454.3 is not as intrinsically bright as many of the Gamma-Ray Bursts observed by telescopes like Swift and Fermi, but it is the consistently brightest source of gamma-rays in the sky right now. Bonning said that, “While both GRBs and blazars are highly beamed toward us, the Lorentz factors (speed of particles in the jet) associated with GRBs are much higher than in blazars, causing them to appear brighter due to special relativistic effects.”

Observations 3C 454.3 are continuing in all wavelengths to capture the light curve of the event, and better understand these periodic flares. Bonning said, “The source has been relatively quiescent since it emerged from behind the Sun, and began to increase in brightness around the end of July. It then entered a bright period of fairly rapid variability, peaking every 20 days or so. The most recent, very intense, flare began around the end of November. Per our [Astronomer's Telegram], since Nov 21, 3C 454 has increased about a factor of 3 in brightness in both optical and infrared. (B, V, and R filters are in optical wavelengths, and J and K are near-infrared).  Similarly, the gamma-ray flux has increased also by a factor of 3 in the 0.1-300 GeV band over the same period.”

The cause of the intermittent flare-ups in 3C 454.3 and other blazars is still a mystery, but this current brightening will give astronomers better data as to what the possible cause could be. There seem to be no periodic events associated with the flares in blazars (with the exception of the possible “supermassive black hole binary” OJ 287).

Bonning said of a potential cause, “This is actually a very active field of research – there are numerous existing models, but no one hypothesis is clearly preferred. Perhaps particles have been shocked at some location in the blazar jet, or the jet may be precessing so that is closer to our line of sight, or there may be some other explanation.”

There will be numerous telescopes around the world zooming in on the current flare-up. According to Bonning:

“Blazars are multi-wavelength objects — their spectral energy distribution covers radio through gamma-rays, so a diverse collection of facilities will be observing 3C 454.3 during this outburst. Besides Fermi, the Italian AGILE satellite has been observing in gamma rays. The Swift X-ray telescope began monitoring in early December.  The blazar monitoring group at Boston University headed by Alan Marscher is observing it with VLBA (radio; 13GHz). There is also a radio astronomy group at Michigan also observing with VLBA, as well one headed by Yuri Kovalev at Max Planck institute in Germany.  There is an optical program with the ATOM telescope associated with the HESS TeV instrument in Namibia. (3C 454.3 is not bright at TeV energies, by the way.)  This is not an exhaustive list by any means, but at any rate numerous facilities across the globe and operating at a wide range of energies will be taking a very close look at 3C 454.3 as it goes through this flare.”

Source: NASA press release, email interview with Erin Wells Bonning

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