The Blazar Group's Work on Gamma-Ray Bright Blazars
The Compton Gamma Ray Observatory (EGRET high-energy instrument) detected roughly 70 blazars in the course of its sky survey and pointed observations. In support of this effort, we undertook a massive campaign to monitor the parsec-scale structure of 42 gamma-ray blazars over a nearly four-year period with the VLBA at 43 GHz. View the results of this project and the images on our web page devoted to VLBA images of gamma-ray bright blazars.
One of the major new NASA space observatory projects is the Gamma-ray Large Area Space Telescope (GLAST), which is scheduled for launch in August 2007. Alan Marscher is a member of the pre-launch Users Committee. The Boston University Blazar Group plans to mount major multiwaveband monitoring campaigns in support of the gamma-ray light curves that GLAST will produce. For the brightest gamma-ray blazars - mostly those detected by EGRET - the gamma-ray flux (brightness) and spectrum should be measured daily, allowing multiwaveband timing with unprecedented accuracy. This will allow blazar researchers to construct multiwaveband emission maps of the relativistic jets of blazars. We expect that this information will provide the clues needed to determine how the jet produces gamma rays, just how close to the speed of light the plasma in the jet flows, the physics of particle acceleration in jets, and how the jet is accelerated and focused into a narrow beam.
New Project: INTEGRAL Observation to Search for Antimatter in 3C 120
We have been awarded 500,000 seconds (about 6 days) of observing time on the INTEGRAL satellite to try to detect the electron-positron annihilation line toward 3C 120. If, as many theories and observations suggest, the jet is composed mainly of electrons and positrons (anti-electrons) as opposed to the electron-proton make-up of "normal" matter, then our calculations indicate that an emission line should be detected at an energy of 495 keV. This is lower than the energy of 511 keV seen in laboratories on Earth because of the redshift of 3C 120, z=0.033. [Energies and frequencies are lowered by a factor of (1+z) because of the expansion of the universe.] The emission line comes from the positrons colliding and annihilating with electrons in the interstellar medium. This should occur in 3C 120, since the jet seems to be interacting strongly with the interstellar gas as the jet propagates through the galaxy.
Various lines of argument suggest that positrons may greatly outnumber protons in jets: how much energy is transported out of a radio galaxy by its jets, how to explain X-rays from jets on scales of hundreds of kiloparsecs, and the observed levels of circular polarization on parsec scales without too much Faraday depolarization. But there are ways out of each of these even if the jets contain mostly normal matter. Our INTEGRAL observations will either detect the annihilation line or place interesting upper limits on the number of positrons produced per second in the jet.
Working with us on this project are Ian
McHardy (U. Southampton, England) and Jose-Luis Gomez (Instituto de Astrofisica
de Andalucia, Spain).
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