The Blazar Group's Work on Gamma-Ray Bright Blazars

The new gamma-ray NASA space observatory the Fermi Gamma-Ray Telescope was launched on June 11, 2008. The Boston University Blazar Group performs multiwaveband monitoring of a sample of gamma-ray blazars in support of the gamma-ray light curves that the Fermi Large Area Telescope ( LAT ) produces. The sample consists of 21 flat-spectrum radio quasars (FSRQs), 12 objects of the BL Lacertae type (BLLacs), and 3 radio galaxies (RGs). We carry out monthly total and polarized intensity imaging of the sources with the Very Long Baseline Array VLBA at 43 GHz and perform optical monitoring in the photometric, BVRI, and polarimetric, R-band, modes. This forms the core of our The VLBA-BU-BLAZAR program. In 2008-2013 we also performed multiwaveband campaigns (1-2 times per year) with 3 epochs of the VLBA observations during two weeks and intense X-ray, optical, and IR monitoring of the most gamma-ray active sources in our sample.

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.

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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