University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Colloquium

Thursday, September 22nd 2016
3:35 pm:
Speaker: Lucy Fortson
Subject: Blazar Observations with VERITAS: expanding the cosmic gamma-ray horizon
Refreshments to be served outside Keller 3-210 after the colloquium.

Blazars are Active Galactic Nuclei (AGNs) that launch collimated jets of relativistic particles towards the Earth. They emit radiation that spans the entire observable electromagnetic spectrum from radio to gamma-ray energies and which varies in intensity over a wide range of timescales, from minutes to years. For the past decade, the VERITAS gamma-ray observatory has been recording very-high-energy (VHE ≥ 100 GeV) photons, yielding a catalog of over 175 VHE gamma-ray sources of which almost 40% are blazars. Pair production interactions between VHE and lower-energy photons strongly attenuate the flux of gamma-rays as they propagate to Earth through a cosmological photon field known as the extragalactic background light (EBL). This is expected to impose a gamma-ray horizon at redshifts well below unity, beyond which VHE gamma-ray sources are rendered undetectable to the current generation of instruments. I will describe recent results from the VERITAS blazar observing program including the exciting and unexpected detection of a particularly high redshift (z~1) blazar. I will then describe efforts to use blazar gamma-ray spectra to constrain the EBL and as a probe for the presence of Ultra-High Energy Cosmic Ray emission from blazars.

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