Blazars: Order and Disorder

Blazars are the most luminous persistent objects in the sky. They emit light across the entire electromagnetic spectrum, from low-energy radio waves to high-energy gamma-rays, and they exhibit variability on timescales that range from years down to minutes. The launch of the Fermi Large Area Telescope (LAT) in 2008 has ushered in a new era of high-energy astrophysics. Fermi provides an unprecedented vantage into the high-energy sky which, outside the Galactic plane, is dominated by blazar emission. One of the major unanswered questions in blazar research concerns the origin of the high-energy emission: precisely where in the jet are the gamma-rays produced? Yet another major unanswered question concerns the composition of blazars: are the high-energy emission processes mainly leptonic or hadronic in nature? In an attempt to address both of these questions, the Boston University Blazar Research Group has made use of the synergy between Fermi and the Very Long Baseline Array (VLBA), a continent spanning radio interferometer. By combining the milliarcsecond resolution of the VLBA with the high-energy coverage of Fermi, we are able to probe deep into the jets of blazars. Prompted by this rich multi-wavelength data set, we have developed two new models of high-energy blazar emission. In contrast with the more commonly accepted view that the gamma-rays originate from the base of the jet, our new models place the sites of gamma-ray production parsecs downstream of the central supermassive black holes from which these jets emanate. In this talk, I will present these two new theoretical models of high-energy blazar emission, which rely on both the ordered and disordered nature of the magnetic fields within the jet. Furthermore, I will show how radio polarimetric data obtained with the VLBA supports the astrophysical plausibility of these models and can potentially be used as a crucial probe of the composition of the jet plasma.

09/06/2016 - 12:30
Prof. Nicholas Macdonald
Boston University, USA