Degree-scale GeV "jets" from active and dead TeV blazars

TL;DR

This paper demonstrates that degree-scale GeV jet-like extensions around TeV blazars, caused by electromagnetic cascades and magnetic deflections, can reveal historical activity of the sources and properties of the extragalactic magnetic field.

Contribution

The study introduces a method to use GeV jet-like extensions to trace the historical activity of TeV blazars and infer properties of the extragalactic magnetic field.

Findings

Degree-scale GeV extensions can reflect TeV blazar activity over 10 million years.

Brightness profiles of these extensions can reveal the structure of the extragalactic magnetic field.

Such GeV jets could be detected from both active blazars and their remnants.

Abstract

We show that images of TeV blazars in the GeV energy band should contain, along with point-like sources, degree-scale jet-like extensions. These GeV extensions are the result of electromagnetic cascades initiated by TeV gamma-rays interacting with extragalactic background light and the deflection of the cascade electrons/positrons in extragalactic magnetic fields (EGMF). Using Monte-Carlo simulations, we study the spectral and timing properties of the degree-scale extensions in simulated GeV band images of TeV blazars. We show that the brightness profile of such degree-scale extensions can be used to infer the lightcurve of the primary TeV gamma-ray source over the past 1e7 yr, i.e. over a time scale comparable to the life-time of the parent active galactic nucleus. This implies that the degree-scale jet-like GeV emission could be detected not only near known active TeV blazars, but also from "TeV blazar remnants", whose central engines were switched off up to ten million years ago. Since the brightness profile of the GeV "jets" depends on the strength and the structure of the EGMF, their observation provides additionally information about the EGMF.