Probing Intergalactic Magnetic Fields in the GLAST Era through Pair Echo Emission from TeV Blazars

TL;DR

This paper investigates how detecting pair echo emissions from TeV blazars with GLAST can reveal properties of intergalactic magnetic fields, using detailed models of gamma-ray interactions and time-dependent spectra.

Contribution

It provides detailed calculations of pair echo spectra from flaring TeV blazars, considering echo geometry and effects, to constrain intergalactic magnetic fields with GLAST data.

Findings

Weak IGMF can enhance echo detectability

Detectable pair echos depend on IGMF strength and structure

Constraints on IGMF can be derived from non-detections

Abstract

More than a dozen blazars are known to be emitters of multi-TeV gamma rays, often with strong and rapid flaring activity. By interacting with photons of the cosmic microwave and infrared backgrounds, these gamma rays inevitably produce electron-positron pairs, which in turn radiate secondary inverse Compton gamma rays in the GeV-TeV range with a characteristic time delay that depends on the properties of the intergalactic magnetic field (IGMF). For sufficiently weak IGMF, such "pair echo" emission may be detectable by the Gamma-ray Large Area Space Telescope (GLAST), providing valuable information on the IGMF. We perform detailed calculations of the time-dependent spectra of pair echos from flaring TeV blazars such as Mrk 501 and PKS 2155-304, taking proper account of the echo geometry and other crucial effects. In some cases, the presence of a weak but non-zero IGMF may enhance the detectability of echos. We discuss the quantitative constraints that can be imposed on the IGMF from GLAST observations, including the case of non-detections.