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

• Published in: The Astrophysical journal Vol. 686; no. 2; pp. L67 - L70
• Main Authors: Murase, Kohta, Takahashi, Keitaro, Inoue, Susumu, Ichiki, Kiyomoto, Nagataki, Shigehiro
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 20.10.2008; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; TeV range; Radiation mechanism; gamma rays: theory; Gamma radiation; Intergalactic magnetic fields; Cosmic radio sources; Blazar; BL Lacertae objects: general; Magnetic fields; BL Lacertae objects; Nonthermal radiation; radiation mechanisms: nonthermal
• ISSN: 1538-4357; 0004-637X; 1538-4357
• DOI: 10.1086/592997

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 echoes 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 nonzero IGMF may enhance the detectability of echo. We discuss the quantitative constraints that can be imposed on the IGMF from GLAST observations, including the case of nondetections.