Time Delay of Cascade Radiation for TeV Blazars and the Measurement of the Intergalactic Magnetic Field

TL;DR

This paper discusses how the measurement of the intergalactic magnetic field strength using cascade radiation from TeV blazars depends on the blazar's activity timescale, leading to more robust lower limits.

Contribution

It introduces a method to set more reliable lower limits on the intergalactic magnetic field by restricting blazar activity duration based on observational data.

Findings

Lower limit of B_IGMF >~ 1e-18 G based on 3-4 years of blazar activity.

The limit could be higher if the blazar's intrinsic flux at >5-10 TeV is strong.

The measurement depends on the assumed timescale of blazar activity.

Abstract

Recent claims that the strength B_IGMF of the intergalactic magnetic field (IGMF) is >~ 1e-15 G are based on upper limits to the expected cascade flux in the GeV band produced by blazar TeV photons absorbed by the extragalactic background light. This limit depends on an assumption that the mean blazar TeV flux remains constant on timescales >~2 (B_ IGMF/1e-18 G)^2 / (E/{10 GeV})^2 yr for an IGMF coherence length ~ 1 Mpc, where E is the measured photon energy. Restricting TeV activity of 1ES 0229+200 to ~3 -- 4 years during which the source has been observed leads to a more robust lower limit of B_IGMF >~ 1e-18 G, which can be larger by an order of magnitude if the intrinsic source flux above ~5 -- 10 TeV from 1ES 0229+200 is strong.