Gamma-ray observations of blazars and the intergalactic magnetic field spectrum

TL;DR

This paper examines how realistic models of intergalactic magnetic fields, considering their spectral properties, influence gamma-ray observations of blazars and the resulting constraints on magnetic field strength.

Contribution

It demonstrates that incorporating the magnetic field's spectral structure significantly alters observational bounds, especially for fields generated causally or during inflation.

Findings

Bounds depend on the magnetic field power spectrum.

Causal fields with $n_B \\geq 2$ tighten constraints.

Red spectra fields relax the bounds.

Abstract

Very-high energy observations of blazars can be used to constrain the strength of the intergalactic magnetic field. A simplifying assumption which is often made is that of a magnetic field of constant strength composed by randomly oriented and identical cells. In this paper, we demonstrate that a more realistic description of the structure of the intergalactic magnetic field is indeed needed. If such a description is adopted, the observational bounds on the field strength are significantly affected in the limit of short field correlation lengths: in particular, they acquire a dependence on the magnetic field power spectrum. In the case of intergalactic magnetic fields which are generated causally, for which the magnetic field large scale spectral index is $n_B\geq 2$ and even, the observational lower bound becomes more constraining by about a factor 3. If instead $-3<n_B<-2$, the lower bound is significantly relaxed. Such magnetic fields with very red spectra can in principle be produced during inflation, but remain up to now speculative.