Fermi/LAT observations of 1ES 0229+200: implications for extragalactic magnetic fields and background light

TL;DR

This paper uses Fermi and HESS observations of the blazar 1ES 0229+200 to constrain the properties of extragalactic magnetic fields and background light, showing their interdependence and providing limits on both.

Contribution

It presents combined constraints on the EBL and EGMF derived from gamma-ray observations, highlighting their mutual dependence and providing a two-dimensional exclusion plot.

Findings

An EBL density twice the Franceschini et al. (2008) value is allowed with a strong EGMF.

An EGMF strength as low as 6x10^-18 G is compatible with a low EBL density.

Constraints depend on the interplay between EBL and EGMF, affecting the interpretation of gamma-ray data.

Abstract

We report the observation in the GeV band of the blazar 1ES 0229+200, which over recent years has become one the primary sources used to put constraints on the Extragalactic Background Light (EBL) and Extragalactic Magnetic Field (EGMF). We derive constraints on both the EBL and EGMF from the combined Fermi-HESS data set taking into account the direct and cascade components of the source spectrum. We show that the limit on the EBL depends on the EGMF strength and vice versa. In particular, an EBL density twice as high as that derived by Franceschini et al. (2008) is allowed if the EGMF is strong enough. On the other hand, an EGMF strength as low as 6x10^-18 G is allowed if the EBL density is at the level of the lower bound from the direct source counts. We present the combined EBL and EGMF limits on as an exclusion plot in two dimensional parameter space: EGMF strength vs. EBL density.