# Bow Ties in the Sky II: Searching for Gamma-ray Halos in the Fermi Sky Using Anisotropy

## TL;DR

This paper introduces a new anisotropy-based stacking method to detect gamma-ray halos around extragalactic sources in Fermi LAT data, improving robustness against systematic uncertainties.

## Contribution

It presents a novel approach using angular power spectra stacking to identify gamma-ray halos, enhancing detection capabilities with existing data.

## Key findings

- Method can detect gamma-ray halos with current Fermi LAT data.
- Approach is robust against instrumental and source contamination.
- Applicable to a broad range of intergalactic magnetic field scenarios.

## Abstract

Many-degree-scale gamma-ray halos are expected to surround extragalactic high-energy gamma ray sources. These arise from the inverse Compton emission of an intergalactic population of relativistic electron/positron pairs generated by the annihilation of >100 GeV gamma rays on the extragalactic background light. These are typically anisotropic due to the jetted structure from which they originate or the presence of intergalactic magnetic fields. Here we propose a novel method for detecting these inverse-Compton gamma-ray halos based upon this anisotropic structure. Specifically, we show that by stacking suitably defined angular power spectra instead of images it is possible to robustly detect gamma-ray halos with existing Fermi Large Area Telescope (LAT) observations for a broad class of intergalactic magnetic fields. Importantly, these are largely insensitive to systematic uncertainties within the LAT instrumental response or associated with contaminating astronomical sources.