Observing small-scale $\gamma$-ray anisotropies with the Cherenkov Telescope Array

TL;DR

This paper explores how the Cherenkov Telescope Array (CTA) can detect small-scale gamma-ray anisotropies in the diffuse gamma-ray background, helping to identify unresolved sources and dark matter contributions at TeV energies.

Contribution

It proposes a method to analyze CTA data for gamma-ray anisotropies and predicts CTA's capability to probe unresolved sources at energies above 30 GeV.

Findings

CTA can detect anisotropies at relative amplitude > 4×10^{-3} sr above 30 GeV

CTA's analysis can distinguish contributions from blazars and active galactic nuclei

The method offers complementary systematic uncertainties to Fermi-LAT measurements.

Abstract

Disentangling the composition of the diffuse gamma-ray background (DGRB) is a major challenge in gamma-ray astronomy. It is presumed that at the highest energies, the DGRB is dominated by relatively few, still unresolved point sources. This conjecture has recently been supported by the measurement of small-scale anisotropies in the DGRB by the Fermi Large Area Telescope (LAT) up to energies of 500 GeV. We show how such anisotropies can be searched for with the forthcoming Earth-bound Cherenkov Telescope Array (CTA) up to the TeV range. We investigate different observation modes to analyse CTA data for small-scale anisotropies and propose the projected extragalactic large-area sky survey as the most promising data set. Relying on an up-to-date model of the performance of the southern CTA, we find that CTA will be able to probe anisotropies in the DGRB from unresolved point sources at a relative amplitude of $C_{\rm P}^I/I^2_{\rm DGRB}\gtrsim 4\times 10^{-3}\,{\rm sr}$ at energies above 30 GeV and angular scales $\lesssim 1.5^{\circ}$. Such DGRB anisotropies have not yet been ruled out by the Fermi-LAT. The proposed analysis would primarily clarify the contribution from blazars and misaligned active galactic nuclei to the very-high-energy regime of the DGRB, as well as provide insight into dark matter annihilation in Galactic and extragalactic density structures. Finally, it constitutes a measurement with complementary systematic uncertainties compared to the Fermi-LAT.