Missing Gamma-ray Halos and the Need for New Physics in the Gamma-ray Sky

TL;DR

This study searches for gamma-ray halos around TeV-bright active galactic nuclei to test intergalactic magnetic field theories, finding no evidence for such halos and suggesting the need for new physics explanations.

Contribution

It provides the first observational constraints on gamma-ray halos around radio galaxies, challenging magnetic field-based explanations for their absence.

Findings

No gamma-ray halos detected around radio galaxies.

Limits on intergalactic magnetic field strength are tightened to below 10^{-15} G.

Purely magnetic explanations for halo absence are excluded.

Abstract

An intergalactic magnetic field stronger than $3\times10^{-13}$~G would explain the lack of a bright, extended degree-scale, GeV-energy inverse Compton component in the gamma-ray spectra of TeV-blazars. A robustly predicted consequence of the presence of such a field is the existence of degree-scale GeV-energy gamma-ray halos -- gamma-ray bow ties -- about TeV-bright active galactic nuclei, corresponding to more than half of all radio galaxies. However, the emitting regions of these halos are confined to and aligned with the direction of the relativistic jets associated with gamma-ray sources. Based on the orientation of radio jets, we align and stack corresponding degree-scale gamma-ray images of isolated Fanaroff-Riley class I and II objects and exclude the existence of these halos at overwhelming confidence, limiting the intergalactic field strength to $<10^{-15}$~G for large-scale fields and progressively larger in the diffusive regime when the correlation length of the field becomes small in comparison to 1 Mpc. When combined with prior limits on the strength of the intergalactic magnetic field, this excludes a purely magnetic explanation for the absence of halos. Thus, it requires the existence of novel physical processes that preempt the creation of halos, e.g., the presence of beam-plasma instabilities in the intergalactic medium or a drastic cutoff of the very high energy spectrum of these sources.