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Search for GeV γ-Ray Pair Halos Around Low Redshift Blazars
Wenlei Chen1, James H Buckley1, Francesc Ferrer1
1Department of Physics and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, USA.
Physical Review Letters
|December 5, 2015
Summary
Researchers found evidence of gamma-ray pair halos around low-redshift blazars using Fermi data. This suggests the presence of intergalactic magnetic fields, crucial for understanding cosmic ray propagation.
Area of Science:
- High-energy astrophysics
- Cosmic ray physics
Background:
- Blazars are active galactic nuclei emitting powerful jets.
- Gamma-ray pair halos are a predicted phenomenon from blazar emissions interacting with intergalactic magnetic fields.
- Previous searches for these halos have yielded inconclusive results.
Purpose of the Study:
- To search for gamma-ray pair halos around low-redshift blazars.
- To investigate the properties of intergalactic magnetic fields.
Main Methods:
- A stacking analysis of gamma-ray data from the Fermi Large Area Telescope was performed.
- 24 isolated high-synchrotron-peaked BL Lacs with redshift z<0.5 were selected based on spatial and spectral criteria.
- Frequentist and Bayesian statistical methods were used to analyze photon distributions.
Main Results:
- An excess of ~1 GeV photons was detected around the stacked blazars, indicating extended emission beyond point sources.
- A frequentist test yielded a p-value of ~0.01, rejecting the point-source hypothesis.
- Bayesian analysis provided Bayes factors log_{10}B_{10}>2, supporting the presence of pair halos.
Conclusions:
- The study provides evidence for the existence of gamma-ray pair halos.
- The results are consistent with pair halos produced in intergalactic magnetic fields of strength 10^{-17}-10^{-15} G.
- This finding has implications for understanding the extragalactic magnetic field and cosmic ray propagation.
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