Constraints on the extragalactic magnetic field strength from blazar spectra based on 145 months of Fermi-LAT observations
Abstract
Properties of the extragalactic magnetic field (EGMF) outside of clusters and filaments of the large-scale structure are essentially unknown. The EGMF could be probed with γ-ray observations of distant (redshift z > 0.1) blazars. TeV γ-rays from these sources are strongly absorbed on extragalactic background light photons; secondary electrons and positrons produce cascade γ-rays with the observable flux dependent on EGMF parameters. We put constraints on the EGMF strength using 145 months of Fermi-LAT observations of the blazars 1ES 1218+304, 1ES 1101-232, and 1ES 0347-121, and imaging atmospheric Cherenkov telescope observations of the same sources. We perform a series of full direct Monte Carlo simulations of intergalactic electromagnetic cascades with the ELMAG 3.01 code and construct a model of the observable spectra inside the point spread functions of the observing instruments for a range of EGMF strengths. We compare the observed spectra with the models for various values of the EGMF strength B and calculate the exclusion statistical significance for every value of B. We find that the values of the EGMF strength B ≤ 10-17 G are excluded at a high level of the statistical significance Z > 4σ for all the four options of the intrinsic spectral shape considered (power law, power law with exponential cutoff, log-parabola, and log-parabola with exponential cutoff). The value of B = 10-16 G is not excluded; it is still a viable option of the EGMF strength. These results were obtained for the case of steady sources.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2022
- DOI:
- 10.1093/mnras/stac2509
- arXiv:
- arXiv:2204.11110
- Bibcode:
- 2022MNRAS.516.5379P
- Keywords:
-
- magnetic fields;
- methods: data analysis;
- methods: numerical;
- gamma-rays: general;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 10 pages, 6 figures, 2 tables. Published in MNRAS. This is the authors' version, not the final typeset. Minor main text changes w.r.t. v2, two appendices added. Supplementary material is available at https://doi.org/10.5281/zenodo.6483355