Constraints on the extragalactic magnetic field strength from blazar spectra based on 145 months of FermiLAT observations
Abstract
Properties of the extragalactic magnetic field (EGMF) outside of clusters and filaments of the largescale structure are essentially unknown. The EGMF could be probed with γray observations of distant (redshift z > 0.1) blazars. TeV γrays from these sources are strongly absorbed on extragalactic background light photons; secondary electrons and positrons produce cascade γrays with the observable flux dependent on EGMF parameters. We put constraints on the EGMF strength using 145 months of FermiLAT observations of the blazars 1ES 1218+304, 1ES 1101232, and 1ES 0347121, 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, logparabola, and logparabola 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;
 gammarays: general;
 Astrophysics  High Energy Astrophysical Phenomena;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 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