Revision of conservative lower bound on intergalactic magnetic field from Fermi and Cherenkov telescope observations of extreme blazars

Abstract

Joint observations of extreme blazars with Fermi Large Area Telescope (LAT) and Imaging Atmospheric Cherenkov telescopes (IACT) have been previously used to derive lower bounds on intergalactic magnetic field (IGMF). We update these previous bounds using a set of extreme blazars that are detected in the Very-High-Energy (VHE, photon energies above 100 GeV) band by both Fermi/LAT and IACTs. We measure IGMF-dependent suppression of secondary delayed gamma-ray flux from electron-positron pairs deposited in the intergalactic medium by VHE gamma-rays interacting with Extragalactic Background Light. From overall 22 extreme blazars detected by Fermi/LAT and IACTs in the VHE band, seven have their spectral characteristics inconsistent with the possibility of zero magnetic field along their lines of sight, even under the most restrictive assumption that the sources have only switched on at the start of VHE band observations. Adopting this assumption, we derive a "conservative" lower bound on the IGMF strength at the level of 2e-17 G. The tightest bound is imposed by the signal of 1ES 0502+675, a source that has not been considered in the IGMF analysis before. Our bound is comparable to the bound derived by MAGIC collaboration, but is weaker than that previously derived from analysis of Fermi/LAT and HESS telescope data, even though our dataset includes that data. We clarify the origin of this discrepancy.

Figures (6)

• Figure 1: Spectral modeling for 1ES 0502+675. Blue data points show Fermi/LAT and VERITAS data. Lighter color data points are corrected for the EBL absorption. Blue butterfly shows the measurement of the source spectrum from 2019ApJ...885..150A. Grey dashed line shows the best-fit intrinsic source cut-off powerlaw expected in the case of strong IGMF. Grey dotted line shows the secondary flux that is suppressed by the IGMF. Black curves show the model with minimal possible IGMF. Dashed line is the intrinsic source spectrum, dotted line is the secondary flux and solid line is the sum of the secondary and attenuated primary components.
• Figure 2: Spectra of sources that provide constraints on IGMF. The data points are combinations of Fermi/LAT data with the IACT data for which references are listed in Table \ref{['tab:sources']}. The models correspond to the minimal possible field strength. The notations are the same as in Fig. \ref{['fig:0502_nofield']}.
• Figure 3: $\Delta \chi^2(B)$ profiles for the sources that constrain IGMF. Vertical blue dashed shows the lower bound on $B$ from 1ES 0502+675, vertical blue dotted line shows the combined lower bound on IGMF from all sources. Horizontal line shows $\Delta\chi^2=2.71$.
• Figure 4: Comparison of $\Delta \chi^2(B)$ profiles for 1ES 0502+675 for two different assumptions about source activity period.
• Figure 5: Comparison of the secondary flux from HESS:2023zwb with our secondary flux model for the same parameters (magnetic field $B=3.2\times 10^{-16}$ G, activity time $T=10$ yr, source 1ES 0229+200). Results from CRbeam (black dotted line) and CRpropa (red dotted line) are both present. Orange data points show the result of Fermi/LAT likelihood analysis by HESS:2023zwb.