A lower bound on intergalactic magnetic fields from time variability of 1ES 0229+200 from MAGIC and Fermi/LAT observations

TL;DR

This work leverages long-term, contemporaneous observations of the TeV blazar 1ES 0229+200 from MAGIC and Fermi/LAT to constrain the intergalactic magnetic field (IGMF) via time-delayed GeV emission from cascades. By modelling the cascade with 3D Monte Carlo codes (CRPropa v3.1.7 and CRbeam) and fitting the GeV–TeV light curves across a range of IGMF strengths and coherence lengths, the authors derive a robust lower bound on the IGMF: B ≥ 1.8×10^{-17} G for long correlation lengths (λB > 0.2 Mpc) and B ≥ 1.8×10^{-17} (λB/0.2)^{-1/2} G for shorter λB, with a cosmological-origin bound of B ≥ 1×10^{-14} G. The bound is strengthened by the use of contemporaneous GeV–TeV data and a conservative intrinsic spectrum, though it remains sensitive to uncertainties in cascade physics and potential plasma instabilities. These results have implications for cosmological magnetogenesis scenarios and baryon asymmetry generation, and illustrate the potential of future facilities (CTA, HERD) for tightening IGMF constraints through energy-dependent time delays.

Abstract

Extended and delayed emission around distant TeV sources induced by the effects of propagation of gamma rays through the intergalactic medium can be used for the measurement of the intergalactic magnetic field (IGMF). We search for delayed GeV emission from the hard-spectrum TeV blazar 1ES 0229+200 with the goal to detect or constrain the IGMF-dependent secondary flux generated during the propagation of TeV gamma rays through the intergalactic medium. We analyze the most recent MAGIC observations over a 5 year time span and complement them with historic data of the H.E.S.S. and VERITAS telescopes along with a 12-year long exposure of the Fermi/LAT telescope. We use them to trace source evolution in the GeV-TeV band over one-and-a-half decade in time. We use Monte Carlo simulations to predict the delayed secondary gamma-ray flux, modulated by the source variability, as revealed by TeV-band observations. We then compare these predictions for various assumed IGMF strengths to all available measurements of the gamma-ray flux evolution. We find that the source flux in the energy range above 200 GeV experiences variations around its average on the 14 years time span of observations. No evidence for the flux variability is found in 1-100 GeV energy range accessible to Fermi/LAT. Non-detection of variability due to delayed emission from electromagnetic cascade developing in the intergalactic medium imposes a lower bound of B>1.8e-17 G for long correlation length IGMF and B>1e-14 G for an IGMF of the cosmological origin. Though weaker than the one previously derived from the analysis of Fermi/LAT data, this bound is more robust, being based on a conservative intrinsic source spectrum estimate and accounting for the details of source variability in the TeV energy band. We discuss implications of this bound for cosmological magnetic fields which might explain the baryon asymmetry of the Universe.

Figures (5)

• Figure 1: Spectral energy distribution of 1ES 0229+200 in the 100 MeV -- 100 TeV energy range. Fermi/LAT and MAGIC data were obtained here; H.E.S.S and VERITAS measurements are taken from HESS_1ES0229 and VERITAS_1ES0229 correspondingly.
• Figure 2: Light curve of 1ES 0229+200 in several energy bands along with an exemplary fit with IGMF of strength of $\mathrm{B=10^{-16}}$ G and coherence scale of $\mathrm{\lambda_B = 1}$ Mpc. Top panel represents the best-fit model light curve (along with its uncertainties), used to make the predictions in the energy bands where the measurements were taken (the panels below). Fermi/LAT and MAGIC data are reported in the text; H.E.S.S. and VERITAS measurements are taken from HESS_VERITAS_lightcurves and VERITAS_1ES0229 correspondingly. The primary, cascade and total source fluxes are denoted with green triangles, orange squares and red circles correspondingly. Solid and dashed lines represent calculations with CRPropa CRPropa and CRbeam Berezinsky:2016feh Monte Carlo codes respectively; the latter use the small point-like markers to distiguish themselves.
• Figure 3: The scan of the cascade power in the $\mathrm{\Gamma-E_{cut}}$ parameter space along with the 68% and 90% confidence contours from the $\mathrm{\chi^2}$ fit. At 90% confidence level the minimal cascade, marked with the yellow dashed lines, corresponds to $\mathrm{\Gamma \approx 1.72}$ and $\mathrm{E_{cut} \approx 6.9}$ TeV.
• Figure 4: IGMF strength scan for several different assumed coherence lengths $\mathrm{\lambda_B}$. The scan is performed via a simultaneous fit of GeV-TeV observations at hand. Calculations with both CRPropa CRPropa and CRbeam Berezinsky:2016feh Monte Carlo codes are presented. Lines of different colors depict the $\mathrm{\chi^2}$ values estimated for different non-zero IGMF strengths. Black solid line represents the case of zero IGMF, in case of which secondary emission dominates in the Fermi/LAT energy range; black dashed line represent the reference fit with the cascade contribution disabled.
• Figure 5: Lower bound on IGMF strength derived from Fermi/LAT and Cherenkov telescope data sets (thick blue curve and red data points). Green dot-dashed and dashed curves show previous Fermi/LAT limits derived for the full source sample and 1ES 0229+200 only fermi_limit. Light-grey shaded upper bound shows previously known limits on the IGMF strength and correlation length from radio telescope data kronberg and CMB planck analysis as well as from theoretical estimates Durrer13. Inclined orange stripe shows the locus of end points of evolution of cosmological magnetic fields banerjee. Red stripe marks possible range of magnetic field produced by the chiral dynamo joyceneronov20. Dark green stripe denotes the range of electroweak phase transition magnetic fields which might explain the observed baryonic asymmetry of the Universe shaposhnikov98fujita16kamada16. Filled vertical green and violet boxes show favored regions of IGMF generated by a frozen-in magnetic field, originating from AGN outflows Furlanetto:2001 or galactic winds Bertone:2006 as labelled in the figure.