Can we constrain the extragalactic magnetic field from very high energy observations of GRB 190114C?

Abstract: Primary very high energy $\gamma$-rays from $\gamma$-ray bursts (GRBs) are partially absorbed on extragalactic background light (EBL) photons with subsequent formation of intergalactic electromagnetic cascades. Characteristics of the observable cascade $\gamma$-ray signal are sensitive to the strength and structure of the extragalactic magnetic field (EGMF). GRB 190114C was recently detected with the MAGIC imaging atmospheric Cherenkov telescopes, for the first time allowing to estimate the observable cascade intensity. We inquire whether any constraints on the EGMF strength and structure could be obtained from publicly-available $\gamma$-ray data on GRB 190114C. We present detailed calculations of the observable cascade signal for various EGMF configurations. We show that the sensitivity of the Fermi-LAT space $\gamma$-ray telescope is not sufficient to obtain such constraints on the EGMF parameters. However, next-generation space $\gamma$-ray observatories such as MAST would be able to detect pair echoes from GRBs similar to GRB 190114C for the EGMF strength below 10^{-17}--10^{-18} G.

• The paper demonstrates that current gamma-ray instruments, like Fermi-LAT, are insufficient to effectively constrain EGMF strengths.
• It employs Monte Carlo simulations with the ELMAG code to reconstruct the intrinsic VHE gamma-ray spectrum and model intergalactic cascades.
• The study highlights future prospects, suggesting that advanced missions such as MAST are crucial for probing EGMF below 10⁻¹⁷ Gauss.

An Analysis of Extragalactic Magnetic Field Constraints from GRB 190114C

The paper by Dzhatdoev, Podlesnyi, and Vaiman evaluates the potential to constrain the extragalactic magnetic field (EGMF) using observations from the high-energy gamma-ray burst GRB 190114C. Recent data from the MAGIC imaging atmospheric Cherenkov telescopes, which observed very high energy (VHE) gamma-ray emissions from this GRB, provides a novel opportunity to infer the characteristics of the EGMF based on the observable cascade gamma-ray signal induced by such events.

Through their analysis, the authors determine that the sensitivity of the current Fermi-LAT space gamma-ray telescope is insufficient to place constraints on EGMF parameters due to the inherent uncertainties associated with EGMF strength and configuration. However, with anticipated advancements in space gamma-ray telescopes, such as the envisioned MAST observatory, the detection of pair echoes from GRBs akin to GRB 190114C might become feasible for EGMF strengths below $10^{-17}$ to $10^{-18}$ Gauss. The results underscore the necessity for enhanced sensitivity in gamma-ray observations to explore and potentially bound the EGMF using intergalactic cascade signals.

Methodology and Key Findings

The paper methodically recalibrates and reconstructs the intrinsic VHE gamma-ray spectrum of GRB 190114C. It employs a model of intergalactic cascades that integrate the uncertainties related to both the primary gamma-ray spectrum and the extragalactic background light (EBL) spectrum. The authors present a cross-check with Fermi-LAT data to set experimental upper limits on cascade intensities. They simulate observable cascade signals under various EGMF configurations through Monte Carlo methods (using the ELMAG code) and contrast these with upper limits inferred from Fermi-LAT data.

One crucial recognition is the lack of statistically significant constraints due to the limitations of current observational tools and the complexity of the underlying physical model. Even when adjusted for potential systematic effects, such as varying EBL intensities, the results remain consistent with a wide range of plausible EGMF strengths, including zero-field hypotheses. This highlights the need to account for systematic uncertainties when probing such faint signals and further clarifies why the anticipated MAST mission may be pivotal for future discoveries.

Implications for Future Research

The paper asserts that while current instrumental capabilities cannot constrain the EGMF effectively, future detectors with increased sensitivity and broader spectral coverage have the potential to enhance our understanding of the intergalactic medium's magnetic properties. The MAST project, in particular, could provide more stringent tests of the EGMF through detection of low-energy cascade gamma-rays. This ability to probe weaker magnetic fields opens a significant window for multi-messenger astronomy and may offer insights into the early universe's magnetogenesis scenarios.

Moreover, the study brings attention to the broader role of plasma instabilities and other exotic processes affecting GRB-induced intergalactic cascades. Further refinement in theoretical modeling, coupled with advanced observational technologies, will serve as catalysts in the quest to reveal the intricacies of cosmic magnetism.

Conclusion

This investigation delineates an essential avenue within astrophysics: the use of GRBs as probes for cosmological structures like the EGMF. As the research indicates, further progress necessitates both advancements in observational capabilities and nuanced modeling approaches. With projects like CTA on the horizon and the potential roll-out of highly sensitive space observatories, the domain is poised to bridge existing gaps between theoretical predictions and empirical evidence regarding extragalactic magnetic fields. This research thereby not only sets the agenda for future investigations but also reiterates the symbiosis between theoretical astrophysics and observational innovations.