EGMF Constraints from Simultaneous GeV-TeV Observations of Blazars

Abstract: Attenuation of the TeV gamma-ray flux from distant blazars through pair production with extragalactic background light leads to the development of electromagnetic cascades and subsequent, lower energy, GeV secondary gamma-ray emission. Due to the deflection of VHE cascade electrons by extragalactic magnetic fields (EGMF), the spectral shape of this arriving cascade gamma-ray emission is dependent on the strength of the EGMF. Thus, the spectral shape of the GeV-TeV emission from blazars has the potential to probe the EGMF strength along the line of sight to the object. We investigate constraints on the EGMF derived from observations of blazars for which TeV observations simultaneous with those by the Fermi telescope were reported. We study the dependence of the EGMF bound on the hidden assumptions it rests upon. We select blazar objects for which simultaneous Fermi/LAT GeV and Veritas, MAGIC or HESS TeV emission have been published. We model the development of electromagnetic cascades along the gamma-ray beams from these sources using Monte Carlo simulations, including the calculation of the temporal delay incurred by cascade photons, relative to the light propagation time of direct gamma-rays from the source. Constraints on EGMF could be derived from the simultaneous GeV-TeV data on the blazars RGB J0710+591, 1ES 0229+200, and 1ES 1218+304. The measured source flux level in the GeV band is lower than the expected cascade component calculated under the assumption of zero EGMF. Assuming that the reason for the suppression of the cascade component is the extended nature of the cascade emission, we find that B>10^{-15} G (assuming EGMF correlation length of ~1 Mpc) is consistent with the data. Alternatively, the assumption that the suppression of the cascade emission is caused by the time delay of the cascade photons the data are consistent with B>10^{-17} G for the same correlation length.

• The paper demonstrates the use of simultaneous GeV and TeV data to derive robust lower bounds on extragalactic magnetic field strength.
• It employs Monte Carlo simulations of electromagnetic cascades to distinguish between angular extension and time delay effects.
• Findings suggest EGMF limits of B ≳ 10⁻¹⁵ G and B ≳ 10⁻¹⁷ G, refining constraints on cosmological magnetogenesis scenarios.

EGMF Constraints from Simultaneous GeV-TeV Observations of Blazars: A Technical Analysis

The examination of extragalactic magnetic fields (EGMF) through multi-wavelength observations of blazars offers valuable insights into the large-scale magnetic structure of the Universe. This paper presents an in-depth investigation of EGMF constraints using simultaneous GeV-TeV observations of blazars by the Fermi Large Area Telescope (LAT) and ground-based Cherenkov telescopes. The interplay between the high-energy gamma rays from these active galactic nuclei and the extragalactic medium forms the crux of the analysis, leveraging the attenuation effects caused by interactions with the extragalactic background light (EBL) leading to electromagnetic cascades.

Methodology and Data Analysis

The paper employs Monte Carlo simulations to model electromagnetic cascades initiated by absorbed TeV gamma-rays from distant blazars, which produce secondary cascade emissions observable in the GeV range. The authors focus on blazars with simultaneous observations by Fermi/LAT (GeV) and Veritas, MAGIC, or HESS (TeV), a methodological improvement over previous studies that faced challenges due to non-simultaneous data. This simultaneous dataset circumvents uncertainties associated with the intrinsic variability of blazar fluxes, enabling a more robust assessment of EGMF-induced deflection effects on very high-energy (VHE) cascade electrons.

Key Findings

The investigation reveals significant constraints on EGMF by studying the spectral shapes of cascade emissions from observed blazars, such as RGB J0710+591, 1ES 0229+200, and 1ES 1218+304. The authors derive two distinct lower bounds on the EGMF strength based on assumed mechanisms of cascade suppression:

• Extension of Cascade Emission: A lower bound of $B \gtrsim 10^{-15}$ G was deduced, assuming EGMF deflects VHE cascade electrons causing the cascade emission to develop over extended regions that elude the point spread function of Fermi/LAT.
• Time Delay of Cascade Photons: Alternatively, accounting for the possibility of time-delayed cascade emissions due to longer light paths, an assumption yields $B \gtrsim 10^{-17}$ G if considering temporal suppression factors.

The bounds are notably scale-dependent on the EGMF correlation length, asserting robustness across potential EGMF configurations over megaparsec scales.

Implications and Future Outlook

This research holds profound implications for understanding the origin and structure of cosmic magnetic fields, potentially bridging gaps in our knowledge of cosmological magnetogenesis scenarios, such as primordial magnetic fields and their evolution through processes like structure formation and dynamo amplification.

The derived constraints enhance our ability to characterize EGMF properties that tie to broader cosmic processes, such as galaxy cluster formation and propagation of UHE cosmic rays. Future developments may further refine these bounds, potentially distinguishing between cascade suppression due to angular extension and time delay, particularly with advancements in gamma-ray observatory sensitivities.

In conclusion, this paper provides substantial contributions to our grasp of intergalactic magnetism, harnessing the unique properties of electromagnetic cascades from blazars. Such meticulous analyses offer tantalizing prospects for not only planetary-scale astrophysical phenomena but also the underlying forces shaping the cosmos.