New Method for Investigating the Presence of Extragalactic Magnetic Fields

TL;DR

The study tackles the uncertain strength and distribution of extragalactic magnetic fields by introducing a cascade-echo signature: an asymmetric gamma-ray halo offset around blazars caused by intergalactic magnetic deflection of cascade photons. It employs a dedicated Monte Carlo model of TeV photon cascades, defines an offset statistic, and analyzes Fermi-LAT data from the brightest BL Lacs to constrain the transverse EGMF, finding that $H$ in the range $10^{-16}$–$10^{-14}$ G is excluded at about $2\sigma$. The results are robust to localization choice (Fermi vs optical) after accounting for catalog biases and background systematics, and the method is competitive with Cherenkov-telescope bounds while offering PSF-independent advantages. This approach opens the door to per-source and stacked EGMF constraints and could be extended to higher energies with upcoming Cherenkov arrays, enhancing sensitivity to weaker or more structured intergalactic fields.

Abstract

The extragalactic magnetic field could be detected by searching for signatures of the electromagnetic cascade initiated by high-energy photons on the intergalactic radiation and deflected by the field. This process produces a time delay and an extended gamma-ray halo around the source, which are looked for. We propose a new signature of electromagnetic echoes: the asymmetry of the gamma-ray distribution around blazars. As a measure of asymmetry, we use the offset of the gamma-ray distribution to the location of the blazar. This offset is due to the tilt of the jet of the blazar relative to the line of sight. Using a subsample of the 10 brightest BL Lacs, we exclude the range of extragalactic magnetic fields from $10^{-16}$ to $10^{-14}$ G, assuming that these objects have maintained a constant average luminosity over hundreds of thousands of years.

Figures (9)

• Figure 1: Diagram illustrating the formation of a "cascade echo". The primary TeV radiation in the jet creates $e^+e^-$ pairs in the intergalactic background light; $e^+e^-$ pairs deflected by EGMF create secondary GeV photons when interacting with the cosmic microwave background radiation. The resulting GeV halo is shifted towards the jet inclination.
• Figure 2: A simulated color-coded map of counts in the plain orthogonal to the line of sight for the case z = 0.1, H=10$^{-15}$ G, jet inclination $\alpha = 3^\circ$ directed to the right.
• Figure 3: Dependence of the image offset on the transverse component of the magnetic field for three different energy thresholds, 1 GeV, 10 GeV, and 100 GeV. The redshift is $z = 0.2$.
• Figure 4: The offset of the source image versus redshift for different energy cuts and magnetic field values. Magenta: E $>$ 1 GeV, H $= 3 \times 10^{-16}$ G, green E $>$ 10 GeV, H $= 3 \times10^{-15}$ G, blue: E $>$ 100 GeV, H = $3 \times 10^{-14}$ G. The jet inclination angle is $2^\circ$.
• Figure 5: Offsets for 310 BL Lacs selected by low background in the field of view $\theta = 1^\circ$ versus source brightness. The horizontal dashed line at $0.02^\circ$ indicates a level typical for EGMF-induced offsets for magnetic field in the range $10^{-16} \lesssim H \lesssim 10^{-14}$, see Figs. \ref{['fig:offset-H']} , \ref{['fig:offset-z']}.