Magnetic Footprints in the Glow of a Distant Explosion express
Radio waves from a gamma-ray burst (the death of a massive star, briefly outshining its galaxy) arrive at us polarized, oscillating in a single plane, like a compass needle frozen in a magnetic field. Astronomers have read the indications of this cosmic needle for the first time.
Using spectroscopy, they traced how polarization increased with frequency: 0.7% at 11 GHz, 3.2% at 25 GHz. But even at its peak, it's far from perfect — the magnetic field in the ejected jet turned out not uniform, but made of many chaotic domains, like thousands of microscopic compasses pointing every which way.
A staggering detail: on the way to Earth, the polarization rotated by –8300 rad/m² — tens of thousands of times stronger than in our Galaxy. This Faraday rotation reveals a dense veil of ionized hydrogen around the explosion site, like a magnetic lens. Now, by tracking the dance of this needle, scientists will be able to peer into the very heart of cosmic cataclysms.
Original abstract
as written by the authors arXiv:2604.27480
We report the detection of linear polarization in the radio afterglow of GRB 260310A, representing the first centimeter-wavelength polarization detection of a gamma-ray burst (GRB) afterglow and the first measurement of Faraday rotation in a GRB environment. We detect linearly polarized emission across $$11-25$$ GHz, with a polarization fraction decreasing monotonically from $$(3.18 \pm 0.18)\%$$ at 25 GHz to $$(0.69 \pm 0.22)\%$$ at 11 GHz. Interpreting the radio data as emission from a reverse shock in a structured, relativistic jet, the observed depolarization toward lower frequencies is consistent with suppression by synchrotron self-absorption, while the low observed polarization at high frequencies relative to the theoretical maximum suggests a patchy magnetic field in the jet with a coherence scale, $$\theta_{\rm B}\approx10^{-2}$$ rad. We identify a frequency-dependent rotation of the polarization angle consistent with Faraday rotation, with a rotation measure of $${\rm RM} = -(8300 \pm 90)~\rm{rad/m^2}$$ at the GRB redshift. The magnitude of the rotation measure is consistent with propagation through a dense, magnetized environment, such as a progenitor HII region. These findings demonstrate that GRB afterglows exhibit measurable linear polarization at centimeter wavelengths, and that their polarimetric properties probe both intrinsic jet magnetization and the surrounding medium. Future multi-frequency polarimetric monitoring over timescales of days to weeks will enable detailed studies of the evolution of magnetic field structure and provide new constraints on the role of magnetic fields in GRB afterglows.
Each 'patch' of the tangled magnetic field in the jet is so tiny that from Earth it appears as a speck no larger than the Moon.