  Gamma-ray bursts (GRBs) are bright extragalactic flashes of gamma-ray
radiation and briefly the most energetic explosions in the Universe. Their
catastrophic origin (the merger of compact objects or the collapse of massive
stars) drives the formation of a newborn compact remnant (black hole or
magnetar) that powers two highly relativistic jets. To distinguish between
magnetized and baryonic jet models and ultimately determine the power source
for these energetic explosions, our team studies the polarization of the light
during the first minutes after the explosion (using novel instruments on fully
autonomous telescopes around the globe) to directly probe the magnetic field
properties in these extragalactic jets. This technology allowed the detection
of highly polarized optical light in GRB 120308A and confirmed the presence of
mildly magnetized jets with large-scale primordial magnetic fields in a reduced
sample of GRBs (e.g. GRB 090102, GRB 110205A, GRB 101112A, GRB 160625B). Here
we discuss the observations of the most energetic and first GRB detected at
very high TeV energies, GRB 190114C, which opens a new frontier in GRB magnetic
field studies suggesting that some jets can be launched highly magnetized and
that the collapse and destruction of these magnetic fields at very early times
may have powered the explosion itself. Additionally, our most recent
polarimetric observations of the jet of GRB 141220A indicate that, when the
jetted ejected material is decelerated by the surrounding environment, the
magnetic field amplification mechanisms at the front shock (needed to generate
the observed synchrotron emission) produce small magnetic domains. These
measurements validate theoretical expectations and contrast with previous
observations that suggest large magnetic domains in collisionless shocks (i.e.
GRB 091208B).
