Abstract:
The composition of relativistic gamma-ray burst (GRB) jets and their emission mechanisms are still debated, and they could be
matter or magnetically dominated. One way to distinguish these mechanisms arises because a Poynting flux dominated jet may
produce low-frequency radio emission during the energetic prompt phase, through magnetic reconnection at the shock front.
We present a search for radio emission coincident with three GRB X-ray flares with the LOw Frequency ARray (LOFAR), in a
rapid response mode follow-up of long GRB 210112A (at z ∼ 2) with a 2 h duration, where our observations began 511 s after
the initial Swift-BAT trigger. Using time-sliced imaging at 120–168 MHz, we obtain upper limits at 3σ confidence of 42 mJy
averaging over 320 s snapshot images, and 87 mJy averaging over 60 s snapshot images. LOFAR’s fast response time means
that all three potential radio counterparts to X-ray flares are observable after accounting for dispersion at the estimated source
redshift. Furthermore, the radio pulse in the magnetic wind model was expected to be detectable at our observing frequency
and flux density limits which allows us to disfavour a region of parameter space for this GRB. However, we note that stricter
constraints on redshift and the fraction of energy in the magnetic field are required to further test jet characteristics across the
GRB population.
