Abstract:
Multipulsed GRB 190530A, detected by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope onboard Fermi,
is the sixth most fluent GBM burst detected so far. This paper presents the timing, spectral, and polarimetric analysis of the
prompt emission observed using AstroSat and Fermi to provide insight into the prompt emission radiation mechanisms. The
time-integrated spectrum shows conclusive proof of two breaks due to peak energy and a second lower energy break. Time integrated (55.43 ± 21.30 per cent) as well as time-resolved polarization measurements, made by the Cadmium Zinc Telluride
Imager (CZTI) onboard AstroSat, show a hint of high degree of polarization. The presence of a hint of high degree of polarization
and the values of low energy spectral index (αpt) do not run over the synchrotron limit for the first two pulses, supporting the
synchrotron origin in an ordered magnetic field. However, during the third pulse, αpt exceeds the synchrotron line of death in
few bins, and a thermal signature along with the synchrotron component in the time-resolved spectra is observed. Furthermore,
we also report the earliest optical observations constraining afterglow polarization using the MASTER (P < 1.3 per cent) and
the redshift measurement (z = 0.9386) obtained with the 10.4 m GTC (Gran Telescopio Canarias) telescopes. The broad-band
afterglow can be described with a forward shock model for an ISM (interstellar medium)-like medium with a wide jet opening
angle. We determine a circumburst density of n0 ∼ 7.41, kinetic energy EK ∼ 7.24 × 1054 erg, and radiated gamma-ray energy
Eγ ,iso ∼ 6.05 × 1054 erg.
