| Authors : | Rahul Gupta, S. Gupta, T. Chattopadhyay, V. Lipunov, A. J. Castro-Tirado, D. Bhattacharya, S. B. Pandey, S. R. Oates, Amit Kumar, Y.-D. Hu, A. F. Valeev, P. Yu. Minaev, H. Kumar, J. Vinko, Dimple, V. Sharma, A. Aryan, A. Castell\'on, A. Gabovich, A. Moskvitin, A. Ordasi, A. P\'al, A. Pozanenko, B.-B. Zhang, B. Kumar, D. Svinkin, D. Saraogi, D. Vlasenko, E. Fern\'andez-Garc\'ia, E. Gorbovskoy, G. C. Anupama, K. Misra, K. S\'arneczky, L. Kriskovics, M. \'A. Castro-Tirado, M. D. Caballero-Garc\'ia, N. Tiurina, P. Balanutsa, R. R. Lopez, R. S\'anchez Ram\'irez, R. Szak\'ats, S. Belkin, S. Guziy, S. Iyyani, S. N. Tiwari, Santosh V. Vadawale, T. Sun, V. Bhalerao, V. Kornilov, and V. V. Sokolov |
| Abstract : | Multi-pulsed GRB 190530A, detected by the GBM and LAT 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 $\pm$ 21.30 \%; 2.60 $\sigma$) as well as time-resolved (53.95 $\pm$ 24.13 \%; 2.24 $\sigma$ for the third pulse) polarization measurements, made by the Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat, show a hint of a high degree of polarization. The presence of a hint of a high degree of polarization and the values of low energy spectral index ($\alpha_{\rm pt}$) 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, $\alpha_{\rm pt}$ exceeds the synchrotron line of death in a 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 telescope(P $<$ 1.3 \%), making it the first GRB for which both the prompt and afterglow polarization have been investigated. Additionally, we present the redshift measurement ($z$= 0.9386) of GRB 190530A obtained with the 10.4m GTC telescope. The broadband afterglow can be described with a forward shock model for an ISM-like medium with a wide jet opening angle. We determine a circumburst density of $n_{0} \sim$ 7.41, kinetic energy $E_{\rm K} \sim$ 7.24 $\times 10^{54}$ erg, and radiated $\gamma$-ray energy $E_{\rm \gamma, iso} \sim$ 6.05 $\times 10^{54}$ erg, respectively. |
