Utkarsh Pathak, Sameer K. Patil, Hitesh Tanenia, Yashowardhan Rai, Viswajeet Swain, Anuraag Arya, Gaurav Waratkar, Anirudh Salgundi, Mehul Goyal, Varun Bhalerao, Igor Andreoni, Sarah Antier, G. C. Anupama, Sudhanshu Barway, Dipankar Bhattacharya, Tanmoy Chattopadhyay, Michael Coughlin, Anirban Dutta, Christoffer Fremling, Nidhal Guessoum, Mansi Kasliwal, Oliver J. Roberts, Gokul P. Srinivasaragavan, Rishabh Singh Teja, Santosh Vadawale, Peter Veres

GRB 230812B is a bright long-duration GRB with a luminous, long-lived afterglow and an AstroSat/CZTI polarization measurement during the prompt phase, enabling a joint study of its prompt spectral evolution, polarization, and broadband afterglow. Time-resolved spectroscopy of the prompt emission shows that during the rising phase, the low-energy Band-function index exceeds the synchrotron line of death, favoring the presence of an additional thermal component. At later times, from $T_0+2$ s to $T_0+32$ s, the prompt spectra are consistent with predominantly non-thermal emission. Polarization analysis of the prompt emission in the $300$-$600$ keV band yields a marginal lower limit on the polarization fraction of $Π\gtrsim 50\%$ at the $1σ$ level. The long X-ray monitoring of the afterglow shows no jet break over the observed baseline. Multiwavelength afterglow modeling favors a wide jet with an inferred half-opening angle of $θ_j = 15^{+6}_{-4}$ degrees observed close to the jet axis with a viewing angle of $θ_v = 0.9^{+1.8}_{-0.6}$ degrees. The inferred circumburst density is low, $n_0 = 1.2^{+0.3}_{-0.1}\times10^{-4}\,\textrm{cm}^{-3}$, and the isotropic-equivalent kinetic energy of the jet is $E_{{\rm k}, iso} = 4.0^{+1.5}_{-0.8} \times 10^{53}$ erg. Taken together, the prompt spectral evolution favors an early phase with a thermal contribution followed by a later phase dominated by non-thermal emission. The polarization constraint in the late prompt phase is consistent with synchrotron emission, although a higher-significance polarization measurement will be required to robustly constrain the magnetic-field geometry and the relative contribution of photospheric emission.