Multi-band analyses of the bright GRB 230812B and the associated SN2023pel

GRB~230812B is a bright and relatively nearby ($z =0.36$) long gamma-rayburst (GRB) that has generated significant interest in the community and hasthus been observed over the entire electromagnetic spectrum. We report over 80observations in X-ray, ultraviolet, optical, infrared, and sub-millimeter bandsfrom the GRANDMA (Global Rapid Advanced Network for Multi-messenger Addicts)network of observatories and from observational partners. Adding complementarydata from the literature, we then derive essential physical parametersassociated with the ejecta and external properties (i.e. the geometry andenvironment) of the GRB and compare with other analyses of this event. Wespectroscopically confirm the presence of an associated supernova, SN2023pel,and we derive a photospheric expansion velocity of v $\sim$ 17$\times10^3$ kms$^{-1}$. We analyze the photometric data first using empirical fits of theflux and then with full Bayesian Inference. We again strongly establish thepresence of a supernova in the data, with a maximum (pseudo-)bolometricluminosity of $5.75 \times 10^{42}$ erg/s, at $15.76^{+0.81}_{-1.21}$ days (inthe observer frame) after the trigger, with a half-max time width of 22.0 days.We compare these values with those of SN1998bw, SN2006aj, and SN2013dx. Ourbest-fit model favours a very low density environment ($\log_{10}({n_{\rmISM}/{\rm cm}^{-3}}) = -2.38^{+1.45}_{-1.60}$) and small values for the jet'score angle $\theta_{\rm core} = 1.54^{+1.02}_{-0.81} \ \rm{deg}$ and viewingangle $\theta_{\rm obs} = 0.76^{+1.29}_{-0.76} \ \rm{deg}$. GRB 230812B is thusone of the best observed afterglows with a distinctive supernova bump.