Multiband analyses of the bright GRB 230812B and the associated SN2023pel

GRB 230812B is a bright and relatively nearby (z = 0.36) long gamma-ray burst (GRB) that has generated significant interest in the community and has thus been observed over the entire electromagnetic spectrum. We report over 80 observations in X-ray, ultraviolet, optical, infrared, and submillimetre bands from the GRANDMA (Global Rapid Advanced Network for Multimessenger Addicts) network of observatories and from observational partners. Adding complementary data from the literature, we then derive essential physical parameters associated with the ejecta and external properties (i.e. the geometry and environment) of the GRB and compare with other analyses of this event. We spectroscopically confirm the presence of an associated supernova, SN2023pel, and we derive a photospheric expansion velocity of v ~ 17 × 10<SUP>3</SUP> km s<SUP>-1</SUP>. We analyse the photometric data first using empirical fits of the flux and then with full Bayesian inference. We again strongly establish the presence of a supernova in the data, with a maximum (pseudo-)bolometric luminosity of 5.75 × 10<SUP>42</SUP> erg s<SUP>-1</SUP>, at $15.76^{+0.81}_{-1.21}$ d (in the observer frame) after the trigger, with a half-max time width of 22.0 d. We compare these values with those of SN1998bw, SN2006aj, and SN2013dx. Our best-fitting model favours a very low density environment ($\log _{10}({n_{\rm ISM}/{\rm cm}^{-3}}) = -2.38^{+1.45}_{-1.60}$) and small values for the jet's core angle $\theta _{\rm core} = 1.54^{+1.02}_{-0.81} \ \rm {deg}$ and viewing angle $\theta _{\rm obs} = 0.76^{+1.29}_{-0.76} \ \rm {deg}$. GRB 230812B is thus one of the best observed afterglows with a distinctive supernova bump.