Characterizing the Ordinary Broad-lined Type Ic SN 2023pel from the Energetic GRB 230812B

Abstract

We report observations of the optical counterpart of the long gamma-ray burst (LGRB) GRB 230812B, and its associated supernova (SN) SN 2023pel. The proximity ($z=0.36$) and high energy ($E_{gamma,rmiso}sim{10}^{53}$ erg) make it an important event to study as a probe of the connection between massive star core-collapse and relativistic jet formation. With a phenomenological power-law model for the optical afterglow, we find a late-time flattening consistent with the presence of an associated SN. SN 2023pel has an absolute peak $r$-band magnitude of $M_r=-19.46pm0.18$ mag (about as bright as SN 1998bw) and evolves on quicker timescales. Using a radioactive heating model, we derive a nickel mass powering the SN of $M_{rmNi}=0.38pm0.01$ $rm{M}_{o}dot$, and a peak bolometric luminosity of $L_{rmbol}sim1.3times{10}^{43}$ $rmerg$ $rm{s}^{-1}$. We confirm SN 2023pel’s classification as a broad-lined Type Ic SN with a spectrum taken 15.5 days after its peak in $r$ band, and derive a photospheric expansion velocity of $v_{rmph}=11,300pm1,600$ $rmkm$ $rm{s}^{-1}$ at that phase. Extrapolating this velocity to the time of maximum light, we derive the ejecta mass $M_{rmej}=1.0pm0.6$ $rm{M}_{o}dot$ and kinetic energy $E_{rm{KE}} = 1.3^{+3.3}{-1.2} times10^{51}$$rm{erg}$.WefindthatGRB230812B/SN2023pelhasSNpropertiesthataremostlyconsistentwiththeoverallGRB-SNpopulation.ThelackofcorrelationsfoundintheGRB-SNpopulationbetweenSNbrightnessand$E{gamma, rm{iso}}$ for their associated GRBs, across a broad range of 7 orders of magnitude, provides further evidence that the central engine powering the relativistic ejecta is not coupled to the SN powering mechanism in GRB-SN systems.