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 ($E_{{\hat{I}}^3,\mathrm{iso}}\sim 10^{53}$ 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 $M_r=-19.46\pm 0.18$ 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 $M_{\mathrm{Ni}}=0.38\pm 0.01$ ${\mathrm{M}}_{\odot }$, and a peak bolometric luminosity of $L_{\mathrm{bol}}\sim 1.3\times 10^{43}$ $\mathrm{erg}$ ${\mathrm{s}}^{-1}$. 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 $v_{\mathrm{ph}}=11,300\pm 1,600$ $\mathrm{km}$ ${\mathrm{s}}^{-1}$ at that phase. Extrapolating this velocity to the time of maximum light, we derive the ejecta mass $M_{\mathrm{ej}}=1.0\pm 0.6$ ${\mathrm{M}}_{\odot }$ and kinetic energy $E_{\mathrm{KE}}=1.3_{-1.2}^{+3.3}\times 10^{51}$ $\mathrm{erg}$. We find that GRB 230812B/SN 2023pel has SN properties that are mostly consistent with the overall GRB-SN population. The lack of correlations found in the GRB-SN population between SN brightness and $E_{{\hat{I}}^3,\mathrm{iso}}$ 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.

21 pages, 7 Figures, 1 Table, Accepted to ApJ Letters