The optical light curve of GRB 221009A: the afterglow and the emerging supernova

Abstract

We present extensive optical photometry of the afterglow of GRB~221009A. Our data cover $0.9-59.9$\,days from the time of \textit{Swift} and \textit{Fermi} GRB detections. Photometry in $rizy$-band filters was collected primarily with Pan-STARRS and supplemented by multiple 1- to 4-meter imaging facilities. We analyzed the Swift X-ray data of the afterglow and found a single decline rate power-law $f(t)\propto t^{-1.556\pm 0.002}$ best describes the light curve. In addition to the high foreground Milky Way dust extinction along this line of sight, the data favour additional extinction to consistently model the optical to X-ray flux with optically thin synchrotron emission. We fit the X-ray-derived power-law to the optical light curve and find good agreement with the measured data up to $5-6$\,days. Thereafter we find a flux excess in the $riy$ bands which peaks in the observer frame at $\sim 20$\,days. This excess shares similar light curve profiles to the type Ic broad-lined supernovae SN~2016jca and SN~2017iuk once corrected for the GRB redshift of $z=0.151$ and arbitrarily scaled. This may be representative of a supernova emerging from the declining afterglow. We measure rest-frame absolute peak AB magnitudes of $M_g=-19.8\pm 0.6$ and $M_r=-19.4\pm 0.3$ and $M_z=-20.1\pm 0.3$. If this is an SN component, then Bayesian modelling of the excess flux would imply explosion parameters of $M_{\mathrm{ej}}=7.1_{-1.7}^{+2.4}$ M${}_{\odot }$, $M_{\mathrm{Ni}}=1.0_{-0.4}^{+0.6}$ M${}_{\odot }$, and $v_{\mathrm{ej}}=33,900_{-5,700}^{+5,900}km{s}^{-1}$, for the ejecta mass, nickel mass and ejecta velocity respectively, inferring an explosion energy of $E_{\mathrm{kin}}\simeq 2.6-9.0\times 10^{52}$ ergs.

arXiv commentSubmitted to ApJL on 23rd January 2023, for consideration for publication in the special issue on GRB 221009A. Accepted on 2nd March 2023. The results of this paper are under press embargo until 28th March 2023. 15 pages, 7 figures, 1 table

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• Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow
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• A Sensitive Search for Supernova Emission Associated with the Extremely Energetic and Nearby GRB 221009A
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• A Narrow Uniform Core with a Wide Structured Wing: Modeling the TeV and Multi-wavelength Afterglows of GRB 221009A
• Carpet-3 detection of a photon-like air shower with estimated primary energy above 100 TeV in a spatial and temporal coincidence with GRB 221009A
• The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151
• The GRBSN webtool: An open-source repository for gamma-ray burst-supernova associations
• Theoretical modeling of the exceptional GRB 221009A afterglow
• GRB 221009A and the Apparently Most Energetic Gamma-Ray Bursts
• Two-component jet model for the afterglow emission of GRB 201216C and GRB 221009A and implications for jet structure of very-high-energy gamma-ray bursts
• TeV Afterglows of Gamma-Ray Bursts: Theoretical Analysis and Prospects for Future Observations
• GRB 180728A and SN 2018fip: the nearest high-energy cosmological gamma-ray burst with an associated supernova
• Probing the interstellar medium toward GRB 221009A through X-ray dust scattering
• Detection of Low-Redshift Excess in Supernova-Linked Gamma-Ray Bursts
• Spectral Hardness as the Primary Discriminator: Unveiling the Collapsar--Merger Boundary with a Gold-Standard Gamma-Ray Burst Sample