The Optical Light Curve of GRB 221009A: The Afterglow and the Emerging Supernova

We present extensive optical photometry of the afterglow of GRB 221009A. Our data cover 0.9-59.9 days from the time of Swift and Fermi gamma-ray burst (GRB) detections. Photometry in rizy-band filters was collected primarily with Pan-STARRS and supplemented by multiple 1-4 m imaging facilities. We analyzed the Swift X-ray data of the afterglow and found a single decline rate power law f(t) proportional to t(-1.556 +/- 0.002) best describes the light curve. In addition to the high foreground Milky Way dust extinction along this line of sight, the data favor 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 that peaks in the observer frame at similar to 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 an SN emerging from the declining afterglow. We measure rest-frame absolute peak AB magnitudes of M-g = -19.8 +/- 0.6 and M-r = - 19.4 +/- 0.3 and M-z = -20.1 +/- 0.3. If this is an SN component, then Bayesian modeling of the excess flux would imply explosion parameters of M-ej = 7.1(-1.7)(+2.4) M-circle dot, M-Ni = 1.0(-0.4)(+0.6) M-circle dot, and nu(ej) = 33,900(-5700)(+5900) km s(-1), for the ejecta mass, nickel mass, and ejecta velocity respectively, inferring an explosion energy of E-kin similar or equal to 2.6-9.0 x 10(52) erg.

Automated summary

We conducted extensive optical photometry of the afterglow of GRB 221009A, collecting data from 0.9-59.9 days after the burst detection. Our analysis revealed a power law decline rate for the light curve and suggested additional extinction to accurately model the optical to X-ray flux. We identified a flux excess in the riy bands, resembling Type Ic broad-lined supernovae, potentially indicating the emergence of a supernova from the declining afterglow.