Late-time HST and JWST Observations of GRB 221009A: Evidence for a Break in the Light Curve at 50 Days

Huei Sears, Ryan Chornock, Peter Blanchard, Raffaella Margutti, V. Ashley Villar, Justin Pierel, Patrick J. Vallely, Kate D. Alexander, Edo Berger, Tarraneh Eftekhari, Wynn V. Jacobson-Galan, Tanmoy Laskar, Natalie LeBaron, Brian D. Metzger, Dan Milisavljevic

Published 2024-12-03Version 1

GRB 221009A is one of the brightest transients ever observed with the highest peak gamma-ray flux for a gamma-ray burst (GRB). A type Ic-BL supernova (SN), SN 2022xiw, was definitively detected in late-time JWST spectroscopy (t = 195 days, observer-frame). However, photometric studies have found SN 2022xiw to be less luminous (10-70%) than the canonical GRB-SN, SN 1998bw. We present late-time Hubble Space Telescope (HST)/WFC3 and JWST/NIRCam imaging of the afterglow and host galaxy of GRB 221009A at t ~ 185, 277, and 345 days post-trigger. Our joint archival ground, HST, and JWST light curve fits show strong support for a break in the light curve decay slope at t = 50 +/- 10 days (observer-frame) and a supernova at $1.4^{+0.37}_{-0.40} \times$ the optical/NIR flux of SN 1998bw. This break is consistent with an interpretation as a jet break when requiring slow-cooling electrons in a wind medium with the electron energy spectral index, p > 2, and $\nu_m < \nu_c$. Our light curve and joint HST/JWST spectral energy distribution (SED) also show evidence for the late-time emergence of a bluer component in addition to the fading afterglow and supernova. We find consistency with the interpretations that this source is either a young, massive, low-metallicity star cluster or a scattered light echo of the afterglow with a SED shape of $f_{\nu} \propto \nu^{2.0\pm1.0}$.