GCN Circular 33747
Subject
GRB 230307A: JWST second-epoch observations
Date
2023-05-08T21:30:32Z (a year ago)
From
Andrew Levan at Radboud University <a.levan@astro.ru.nl>
A.J. Levan (Radboud), N. R. Tanvir (U. Leicester), E. Burns (LSU), G.P. Lamb (LJMU), J. P. U. Fynbo (DAWN/NBI), A.S. Fruchter (STScI), K. Bhirombhakdi (STScI), P O’Brien (U. Leicester), P. Jakobsson (U. Iceland), D. B. Malesani (Radboud and DAWN/NBI), B. P. Gompertz (U. Birmingham), A. Martin-Carrillo (UCD), D. Watson (DAWN/NBI), S. Smartt (Oxford) , J. Hjorth (DARK/NBI) report for a larger collaboration:
We obtained a second epoch of JWST observations of the exceptionally bright GRB 230307A (Fermi GBM team, GCN 33405; Xiong et al., GCN 33406; Xiao & Krucker, GCN 33410; Cosentini et al., GCN 33412; Navaneeth et al., GCN 33415) on 8 May 2023 (about 62 days post-burst). Imaging was obtained with NIRCam in the F115W, F150W, F277W and F444W filters and spectroscopy with the NIRSpec prism.
The source at the location of the optical/IR afterglow (Levan et al., GCNs 33439, 33569) is still detected but has faded significantly, particularly in the redder bands. In the F444W filter, it faded by 2.7 mag.
This result confirms that the very red continuum noted in the first epoch by Levan et al. (GCN 33569) is due to a variable source and does not contain a significant contribution from any underlying host. However, we do note the presence of a faint galaxy, approximately 0.3” from the burst position, which is plausibly the source of the emission lines seen in previous NIRSpec spectroscopy (Levan et al., GCN 33580). Although further analysis is required, the rapid IR fading is consistent with the expectations of kilonova emission. If that interpretation is correct, the higher redshift source has to be an unrelated background galaxy, aligned by chance with the GRB. Alternatively, if associated with the galaxy at z = 3.87, the counterpart would have exhibited unprecedented temporal and chromatic behaviour.