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GCN Circular 33569

GRB 230307A: JWST observations consistent with the presence of a kilonova
2023-04-05T19:08:10Z (a year ago)
Andrew Levan at U.of Leicester <>
A. J. Levan (Radboud), B. P. Gompertz (U. Birmingham), D. B. Malesani (Radboud/DAWN NBI), N. R. Tanvir (Leicester), E. Burns (LSU), R. Salvaterra (INAF/IASF-Mi), K. Ackley (U. Warwick), G. P. Lamb (LJMU), J. Fynbo (DAWN NBI), B. Schneider (MIT), P. Jakobsson (U. Iceland), L. Izzo (DARK/NBI), A. Fruchter (STScI), D. Watson (DAWN NBI), M. Kennedy (UCC), J. Hjorth (DARK/NBI), G. Pugliese (API UvA), K. Bhirombhakdi (STScI), V. S. Dhillon (Sheffield/IAC)  report for a larger collaboration.
"We obtained observations of the exceptionally bright, long-duration 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) with the James Webb Space Telescope on 5 April 2023 (about 28.8 days after the GRB). Observations were obtained with NIRCam in the F070W, F115W, F150W, F277W, F356W and F444W filters.
At the location of the optical afterglow (Levan et al., GCN 33439) we find a faint source with F150W(AB) ~ 28.4 +/- 0.3. From our provisional analysis, the source appears point-like, without any evident extension. It is thus unlikely to be due to an underlying host galaxy. The lack of a host galaxy in observations of this depth is unusual for a long GRB, particularly for one as bright as GRB 230307A. 

In addition, observations in the redder bands show a much brighter source, with F444W(AB)~24.5 +/- 0.1, consistent with a power-law slope of approximately nu^-3 through the redder bands. 

We suggest that the very red colour and the absence of a host galaxy make a kilonova the most likely interpretation. In this case the burst may arise from a compact binary ejected from the nearby galaxy at z=0.065, which is ~40 kpc away in projection (Gillanders et al. GCN 33485).3

Further analysis is ongoing.
We thank the staff of STScI for their work to get these observations rapidly scheduled, in particular Katey Alatalo, Alaina Henry, Armin Rest and Wilson Skipper."
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