TITLE:   GCN CIRCULAR
NUMBER:  33578
SUBJECT: GRB 230307A: good match with kilonova models
DATE:    23/04/06 14:41:22 GMT
FROM:    Cristiano Guidorzi at Ferrara U,Italy  <guidorzi@fe.infn.it>

M. Bulla, A.E. Camisasca, C. Guidorzi (Ferrara U.), L. Amati, A. Rossi 
(INAF-OAS), G. Stratta, P. Singh (Goethe U. Frankfurt) on behalf of a 
larger collaboration report: We compared kilonova models computed with 
the latest version of the radiative transfer code POSSIS (Bulla 2023, 
MNRAS, 520, 2558) to photometric data of a faint source associated with 
GRB 230307A as reported by Levan et al. (GCN 33569). Assuming a redshift 
z=0.065 (Gillanders et al. GCN 33485) and a Galactic extinction of 
E(B-V)=0.0743 mag (Schlafly & Finkbeiner 2011, ApJ, 737, 103), we find a 
reasonably good match to observations in both the F150W and F444W 
filters for a model viewed along the jet axis (face-on) and with two 
ejecta components: 1) a dynamical ejecta component with a mass of 0.005 
Msun, an average velocity of 0.25c and an average electron fraction 
Ye=0.15; 2) and a disk-wind ejecta component with a mass of 0.05 Msun, 
an average velocity of 0.1c and an average electron fraction Ye=0.3 (see 
figure at the link below). This supports the claim of kilonova emission 
by Levan et al. (GCN 33569) and strengthens the interpretation of GRB 
230307A as a merger event, as also suggested by the properties of the 
gamma-ray prompt emission (Camisasca et al. GCN 33577). Further analysis 
is ongoing. Figure: 
https://drive.google.com/file/d/1XhpfwCXQxWWS_G7WQ1fUSZKSiZXnwXiJ/view?usp=sharing