TITLE:   GCN CIRCULAR
NUMBER:  33577
SUBJECT: GRB 230307A: short mininum variability timescale compatible with a merger origin
DATE:    23/04/06 11:06:22 GMT
FROM:    Cristiano Guidorzi at Ferrara U,Italy  <guidorzi@fe.infn.it>

A.E. Camisasca, C. Guidorzi, M. Bulla (Ferrara U.), L. Amati, A. Rossi 
(INAF-OAS), G. Stratta, P. Singh (Goethe U. Frankfurt) on behalf of a 
larger collaboration report:

"We determined the minimum variability timescale (MVT) of GRB230307A 
(Fermi GBM team, GCN 33405; Xiong et al., GCN 33406; Xiao & Krucker, GCN 
33410; Cosentini et al., GCN 33412; Navaneeth et al., GCN 33415) from 
the Fermi/GBM light curve (NaI detectors 10 and 6) following the 
prescriptions of Camisasca et al. (2023), i.e. as the minimum full width 
half maximum (FWHM_min) of all statistically significant pulses, and 
found FWHM_min = 28 (-7, +10) ms. Combined with the T90=34.56 +- 0.6 s 
(Fermi GBM team, GCN 33405), in the T90-FWHM_min plot GRB230307A (purple 
star in the Figure below) lies in the region populated by other 
long-lasting merger candidates, such as also GRB191019A (green star in 
Figure; Levan et al. 2023; Lazzati et al. 2023). In particular, it lies 
very close to the merger GRB211211A (Gompertz et al. 2023; Mei et al. 
2022; Rastinejad et al. 2022; Troja et al 2022; Yang et al 2022).
Despite its long duration and complex light curve, its short MVT 
therefore supports the merger origin for GRB230307A, as suggested by the 
possible evidence for kilonova emission reported by Levan et al. (GCN 
33569).
In addition, assuming a redshift z=0.065 (Gillanders et al. GCN 33485), 
a fluence of 3.6e-3 erg cm-2 (Svinkin et al. GCN 33427) would correspond 
to Eiso=3.7e52 erg. Combined with Ep~1 MeV (Svinkin et al.), the 
position of GRB230307A in the Ep-Eiso plane seems to be more compatible 
with short rather than long GRBs.

http://www.fe.infn.it/u/guidorzi/T90_vs_FWHMmin_neu.pdf   (Figure 
adapted from Camisasca et al. 2023)

References
- Camisasca et al., 2023, A&A, 671, A112, 
https://doi.org/10.1051/0004-6361/202245657
- Gompertz et al., 2023, Nature Astronomy, 7, 67
- Lazzati et al., 2023, arXiv:2303.12935
- Levan et al., 2023, arXiv:2303.12912
- Mei et al., 2022, Nature, 612, 236
- Rastinejad et al., 2022, Nature, 612, 223
- Troja et al., 2022, Nature, 612, 228
- Yang et al., 2022, Nature, 612, 232