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GCN Circular 33485

GRB 230307A: Continued Gemini-South observations confirm rapid optical fading
2023-03-17T16:06:24Z (a year ago)
James Gillanders at University of Rome Tor Vergata <>
J. Gillanders (UTV), B. O'Connor (UMD, GWU), S. Dichiara (PSU), and E. 
Troja (UTV, ASU) report on behalf of a larger team:

We re-observed the field of GRB 230307A (GBM team GCN 33405, Xiong et al.
GCN 33406) with the GMOS-S spectrograph at Gemini-South through Director's
Discretionary Time (PI: O���Connor). 

Our initial epoch was carried out at 2.4 d post-burst. We performed 4x1000 s
exposures with the R400 grating covering wavelengths 4100-9200 angstroms.
The brightness in the initial acquisition image (r~22 AB mag) was reported
in O'Connor et al. (GCN 33447). A weak trace is visible from this position
down to ~5300 angstroms, which sets an upper limit of z<4.3 to the GRB
redshift. No obvious emission or absorption features are visible in the

Our slit also covered a nearby bright galaxy at an offset of ~30���. We estimate
a redshift z~0.065 from Halpha, N II, and S II emission lines. If this galaxy
(RA=+60.8280, DEC=-75.3819) is the host, then the GRB would have a projected
offset of ~40 kpc. The probability of chance coincidence is ~0.08.

Our latest observations were carried out in z-band at approximately 8.4 d
post-burst. A faint source is significantly detected at the location of the
optical counterpart (Levan et al. GCN 33439,  O'Connor et al. GCN 33447), and
indicates a rapid fading of the afterglow by approximately 2 mag with respect
to earlier measurements. This suggests that the source reported by Bom et al.
(GCN 33459) is not the GRB host galaxy. Additionally, the observed power-law
temporal slope of ~-2 appears consistent with a jet-break.

We thank Andrew Levan for providing an initial finding chart and the staff of
the Gemini Observatory for rapidly approving and executing these observations.
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