{
  "bibcode": "2023GCN.33485....1G",
  "body": "J. Gillanders (UTV), B. O'Connor (UMD, GWU), S. Dichiara (PSU), and E. \nTroja (UTV, ASU) report on behalf of a larger team:\n\nWe re-observed the field of GRB 230307A (GBM team GCN 33405, Xiong et al.\nGCN 33406) with the GMOS-S spectrograph at Gemini-South through Director's\nDiscretionary Time (PI: O���Connor). \n\nOur initial epoch was carried out at 2.4 d post-burst. We performed 4x1000 s\nexposures with the R400 grating covering wavelengths 4100-9200 angstroms.\nThe brightness in the initial acquisition image (r~22 AB mag) was reported\nin O'Connor et al. (GCN 33447). A weak trace is visible from this position\ndown to ~5300 angstroms, which sets an upper limit of z<4.3 to the GRB\nredshift. No obvious emission or absorption features are visible in the\nspectrum.\n\nOur slit also covered a nearby bright galaxy at an offset of ~30���. We estimate\na redshift z~0.065 from Halpha, N II, and S II emission lines. If this galaxy\n(RA=+60.8280, DEC=-75.3819) is the host, then the GRB would have a projected\noffset of ~40 kpc. The probability of chance coincidence is ~0.08.\n\nOur latest observations were carried out in z-band at approximately 8.4 d\npost-burst. A faint source is significantly detected at the location of the\noptical counterpart (Levan et al. GCN 33439,  O'Connor et al. GCN 33447), and\nindicates a rapid fading of the afterglow by approximately 2 mag with respect\nto earlier measurements. This suggests that the source reported by Bom et al.\n(GCN 33459) is not the GRB host galaxy. Additionally, the observed power-law\ntemporal slope of ~-2 appears consistent with a jet-break.\n\nWe thank Andrew Levan for providing an initial finding chart and the staff of\nthe Gemini Observatory for rapidly approving and executing these observations.",
  "circularId": 33485,
  "createdOn": 1679069184000,
  "email": "jhgillanders.astro@gmail.com",
  "subject": "GRB 230307A: Continued Gemini-South observations confirm rapid optical fading",
  "submitter": "James Gillanders at University of Rome Tor Vergata  <jhgillanders.astro@gmail.com>",
  "eventId": "GRB 230307A"
}