GCN Circular 37476

I. Pérez-Fournon and F. Poidevin (IAC and ULL)  

Report on LCOGT follow-up observations of the optical afterglow of GRB 240912A, detected by 
the Fermi Gamma-ray Burst Monitor (GBM) (Fermi GBM team, GCN 37465
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); Swift BAT, XRT, and UVOT (Evans et al., GCN 37466
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; an enhanced Swift XRT position is given by Goad et al., GCN 37470
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); and with INTEGRAL ACS SPI (Pawar, GCN 37473
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). Spectroscopy of the optical afterglow reported by de Ugarte Postigo et al. (GCN 37469
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) and An et al. (GCN 37471
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) provides a redshift of z = 1.234, measured from absorption features.

We observed the field of GRB 240912A with one of the 1-meter telescopes at the Las Cumbres Observatory node located at McDonald Observatory (Texas), with a single 500-sec exposure in the SDSS-r' band starting at 2024-09-12T11:13:18 (UTC), ~ 9.44 hours after the Fermi GBM trigger.
We detect a point-like source at RA = 08:18:32.39, Dec = +33:59:56.1 (J2000) with an uncertainty of ~ 0.4", after astrometric calibration with Gaia DR3 stars, at a magnitude r' = 19.88 +/- 0.05,  calibrated with PanSTARRS and not corrected for Galactic extinction.

We report also the identification of the likely GRB host galaxy in archival Legacy Surveys DR10 imaging at RA = 08:18:32.38, Dec = +33:59:55.3 (J2000) and magnitudes g = 23.08, r = 22.88; and z = 22.01. 

The position of the optical afterglow on our LCOGT image is at about 0.2" from the Swift UVOT afterglow position and 0.8" from the DESI galaxy position.


This work makes use of observations from the Las Cumbres Observatory global telescope network (program IAC2024B-004).