TITLE:   GCN CIRCULAR
NUMBER:  36442
SUBJECT: GRB 240511A: optical afterglow detection from the NOT
DATE:    24/05/12 07:55:52 GMT
FROM:    Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>

Daniele B. Malesani (DAWN/NBI and Radboud Univ.) and Bo Milvang-Jensen (DAWN/NBI) report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 240511A (Laha et al., GCN 36430; Fermi GBM team, GCN 36428) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. Our observations started on 2024 May 12.186 UT (10.35 hr after the trigger) using the SDSS r and z filters. The seeing was moderate, about 1.5" in the r band.

In the stack of three r-band frames (900 s exposure), a faint object can be seen at a position consistent with the XRT error circle (Evans, GCN 36431). At the same location a faint source can be glimpsed also in the z images (1000 s exposure). Its coordinates are:

RA = 22:26:42.23
Dec = 08:30:46.6

with an estimated 0.5" uncertainty.

Calibrated against nearby objects from the Pan-STARRS catalog, we measure r = 23.36 +- 0.22 (AB), at a mean epoch of May 12.191 UT. Inspection of the Legacy Survey frames reveals no bright object at the source location, confirming that this is most likely the optical afterglow of GRB 240511A.