GCN Circular 36498

A. de Ugarte Postigo (CNRS, OCA/LAM), J. F. Agui Fernandez (CAHA), N. A. Rakotondrainibe (LAM), C. C. Thoene (ASU/CAS), D. B. Malesani (DAWN/NBI and Radboud Univ.), N. R. Tanvir (U. Leicester), J. P. U. Fynbo (DAWN/NBI), L. Izzo (INAF-OACn & DARK/NBI), S. Geier (GRANTECAN), R. Scarpa (GRANTECAN) report:

We observed the counterpart	of GRB 240511A (Laha et al., GCN 36430
Not found
 
 
; Scotton & Meegan, GCN 36460
Not found
 
 
) with OSIRIS+ on the 10.4 m GTC, at the Roque de los Muchachos Observatory (La Palma, Spain). The observations started with acquisition images in the r-band, where the object reported by Malesani et al. (GCN 36442
Not found
 
 
) is detected as a faint source. The stack of these images, with a total exposure of 320 s and a mean time of 10.42 hr after the burst shows the afterglow at r(AB) = 23.80 +/- 0.16 mag, as compared to field stars of the SDSS catalogue.

We later proceeded to perform long slit spectroscopy with grism R1000B, covering the spectral range between 3600 and 7880 AA at a resolving power of ~600. Unfortunately the observing window was very short and we only managed to obtain 2 exposures of 900 s. The object is detected as a very weak trace, visible redder than 4700 AA, which implies a redshift larger than 4.1 if we consider the Lyman limit as the constraining wavelength.

Two further nearby objects are covered within the slit. The first one, at 5.9" North West of the afterglow and the second 10.4" South East of it. The spectrum of the second object allows us to identify it as a galaxy at redshift of z=0.416 through the detection of the [OII] doublet and H-beta. The first object has an emission feature at a similar wavelength as the [OII] of the other source. If we assume that this is also [OII] the redshift of the first galaxy would be z=0.420. At these redshifts, the afterglow would be at a projected distance of 33 and 58 kpc, making them likely unrelated to the GRB, assuming a collapsar origin.