GCN Circular 35747

P. D’Avanzo, M. Ferro, R. Brivio, Y.-D. Hu, D. Fugazza, S. Covino (INAF-OAB) on behalf of the REM team, report:


We observed the field of GRB 240218A (Page et al., GCN 35742
Not found
 
 
) with the REM 60cm robotic telescope located at the ESO Observatory of La Silla (Chile). 
The observations were carried out in the g, r, i, z, H bands, starting on 2024 Feb 18 at 02:01:08 UT (i.e. 68 seconds after the burst T0) and lasted for about 3 hours.

An uncatalogued, fading source is detected in the initial H REM images. The source is not detected in the optical filters. 
From preliminary astrometry we obtain the following position:
RA (J2000) = 10:47:11.50
Dec (J2000) = +01:16:38.0

(+/- 0.5”). 

This position is located at the South-East edge of the 3.8” radius XRT error circle (https://www.swift.ac.uk/xrt_unenh_positions/01215912/ <https://www.swift.ac.uk/xrt_unenh_positions/01215912/>)

From preliminary analysis we derive:
H = 14.7 +/- 0.2 mag, at a mid time of t-t0 ~ 103 s after the GRB T0.
H = 15.8 +/ -0.2 mag, at a mid time of t-t0 ~ 511 s after the GRB T0.

Magnitudes are in the Vega system and calibrated against the 2MASS catalogue. 

The optical magnitude limits reported by Malesani et al. (GCN Circ. 35743
Not found
 
 
), although obtained about 1 hour after the REM detection, may imply a significant red color 
for the afterglow (r - H > 4 mag, assuming the same decay from ~ 100 s to 3600 s after the GRB T0). In light of the low Galactic extinction along the GRB line of sight 
and of the absence of evidence for intrinsic N(H) excess in the X-ray afterglow spectrum  obtained by Swift/XRT (https://www.swift.ac.uk/xrt_spectra/01215912/ <https://www.swift.ac.uk/xrt_spectra/01215912/>), 
we suggest that GRB 240218 can be a high-redshift event. 
Further follow-up, especially in the NIR bands, is encouraged.