Short GRB 210323A: GTC Observations

We observed the afterglow (Malesani et al., GCN #29703, Pozanenko et al., GCN #29708) of the short/hard GRB 210323A (Swift detection: Gropp et al., GCN #29699, Fermi GBM detection: Hamburg & Meegan, GCN #29709; Konus-Wind detection: Frederiks et al., GCN #29713) with OSIRIS at the 10.4m GTC telescope at Roque de los Muchachos Observatory, in La Palma (Spain). The observation started at 05:31 UT (0.3118 days after the burst), and consisted of 3 x 600s with grism R1000B, covering the spectral range from 3700 to 7800 AA. Observations were taken at high airmass, near dawn. The 60 s acquisition image, with a seeing of 1".3, shows the afterglow at a magnitude of r' = 23.03 +/- 0.15 mag, using a nearby Pan-STARRS field star as photometric reference. This is in agreement with the NOT measurement. Spectral continuum is faintly detected above 5000 AA, and shows no clear evidence for any absorption or emission lines. We therefore place a weak upper limit of z < 3.1 on the redshift of GRB 210323A. There may be evidence for a low-significance emission line at 5110 AA, which is slightly offset from the continuum trace in the 2D spectrum. It is found using multiple reductions and analyses. Interpreting this line as [OII], the corresponding redshift would be z = 0.37. However, we do not find any evidence for other emission lines, such as Hbeta and [OIII], at the corresponding redshift, placing its reality in doubt. Further observations will be required to verify the reality of this feature and to attempt a redshift determination through host galaxy spectroscopy.