GCN Circular 29717

A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann 
(HETH/IAA-CSIC), D. B. Malesani (DTU Space), N. R. Tanvir (Univ. 
Leicester), C. Thoene, M. Blazek, J. F. Agui Fernandez (all 
HETH/IAA-CSIC), and S. Geier (GTC, IAC) report:

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.