GCN Circular 30392

A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann, 
C. Thoene, M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC),
J. P. U Fynbo (DAWN/NBI), L. Izzo (DARK/NBI), N. R. Tanvir (U. Leicester), 
G. Lombardi, A. Marante (both GTC) report:

We observed the afterglow of GRB 210704A (BALROG localization: 
Kunzweiler et al., GCN #30369; Fermi/LAT detection: Berretta et al., 
GCN #30375; Fermi/GBM detection: Malacaria & Meegan, GCN #30382; 
AGILE/MCAL detection: Ursi et al., GCN #30372; AstroSat/CZTI detection: 
Prasad et al., GCN #30378; Konus-Wind detection: Ridnaia et al., GCN 
#30388; Swift/XRT afterglow detection: D'Ai et al., GCN #30379; optical 
afterglow: Kim et al., GCN #30384, D'Avanzo et al., GCN #30385, Kann et 
al. GCN #30391) with OSIRIS, mounted on the 10.4 m GTC telescope, at 
the Roque de los Muchachos observatory (La Palma, Spain). The 
observation consisted of 2x60s acquisition images in r-band followed by 
4x600s spectra with grism R1000B, covering the spectral range between 
3700 and 7800 AA. The observation started at 21:48:57 UT (26.26 hr after 
the burst) and the combined spectrum has an average epoch of 22:18:25 
UT (26.75 hr after the burst).

The afterglow is well detected in the acquisition image at an r-band 
magnitude of 22.40 +/- 0.07 as compared to several SDSS field stars. The 
spectrum shows continuum above 3800 AA with a typical SNR of 5 above 
5000 AA. 

The detection of continuum down to 3800 AA implies an upper limit on the 
redshift of z < 3.15.

We do not detect any absorption or emission features at the redshift of the 
field galaxy (z = 0.0817) or field cluster (z = 0.203) reported in by Levan et 
al. in GCN #30381. However, a featureless spectrum would be expected 
if this event were a short GRB in an intracluster environment.

We note that there is a tentative detection of a broad dip in the continuum at 
~ 4050 AA, that could be identified as due to Ly-alpha. However, we caution 
that the SNR in this part of the spectrum is very low. This would imply a 
redshift of z = 2.34, and could be sustained by further low-significance 
detections of OI, SiII and CII. However, we also note that at this redshift 
there is no detection of CIV, nor SiIV, which is uncommon but not 
unprecedented. 

The spectrum is publicly available for inspection at GRBSpec.eu.