GCN Circular 30392
Subject
GRB 210704A: OSIRIS/GTC spectroscopy
Event
Date
2021-07-06T14:24:06Z (4 years ago)
From
Antonio de Ugarte Postigo at IAA-CSIC <deugarte@iaa.es>
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.