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GCN Circular 21298

GRB 170705A: Redshift from 10.4m GTC/OSIRIS
2017-07-05T23:48:25Z (7 years ago)
Antonio de Ugarte Postigo at IAA-CSIC <>
A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), D.A. Kann (IAA-CSIC), 
L. Izzo (IAA-CSIC), C.C. Thoene (IAA-CSIC) report on behalf of the 
HETH group:

We observed the afterglow of GRB 170705A (Starling et al. GCN 21289, 
Lipunov et al. GCN 21291, Butler et al. GCN 21293, Emery et al. 
GCN 21294) with OSIRIS at the 10.4m GTC telescope. The observation 
consisted of 3x900s using grism R1000B, covering the range between 
3700 and 7880 AA at a mean epoch of July 5.9350 2017 UT (19.68 hr 
after the burst).

The acquisition shows the afterglow at a magnitude of r(AB) = 20.20 
+/- 0.10 mag, as compared to SDSS field stars. The spectrum shows 
continuum across all the spectral range, although the signal to noise 
ratio of the bluest part is low because of the bright sky due of the nearby 
almost full Moon. We detect weak spectral absorption features due to 
CIV, AlII, AlIII, FeII and FeII* at a common redshift of 2.010, which we 
identify as the redshift of the GRB.
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