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GCN Circular 21177

GRB 170531B: Redshift from GTC/OSIRIS
2017-06-01T06:34:45Z (7 years ago)
Antonio de Ugarte Postigo at IAA-CSIC <>
A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), L. Izzo (IAA-CSIC), 
D.A. Kann (IAA-CSIC), C.C. Thoene (IAA-CSIC), Z. Cano 
(IAA-CSIC), J.P.U. Fynbo (DARK/NBI), D. Garcia Alvarez 
(GTC, IAC), D. Perez Valladares (GTC), A. Nu��ez (GTC), M. Huertas (GTC)
report on behalf of a larger collaboration:

We observed the afterglow of GRB 170531B (Evans et al. GCN 
21171, Lipunov et al. GCN 21173, Pozanenko et al. GCN 21174) 
using OSIRIS at the 10.4 m GTC telescope in La Palma (Spain). 
Observation consisted in 3x900s exposures using the R1000B 
grism, covering the range between 3700 and 7880 AA. The mean 
epoch of the combined spectrum is 03:09:29 UT (5.12 hr after the 
GRB onset). Additionally, g, r, i, and z-band observations were 
obtained. We note that the coordinates of the afterglow are 
19:07:32.09, -16:25:05.8 (J2000 +/-0.5���), which places the GRB 
approximately 5��� away from those of the source identified by 
Lipunov et al. (GCN 21173) and Pozanenko et al. (GCN 21174). 
The afterglow can be also detected in the early Swift/UVOT 
images. In the acquisition image we measure a magnitude of 
r_AB = 21.9 for the afterglow, as compared to PANSTARRS 
catalogue field stars.

The afterglow spectrum shows continuum throughout the 
wavelength range and several strong absorption features 
corresponding to Lyman-alpha, SiII, OI, CII, SiIV, CIV, AlII and AlII 
at a common redshift of 2.366, which we identify as the redshift of 
the GRB.

[GCN OPS NOTE(01jun17): Per author's request, authors DPV, AN, & MH
were added to the list.]
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