[vsnet-grb-info 19915] GRB 171010A: ePESSTO NTT spectroscopic redshift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22002
SUBJECT: GRB 171010A: ePESSTO NTT spectroscopic redshift
DATE:    17/10/12 11:14:14 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <paolo.davanzo at brera.inaf.it>

E. Kankare (QUB), D. O'Neill (QUB), L. Izzo (IAA-CSIC), V. D'Elia (ASI/ASDC), S. D. Vergani (GEPI/Obs. Paris), 
D. Malesani (DARK/NBI), K. E. Heintz (Univ. Iceland & DARK/NBI), P. Schady (MPE), A. Melandri, P. D'Avanzo, 
S. Campana, S. Covino (INAF-OAB), M. Magee (QUB), T.-W Chen (MPE), L. Galbany (U. Pittsburgh), C. Inserra 
(Southampton), K. Maguire (QUB), S. Smartt (QUB), O. Yaron (Weizmann), D. Young (QUB), I. Manulis (Weizmann) report:


We observed the optical afterglow of GRB 171010A (Thorstensen & Halpern, GCN 21987; Izzo & Malesani, 
GCN 21988; Guidorzi et al., GCN 21991; Malesani et al. GCN 22000; Harita et al. GCN 22001) under the 
extended Public ESO Spectroscopic Survey for Transient Objects (ePESSTO; see Smartt et al. 2015, A&A, 
579, 40 http://www.pessto.org <http://www.pessto.org/> ). The observations were performed on the ESO New Technology Telescope 
at La Silla with the EFOSC2 instrument in spectroscopic mode starting from 2017-10-12 at 07:05:01 UT 
(i.e. 36.1 hours from the burst) and using the Grism 13 (3985-9315 AA, 18 AA resolution). Both the optical 
afterglow and the object visible in the Pan-STARRS archival images lying 1.3” away (suggested as the 
possible host galaxy by Thorstensen & Halpern, GCN 21987 and Izzo & Malesani, GCN 21988) were included 
in the slit. 

In the acquisition image, the optical afterglow is clearly detected with a magnitude of R(AB)~19.3.

The optical spectrum shows emission features which we identify as [O II], Hbeta, [O III], and Halpha at a 
common redshift z=0.33. No clear absorption features are detected, in agreement with the report by 
Thorstensen & Halpern (GCN 21987). The lack of detection of absorption features in high-S/N spectra is 
strongly suggestive of a low redshift for the afterglow. Combined with the proximity to the archival object, 
we suggest that z = 0.33 is the redshift of GRB 171010A. Further analysis is in progress.