[vsnet-grb-info 5293] GRB 071020: BTA spectroscopy

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6984
SUBJECT: GRB 071020: BTA spectroscopy 
DATE:    07/10/24 15:00:41 GMT
FROM:    Alberto Castro-Tirado at Inst.de Astro. de Andalucia  <ajct at iaa.es>

T. A. Fatkhullin, V. V. Sokolov (SAO-RAS Nizhnij Arkhyz), S. Guziy 
(Nikolaev St. Univ.), A. de Ugarte Postigo (ESO Santiago), D. 
Pérez-Ramírez (Univ. de Jaén and U. Leicester), J. Gorosabel, M. Jelínek 
and A. J. Castro-Tirado (IAA-CSIC Granada), on behalf of a larger 
collaboration, report:

"Following the detection of GRB 071020 by SWIFT (Holland et al. GCN 
Circ. 6949)  we obtained optical spectroscopy with the 6.0m BTA 
telescope (+SCORPIO) of the SAO-RAS.  In spite of poor weather 
conditions (dense cirrus) we managed to get 4 x 1200s spectra of the 
proposed optical afterglow (Schaefer et al. GCN Circ. 6948). The data 
were taken on Oct 21.05 U.T. (i.e. 18 hr after the event), with the VPHG 
400 grism (range 3700-9000 A).

We identify absorption lines from Fe II (2344, 2586 and 2600) and 
possibly C IV (1548-1550) and Fe II (1608), at z = 2.142 ± 0.002. This 
confirms the lower limit to the GRB 071020 redshift derived from VLT 
spectroscopy (Jakobsson et al., GCN Circ.  6952). Moreover, we do not 
see any flux drop in the OA spectrum around 4300 A as the VLT spectrum 
may suggest. However, there are indications of a flux drop at ~3870A but
the low S/N ratio in this range prevents us to draw any further 
conclusion."

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