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GCN Circular 6071

Subject
GRB 070125: Redshift z>~1.54 from Gemini Afterglow Spectrum
Date
2007-02-03T21:13:05Z (17 years ago)
From
Derek Fox at PSU <dfox@astro.psu.edu>
D. B. Fox (PSU), E. Berger (Carnegie), P. A. Price (Hawaii) and
S. B. Cenko (Caltech) report on behalf of a larger collaboration:

"We observed the optical afterglow (Cenko & Fox, GCN 6028) of GRB
070125 (Hurley et al., GCN 6024) with the Gemini-North telescope +
Gemini Multi-Object Spectrograph in four 1800-s integrations with mean
epoch 06:16 UT on 26 Jan 2007 (approximately 23 hours post-burst).
Our spectral coverage is 6000-10,000 A for the first hour and
4000-8000 A for the second hour.

We confirm the absence of prominent absorption features noted in
contemporaneous optical spectra from the Lick 3m (Prochaska et al.,
GCN 6031) and Keck-I (Prochaska et al., GCN 6032) telescopes.

At the same time, we identify a single weak (EW < 1 A) absorption
doublet at observer-frame wavelengths 7122.9 A + 7140.9 A.  Distinct
components of the doublet are well detected in separate spectra from
the first and second hour of observations, and the relative strength
of the two systems is consistent with their identification as the Mg
II (2796.4 A, 2803.5 A) doublet in absorption at redshift z=1.547.

We therefore conclude that GRB 070125 is at redshift z>1.547.  As
noted by Prochaska et al. (GCN 6032), the absence of Lyman-alpha
forest absorption at short wavelengths simultaneously suggests that
the burst redshift is not much greater than this."
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