GCN Circular 1605
Subject
GRB 021004 redshift
Event
Date
2002-10-08T16:49:46Z (24 years ago)
From
Alex Filippenko at UC Berkeley <alex@astro.berkeley.edu>
R. Chornock and A. V. Filippenko, University of California, Berkeley,
report that optical spectra (range 300-940 nm) of the GRB 021004
afterglow (GCN 1564, 1565) were obtained with the Low Resolution
Imaging Spectrometer on the Keck-I 10-m telescope on Oct. 8 UT. We
detect the absorption lines due to Mg I, Mg II, and Fe II associated
with systems at z=1.381 and 1.603 previously noted (GCN 1569). We
also detect the unidentified absorption lines at 463, 466, 510, and
514.5 nm reported in GCN 1579, as well as the narrow emission line at
405 nm and the absorption lines between 400 and 404 nm (GCN 1599).
We identify the emission line as Lyman-alpha at z=2.323, in
contradiction to the identification as C IV in GCN 1599. We note that
P-Cygni absorption line profiles from a hypothetical starburst host
galaxy should not be prominent above the bright continuum of the
transient. In addition, we feel that a lone UV emission line is
significantly more likely to be Lyman-alpha than C IV. The
previously unidentified absorption lines can now be identified as the
Si IV 1393.8, 1402.8 Angstrom doublet and C IV 1548.2, 1550.8
Angstrom doublet. These absorption lines have multiple velocity
components, as does the Lyman-alpha absorption. We note the
wavelength coincidence of one of the components of Lyman-alpha with
the expected C IV absorption from the Mg II system at z=1.603, but
believe it only to be a coincidence.
An additional point in favor of the z=2.323 interpretation is the
appearance of the continuum to the blue of the Lyman-alpha line.
While there is no substantial Lyman break, there are many more
absorption lines blueward of Lyman-alpha than redward. A previously
unnoted absorption feature with multiple components is present at
341 nm, consistent with Lyman-beta at this redshift.
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