GCN Circular 605
Subject
GRB 000301C: A Precise Redshift Determination
Date
2000-03-10T09:41:17Z (25 years ago)
From
George Djorgovski at Caltech/Palomar <george@oracle.caltech.edu>
GRB 000301C: A Precise Redshift Determination
S. M. Castro, A. Diercks, S. G. Djorgovski, S. R. Kulkarni, T. J. Galama,
J. S. Bloom, F. A. Harrison (Caltech), and D. A. Frail (NRAO), report on
behalf of the Caltech-CARA-NRAO GRB collaboration:
Moderately high resolution spectra (FWHM ~ 1 Ang) of the optical transient
(OT) associted with GRB 000301C (Fynbo et al., GCN 570) were obtained on UT
2000 March 04, by W. L. W. Sargent, A. Boksenberg, and M. Rauch, using the
ESI Echelle spectrograph on the Keck-II 10-m telescope on Mauna Kea, Hawaii.
Two exposures of 1800 sec each were obtained, over an effective wavelength
range from ~3800 Ang, to ~10800 Ang. The OT continuum is well detected.
No strong absorption systems were found in the spectrum. However, a number
of possible weak lines are present. We identify a subset of them with the
following lines: Fe II 2260, 2344, 2374, 2382, 2586, and 2600, and Mg II 2796
and 2803; and a less reliable set of O I 1302, C II 1334, Si IV 1393,
Si II 1526, C IV 1550, Fe II 1608, Al II 1670. The weighted mean absorption
redshift of this 16-line system is z = 2.0335 +- 0.0003.
This is fully consistent with the redshift estimate from the HST spectroscopy,
z = 1.95 +- 0.1 (Smette et al., GCN 603), based on the ostensible Lyman break.
We thus consider our redshift determination to be secure, and interpret it as
the redshift of the GRB host galaxy, with no intervening foreground systems.
The restframe equivalent widths of the absorption lines are in the range of
interstellar medium in the host galaxy.
Assuming z = 2.0335, and a simple Friedmann cosmology with H_0 = 65 km/s/Mpc,
Omega_0 = 0.2, and Lambda_0 = 0, the luminosity distance is D_L = 5.11e28 cm,
the distance modulus is (m-M) = 49.1 mag, and the relativistic (1+z)**4
surface brightness dimming factor is 4.8 mag. This may account for the lack
of a detection of the host galaxy in the HST images (Fruchter et al., GCN 602).
This note can be cited.