TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1620
SUBJECT: GRB 021004:  Host Galaxy Star Formation Rate
DATE:    02/10/11 05:26:55 GMT
FROM:    George Djorgovski at Caltech/Palomar  <george@astro.caltech.edu>

S. G. Djorgovski, A. Barth, P. Price, D. Fox, W.L.W. Sargent, R. Simcoe,
A. Soderberg, S. Yost, E. Berger, S.R. Kulkarni, F.A. Harrison (Caltech) and
D. Frail (NRAO) report on behalf of the Caltech-NRAO-CARA GRB collaboration:

Analysis of spectra of the OT + host galaxy associated with GRB 021004,
obtained by W. Sargent and R. Simcoe using the LRIS spectrograph at the
WMKO Keck-1 telescope, confirms the exitence of the multi-component z ~ 2.3
redshift systems reported by Chornock & Filippenko (GCN 1605), Salamanca
et al. (GCN 1611) and Mirabal et al. (GCN 1618).

For the emission component of the Ly-alpha line we measure the flux of
1.7e-16 cgs, which after the correction for the Galactic extinction becomes
2.2e-16 cgs.

This places the host galaxy of GRB 021004 in the top 0.5 to 1 percent of the
field galaxy population at comparable redshifts, in terms of the Ly-alpha
line flux (on the basis of data by A. Shapley, C. Steidel, et al., priv.
comm.)

In the currently popular cosmology with h = 0.65, Omega_0 = 0.3, and
Lambda_0 = 0.7, the luminosity distance is 6.2e28 cm, and the inferred
Ly-alpha luminosity is 1.1e+43 erg/s.  Using the standard conversion, this
corresponds to an unobscured star formation rate of 15 M_sun/yr.  However,
this is certainly a lower limit, since the blue half of the Ly-alpha line is
absorbed, there is an unknown atennuation factor due to the resonant
scattering and absorption, and an unknown, fully obscured star formation
component.  Thus, the true SFR in the host is at least twice as high, and
could be an order of magnitude higher (or more).

This note can be cited.