GCN Circular 289

GRB 970228: Redshift and properties of the host galaxy

S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom (Caltech), and D. A. Frail
(NRAO) report on behalf of the Caltech-CARA-NRAO GRB Collaboration:

Spectra of the host galaxy of GRB 970228 were obtained in the course of
several runs at the Keck-II 10-m telescope.  A prominent emission line at 
6319 A was observed in at least 3 independent data sets, along with a very 
blue continuum, showing no apparent breaks between about 4000 and 8900 A.  
We interpret this emission line as [O II] 3727 at z = 0.695 +- 0.002.
Moreover, possible emission lines are detected corresponding to [Ne III] 
3869 and [O III] 5007 at the same redshift.  We thus consider this redshift
to be practically secure.

Assuming a cosmology with H_0 = 65 km/s/Mpc, Omega_0 = 0.2, and Lambda_0 = 0, 
we derive the luminosity distance of 1.28 e28 cm.  The measured fluence from
Ulysses (cf. Hurley et al. 1997, ApJ, 485, L1) of 4.3 e-6 erg/cm2 then implies 
E(gamma,isotropic) = 5.2 e51 erg.

We detect continuum flux in the V band of about 0.12 uJy, uncertain by at 
least 20%.  This is in an excellent agreement with the published ground-based 
and HST photometry, corresponding to V ~ 26 mag.

The [O II] line has the equivalent width 75 +- 15 A (observed), or 44 +- 9 A 
(restframe), typical for star-forming galaxies at comparable redshifts.  The
very blue continuum suggests little restframe extinction in this galaxy.

In the following estimates, we assume a Galactic extinction correction A(V) = 
0.75 mag.  From the [O II] line flux, we estimate SFR = 0.47 Msun/yr, and from
the continuum power at 2800 A (rest), SFR = 0.35 Msun/yr.  Both of these are 
uncertain by at least 30%.  The absolute restframe B band luminosity of the 
galaxy corresponds to about 0.05 to 0.1 L_* today.  Thus, neither the SFR 
nor the SFR per unit mass are especially high.

This note can be cited.

[GCN OPERATOR'S NOTE (04 May 99):  This archived copy was corrected
for the mistaken citation that BATSE yielded the fluence.  The true
source of the fluence for the burst came from Ulysses.]