GRB 970228

Shri Kulkarni et al. report the following (also submitted to the IAUCs):

   S. R. Kulkarni, S. G. Djorgovski, and J. C. Clemens, California
   Institute of Technology, report: "On 13 August 1997 UT, we used the Low
   Resolution Imaging Spectrograph mounted on the Keck II telescope to
   acquire spectra at the position of the GRB 970228 optical transient
   reported in IAUC 6584.  We placed a 1.5-arcsec slit (PA=87 degrees) at
   the location specified in IAUC 6588 (Metzger et al.), and used a
   low resolution grating (300 l/mm blazed at 5000 A) to acquire exposures
   totalling 1800 s.  A spectral trail is well detected in this location.
   The spectrum has some weak features but no strong emission lines.
   The detected flux is consistent with an object of R magnitude
   between 24 and 25." 



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Ed. Note April 2024: Title is updated in the new GCN system using the moderation tool.
Ed. Note 22Mar98:  This is the initial GCN Circular.  It has the -4 serial
number because of a mistake I made in making the initial number assignments
when I started the assignments at around circular #15.  See the explanation
at the bottom of the Circular Archive List web page.

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.]