GCN Circular 189

GRB 980613:  Spectroscopy of the host galaxy

S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom (Caltech), D. Frail (NRAO),
F. Chaffee and R. Goodrich (CARA/WMKO), on behalf of the Caltech-NRAO-CARA
GRB collaboration, report:

We obtained spectra of the host galaxy of GRB 980613 (see GCN Circ. 114
Not found
 
 
 and
117
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) at the Keck-I telescope, on 17 Dec 1998 UT.  The reduced spectrum shows
a single strong emission line at 7814 Angstroms, which we interpret as the 
[O II] 3727 line at z = 1.0964 +- 0.0003, a probable 4000-A break just to
the red of it, and a very blue, featureless continuum blueward of the line.  
Other plausible spectroscopic identifications of this line (e.g., Ly alpha,
Balmer lines, [O III], etc.) would predict appearance of other strong 
emission lines or continuum breaks, which are clearly not seen in our data.  
We thus consider our redshift interpretation to be secure.  The spectrum is 
typical of star forming galaxies at comparable redshifts.

Both the continuum and the line emission are clearly extended.  The images
of the galaxy (which is coincident with the OT discovered by Hjorth et al.,
1998, GCN Circ. 109
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), show a patchy or asymmetric morphology, suggestive of
an interacting system.

In what follows, we assume the Galactic foreground extinction A(V) = 0.27 mag
from Schlegel et al. (1998, ApJ 500, 525), and a simple Friedman model 
cosmology with H_0 = 65 km/s/Mpc, Omega_0 = 0.2, and Lambda_0 = 0.  At this 
redshift, the luminosity distance is 2.256e28 cm.

The extinction-corrected spectroscopic magnitudes of the host are: B = 24.4, 
V = 24.1, and R = 23.85 mag, with the zero-point uncertain by at least 0.2 
mag.  This is in an excellent agreement with the R-band magnitude in a 
comparable aperture, derived from direct images obtained at the Keck-II 
telescope on 28 Nov 1998 UT, and consistent with our earlier estimate (GCN 
Circ. 117
Not found
 
 
).  

From the BATSE fluence of 1.71e-6 erg/cm2 (Woods et al. 1998, GCN Circ. 112
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) 
the implied isotropic gamma-ray energy of the burst was 5.2e51 erg.

The extinction-corrected [O II] line flux is 4.4e-17 erg/cm2/s, giving the
line luminosity of 2.8e41 erg/s.  The implied unobscured SFR, using the
conversion from Kennicutt (1998, ARAA 36) is 3.9 Msun/yr.  The observed, 
extinction-corrected monochromatic flux at the rest wavelength of 2800 A is 
0.9 microJy, corresponding to the restframe power of 2.76e28 erg/s/Hz.  The 
implied unobscured SFR, using the estimator from Madau et al. (1998, ApJ 498,
106) is 3.5 Msun/yr, in an excellent agreement with the [O II] line estimate, 
although both may be uncertain by as much as 20-50%.  This agreement suggests 
that the host galaxy as a whole is not heavily obscured.  We note that this 
SFR is rather moderate, despite the very blue color of the host galaxy.

Further observations and analysis are in progress.  This report may be cited.