GRB 980703: Spectrum of the proposed optical counterpart
S. G. Djorgovski, S. R. Kulkarni (Caltech), R. Goodrich (WMKO),
D. A. Frail (NRAO), and J. S. Bloom (Caltech), report on behalf
of the Caltech GRB collaboration:
Analysis of the spectra of the proposed optical/radio counterpart of
GRB 980703 (see GCN Circ. 128, 136, 137) gives the following results:
The strong emission line is definitively identified as [O II] 3727 at
the redshift z_em = 0.9660 +- 0.0002. It has the equivalent width of
90 +- 2 Angstroms (observed), or 45.7 +- 1 A in the restframe. This
is somewhit higher than typical for galaxies at comparable redshifts
and magnitudes. After correcting for the as-yet unknown contribution
of the GRB afterglow to the continuum flux, these numbers will be higher.
The observed line flux is 2.7e-16 erg/cm2/s.
There are several absorption features identified as Fe II (2344, 2374,
2383, 2587, and 2600 A) and Mg II (2796 and 2803) lines, with the mean
redshift z_abs = 0.9653 +- 0.0007, superposed on an otherwise featureless
UV continuum. This too is typical of actively star-forming galaxies at
similar redshifts. We also see the 4000-Angstrom break.
The observed spectrophotometric magnitude at the epoch of these
observations (July 7.6 UT) are: B = 23.3, V = 22.65, and R = 21.9 mag,
with the zero-point uncertain by a few tenths of a magnitude, due to the
unknown slit losses. The UV continuum blueward of the [O II] line can
be described with a power-law F_nu ~ nu**alpha, with alpha = -1.5 +- 0.2,
possibly indicative of a modest extinction.
Assuming a Friedmann cosmology with H_0 = 65 km/s/Mpc, Omega_0 = 0.2, and
Lambda_0 = 0, the luminosity distance at this redshift is 1.9208e28 cm.
Our observed [O II] line flux then translates to the restframe line
luminosity of 1.25e42 erg/s. Assuming the conversion from Kennicutt
(1992), the implied star formation rate is about 63 M_sun/yr, even
without any extinction or slit-loss corrections.
We thus conclude that the host galaxy of GRB 980703 is a compact starburst
object. This gives some support to the models which associate origins of
GRBs with massive star formation.
Further observations of the object are in progress.
This note can be cited.