GCN Circular 2117
Subject
GRB030329: HET spectroscopic monitoring
Date
2003-04-08T03:37:04Z (22 years ago)
From
Mike Eracleous at PSU, Astro.Dept. <mce@astro.psu.edu>
Mike Eracleous (Penn State), Brad Schaefer, and Chris Gerardy
(U. Texas) on behalf of the Hobby-Eberly Telescope GRB followup team
report:
We have obtained several spectra (410-900 nm, resolution 1.6 nm) of
the optical counterpart of GRB030329 (Peterson & Price, GCN 1985;
Torii: GCN 1986) with the Hobby-Eberly Telescope and Marcario
Low-Resolution spectrograph. Our spectra of March 31, April 2, and
April 5 (UT) show the following evolution of the spectral properties:
(a) the equivalent widths of the [O III] and H-alpha emission lines is
increasing in a manner consistent with a constant line flux and a
decline of the continuum, and (b) the spectrum of April 5 (UT) shows
evidence of a broad feature underlying the [O III] lines, similar
tresembling that reported by Matheson et al. (GCN 2107), who interpret
it as due to an emerging SN spectrum. However, the contrast of this
feature in our spectrum is not as high as in the spectrum of Matheson
et al. (taken a day later than ours). As a consequence, we cannot rule
out alternative interpretations, such as the emergence of a host
galaxy spectrum that manifests itself as a break in the power-law
continuum. Analysis of more recent spectra is under way to clarify
this issue.
Using the [O II] and H-alpha emission line fluxes of Caldwell et
al. (GCN 2053), we estimate luminosities of 1x10^40 and 4x10^40 erg/s,
respectively (for z=0.168, i.e., a luminosity distance of 880 Mpc),
and infer star formation rates of 0.15 and 0.32 solar masses/yr
(following Kennicutt 1998, ARAA, 36, 189). The upper limit on the host
galaxy from historical images reported by Wood-Vasey et al. (GCN
1998; R > 22.28) implies a limit on the absolute magnitude of M_R >
-17.4, which is comparable to that of the LMC. The [OIII]/H-beta ratio
of 2.7 (Caldwell et al.; GCN 2053) corresponds to an LMC-like faint
starburst galaxy (Ho, et al. 1997, ApJ, 487, 579, Figure 6; Hunter &
Gallagher, 1997, ApJ, 475, 65; Hunter et al. 2001 ApJ, 553, 121),
while the H-alpha luminosity is characteristic of starburst galaxies
(Ho. et al. Figure 2). High-dispersion spectroscopy of these narrow
lines may give more information on the nature of star formation in the
host galaxy.