GRB 990712 Optical Decay: Indication of Bright Host Galaxy (GCN Circular #389)
J. Hjorth (University of Copenhagen) and F. Courbin, J. Cuadra, D. Minniti
(Universidad Catolica de Chile) report:
"We have obtained a 5-min R-band exposure of the optical afterglow of
GRB 990712 (Frontera, GCN #385; Bakos et al., GCN #387) with the
ESO 3.5-m NTT on 16.403 July 1999 UT. We detect an unresolved (seeing
FWHM = 1.8") object at RA (2000) = 22 31 53.03, Dec (2000) = -73 24 28.3
(with a positional uncertainty of +- 0.6" relative to the USNO-A2.0 system),
consistent with the position of the bright decaying source discovered by
Bakos et al. (IAUC 7225). We have tied our photometry to the PLANET
photometric zeropoint (K. Sahu, personal communication) and find that the
object has continued to fade to R = 21.48 +- 0.02 (systematic) +- 0.05
(random). The combined SAAO data (Bakos et al., IAUC 7225) and NTT data
indicate that the light curve is leveling off relative to a power law
decline. Assuming that the light curve can be modeled as the combined effects
of a power law decline of the OT and a constant contribution from the host
galaxy we find an OT decay slope of -0.81 (i.e. a rather slow decay) and a
bright host galaxy with R = 22.0. Such a bright host galaxy would be
consistent with its fairly low redshift (z = 0.43) and would possibly even
account for the prominent emission lines seen in the VLT spectrum
(Galama et al., GCN #388). We caution however that the hypothesis of a bright
host galaxy is based on just a few data points. To test this hypothesis
continued monitoring of the system is therefore urged. The NTT image and the
R-band light curve are posted at http://www.astro.ku.dk/~jens/grb990712/ ."
5-min R-band image obtained with the ESO 3.5-m New Technology Telescope.
The 2 arcmin (radius) BeppoSAX error circle is shown in blue, the OT is
circled in the center of the image. North is up and East is to the left.
The R-band light curve of the optical afterglow of GRB 990712. The three first
points are from SAAO, the last point is based on the NTT image. Error bars
are about the size of the data points. The dotted line shows the contribution
from the purported host galaxy and the dashed curve is the assumed power law
decline of the optical afterglow emission. The solid curve is the combined
effect of these contributions and gives a good fit to the observations.