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GCN Circular 389

Subject
GRB 990712 Optical Decay: Indication of Bright Host Galaxy
Date
1999-07-17T18:36:16Z (25 years ago)
From
Jens Hjorth at U.Copenhagen <jens@astro.ku.dk>
GRB 990712 Optical Decay: Indication of Bright Host Galaxy

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/ ."
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