TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  403
SUBJECT: GRB 990712 Late Optical Decay and Host Galaxy
DATE:    99/08/19 17:11:21 GMT
FROM:    Jens Hjorth at U.Copenhagen  <jens@astro.ku.dk>

GRB 990712 Late Optical Decay and Host Galaxy


J. Hjorth (University of Copenhagen), J. Fynbo (University of Aarhus and ESO),
A. Dar (Technion, Haifa and CERN), F. Courbin (Universidad Catolica de Chile)
and P. Moller (ESO) report:

"We have obtained a 4-min R-band exposure of the optical afterglow + host
galaxy of GRB 990712 (Bakos et al., GCN #387; IAUC 7225; Hjorth et al., 
GCN #389; Kemp & Halpern GCN #402) with the ESO 8.2-m Antu telescope on 
12.232 August 1999 UT. The OT + host has R = 21.71 +- 0.03 on the PLANET 
photometric system and is slightly extended in seeing FWHM = 1.2" as revealed 
by (i) faint but significant residuals after fitting and subtraction of a 
point source (ii) deconvolution (Magain et al., ApJ, 494, 452, 1998). The host 
galaxy is oriented roughly E-W. The decay slope of the optical transient 
(-1.03) and host galaxy magnitude (R = 21.76) are consistent with the 
prediction of Hjorth et al. (GCN #389) when correcting for the erroneous date 
of the first data point of Bakos et al. (IAUC 7225) and with the findings of 
Kemp & Halpern (GCN #402). The R band light curve can also be fit by a galaxy 
(R = 21.92) and an OT with initial power-law decay (-1.00) that is taken over 
later by a ``standard candle'' SN (Dar, astro-ph/9902017; Bloom et al., 
astro-ph/9905301; Dar, GCN #346; Reichart, astro-ph/9906079; Galama et al., 
astro-ph/9907264) like 1998bw (Galama et al., Nature, 395, 670, 1998; 
McKenzie & Schaefer, astro-ph/9904397) at the redshift of the GRB (z = 0.43; 
Galama et al., GCN #388). The HST observation of GRB 990712 scheduled for 
August 29 can distinguish between the two models which predict OT magnitudes 
of R = 25.80 in the simple power-law model and R = 24.20 in the power law + SN 
model (or R(SN) = 24.58 if the power law decline steepened before August 29). 
The takeover by a SN light curve would also result in a dramatic reddening of 
the afterglow colours. Alternatively, the two models can be discriminated by 
high-precision R band observations obtained around 1 August 1999 UT or by
late-time observations of the host galaxy brightness. The VLT image (before and 
after point source subtraction or deconvolution) and the R-band light curves 
for the two models are posted at http://www.astro.ku.dk/~jens/grb990712/ ."