GCN Circular 403
Subject
GRB 990712 Late Optical Decay and Host Galaxy
Date
1999-08-19T17:11:21Z (25 years ago)
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/ ."