GRB 990712 Late Optical Decay and Host Galaxy (GCN Circular #403)
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. Møller (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/ ."
4-min R-band image obtained with the ESO VLT 8.2-m ANTU unit telescope.
The OT + host is circled in blue. North is up and East is to the left.
Same image as above with 3 stars subtracted. Also a point source has
been fit and subtracted from GRB 990712. There are no residuals after
the stars but significant residuals after the GRB 990712 despite it
being fainter. This is evidence that the object is extended and hence
contains a significant contribution from the GRB 990712 host galaxy.
Zoom-in on a smoothed version of the PSF subtracted image. The residuals
are extended roughly E-W.
Deconvolved image of the host galaxy + OT using the code of
Magain, Courbin & Sohy (ApJ, 494, 452, 1998).
The updated R-band light curve of the optical afterglow of GRB 990712.
The three first points are from SAAO, the fourth point is based on our
NTT image. The fifth point is based on our VLT image. The last point
is Kemp & Halpern's CTIO data point. Error bars are about the size of
the data points. In the simplest model, the dotted line shows the contribution
from the host galaxy (R = 21.76) and the dashed curve is the assumed power
law decline (index = -1.03) of the optical afterglow emission. The solid
curve is the combined effect of these contributions and gives a good
fit to the observations. The dashed-dotted curve shows the light curve of
the host galaxy (R = 21.92) + power law (index = -1.00) + SN model.
This model also gives a good fit to the observations.
More details about the predictions of the SN model
The predicted spectrum of the SN model after the SN peak