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 here.
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
here".
