We first fitted a broken power-law
The
per degree of freedom is smallest for the broken power-law fit.
The data favor a very large value of n as the
is monotonically
decreasing as n is increased even for n > 100. The 2
lower limit
on n is 7 in the sense that the difference between
for a fit with
n = 7 and the broken power-law is 2. This indicates that the break in the
light-curve is very abrupt (within a few hours).
In conclusion, the data
are best fit by a sharp break around
days after the burst.
In Fig. 2 we show the R-band
light-curve of the OT together with the three fits and the residuals
around the fits.
Fit | ![]() |
![]() |
other | ![]() |
b. p.-l. |
![]() |
![]() |
![]() |
1.000 |
n=1 |
![]() |
![]() |
n=1 (fixed) | 1.263 |
n free |
![]() |
![]() |
n>7 (2![]() |
1.084 |
To precisely determine the broad band colours of the OT we used the
UBV and I band observations obtained at NOT and INT. The CA points were excluded
since Johnson R and I filters are significantly different from Bessel,
Cousins and
Harris. We determined the colours as the offset of the broken power-law fit
to the R-band light-curve that minimized the
of the fit.
Due to the lower error bars the magnitudes obtained on Sep. 27 have
the largest weight in the fits. Therefore we do not expect a
large systematic uncertainty due to emission from the underlying
host galaxy. In order to minimize the effect of the host galaxy
only points obtained earlier than and including September 29 were used
in the fits. The 1
errors on the
colours were determined as the colours that increased the value
of
by 1, but the true uncertainty including calibration and
systematic errors is most likely somewhat
larger (
5%). For all filters U,B,V and I the fits were
consistent with the (offset) broken power-law fit, which shows that
the data within the errors (few percent) are consistent with an
achromatic optical afterglow. The results are given in
Table 4.
Colour | GRB 000926 | ![]() |
GRB 000301C | |
U-R |
![]() |
0.59 |
![]() |
|
B-R |
![]() |
0.53 |
![]() |
|
V-R |
![]() |
0.34 |
![]() |
|
R-I |
![]() |
0.48 |
![]() |
|
R-K |
![]() |
1.87 |
![]() |
The afterglow was also observed in the J, H and K bands with the UFTI imager on UKIRT on September 30. The final frames were accumulated in 26, 24 and 9 dithered exposures of 60s duration for J, H and K respectively resulting in a total on-source integration time of 1.56 ks, 1.44 ks and 540s, all in photometric conditions. Employing standard procedures these frames were reduced, combined and calibrated using observations of UKIRT faint standards bracketing the science exposures. The final frames have only a (for this instrument) modest seeing of 0.55-0.60'' FWHM due to the relatively high airmass of the observations, 1.5-2, but clearly detect the OT in all three passbands. The magnitude of the OT was again measured using aperture photometry. The results of the IR photometry is presented in Table 5.
Using the standard star calibrated UKIRT K-band observations we confirmed from faint objects visible in both the UKIRT and SUBARU images that the calibration of the SUBARU images is consistent with that of the UKIRT K-band observations.
UT (Sep) | filter/Obs. | mag | Seeing | Exp. time |
(arcsec) | (sec) | |||
30.276 | J/UKIRT |
![]() |
0.6 | 1560 |
30.250 | H/UKIRT |
![]() |
0.6 | 1440 |
29.24 | K'/SUBARU |
![]() |
0.7 | 1800 |
30.301 | K/UKIRT |
![]() |
0.6 | 540 |
Copyright ESO 2001