TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 850
SUBJECT: V,R,I band observations of GRB000926
DATE: 00/10/14 18:11:47 GMT
FROM: Evert Rol at U.Amsterdam
E. Rol, P. M. Vreeswijk (U. of Amsterdam) and N. Tanvir (U. of
Hertfordshire), on behalf of a larger collaboration, report:
We have obtained Sloan i and Harris R & V band images of the field of
GRB 000926 (GCN #801), using the Wide Field Camera on the 2.5m Isaac
Newton Telescope at La Palma. Using aperture photometry (with a radius
twice the FWHM), we find the magnitude of the OT, relative
to the reference star of Halpern et al. (GCN #824), to be as follows:
date (UT) filter rel-mag error exp-time
(Sept 2000) (secs)
- ------------------------------------------------
27.919 i 2.406 0.007 300
27.924 i 2.439 0.007 300
27.929 i 2.454 0.008 300
30.850 i 5.403 0.009 600
27.892 R 2.369 0.007 300
27.896 R 2.386 0.007 300
27.901 R 2.396 0.008 300
27.906 V 2.271 0.009 300
27.910 V 2.298 0.009 300
27.915 V 2.301 0.010 300
30.839 V 5.352 0.096 600
(errors are statistical only)
We have fitted the R band data, together with all published data (GCNs
#809, #814, #816, #819, #820, #824, #825, GCN #829, #831, #840), with a
smoothly broken power-law, and a constant flux (presumably from the
underlying host galaxy). The fitted function is
F(t) = { F_1^(-n) + F_2^(-n) }^(-1/n) + F_host,
with F_i = k_i t ^(-a_i), n > 0; i = 1, 2.
a_1 and a_2 correspond to the early and late time power-law indices,
respectively. The break time t_0 corresponds to the time when F_1 = F_2.
(see e.g. Beuermann, K., et al 1999, A&A, 352, L26-L30)
We obtain the following parameters for the R band data, with n fixed at 1:
a_1 = 1.1 +- 0.2
a_2 = 3.2 +- 0.4
t_0 = 2.0 +- 0.4
host: R = 24.2 +- 0.3
Errors are 1 sigma; chi^2/DOF = 69/31. Excluding the host,
we find a chi^2/DOF = 83/32.
The fit shows a rather large increase in the decay-rate of the OT flux
around two days after the burst (from a power-law index of 1.1 to
3.2). Fitting for n as well (which can be used to define the
sharpness of the break) gives the same results as above, with n = 1.1
+- 0.8.
Graphs of these fits can be found at
http://www.astro.uva.nl/~evert/grb000926/
We acknowledge the help of the observers Chris Blake and Steve Rawlings
(U. of Oxford), and the ING staff.