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  <evert@astro.uva.nl>

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.