Complicated variations of early optical afterglow of GRB 090726
  We report on a detection of an early rising phase of optical afterglow (OA)
of a long GRB 090726. We resolve a complicated profile of the optical light
curve. We also investigate the relation of the optical and X-ray emission of
this event. We make use of the optical photometry of this OA obtained by the
0.5 m telescope of AI AS CR, supplemented by the data obtained by other
observers, and the X-ray Swift/XRT data.
  The optical emission peaked at ~ 17.5 mag (R) at t-T0 ~ 500 s. We find a
complex profile of the light curve during the early phase of this OA: an
approximately power-law rise, a rapid transition to a plateau, a weak flare
superimposed on the center of this plateau, and a slowly steepening early
decline followed by a power-law decay. We discuss several possibilities to
explain the short flare on the flat top of the optical light curve at t-T0 ~
500 s; activity of the central engine is favored although reverse shock cannot
be ruled out. We show that power-law outflow with Theta_obs/Theta_c > 2.5 is
the best case for OA of GRB 090726. The initial Lorentz factor is Gamma_0 ~
230-530 in case of propagation of the blast wave in a homogeneous medium, while
propagation of this wave in a wind environment gives Gamma_0 ~ 80-300. The
value of Gamma_0 in GRB 090726 thus falls into the lower half of the range
observed in GRBs and it may even lie on the lower end. We also show that both
the optical and X-ray emission decayed simultaneously and that the spectral
profile from X-ray to the optical band did not vary. This OA belongs to the
least luminous ones in the phase of its power-law decay corresponding to that
observed for the ensemble of OAs of long GRBs.
