Early optical and millimeter observations of GRB 030226 afterglow

Abstract

The CCD magnitudes in Johnson UBV and Cousins RI photometric passbands for the afterglow of the long duration GRB 030226 are presented. Upper limits of a few mJy to millimeter wave emission at the location of optical are obtained over the first two weeks. The optical data presented here, in combination with other published data on this afterglow, show an early R band flux decay slope of 0.77 ± 0.04, steepening to 2.05 ± 0.04 about 0.65 ± 0.03 day after the burst. Interpreted as the “jet break”, this indicates a half opening angle of ∼3.2 degree for the initial ejection, for an assumed ambient density of ∼1 cm−³. Broadband spectra show no appreciable evolution during the observations, and indicate the presence of synchrotron cooling frequency Vc near the upper edge of the optical band. From the broadband spectra we derive an electron energy distribution index p = 2.07 ± 0.06 and an intrinsic extinction E(B − V) ∼ 0.17. Millimeter upper limits are consistent with these derived parameters.