2005A&A...439..527P

We present a photometric study of the optical counterpart of the long-duration Gamma Ray Burst (GRB) 030725, which triggered the HETE FREGATE and WXM instruments on July 25th, 2003, and lasted more than 160s. An optical counterpart was identified at the Bronberg Observatory in South Africa about 7h after the burst occurred. The optical afterglow (OA) was observed between 4 and 15 days after the burst with the 1.54m Danish telescope at La Silla in the V, R_c, and I_c bands. We fit a broken power law to the data and determine a break time in the light curve between 16 hours and 4.7 days after the first detection of the burst. The decay slope is α₁=-0.59^+0.59_–0.44 before and α₂=-1.43±0.06 after the break. A bump may be present in the light curve, only significant at the 2σ level, 13.9 days after the main burst. The spectral slope of the OA, measured 12 days after the burst, is -2.9±0.6, i.e. it falls in the extreme red end of the distribution of previous OA spectral slopes. Observations of the field 8 months after the burst with the EMMI instrument on the NTT telescope (La Silla) resulted in an upper limit of R_c=24.7mag for the host galaxy of GRB 030725. The OA of GRB 030725 was discovered at a private, non-professional observatory and we point out that with the current suite of gamma ray satellites, an effort to organize future contributions of amateur observers may provide substantial help in GRB light curve follow up efforts.