The red optical afterglow of GRB 030725

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: gpuglies@eso.org
² IAA-CSIC, PO Box 03004, 18080 Granada, Spain
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark
⁵ Bronberg Observatory, CBA Pretoria, PO Box 11426, Tiegerpoort 0056, Rep. South Africa

Received: 28 February 2005
Accepted: 4 May 2005

Abstract

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 160 s. An optical counterpart was identified at the Bronberg Observatory in South Africa about 7 h after the burst occurred. The optical afterglow (OA) was observed between 4 and 15 days after the burst with the 1.54 m Danish telescope at La Silla in the V, , and  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 before and  ± 0.06 after the break. A bump may be present in the light curve, only significant at the 2 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  = 24.7 mag 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.