EDP Sciences
Free access
Volume 417, Number 3, April III 2004
Page(s) 919 - 924
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034515

A&A 417, 919-924 (2004)
DOI: 10.1051/0004-6361:20034515

Early optical and millimeter observations of GRB 030226 afterglow

S. B. Pandey1, R. Sagar1, 2, G. C. Anupama2, D. Bhattacharya3, D. K. Sahu2, 4, A. J. Castro-Tirado5 and M. Bremer6

1  State Observatory, Manora Peak, Nainital - 263 129, Uttaranchal, India
2  Indian Institute of Astrophysics, Bangalore - 560 034, India
3  Raman Research Institute, Bangalore - 560 080, India
4  Center for Research & Education in Science & Technology, Hosakote, Bangalore - 562 114, India
5  Instituto de Astrofísica de Andalucía, PO Box 03004, 18080 Granada, Spain
6  IRAM, 300 rue de la Piscine, Dom. Universitaire, 38406 Saint Martin d'Hères, France

(Received 15 October 2003 / Accepted 7 January 2004)

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 $0.77\pm0.04$, steepening to $2.05\pm0.04$ about $0.65\pm0.03$ day after the burst. Interpreted as the "jet break", this indicates a half opening angle of $\sim $3.2 degree for the initial ejection, for an assumed ambient density of $\sim $ $1~{\rm cm}^{-3}$. Broadband spectra show no appreciable evolution during the observations, and indicate the presence of synchrotron cooling frequency $\nu_{\rm c}$ near the upper edge of the optical band. From the broadband spectra we derive an electron energy distribution index $p = 2.07\pm0.06$ and an intrinsic extinction $E(B - V)\sim0.17$. Millimeter upper limits are consistent with these derived parameters.

Key words: gamma rays: bursts -- gamma rays: observations

Offprint request: S. B. Pandey, shashi@upso.ernet.in

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