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Issue A&A Volume 427, Number 3, December I 2004 Page(s) 901 - 905 Section Stellar structure and evolution DOI 10.1051/0004-6361:20041548

A&A 427, 901-905 (2004)
DOI: 10.1051/0004-6361:20041548

The color evolution of the optical afterglow of GRB 030329 and the implications for the underlying supernova SN 2003dh

V. Simon¹, R. Hudec¹ and G. Pizzichini<sup>2

1</sup>  Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 Ondrejov, Czech Republic
    e-mail: simon@asu.cas.cz
²  IASF/CNR, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy

(Received 4 February 2004 / Accepted 30 June 2004 )

Abstract
We find that the color indices (B-V)₀, (V-R)₀, (R-I)₀, (I-J)₀ in the observer frame of the optical afterglow (OA) of GRB 030329 during t-T₀<10 days are consistent with those of a uniform group of 25 OAs of GRBs, previously analyzed by Simon et al. (2001; 2003a). The synchrotron component with parameters similar to those of the other 25 OAs was thus initially dominant in the optical and IR spectrum of the OA of GRB 030329. Following the arguments of Simon et al. (2001), it also appears that either GRB 030329 was not located deep inside a star-forming region or that the local dust was destroyed by the intense initial flash. The absolute R magnitude of this event  mag in the rest frame at days is in good agreement with the typical brightness of the OAs analyzed by Simon et al. (2001). We detected large color variations of the OA of GRB 030329 only for t-T₀>12 days, which suggest a rapid evolution of the spectrum of the underlying SN 2003dh. We compare it with the individual type Ic SNe and find that its color evolution is plausibly matched only by that of SN 1998bw although the indices still reveal spectral differences between the two objects. The agreement is particularly good only at t-T₀= 26 and 27 days. The evolution of SN 2003dh becomes faster than that of SN 1998bw later on and SN 2003dh develops an excess light in the red/IR spectral region. We also emphasize the discordance with other type Ic SNe which are not associated with GRBs.

Key words: gamma rays: bursts -- radiation mechanisms: non-thermal  -- ISM: jets and outflows  -- galaxies: ISM  -- stars: supernovae: general  -- stars: supernovae: individual: SN 2003dh

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