Based on the temporal and positional coincidence of the transient object with GRB 000131, and the fact that it was located at a cosmological distance, we conclude that this transient object was the optical afterglow of GRB 000131. Assuming a Hubble constant of 65 km s-1 Mpc-1, and , a redshift of 4.50 implies a distance modulus of 48.24. Hence, the total energy release assuming isotropic emission is 1.11054 erg. This is only a factor of 3 smaller than the isotropic equivalent energy for the most energetic event so far, GRB 990123 (Andersen et al. 1999; Kulkarni et al. 1999). However, the combination of the decay slope and the constraints on the spectral slope indicate that the afterglow was due to a collimated jet. The upper limit on the break epoch, 3.5 days, implies a lower limit on the jet opening angle of 7o 7o, where n is the density of the ambient medium (in units of cm-3), which is larger than 1 in star-forming regions. From the lower limit on the opening angle we infer a lower limit on the released energy of erg.
This work shows that 8-m class telescopes may be used successfully to detect faint R optical afterglows even in fields with very bright stars. This is necessary in order to resolve why of all attempts to detect optical afterglows of GRBs are currently unsuccessful (Fynbo et al. 2000). The apparent brightness of the GRB 000131 afterglow was similar to that of GRB 990123. GRB 000131 does therefore provide the first observational evidence that it is possible to obtain high resolution optical spectra of GRBs at very high redshift, if the afterglow is identified at an early time. High redshift GRBs will therefore no doubt prove to be an extremely valuable tool for not only the understanding of the GRB environment but also for the study of the Lyman forest and of cosmology.
© European Southern Observatory (ESO) 2000
Online publication: December 15, 2000