Astron. Astrophys. 355, L43-L46 (2000)

4. Discussion and conclusions

Determining the existence of beaming in GRBs will be helpful to our understanding of the GRB "central engines". Based on our refined dynamical model for beamed GRB remnants (Huang et al. 2000a), we have examined those GRBs with rapidly decaying optical afterglows closely. Detailed numerical results show that afterglows from GRB 970228, 980326, 980519, 990123, 990510 and 991208 can be satisfactorily fitted: the obvious break in the optical light curve of GRB 990123 is due to the relativistic-Newtonian transition of the beamed ejecta, and the rapid fading of afterglows from other GRBs is due to the relatively large values of and n. We thus strongly suggest that these GRBs be highly collimated. Note that in all cases, synchrotron radiation during the mildly relativistic and non-relativistic phases plays an important role in explaining the rapid decaying of optical afterglows.

In our calculations, the values distribute in a narrow range, i.e., between 0.1 and 0.2. This has given some support to Freedman & Waxman's (1999) suggestion that has a universal value of . However our results do not support their proposal that p has a universal value of 2.2. Our values distributed in a narrow range, . This may provide important clues to our understanding of the central engine. We note that Woosley et al. (1999) have obtained such a small value of after numerically studying the collapsar models of GRBs. The range of our n values ( - 1000 cm^-3) indicates that some GRBs may be in gaseous environments, also giving some hints on GRB central engines.

The jet model greatly relaxes the energy crisis for GRB 990123 and 990510. However, we should keep in mind that other GRBs such as GRB 970508, 971214, 980329 and 980703 do not have rapidly fading afterglows. They should not be highly collimated (Huang, Dai & Lu 2000b). Then the energy crisis is really a problem: GRB 971214 and 980703 have indicated isotropic -ray energies of and respectively!

The rapid fading of afterglows from GRB 970228, 980326, 980519 has also been explained as being due to the interaction of an isotropic blastwave with a wind environment (Dai & Lu 1998; Chevalier & Li 1999). However the wind environment model could not explain the light curve break observed for GRB 990123 and 990510. As has been shown clearly in this paper, the jet model can naturally explain all these bursts, and should be more reasonable.

Another possibility was proposed recently by Dai & Lu (1999, 2000). They suggested that the light curve break is due to the relativistic-Newtonian transition of an isotropic blastwave in a dense medium ( cm^-3), and the rapidly fading afterglows can be explained as emissions in the non-relativistic phase. It is obviously an interesting proposition and should also be paid attention to.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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