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Next: Acknowledgements Up: Evolution of the double Previous: Effect of sources evolution

Conclusions

Our calculations show that if GRB indeed are due to binary neutron star coalescence, they can potentially constrain the cosmological parameters as well as the early star formation history. Even for the simplest models we used, late epochs of galaxy formation ( tex2html_wrap_inline363 ) do not seem to be consistent with the observed BATSE tex2html_wrap_inline175 - tex2html_wrap_inline177 distribution. The best-fit model we obtained by fitting to the 411 GRB available from the 2nd BATSE catalogue (Meegan et al. 1994) corresponds to tex2html_wrap_inline371 , tex2html_wrap_inline337 and tex2html_wrap_inline335 , with a tex2html_wrap_inline327 agreement according to tex2html_wrap_inline323 -test (we also used a Kolmogorov-Smirnov test; both criteria give qualitatively similar results but we present results for the tex2html_wrap_inline323 -test as it gives a more smooth likelihood function). According to the best-fit, the most distant BATSE GRB come from tex2html_wrap_inline383 (compared to Cohen & Piran 1995 who obtained tex2html_wrap_inline385 ; however, we made no data selection). The results are also sensitive to the accepted mean spectral index of the GRB (s=1.5 in our case) and favours earlier star formation tex2html_wrap_inline355 for flatter spectra (s=1). We also note that the earlier epochs of the primordial star formation are not favored by other cosmological grounds (see calculations by Cen et al. 1994).

One may also wonder how the assumption about the total tex2html_wrap_inline393 change the results. Obviously, we can fit the observations for a wide range of tex2html_wrap_inline393 by varying other parameters ( tex2html_wrap_inline225 , s, tex2html_wrap_inline315 etc.). The dependence of tex2html_wrap_inline175 - tex2html_wrap_inline177 curves on tex2html_wrap_inline393 was found to be rather small (Yi 1994), and our main conclusion still holds - the tex2html_wrap_inline175 - tex2html_wrap_inline177 should show a dramatic turnup at low count rates due to early evolutionary effects.

The total merging event rate predicted by our evolutionary model is tex2html_wrap_inline413 per year per tex2html_wrap_inline239 , that is tex2html_wrap_inline417 events/yr for the entire Universe, implying a factor of tex2html_wrap_inline419 overproduction relative to the presently observed BATSE GRB rate of 0.8 events per day. This could be explained, for example, by a relativistic beaming in GRB sources (Paczynski 1994). The corresponding angle required to explain the GRB anisotropy that high is about tex2html_wrap_inline421 (Mao & Yi 1994). Taking this factor into account yields the expected total GRB rate tex2html_wrap_inline423 events per year for a limiting sensitivity lower by a factor of 3-10 than the presently exisitng BATSE limit. We note that use of another mean galactic density tex2html_wrap_inline425 in the Universe would accordingly change the overall GRB rate tex2html_wrap_inline427 in the Universe, but does not change the tex2html_wrap_inline175 - tex2html_wrap_inline177 curve shape. Taking it less than 0.08 per cubic megaparsec would somewhat decrease the anisotropy required ( tex2html_wrap_inline433 ).

We conclude that the crucial test of the cosmological origin of GRBs would be observing the predicted increase of tex2html_wrap_inline175 - tex2html_wrap_inline177 slope at smaller fluxes, inevitable due to early evolutionary effects. If the cosmological origin of GRB will be confirmed, the tex2html_wrap_inline175 - tex2html_wrap_inline177 and tex2html_wrap_inline197 test could be used to independently estimating the cosmological parameters and tracing star formation history in galaxies.


next up previous
Next: Acknowledgements Up: Evolution of the double Previous: Effect of sources evolution

Mike E. Prokhorov
Tue Aug 20 18:36:42 MSD 1996