A Comparative Study of Luminosity Functions and Event Rate Densities of Long GRBs with Non-parametric Method

Abstract

In this work, we restudy the dependence of luminosity function and event rates for different gamma-ray burst samples on the criteria of sample selection and threshold effect. To compare with many previous studies, we have chosen two samples including 88 and 118 long bursts with known redshift and peak flux over 2.6 ph cm$^{-2}$ s$^{-1}$, from which 79 bursts are picked out to constitute our complete sample. It is found that the evolution of luminosity with redshift can be expressed by $L\propto(1+z)^k$ with a diverse $k$ relied more on the sample selection. Interestingly, the cumulative distributions of either non-evolving luminosities or redshifts are found to be also determined by the sample selection rather than the instrumental sensitivity. Nevertheless, the non-evolving luminosities of our samples are similarly distributed with a comparable break luminosity of $L_0\sim10^{51}$ erg s$^{-1}$. Importantly, we verify with a K-S test that three cases of event rates for the two burst samples evolve with redshift similarly except a small discrepancy due to sampling differences at low-redshift of $z<1$, in which all event rates show an excess of Gaussian profile instead of monotonous decline no matter whether the sample is complete. Most importantly, it is found that the burst rates violate the star formation rate at low redshift, while both of them are good in agreement with each other in the higher-redshift regions as many authors discovered previously. Therefore, we predict that two types of long bursts are favored in terms of their associations with both the star formation and the cosmic metallicity.