Fermi Gamma-ray Space Telescope

Excess of GRBs in the Swift Era below redshift of 2

Truong Le
(Vedant Mehta and Cecilia Ratke)


Utilizing more than 100 GRBs in the Swift sample that include both the observed estimated redshifts and jet opening angles, we obtain a GRB formation rate functional form that gives acceptable fits to the pre-Swift and Swift redshift and jet opening angle distributions with an indication that an excess of GRBs exists at low redshift below z = 2. The mean redshifts and jet opening angle for pre-Swift (Swift) are z ~ 1.3 (1.7) and theta ~ 7 (11 degree), respectively. From this GRB rate, the fraction of high-redshift GRBs is estimated to be below 10% and 5% at z > 4 and z > 5, respectively, and below 10% at z < 1.