Modelling and predicting the rate of transients and associated events

Astrophysical transients are a signature of cataclysmic events, often associated with death of a system and the formation of compact objects. Gamma-ray bursts (GRBs) are transients associated with the collapse of a massive star into a black hole, and the merger of neutron star binaries to form a more massive neutron star or a low-mass black hole along with the emission of gravitational waves. Although it is impossible to predict individual GRBs, it is nonetheless important to make statistical predictions of the number detectable by a GRB monitor in the sky, and the true GRB rate of the universe. In this talk, I will present predictions of the event rate of the AstroSat/CZTI and the Athena/WFI via careful studies of the luminosity function of both the long and short GRBs. Reasonable physical assumptions are made in modelling the true event rate, and simplifications are made whenever possible by using data from the existing GRB catalogues. Using the maximum distance to which the gravitational networks are sensitive in the past, present and future runs, stringent lower limits on the rate of binary neutron star mergers (BNSMs) are calculated by assuming that each BNSM produces a short GRB. In light of the detection of GW/EM170817, it is shown that the number will go up significantly in the next observing runs of aLIGO/VIRGO. Comparison of the short GRB rate with the BNSM rate calculated independently from the single source statistics of GW170817 reveals the presence of a slight tension that can have significant implications on the physics of the merger ejecta; however the scenario that each BNSM indeed produces a short GRB, cannot be ruled out.