The rate of supernovae from the combined sample of five searches

With the purpose to obtain new estimates of the rate of supernovaewe joined the logs of five SN searches, namely the Asiago, Crimea, Calán-Tololo and OCA photographic surveys and the visual search by Evans. In this way we improved the SN statistics (the sample counts 110 SNe) and hence, reduced the uncertainties. The computation was based on the control time method which allowed the proper merging of the observations of each galaxy in the various searches. In addition to discussing the choice for the various input parameters, we verified the existence of two biases against SN discoveries, one in the nuclear regions of distant galaxies, most severe for deep photographic surveys, and a second in inclined spirals, in particular late spirals. After correction for these two biases we obtained the rates of each SN type in the different types of galaxies. We found that the most prolific galaxies are late spirals in which most SNe are of type II (0.88 SNu). SN Ib/c are rarer than SN Ia (0.16 and 0.24 SNu, respectively), ruling out previous claims of a very high rate of SNIb/c. We also found that the rate of SN Ia in ellipticals (0.13 SNu) is smaller than in spirals, supporting the hypothesis of different ages of the progenitor systems in early and late type galaxies. Finally, we estimated that even assuming that separate classes of faint SN Ia and SN II do exist (SNe 1991bg and 1987A could be the respective prototypes) the overall SN rate is raised only by 20-30%, therefore excluding that faint SNe represent the majority of SN explosions. Also, the bright SNIIn are intrinsically very rare (2 to 5 % of all SNII in spirals).