How much radioactive nickel does ASASSN-15lh require?

The discovery of the most luminous supernova ASASSN-15lh triggered a shock-wave in the supernova community. The three possible mechanisms proposed for the majority of other superluminous supernovae do not produce a realistic physical model for this particular supernova. In the present study we show the limiting luminosity available from a nickel-powered pair-instability supernova. We computed a few exotic nickel-powered explosions with a total mass of nickel up to 1500 solar masses. We used the hydrostatic configurations prepared with the GENEVA and MESA codes, and the STELLA radiative-transfer code for following the explosion of these models. We show that 1500 solar masses of radioactive nickel is needed to power a luminosity of 2x10^45 erg/s. The resulting light curve is very broad and incompatible with the shorter ASASSN-15lh time-scale. This rules out a nickel-powered origin of ASASSN-15lh. In addition, we derive a simple peak luminosity - nickel mass relation from our data, which may serve to estimate of nickel mass from observed peak luminosities.