Super Luminous Ic Supernovae: catching a magnetar by the tail
  We report extensive observational data for five of the lowest redshift
Super-Luminous Type Ic Supernovae (SL-SNe Ic) discovered to date, namely
PTF10hgi, SN2011ke, PTF11rks, SN2011kf and SN2012il. Photometric imaging of the
transients at +50 to +230 days after peak combined with host galaxy subtraction
reveals a luminous tail phase for four of these SL-SNe. A high resolution,
optical and near infrared spectrum from xshooter provides detection of a broad
He I $\lambda$10830 emission line in the spectrum (+50d) of SN2012il, revealing
that at least some SL-SNe Ic are not completely helium free. At first sight,
the tail luminosity decline rates that we measure are consistent with the
radioactive decay of \co, and would require 1-4M of \ni to produce the
luminosity. These \ni masses cannot be made consistent with the short diffusion
times at peak, and indeed are insufficient to power the peak luminosity. We
instead favour energy deposition by newborn magnetars as the power source for
these objects. A semi-analytical diffusion model with energy input from the
spin-down of a magnetar reproduces the extensive lightcurve data well. The
model predictions of ejecta velocities and temperatures which are required are
in reasonable agreement with those determined from our observations. We derive
magnetar energies of $0.4\lesssim E$($10^{51}$erg) $\lesssim6.9$ and ejecta
masses of $2.3\lesssim M_{ej}$(\M) $\lesssim 8.6$. The sample of five SL-SNe Ic
presented here, combined with SN 2010gx - the best sampled SL-SNe Ic so far -
point toward an explosion driven by a magnetar as a viable explanation for all
SL-SNe Ic.
