Rates of superluminous supernovae at z ∼ 0.2

We calculate the volumetric rate of superluminous supernovae (SLSNe) based on five events discovered with the Robotic Optical Transient Search Experiment-IIIb (ROTSE-IIIb) telescope. We gather light curves of 19 events from the literature and our own unpublished data and employ crude k-corrections to constrain the pseudo-absolute magnitude distributions in the rest-frame ROTSE-IIIb (unfiltered) bandpass for both the hydrogen-poor (SLSN-I) and hydrogen-rich (SLSN-II) populations. We find that the peak magnitudes of the available SLSN-I are narrowly distributed (M = -21.7 +/- 0.4) in our unfiltered band pass and may suggest an even tighter intrinsic distribution when the effects of dust are considered, although the sample may be skewed by selection and publication biases. The presence of O II features near maximum light may uniquely signal a high-luminosity event, and we suggest further observational and theoretical work is warranted to assess the possible utility of such SN 2005ap-like SLSN-I as distance indicators. Using the pseudo-absolute magnitude distributions derived from the light-curve sample, we measure the SLSN-I rate to be about (32(-26)(+77)) events Gpc(-3) yr(-1) h(71)(3) at a weighted redshift of (z) over bar = 0.17, and the SLSN-II rate to be about (151(-82)(+151)) events Gpc(-3) yr(-1) h(71)(3) at (z) over bar = 0.15. Given that the exact nature and limits of these populations are still unknown, we discuss how it may be difficult to distinguish these rare SLSNe from other transient phenomena such as active galactic nucleus activity and tidal disruption events even when multiband photometry, spectroscopy or even high-resolution imaging are available. Including one spectroscopically peculiar event, we determine a total rate for SLSN-like events of (199(-86)(+137)) events Gpc(-3) yr(-1) h(71)(3) at (z) over bar = 0.16.