PS1-14bj: A Hydrogen-Poor Superluminous Supernova With a Long Rise and Slow Decay

We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor superluminous supernova (SLSN) at redshift $z=0.5215$ discovered in the last months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out by its extremely slow evolution, with an observed rise to maximum light $\gtrsim 125$ days in the rest frame, and exponential decline out to $\sim 250$ days past peak at a measured rate of $9.75\times 10^{-3}$ mag day$^{-1}$, consistent with fully-trapped $^{56}$Co decay. This is the longest rise time measured in a SLSN to date, and the first SLSN to show a rise time consistent with pair-instability supernova (PISN) models. Compared to other slowly-evolving SLSNe, it is spectroscopically similar to the prototype SN 2007bi at maximum light, though somewhat lower in luminosity ($L_{\rm peak} \simeq 4.4 \times 10^{43}~{\rm erg~s}^{-1}$) and with a flatter peak than previous events. In addition to its slow evolution, PS1-14bj shows a number of peculiar properties, including a near-constant color temperature for $>200$ days past peak, and strong emission lines from [O III] $\lambda$5007 and [O III] $\lambda$4363 with a velocity width of $\sim$3400 km s$^{-1}$, in its late-time spectra. These both suggest there is a sustained source of heating over very long timescales, and are incompatible with a simple $^{56}$Ni-powered/PISN interpretation, or any model that predicts a monotonically decreasing temperature (such as the simple magnetar spin-down model). A modified magnetar model including emission leakage at late times can reproduce the light curve, in which case the blue continuum and [O III] features would be interpreted as material heated and ionized by the inner pulsar wind nebula becoming visible at late times. Alternatively, the late-time heating could be due to interaction with a shell of H-poor circumstellar material.