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 because of its extremely slow evolution, with an observed rise of ≳125 rest-frame days, and exponential decline out to ∼250 days past peak at a measured rate of 0.01 mag day-1, consistent with fully trapped 56Co decay. This is the longest rise time measured in an 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, although lower in luminosity (Lpeak≃4.6×1043 erg s-1) and with a flatter peak than previous events. PS1-14bj shows a number of peculiar properties, including a near-constant color temperature for days past peak, and strong emission lines from [O iii] λ5007 and [O iii] λ4363 with a velocity width of ∼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 56Ni-powered/PISN interpretation. 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 are 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.
Lunnan, R; Chornock, R; Berger, E; Milisavljevic, D; Jones, DO; Rest, A; Fong, W; Fransson, C; Margutti, R; Drout, MR; Blanchard, PK; Challis, P; Cowperthwaite, PS; Foley, RJ; Kirshner, RP; Morrell, N; Riess, AG; Roth, KC; Scolnic, D; Smartt, SJ; Smith, KW; Villar, VA; Chambers, KC; Draper, PW; Huber, ME; Kaiser, N; Kudritzki, RP; Magnier, EA; Metcalfe, N; Waters, C
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