We report on our discovery and observations of the Pan-STARRS1 supernova ( SN ) PS1-12sk , a transient with properties that indicate atypical star formation in its host galaxy cluster or pose a challenge to popular progenitor system models for this class of explosion .
The optical spectra of PS1-12sk classify it as a Type Ibn SN ( c.f .
SN 2006jc ) , dominated by intermediate-width ( 3 \times 10 ^ { 3 } ~ { } { km~ { } s } ^ { -1 } ) and time variable He i emission .
Our multi-wavelength monitoring establishes the rise time dt \sim 9 - 23 days and shows an NUV-NIR SED with temperature \gtrsim 17 \times 10 ^ { 3 } K and a peak magnitude of M _ { z } = -18.88 \pm 0.02 mag .
SN Ibn spectroscopic properties are commonly interpreted as the signature of a massive star ( 17 - 100 ~ { } M _ { \odot } ) explosion within a He-enriched circumstellar medium .
However , unlike previous Type Ibn supernovae , PS1-12sk is associated with an elliptical brightest cluster galaxy , CGCG 208-042 ( z = 0.054 ) in cluster RXC J0844.9+4258 .
The expected probability of an event like PS1-12sk in such environments is low given the measured infrequency of core-collapse SNe in red sequence galaxies compounded by the low volumetric rate of SN Ibn .
Furthermore , we find no evidence of star formation at the explosion site to sensitive limits ( \Sigma _ { H \alpha } \lesssim 2 \times 10 ^ { -3 } ~ { } { M } _ { \odot } ~ { } { yr } ^ { -1 % } ~ { } kpc ^ { -2 } ) .
We therefore discuss white dwarf binary systems as a possible progenitor channel for SNe Ibn .
We conclude that PS1-12sk represents either a fortuitous and statistically unlikely discovery , evidence for a top-heavy IMF in galaxy cluster cooling flow filaments , or the first clue suggesting an alternate progenitor channel for Type Ibn SNe .