OGLE-2014-SN-131: A long-rising Type Ibn supernova from a massive progenitor

Context. Type Ibn supernovae (SNe Ibn) are thought to be thecore-collapse explosions of massive stars whose ejecta interact withHe-rich circumstellar material (CSM). Aims: We report thediscovery of a SN Ibn, with the longest rise-time ever observed,OGLE-2014-SN-131. We discuss the potential powering mechanisms and theprogenitor nature of this peculiar stripped-envelope (SE),circumstellar-interacting SN. Methods: Optical photometry andspectroscopy were obtained with multiple telescopes including VLT, NTT,and GROND. We compare light curves and spectra with those of other knownSNe Ibn and Ibc. CSM velocities are derived from the spectral analysis.The SN light curve is modeled under different assumptions about itspowering mechanism (56Ni decay, CSM-interaction, magnetar) inorder to estimate the SN progenitor parameters. Results:OGLE-2014-SN-131 spectroscopically resembles SNe Ibn such as SN 2010al.Its peak luminosity and post-peak colors are also similar to those ofother SNe Ibn. However, it shows an unprecedentedly long rise-time and amuch broader light curve compared to other SNe Ibn. Its bolometric lightcurve can be reproduced by magnetar and CSM-interaction models, but notby a 56Ni-decay powering model. Conclusions: Toexplain the unusually long rise-time, the broad light curve, the lightcurve decline, and the spectra characterized by narrow emission lines,we favor a powering mechanism where the SN ejecta are interacting with adense CSM. The progenitor of OGLE-2014-SN-131 was likely a Wolf-Rayetstar with a mass greater than that of a typical SN Ibn progenitor, whichexpelled the CSM that the SN is interacting with.The photometry is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A93.