SN 2008iy circumstellar interaction: Bright and lesser light effect

Optical photometry and spectra of the luminous type IIn supernova SN~2008iy are analysed in detail with implications for cosmic ray acceleration and the radio emission. The light curve and expansion velocities indicate ejecta with the kinetic energy of $3\times10^{51}$ erg to collide with the $\sim10$ Msun circumstellar envelope. The luminous Ha is explained as originated primarily from circumstellar clouds interacting with the forward shock. For the first time the fluorescent OI 8446A emission is used to demonstrate that the cloud fragmentation cascade spans a scale range > 2.3 dex. The narrow circumstellar Ha permitted us to estimate the acceleration efficiency of cosmic rays. The found value is close to the efficiency inferred in the same way for other two SNe~IIn, SN~1997eg and SN~2002ic. The efficiency of cosmic ray acceleration is utilized to reproduce the radio flux from SN~2008iy for the amplified magnetic field consistent with the saturated turbulent magnetic field in the diffusive shock acceleration mechanism.