Radio evolution of supernova SN 2008iz in M 82 ^⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: nkimani@mpifr-bonn.mpg.de
² Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
³ Onsala Space Observatory, Chalmers Univ. of Technology, 43992 Onsala, Sweden
⁴ Astron. Dept., King Abdulaziz University, PO Box 80203, 21589 Jeddah Saudi Arabia
⁵ Department of Astrophysics, Radboud University Nijmegen, Postbus 9010, 6500 GL Nijmegen, The Netherlands
⁶ ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁷ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The university of Manchester, Oxford Road, Manchester M13 9PL, UK
⁸ Academia Sinica Institute of Astronomy and Astrophysics, 645 N. Aóhoku Place, Hilo, HI 96720, USA

Received: 26 April 2016
Accepted: 20 June 2016

Abstract

We report on multi-frequency Very Large Array (VLA) and Very Long Baseline Interferometry (VLBI) radio observations for a monitoring campaign of supernova SN 2008iz in the nearby irregular galaxy M 82. We fit two models to the data, a simple time power-law, S ∝ t^β, and a simplified Weiler model, yielding decline indices of β = −1.22 ± 0.07 (days 100−1500) and −1.41 ± 0.02 (days 76−2167), respectively. The late-time radio light-curve evolution shows flux-density flares at ~970 and ~1400 days that are a factor of ~2 and ~4 higher than the expected flux, respectively. The later flare, except for being brighter, does not show signs of decline at least from results examined so far (2014 January 23; day 2167). We derive the spectral index, α, S ∝ ν^α for frequencies 1.4 to 43 GHz for SN 2008iz during the period from ~430 to 2167 days after the supernova explosion. The value of α shows no signs of evolution and remains steep ≈–1 throughout the period, unlike that of SN 1993J, which started flattening at ~day 970. From the 4.8 and 8.4 GHz VLBI images, the supernova expansion is seen to start with a shell-like structure that becomes increasingly more asymmetric, then breaks up in the later epochs, with bright structures dominating the southern part of the ring. This structural evolution differs significantly from SN 1993J, which remains circularly symmetric over 4000 days after the explosion. The VLBI 4.8 and 8.4 GHz images are used to derive a deceleration index, m, for SN 2008iz, of 0.86 ± 0.02, and the average expansion velocity between days 73 and 1400 as (12.1 ± 0.2) × 10³ km s^-1. From the energy equipartition between magnetic field and particles, we estimate the minimum total energy in relativistic particles and the magnetic fields during the supernova expansion and also find the magnetic field amplification factor for SN 2008iz to be in the range of 55−400.