Performing a stellar autopsy using the radio-bright remnant of SN 1996cr

Abstract

We present newly reduced archival radio observations of SN 1996cr in the Circinus Galaxy from the Australia Telescope Compact Array (ATCA) and the Molonglo Observatory Synthesis Telescope (MOST), and attempt to model its radio light curves using recent hydrodynamical simulations of the interaction between the SN ejecta and the circumstellar material (CSM) at X-ray wavelengths. The radio data within the first 1000 days show clear signs of free-free absorption (FFA), which decreases gradually and is minimal above 1.4 GHz after day ∼3000. Constraints on the FFA optical depth provide estimates of the CSM free electron density, which allows insight into the ionisation of SN 1996cr's CSM and offers a test on the density distribution adopted by the hydrodynamical simulation. The intrinsic spectral index of the radiation shows evidence for spectral flattening, which is characterised by α = 0.852 ± 0.002 at day 3000 and a decay rate of ∆α = -0.014 ± 0.001 yr −1 . The striking similarity in the spectral flattening of SN 1987A, SN 1993J, and SN 1996cr suggests this may be a relatively common feature of SNe/CSM shocks. We adopt this spectral index variation to model the synchrotron radio emission of the shock, and consider several scalings that relate the parameters of the hydrodynamical simulation to the magnetic field and electron distribution. The simulated light curves match the large-scale features of the observed light curves, but fail to match certain tightly constraining sections. This suggests that simple energy density scalings may not be able to account for the complexities of the true physical processes at work, or alternatively, that the parameters of the simulation require modification in order to accurately represent the surroundings of SN 1996cr.

Pulsational Pair-instability Supernovae

“…The collision may produce x-rays and radio emission as well as optical emission. An example of a Type IIn supernova in this state, although not necessarily a PPISN itself, is SN 1996er (Meunier et al 2013), which is currently interacting with several solar masses of ejecta at about 10 17 cm. The qualitative nature of these events can be inferred from the time scales for pulsational duration given in Table 2.…”

The exceptional X-ray evolution of SN 1996cr in high resolution

“…SN 1996cr has remained bright at X-ray, optical, and radio wavelengths for nearly two decades, placing it amongst the remarkable handful of long-lived CCSNe attributed to strong ejecta-CSM interactions: e.g., SNs 1978K, 1979C, 1986J, 1988Z, 1993J, 2005kd, 2007bg, 2010jl, 2009ip, 1998S, and 1987A (e.g., respectively, Chandra et al 2012bSmith et al 2014;Leonard et al 2000;Margutti et al 2017;Dwarkadas et al 2016;Michael et al 2002;Salas et al 2013;Zhekov et al 2006;Dewey et al 2008). Due to its relative proximity at d ≈3.7 Mpc, SN 1996cr affords us an exceptional opportunity to study its features (Bauer et al 2008;Dwarkadas et al 2010;Dewey et al 2011;Meunier et al 2013) and evolution in great detail. SN 1996cr's radio emission shows an initial rise which is attributed to a combination of increasing CSM density and decreasing free-free absorption, which provides estimates of the CSM free electron density and hence insight into the ionization of SN 1996cr's CSM (Meunier et al 2013).…”

“…Due to its relative proximity at d ≈3.7 Mpc, SN 1996cr affords us an exceptional opportunity to study its features (Bauer et al 2008;Dwarkadas et al 2010;Dewey et al 2011;Meunier et al 2013) and evolution in great detail. SN 1996cr's radio emission shows an initial rise which is attributed to a combination of increasing CSM density and decreasing free-free absorption, which provides estimates of the CSM free electron density and hence insight into the ionization of SN 1996cr's CSM (Meunier et al 2013). The X-ray luminosity likewise exhibits an initial increase with time, seen previously in the famous SN 1987A (e.g., Michael et al 2002;Frank et al 2016) and see later in SN 2014C (Margutti et al 2017).…”

ALMA, ATCA, and Spitzer Observations of the Luminous Extragalactic Supernova SN 1978K

“…Only three extragalactic supernovae have been detected at late times at millimeter wavelengths: SN 1978K, SN 1987A (Lakićević et al 2012Matsuura et al 2011Matsuura et al , 2015, and SN 1996cr (Meunier et al 2013). SN 1978K was first detected at 34 and 94 GHz in our ATCA observations in 2014 September (Ryder et al 2016).…”