The peculiar mass-loss history of SN 2014C as revealed through AMI radio observations

Abstract

We present a radio light curve of supernova (SN) 2014C taken with the Arcminute Microkelvin Imager (AMI) Large Array at 15.7 GHz. Optical observations presented by Milisavljevic et al. demonstrated that SN 2014C metamorphosed from a strippedenvelope Type Ib SN into a strongly interacting Type IIn SN within 1 year. The AMI light curve clearly shows two distinct radio peaks, the second being a factor of 4 times more luminous than the first peak. This double bump morphology indicates two distinct phases of mass-loss from the progenitor star with the transition between density regimes occurring at 100-200 days. This reinforces the interpretation that SN 2014C exploded in a low density region before encountering a dense Hydrogenrich shell of circumstellar material that was likely ejected by the progenitor prior to the explosion. The AMI flux measurements of the first light curve bump are the only reported observations taken within ∼ 50 to ∼ 125 days post-explosion, before the blastwave encountered the Hydrogen shell. Simplistic synchrotron self-absorption (SSA) and free-free absorption (FFA) modelling suggest that some physical properties of SN 2014C, such as the mass-loss rate, are consistent with the properties of other Type Ibc and IIn SNe. However, our single frequency data does not allow us to distinguish between these two models, which implies they are likely too simplistic to describe the complex environment surrounding this event. Lastly, we present the precise radio location of SN 2014C obtained with eMERLIN, which will be useful for future VLBI observations of the SN.

SN 2014C: VLBI images of a supernova interacting with a circumstellar shell

“…By fitting the 16-GHz lightcurve, Anderson et al (2017) derive r16 0.3 at t 80 d, implying a velocity of ∼5000 km s −1 . Considerably higher velocities are suggested by spectroscopy, with Milisavljevic et al (2015) finding velocities of 13000 km s −1 (from Fe II, He I and Ca II absorption lines) at t = 10 d, which suggesting that the forward shock must be moving at very least at 13000 km s −1 .…”

“…The dense H-rich CSM must be at least partly transparent to radio waves, otherwise the early radio emission, first detected at t = 12 d after shock breakout and at t = 17 d at 15 GHz (Anderson et al 2017) would have been absorbed. If the dense CSM were distributed in a uniform spherical shell, with mass ∼1 M (Margutti et al 2017), then the densities must be quite high, and strong freefree absorption would be expected of the shell were ionized.…”

“…The other possibility is that the dense CSM was not spherically distributed (note that Anderson et al 2017, have already suggested this possibility for SN 2014C). The CSM might be non-spherically distributed on small scales, in other words fragmented.…”

“…A 16-GHz radio lightcurve from the Arcminute Microkelvin Imager was presented by Anderson et al (2017), who found a very unusual two-peaked lightcurve, with the first peak at t 80 d, and a second at t 400 d. A paper on our multi-frequency flux-density monitoring of SN 2014C with the Jansky Very Large Array is in preparation (Kamble et al). Milisavljevic et al (2015) and Margutti et al (2017) suggest a scenario where SN 2014C's progenitor exploded inside a relatively low-density cavity, but within a year, the expanding shock encountered a massive H-rich shell (probably ∼ 1 M ) at radius, r16 ∼ 6, where r16 is a dimensionless radius, and r16 = r/(10 16 cm).…”

“…Very contradictory values have been proposed: Anderson et al (2017) suggest values of r16 of 0.332 ± 0.007, based on modelling the lightcurve bumps on the assumption that the dominant absorption mechanism is synchrotron self-absorption (SSA), although they do warn that this assumption may not be appropriate for SN 2014C. The size reported by Anderson et al would imply relatively low average expansion velocities of < ∼ 5000 km s −1 for t = 80 d, whereas typically Type Ib SNe have velocities several times higher (e.g.…”