Radio synchrotron emission from secondary electrons in interaction-powered supernovae

Abstract

Several supernovae (SNe) with an unusually dense circumstellar medium (CSM) have been recently observed at radio frequencies. Their radio emission is powered by relativistic electrons that can be either accelerated at the SN shock (primaries) or injected as a by-product (secondaries) of inelastic proton-proton collisions. We investigate the radio signatures from secondary electrons, by detailing a semi-analytical model to calculate the temporal evolution of the distributions of protons, primary and secondary electrons. With our formalism, we track the cooling history of all the particles that have been injected into the emission region up to a given time, and calculate the resulting radio spectra and light curves. For a SN shock propagating through the progenitor wind, we find that secondary electrons control the early radio signatures, but their contribution decays faster than that of primary electrons. This results in a flattening of the light curve at a given radio frequency that depends only upon the radial profiles of the CSM density and of the shock velocity, υ 0 . The relevant transition time at the peak frequency is ∼ 190 d K −1 ep,−3 A w,16 /β 2 0,−1.5 , where A w is the wind mass-loading parameter, β 0 = υ 0 /c and K ep is the electron-to-proton ratio of accelerated particles. We explicitly show that late peak times at 5 GHz (i.e., t pk 300 − 1000 d) suggest a shock wave propagating in a dense wind (A w 10 16 − 10 17 gr cm −1 ), where secondary electrons are likely to power the observed peak emission.

High-energy Emission from Interacting Supernovae: New Constraints on Cosmic-Ray Acceleration in Dense Circumstellar Environments

“…(a) We numerically calculated broadband spectra of CR-induced non-thermal emission from interacting SNe, taking into account various processes such as electromagnetic cascades in the post-shock CSM as well as the attenuation in the pre-shock CSM. The electromagnetic cascade is unavoidable, although its detailed effect was not included in the previous literature (Murase et al 2011;Zirakashvili & Ptuskin 2016;Petropoulou et al 2016). We found that GeV gamma-ray spectra are insensitive to s p thanks to the cascade, and the attenuation effect can reduce the flux only modestly, which ensures that gamma rays can be used as a probe of shock interactions in dense environments that are difficult to directly observe in visible light, X rays, and radio waves.…”

“…The escape fraction is phenomenologically introduced to represent effects of various low-energy photon absorption processes (Murase et al 2014). Follow-up observations at high-frequency radio bands are important (Murase et al 2014;Petropoulou et al 2016), and the hadronic scenario predicts…”

Point-source and diffuse high-energy neutrino emission from Type IIn supernovae

“…The production rates of accelerated protons and neutrinos (at a fixed energy) are expected to decrease beyond the deceleration radius, since they scale, respectively, as v 3 sh and v 2 sh (e.g. Petropoulou et al 2016). Furthermore, the neutrino production rate is expected to decrease significantly beyond r w , where the density of the medium is much lower than n(r w ).…”

“…Mastichiadis 1996;Kirk et al 1998). Neutrinos are produced at a rate dictated by the relativistic proton distribution and the non-relativistic proton density of the shocked CSM, which is assumed to be uniform and equal to 4n(r) (for details, see Petropoulou et al 2016), and they escape from the shell without any losses. The equations for the proton and neutrino distributions may be written as:…”

“…Some of these estimates are obtained assuming that the observed radio emission is produced by shock-accelerated electrons and without taking into account electron cooling. The first assumption may not be valid for the dense environments of SNe IIn, where relativistic electrons produced via the decay of charged pions from p-p collisions, are expected to contribute to the observed radio emission (Murase et al 2014;Petropoulou et al 2016). Furthermore, electron cooling cannot be, in general, neglected for the inferred B values and the high CSM densities of interaction-powered SNe (see e.g.…”