Broadband Radiation from Primary Electrons in Very Energetic Supernovae

A class of very energetic supernovae (hypernovae) is associated with long gamma-ray bursts, in particular with a less energetic but more frequent population of gamma-ray bursts. Hypernovae also appear to be associated with mildly relativistic jets or outflows, even in the absence of gamma-ray bursts. Here we consider radiation from charged particles accelerated in such mildly relativistic outflows with kinetic energies of ~10⁵⁰ ergs. The radiation processes of the primarily accelerated electrons considered are synchrotron radiation and inverse Compton scattering of synchrotron photons (synchrotron self-Compton; SSC) and of supernova photons (external inverse Compton; EIC). In the soft X-ray regime, both the SSC and EIC flux can be the dominant component, but due to their very different spectral shapes, it should be easy to distinguish between them. When the fraction of the kinetic energy going into the electrons (epsilon_e) is large, the SSC is expected to be important; otherwise, the EIC will dominate. The EIC flux is quite high, almost independently of epsilon_e, providing a good target for X-ray telescopes such as XMM-Newton and Chandra. In the GeV gamma-ray regime, the EIC would be the dominant radiation process and the Gamma-Ray Large Area Space Telescope (GLAST, which has been renamed the Fermi Gamma-ray Space Telesope) should be able to probe the value of epsilon_e, the spectrum of the electrons, and their maximum acceleration energy. Accelerated protons also lead to photon radiation through the secondary electrons produced by the photopion and photopair processes. We find that over a significant range of parameters the proton component is generally less prominent than the primary electron component. We discuss the prospects for the detection of the X-ray and GeV signatures of the mildly relativistic outflow of hypernovae.