2016MNRAS.463..394V

The discovery of GeV gamma-rays from classical novae indicates that shocks and relativistic particle acceleration are energetically key in these events. Further evidence for shocks comes from thermal keV X-ray emission and an early peak in the radio light curve on a time-scale of months with a brightness temperature which is too high to result from freely expanding photoionized gas. _B and the efficiency ε_e of accelerating relativistic electrons of characteristic Lorentz factor γ ∼ 100. If the shocks are radiative (low velocity v_sh <= 1000 km s^–1) and cover a large solid angle of the nova outflow, as likely characterize those producing gamma-rays, then values of ε_e ∼ 0.01-0.1 are required to achieve the peak radio brightness for ε_B = 10^–2. Such high efficiencies exclude secondary pairs from pion decay as the source of the radio-emitting particles, instead favouring the direct acceleration of electrons at the shock. If the radio-emitting shocks are instead adiabatic (high velocity), as likely characterize those responsible for the thermal X-rays, then much higher brightness temperatures are possible, allowing the radio-emitting shocks to cover a smaller outflow solid angle.