Shocks in nova outflows – I. Thermal emission

Abstract

Growing evidence for shocks in nova outflows include (1) multiple velocity components in the optical spectra; (2) hard X-ray emission starting weeks to months after the outburst;(3) an early radio flare on timescales of months, in excess of that predicted from the freely expanding photo-ionized gas; and, perhaps most dramatically, (4) ∼ GeV gamma-ray emission. We present a one dimensional model for the shock interaction between the fast nova outflow and a dense external shell (DES) and its associated thermal X-ray, optical, and radio emission. The lower velocity DES could represent an earlier stage of mass loss from the white dwarf or ambient material not directly related to the thermonuclear runaway. The forward shock is radiative initially when the density of shocked gas is highest, at which times radio emission originates from the dense cooling layer immediately downstream of the shock. Our predicted radio light curve is characterized by sharper rises to maximum and later peak times at progressively lower frequencies, with a peak brightness temperature that is approximately independent of frequency. We apply our model to the recent gamma-ray producing classical nova V1324 Sco, obtaining an adequate fit to the early radio maximum for reasonable assumptions about the fast nova outflow and assuming the DES possesses a characteristic velocity ∼ 10 3 km s −1 and mass ∼ few 10 −4 M ⊙ ; the former is consistent with the velocities of narrow line absorption systems observed previously in nova spectra, while the total ejecta mass of the DES and fast outflow is consistent with that inferred independently by modeling the late radio peak as uniformly expanding photo-ionized gas. Rapid evolution of the early radio light curves require the DES to possess a steep outer density profile, which may indicate that the onset of mass loss from the white dwarf was rapid, providing indirect evidence that the DES was expelled as the result of the thermonuclear runaway event. Reprocessed X-rays from the shock absorbed by the DES at early times are found to contribute significantly to the optical/UV emission, which we speculate may be responsible for the previously unexplained 'plateaus' and secondary maxima in nova optical light curves.

A Detailed Observational Analysis of V1324 Sco, the Most Gamma-Ray-luminous Classical Nova to Date

“…Radiative shocks can also explain the observed gamma-ray luminosity and nondetection in X-rays (Section 5; Metzger et al 2014). When compared to other gamma-ray-detected novae in the AAVSO database (Kafka 2016), V1324Sco had an unusually dramatic dust event.…”

The X-Ray Through Optical Fluxes and Line Strengths of Tidal Disruption Events

“…However, for high luminosity TDEs, the intense radiation field may completely ionize helium to He III, allowing only a small fraction of the source luminosity to be absorbed. Such an ionization effect has been explored in Metzger et al (2014) and Metzger & Stone (2015). Determining the ionization state is therefore crucial for estimating the reprocessing efficiency of a TDE envelope.…”

Novae as Tevatrons: prospects for CTA and IceCube

“…2). The FS propagates at a velocity v fs = v c − v ej v ej , while the velocity of the reverse shock is v rs = v f − v c , where v c is the velocity of the cold central shell, which only moderately exceeds the velocity of the slow ejecta due to the large inertia of the latter (see Metzger et al 2014). Hereafter, the velocities of both shocks are parametrized as v sh = 10 8 v 8 cm s −1 = ηv ej , where η < 1 for the FS and η ∼ 1 for RS.…”

“…where X Z is the mass fraction of element Z and mass number A and we have taken a characteristic cross section σ Z thr /2 as half that of the threshold value (see Appendix A of Metzger et al 2014 for justification). For ∆ Z R ej we thus have (in the λ Z 1 limit)…”