A Strong Shock During a Nova Outburst: An Origin of Multiple Velocity Systems in Optical Spectra and of High-energy Emissions

Abstract

We propose a theoretical explanation of absorption/emission line systems in classical novae based on a fully self-consistent nova explosion model. We found that a reverse shock is formed far outside the photosphere (≳1013 cm) because later-ejected mass with a faster velocity collides with earlier-ejected matter. Optically thick winds blow continuously at a rate of ∼10−4 M ☉ yr−1 near the optical maximum, but its velocity decreases toward the optical maximum and increases afterward, so that the shock arises only after the optical maximum. The nova ejecta is divided by the shock into three parts, the outermost expanding gas (earliest wind before maximum), shocked shell, and inner fast wind, which respectively contribute to pre-maximum, principal, and diffuse-enhanced absorption/emission line systems. A large part of nova ejecta is eventually confined to the shocked shell. The appearance of the principal system is consistent with the emergence of a shock. This shock is strong enough to explain thermal hard X-ray emissions. The shocked layer has a high temperature of kT sh ∼ 1 keV × ( ( v wind − v shock ) / 1000 km s − 1 ) 2 = 1 keV × ( ( v d − v p ) / 1000 km s − 1 ) 2 , where v d − v p is the velocity difference between the diffuse-enhanced (v d) and principal (v p) systems. We compare a 1.3 M ☉ white dwarf model with the observational properties of the GeV gamma-ray detected classical nova V5856 Sgr (ASASSN-16ma) and discuss what kind of novae can produce GeV gamma-ray emissions.

Shock shaping? Nebular Spectroscopy of Nova V906 Carinae

“…V5668 Sgr Cheung et al (2016)) gamma (>100 MeV) episodes. Where some have argued for a white dwarf spin correlation to the gamma periodicity (ASASSN-16ma Li et al (2017)), here we posit several forward and reverse shocks in both polar and equatorial nova shell regions to give the observed structured optical and gamma-ray light curve of V906 Car, similar to the theoretical work of Hachisu & Kato (2022), effectively leading to the shaping of the nova shell as observed in its nebular phase. Steinberg & Metzger (2020) discuss varying velocity outflows over the course of a nova eruption leading to flares and complex velocity evolution of absorption and emission components of spectral lines, which is what we propose here being observed in V906 Car.…”

“…Supporting the observations are a series of theoretical works, e.g. Metzger et al (2014); Derdzinski et al (2017); Steinberg & Metzger (2020); Hachisu & Kato (2022). Slow evolving dust producing novae, similar to V906 Car, have a tendency to feature the brightest (V1324 Sco Finzell et al (2018)) and longest duration (e.g.…”

A Multiwavelength Light-curve Model of the Classical Nova V339 Del: A Mechanism for the Coexistence of Dust Dip and Supersoft X-Rays

“…Such a large variety in wavelengths (or energies) of emissions indicates that various emissions originate from different places or times of the nova ejecta. Hachisu & Kato (2022) and Hachisu & Kato (2023) proposed nova ejecta evolution models based on the fully selfconsistent nova outburst model of Kato et al (2022a). The authors found that a strong shock naturally arises in nova ejecta far outside the WD photosphere without ad hoc or arbitrary settings on the nova ejecta, and elucidated the origin of nova absorption/emission line systems raised by McLaughlin (1942).…”

“…In the present work, we elucidate the nature of the classical nova V339 Del based on our nova shock model (Hachisu & Kato 2022. The outburst of V339 Del was optically discovered by K. Itagaki at 6.8 mag on UT 2013 August 14.584 (JD 2,456,519.084;Nakano & Samus 2013).…”

“…Hydrogen ignites to trigger an outburst when the mass of the envelope reaches a critical value (see, e.g., Kato et al 2022a for a recent fully self-consistent nova explosion model). Hachisu & Kato (2022) theoretically found a strong shock formation far outside the photosphere in nova outbursts based on Kato et al (2022a)ʼs nova explosion model. A strong shock inevitably arises after the optical maximum because the photospheric wind velocity increases after the optical maximum so that the wind ejected later catches up with the wind ejected earlier.…”