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Classical novae are runaway thermonuclear burning events on the surfaces of accreting white dwarfs in close binary star systems, sometimes appearing as new naked-eye sources in the night sky. The standard model of novae predicts that their optical luminosity derives from energy released near the hot white dwarf which is reprocessed through the ejected material. Recent studies with the Fermi Large Area Telescope have shown that many classical novae are accompanied by gigaelectronvolt gamma-ray emission. This emission likely originates from strong shocks, providing new insights into the properties of nova outflows and allowing them to be used as laboratories to study the unknown efficiency of particle acceleration in shocks. Here we report gamma-ray and optical observations of the Milky Way nova ASASSN-16ma, which is among the brightest novae ever detected in gamma-rays. The gamma-ray and optical light curves show a remarkable correlation, implying that the majority of the optical light comes from reprocessed emission from shocks rather than the white dwarf. The ratio of gamma-ray to optical flux in ASASSN-16ma directly constrains the acceleration efficiency of non-thermal particles to be ~0.005, favouring hadronic models for the gamma-ray emission. The need to accelerate particles up to energies exceeding 100 gigaelectronvolts provides compelling evidence for magnetic field amplification in the shocks.
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We present a bolometric light curve model of Type IIn supernovae powered by supernova ejecta colliding with a circumstellar medium. We estimate the conversion efficiency of the ejectas kinetic energy to radiation at the reverse and forward shocks and find that a large density contrast makes a difference in the efficiency. The emission from the reverse shock can maintain high efficiency for a long time, and becomes important at the late phase of the light curve. We first construct a semi-analytical model that is applicable to the late phase of the light curve when the diffusion time of photons in the shocked region becomes negligible. We further develop radiation transfer simulations that incorporate these physical processes into the light curve. The numerical calculations predict light curves at early phases, which are testable by present and future short-cadence surveys. We compare our model with the bolometric light curve constructed from observations for a type IIn supernova 2005ip. Due to the reduced efficiency at the forward shock, we find from our model that the mass-loss rate of the progenitor star was $approx 1times 10^{-2} {rm M_odot yr^{-1}}$ for a wind velocity of $100 {rm km s^{-1}}$, an order of magnitude higher compared to previous work that used simple assumptions of the efficiency. This highlights the importance of taking these two components into account when extracting the physical parameters from observations.
Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems (Bode & Evans 2008). It has long been thought that the luminosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway (Gallaher & Starrfield 1978). However, recent observations of GeV $gamma$-rays from classical novae have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminosity of events as diverse as stellar mergers (Metzger & Pejcha 2017), supernovae (Moriya et al. 2018), and tidal disruption events (Roth et al. 2016), but observational confirmation has been lacking. Here we report simultaneous space-based optical and $gamma$-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and $gamma$-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock-powered. Furthermore, we detect concurrent but weak X-ray emission from deeply embedded shocks, confirming that the shock power does not appear in the X-ray band and supporting its emergence at longer wavelengths. Our data, spanning the spectrum from radio to $gamma$-ray, provide direct evidence that shocks can power substantial luminosity in classical novae and other optical transients.
Evidence for shocks in nova outflows include (1) multiple velocity components in the optical spectra; (2) keV X-ray emission weeks to months after the outburst; (3) early radio flare on timescales of months, in excess of that predicted from the freely expanding photo-ionized gas; and (4) ~ GeV gamma-rays. We present a 1D 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 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. The 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 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 velocity ~1e3 km/s and mass ~ 2e-4 M_sun; 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. Rapid evolution of the early radio light curves require the DES 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 by the thermonuclear runaway event. Reprocessed X-rays from the shock absorbed by the DES at early times may contribute significantly to the optical/UV emission, which we speculate is responsible for the previously unexplained `plateaus and secondary maxima in nova optical light curves.
The discovery that many classical novae produce detectable GeV $gamma$-ray emission has raised the question of the role of shocks in nova eruptions. Here we use radio observations of nova V809 Cep (Nova Cep 2013) with the Jansky Very Large Array to show that it produced non-thermal emission indicative of particle acceleration in strong shocks for more than a month starting about six weeks into the eruption, quasi-simultaneous with the production of dust. Broadly speaking, the radio emission at late times -- more than a six months or so into the eruption -- is consistent with thermal emission from $10^{-4} M_odot$ of freely expanding, $10^4$~K ejecta. At 4.6 and 7.4 GHz, however, the radio light-curves display an initial early-time peak 76 days after the discovery of the eruption in the optical ($t_0$). The brightness temperature at 4.6 GHz on day 76 was greater than $10^5 K$, an order of magnitude above what is expected for thermal emission. We argue that the brightness temperature is the result of synchrotron emission due to internal shocks within the ejecta. The evolution of the radio spectrum was consistent with synchrotron emission that peaked at high frequencies before low frequencies, suggesting that the synchrotron from the shock was initially subject to free-free absorption by optically thick ionized material in front of the shock. Dust formation began around day 37, and we suggest that internal shocks in the ejecta were established prior to dust formation and caused the nucleation of dust.
We analyse here four observations of nova KT Eri (Nova Eri 2009) done with the Chandra High Resolution Camera Spectrometer (HRC-S) and the Low Energy Transmission Grating (LETG) in 2010, from day 71 until day 159 after the optical maximum, in the luminous supersoft X-ray phase. The spectrum presents many absorption features with a large range of velocity, from a few hundred km s$^{-1}$ to 3100 km s$^{-1}$ in the same observation, and a few prominent emission features, generally redshifted by more than 2000 km s$^{-1}$. Although the uncertainty on the distance and the WD luminosity from the approximate fit do not let us rule out a larger absolute luminosity than our best estimate of $simeq 5 times 10^{37}$ erg s$^{-1}$, it is likely that we observed only up to $simeq$40% of the surface of the white dwarf, which may have been partially hidden by clumpy ejecta. Our fit with atmospheric models indicate a massive white dwarf in the 1.15-1.25 M$_odot$ range. A thermal spectrum originating in the ejecta appears to be superimposed on the white dwarf spectrum. It is complex, has more than one component and may be due to a mixture of photoionized and shock ionized outflowing material. We confirm that the $simeq$35 s oscillation that was reported earlier, was detected in the last observation, done on day 159 of the outburst.
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