The Fermi LAT discovery that classical novae produce ∼ > 100 MeV gamma-rays establishes that shocks and relativistic particle acceleration are key features of these events. These shocks are likely to be radiative due to the high densities of the nova ejecta at early times coincident with the gamma-ray emission. Thermal X-rays radiated behind the shock are absorbed by neutral gas and reprocessed into optical emission, similar to Type IIn (interacting) supernovae. Gamma-rays are produced by collisions between relativistic protons with the nova ejecta (hadronic scenario) or Inverse Compton/bremsstrahlung emission from relativistic electrons (leptonic scenario), where in both scenarios the efficiency for converting relativistic particle energy into LAT gamma-rays is at most a few tens of per cent. The measured ratio of gamma-ray and optical luminosities, L γ /L opt , thus sets a lower limit on the fraction of the shock power used to accelerate relativistic particles, nth . The measured value of L γ /L opt for two classical novae, V1324 Sco and V339 Del, constrains nth ∼ > 10 −2 and ∼ > 10 −3 , respectively. Leptonic models for the gamma-ray emission are disfavored given the low electron acceleration efficiency, nth ∼ 10 −4 − 10 −3 , inferred from observations of Galactic cosmic rays and particle-in-cell (PIC) numerical simulations. A fraction f sh ∼ > 100( nth /0.01) −1 and ∼ > 10( nth /0.01) −1 per cent of the optical luminosity is powered by shocks in nova Sco and nova Del, respectively. Such high fractions challenge standard models that instead attribute all nova optical emission to the direct outwards transport of thermal energy released near the white dwarf surface. We predict hard ∼ 10 − 100 keV X-ray emission coincident with the LAT emission, which should be detectable by NuSTAR or ASTRO-H, even at times when softer ∼ < 10 keV emission is absorbed by neutral gas ahead of the shocks.

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 1 . 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 [2][3][4][5] . Recent studies with the Fermi Large Area Telescope have shown that many classical novae are accompanied by gigaelectronvolt γ-ray emission 6, 7 . 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 γ-ray and optical observations of the Milky Way nova ASASSN-16ma, which is among the brightest novae ever detected in γ-rays. The γ-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 8 . The ratio of γ-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 γ-ray emission 9 .The need to accelerate particles up to energies exceeding 100 gigaelectronvolts provides compelling evidence for magnetic field amplification in the shocks.ASASSN-16ma (a.k.a. PNV J18205200−2822100, Nova Sgr 2016d, and V5856 Sgr) is an optical transient source in the constellation Sagittarius, discovered by the All Sky Automated Survey for SuperNovae (ASAS-SN 10 ), on 25.02 October 2016 UT 11 (a corresponding Modified Julian Day of MJD 57686.02) and identified as a normal classical nova with optical spectroscopy 12, 13 .The optical light curve of the nova after its discovery shows three distinct phases (Figure 1). In Phase I, the nova slowly rose to m V ∼ 8 mag over two weeks. It then showed a rapid brightening by a factor of ∼ 10 over just two days (Phase II), reaching a naked-eye peak visual magnitude of 5.4 (MJD 57700). This was followed by a relatively stable decline lasting for several weeks (Phase III; see Figure 1 and Methods).Immediately following the optical peak, our Fermi target-of-opportunity (ToO) observation detected strong γ-ray emission from the nova with a very high photon flux of F ph,γ ≈ 2 10 −6 ph cm −2 s −1 (Methods). The γ-ray emission faded rapidly over the next nine days, with only marginal γ-ray detections in the following week. This is among the fastest-evolving γ-ray light curves seen to date from a nova. The optical and γ-ray light curves are tightly correlated, declining at the same rate and showing a simultaneous dip in the emission around MJD 57705 (Figure 1). The ratio of the γ-ray to optical luminosity (∼ 0.002) remains constant while the γ-rays are detectable (Figure 1; see also Supplementary Information, SI hereafter).The clear correlation between the γ-ray and optical light in ASASSN-16ma leads us to reconsider the standard mod...

We present multiwavelength observations of the persistent Fermi-LAT unidentified γ-ray source 1FGL J1417.7−4407, showing it is likely to be associated with a newly discovered X-ray binary containing a massive neutron star (nearly 2M ⊙ ) and a ∼ 0.35M ⊙ giant secondary with a 5.4 day period. SOAR optical spectroscopy at a range of orbital phases reveals variable double-peaked Hα emission, consistent with the presence of an accretion disk. The lack of radio emission and evidence for a disk suggests the γ-ray emission is unlikely to originate in a pulsar magnetosphere, but could instead be associated with a pulsar wind, relativistic jet, or could be due to synchrotron self-Compton at the disk-magnetosphere boundary. Assuming a wind or jet, the high ratio of γ-ray to X-ray luminosity (∼ 20) suggests efficient production of γ-rays, perhaps due to the giant companion. The system appears to be a low-mass X-ray binary that has not yet completed the pulsar recycling process. This system is a good candidate to monitor for a future transition between accretion-powered and rotational-powered states, but in the context of a giant secondary.

The importance of shocks in nova explosions has been highlighted by Fermi's discovery of γ-ray producing novae. Over three years of multi-band VLA radio observations of the 2010 nova V1723 Aql show that shocks between fast and slow flows within the ejecta led to the acceleration of particles and the production of synchrotron radiation. Soon after the start of the eruption, shocks in the ejecta produced an unexpected radio flare, resulting in a multi-peaked radio light curve. The emission eventually became consistent with an expanding thermal remnant with mass 2 × 10 −4 M ⊙ and temperature 10 4 K. However, during the first two months, the ∼ >10 6 K brightness temperature at low frequencies was too high to be due to thermal emission from the small amount of X-ray producing shock-heated gas. Radio imaging showed structures with velocities of 400 km s −1 (d/6 kpc) in the plane of the sky, perpendicular to a more elongated 1500 km s −1 (d/6 kpc) flow. The morpho-kinematic structure of the ejecta from V1723 Aql appears similar to nova V959 Mon, where collisions between a slow torus and a faster flow collimated the fast flow and gave rise to γ-ray producing shocks. Optical spectroscopy and X-ray observations of V1723 Aql during the radio flare are consistent with this picture. Our observations support the idea that shocks in novae occur when a fast flow collides with a slow collimating torus. Such shocks could be responsible for hard X-ray emission, γ-ray production, and double-peaked radio light curves from some classical novae.

It has recently been discovered that some, if not all, classical novae emit GeV gamma-rays during outburst, but the mechanisms involved in the production ofgamma-rays are still not well understood. We present here a comprehensive multiwavelength data set-from radio to X-rays-for the most gamma-ray-luminous classical nova to date, V1324 Sco. Using this data set, we show that V1324 Sco is a canonical dusty Fe II-type nova, with a maximum ejecta velocity of 2600 km s −1 and an ejecta mass of a few´-10 5  M . There is also evidence for complex shock interactions, including a double-peaked radio light curve which shows high brightness temperatures at early times. To explore why V1324Sco was so gamma-ray luminous, we present a model of the nova ejecta featuring strong internal shocks and find that higher gamma-ray luminosities result from higher ejecta velocities and/or mass-loss rates. Comparison of V1324Sco with other gamma-ray-detected novae does not show clear signatures of either, and we conclude that a larger sample of similarly well-observed novae is needed to understand the origin and variation of gamma-rays in novae.