A nova outburst powered by shocks

Abstract

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...

Internal shocks from variable outflows in classical novae

“…Metzger et al (2015) show how this behavior allows for an estimate of the efficiency of relativistic particle acceleration at the shocks, γ ∼ L γ /L sh , where L γ and L sh are, respectively, the gamma-ray luminosity and the portion of the optical luminosity powered by shocks. Application of this method to gamma-ray detected novae results in typical values of γ ≈ 3 − 5 × 10 −3 (Li et al 2017), consistent with theoretical expectations for the efficiency of diffusive shock acceleration. 3 The gamma-ray light curves in Figure 10 were calculated by multiplying the kinetic power of the shocks from the simulation by a constant value γ = 3×10 −3 .…”

“…(2) gamma-rays from pion decay (and relativistic bremsstrahlung from secondary pairs) generated by relativistic ions accelerated at the shocks (the spectrum shown was taken from the best-fit model to the LAT emission from ASASSN15-lh from Li et al 2017 following Vurm & Metzger 2018).…”

“…In fact, the required shock powers are so high that it led Metzger et al (2014) to conclude that a significant fraction of the total optical luminosity of the nova may be powered (indirectly) by shocks (see also Metzger et al 2015;Martin et al 2018) rather than the white dwarf. This possibility has received strong support from the observed temporal correlations between the optical and gamma-ray light curves (Li et al 2017;Aydi et al, in prep). High gas densities behind the shocks also imply that primary and secondary relativistic leptons (electrons or positrons) accelerated at the shock transfer their energy through Coulomb scattering to the thermal plasma faster than it can be radiated (e.g.…”

“…Figure 5 shows that, in both single-and multiple-transition models, the shocks spend most of the outburst duration above the gamma-ray photosphere. Nevertheless, the shocks can be deeply embedded at early times after the fast outflow is launched (τ γ = Σκ γ > 1; Σ 10 26 cm 2 g −1 ) and this could generate a modest delay in the onset of gamma-ray emission relative to the optical peak of a flare (Li et al 2017).…”

Kinematics of mass-loss from the outer Lagrange point L2

“…Recently, Pejcha (2014) and Pejcha et al (2017) argued that L2 mass loss lasting many hundreds and thousands of orbital periods was responsible for the pre-merger behaviour of V1309 Sco -a contact binary that exhibited accelerating orbital period decay accompanied by changes in the light curve shape and overall brightening preceding its luminous red nova outburst (Tylenda et al 2011). L2 mass loss or a similar process could lead to an equatoriallyconcentrated mass distribution that is later overtaken by a faster and more spherical explosion as is implicated in many supernovae, classical novae, luminous blue outbursts, and other transients (e.g., Li et al 2017;Andrews & Smith 2018;Smith et al 2018). Furthermore, mass loss from the vicinity of L2 can feed the circumbinary disk of microquasar SS433 (Fabrika 1993) and may significantly affect the gravitational wave frequency and amplitude of extreme mass-ratio inspirals (Linial & Sari 2017).…”