Fermi establishes classical novae as a distinct class of gamma-ray sources

Abstract

A classical nova results from runaway thermonuclear explosions on the surface of a white dwarf that accretes matter from a low-mass main-sequence stellar companion. In 2012 and 2013, three novae were detected in γ rays and stood in contrast to the first γ-ray detected nova V407 Cygni 2010, which belongs to a rare class of symbiotic binary systems. Despite likely differences in the compositions and masses of their white dwarf progenitors, the three classical novae are similarly characterized as soft spectrum transient γ-ray sources detected over 2−3 week durations. The γ-ray detections point to unexpected high-energy particle acceleration processes linked to the mass ejection from thermonuclear explosions in an unanticipated class of Galactic γ-ray sources.The Fermi-LAT [Large Area Telescope; (1)], launched in 2008, continuously scans the sky in γ rays, thus enabling searches for transient sources. When a nova explodes in a symbiotic binary system, the ejecta from the white dwarf surface expand within the circumstellar wind of the red giant companion and high-energy particles can be accelerated in a blast wave driven in the high-density environment (2) so that variable γ-ray emission can be produced, as was detected at >100 MeV energies by the LAT in V407 Cygni 2010 (V407 Cyg) (3). In a classical nova, by contrast, the ejecta quickly expand beyond the confines of the compact binary into a much lower density environment. High-energy particle acceleration could therefore be related to a bow shock driven by the ejecta in the interstellar medium, or to turbulence and eventually weaker internal shocks formed in the inhomogeneous ejecta itself. The contribution of such expanding nova shells to cosmic-ray acceleration had been considered (4), but no predictions have so far been made for >100 MeV γ-rays. The classical novae (or simply "novae" where appropriate) detected by the LAT with 12−20σ significances (Table 1, Fig. 1) -V959 Monocerotis 2012 (V959 Mon), V1324 Scorpii 2012 (V1324 Sco), and V339 Delphini 2013 (V339 Del) -were unanticipated. These observed γ rays have higher energies than nuclear line emission by * All authors with their affiliations appear at the end of this paper. †To whom correspondence should be addressed. E-mail: Teddy.Cheung@nrl.navy.mil (C. C. Cheung); E-mail: Pierre.Jean@irap.omp.eu (P. Jean); E-mail: shore@df.unipi.it (S. N. Shore) 1 arXiv:1408.0735v1 [astro-ph.HE] 4 Aug 2014 radioactive decay at ∼MeV energies that remain undetected in individual novae (5) and < ∼ 0.1 MeV emission detected in isolated cases (6).V959 Mon was detected as a transient γ-ray source in June 2012 by the LAT while close (∼ 20• separation) to the Sun (7) and then optically in August (8). Ultraviolet spectroscopy revealed an oxygen-neon nova (9), recognized as the class with the most massive white dwarfs ( > ∼ 1.1 M ) with massive ( > ∼ 8 M ) progenitors [e.g., (10)]. The expected peak visual magnitude of ∼5 would have been observable with the naked eye ∼50 days earlier, when the γ-ray transient was detected (9). V339 D...

Understanding the γ -ray emission from the globular cluster 47 Tuc: Evidence for dark matter?

“…While CVs were discovered to be γ-ray bright by the Fermi-LAT [32][33][34], the γ-ray emission is transient, on the timescale of days, and only CVs in our local Galactic neighborhood have been observed to be γ-ray bright, suggesting that their γ-ray flux is observed simply because of their proximity to us [35]. LMXBs are not known to be γ-ray emitters; all confirmed γ-ray emitting binary systems are wind-driven systems and are classed as high-mass x-ray binaries.…”

COLLIMATION AND ASYMMETRY OF THE HOT BLAST WAVE FROM THE RECURRENT NOVA V745 Sco

“…The γ-rays are thought to be produced either from the decay of neutral pions that arise in energetic proton collisions within the shock acceleration region, analogous to similar processes in supernovae, or from inverse Compton scattering and bremsstrahlung due to accelerated electrons (Abdo et al 2010). The unexpected γ-rays found in nonsymbiotic classical novae likely originate in the interaction of the blast with the immediate circumstellar environment, such as the accretion disk, with a period of a few days required from the explosion onset to engender sufficient particle acceleration to induce an observable γ-ray flux (Ackermann et al 2014). The γ-ray detection in V745 Sco coincided with the nova onset, suggesting that conditions in the immediate vicinity (in the stellar wind, equatorial density enhancement, or an accretion disk) were conducive to rapid particle acceleration.…”

Internal shocks from variable outflows in classical novae

“…Furthermore, X-ray observations reveal the presence of hot 10 7 K gas indicative of shocks (O'Brien et al 1994;Mukai & Ishida 2001;Nelson et al 2019). However, the prevalance, and energetic importance, of these shocks were not fully appreciated until the discovery by Fermi/LAT of 100 MeV gamma-rays from Galactic classical novae (Ackermann et al 2014;Cheung et al 2016;Franckowiak et al 2018;Siegert et al 2018).…”