The multiwavelength view of shocks in the fastest nova V1674 Her

Abstract

Classical novae are shock-powered multi-wavelength transients triggered by a thermonuclear runaway on an accreting white dwarf. V1674 Her is the fastest nova ever recorded (time to declined by two magnitudes is t2 = 1.1 d) that challenges our understanding of shock formation in novae. We investigate the physical mechanisms behind nova emission from GeV γ-rays to cm-band radio using coordinated Fermi-LAT, NuSTAR, Swift and VLA observations supported by optical photometry. Fermi-LAT detected short-lived (18 h) 0.1-100 GeV emission from V1674 Her that appeared 6 h after the eruption began; this was at a level of (1.6 ± 0.4) × 10−6 photons cm−2 s−1. Eleven days later, simultaneous NuSTAR and Swift X-ray observations revealed optically thin thermal plasma shock-heated to kTshock = 4 keV. The lack of a detectable 6.7 keV Fe Kα emission suggests super-solar CNO abundances. The radio emission from V1674 Her was consistent with thermal emission at early times and synchrotron at late times. The radio spectrum steeply rising with frequency may be a result of either free-free absorption of synchrotron and thermal emission by unshocked outer regions of the nova shell or the Razin-Tsytovich effect attenuating synchrotron emission in dense plasma. The development of the shock inside the ejecta is unaffected by the extraordinarily rapid evolution and the intermediate polar host of this nova.

The RS Oph Outburst of 2021 Monitored in X-Rays with NICER

“…However, there is no signature for RS Oph of the predicted, accompanying nonthermal emission in the NICER and NuSTAR ranges (see Luna et al 2021, for the latter), namely the "tail" of the gamma-ray flux at lower energy. This implies that, despite the high thermal X-ray flux compared to the gamma-ray flux, the lower limit ratio of nonthermal X-rays to gamma rays flux (measured in the Fermi energy range) is orders of magnitude lower than indicated by upper limits and detections measured for many classical novae (Metzger et al 2015;Nelson et al 2018aNelson et al , 2018bNelson et al , 2019Sokolovsky et al 2023) and even less than the factor of 10 −4 -10 −3 expected for the secondary emission above 3 keV. It is thus possible that the X-rays we observed were altogether from a completely different region than the original shocks that caused particle acceleration.…”

“…The line ratios' diagnostics in the X-ray high-resolution spectrum RS Oph on day 18 reveals that the emission originated in clumps of material with electron density possibly as high 10 12 cm −3 (Orio et al 2022a). Typical values of the electron density in the nova ejecta in the first days have been estimated to be even close to 10 11 cm −3 both in X-rays (e.g., the recent nova V1674 Her, Sokolovsky et al 2023) and in optical spectra (e.g., Neff et al 1978, who found a value of 7.4 × 10 −11 cm −3 for V1500 Cyg). Other published values from optical lines in novae span orders of magnitudes, but were mostly obtained at later post-outburst epochs, in the nebular phase.…”

Antistars as possible sources of antihelium cosmic rays

“…Recent detection of gamma-rays in the 60 GeV to 250 GeV energy range from an outburst of the well known recurrent nova RS Ophiuchi (a symbiotic binary system with a white dwarf accreting mass from a red giant donor star) by the MAGIC telescope [56] proves that the novae outbursts can accelerate CRs well above the GeV range (see also the Fermi/LAT spectra of the event in the 50 MeV-23 GeV range [57]). In 2021, Fermi/LAT telescope also detected a signal in 0.1-100 GeV range from nova V1674 Her that appeared 6 h after the outburst detection and lasted for 18 h with an integrated energy flux of ∼ 10 −9 erg cm −2 s −1 [58] suggesting a gamma-ray luminosity of above 10 36 erg s −1 at a distance of about 6 kpc advocated by the authors. From the analysis of the temporal evolution of the very high energy radiation from the outburst of RS Ophiuchi detected by the H.E.S.S.…”

“…Her [58] the authors concluded that the shock responsible for the emission was likely propagating within the nova ejecta. The shock in this case can be formed at the interaction region of the slowly moving common envelope and fast radiation-driven wind from the hot nuclearburning white dwarf [58] and therefore propagate into the ejected envelope. Fast shocks with a velocity of 1, 000 km s −1 would accelerate the antimatter-dominated CRs to GeV energies.…”

Classifying Optical (Out)bursts in Cataclysmic Variables: The Distinct Observational Characteristics of Dwarf Novae, Micronovae, Stellar Flares, and Magnetic Gating

“…In order to estimate the energies of bursts in each system we assumed distances based on a Gaia Data Release 3 (DR3) parallax (Bailer-Jones et al 2021). We also exclude classical novae from our analysis, even though they can occur on short timescales (e.g., Sokolovsky et al 2023). This is because classical novae have luminosities several orders of magnitude larger compared to other bursts in CVs.…”