The multi-wavelength view of shocks in the fastest nova V1674 Her

TL;DR

This study provides a comprehensive multi-wavelength analysis of the fastest recorded nova V1674 Her, revealing shock formation, gamma-ray emission, and thermal and synchrotron radio emissions, challenging existing models of nova shocks.

Contribution

It offers the first detailed multi-wavelength observational characterization of V1674 Her, highlighting shock mechanisms in an extremely fast nova and their emission signatures.

Findings

Detected 18-hour GeV gamma-ray emission post-eruption

Observed shock-heated plasma at 4 keV with no Fe K_alpha emission

Radio emission transitioned from thermal to synchrotron over time

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 t_2=1.1 d) that challenges our understanding of shock formation in novae. We investigate the physical mechanisms behind nova emission from GeV gamma-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)x10^-6 photons cm^-2 s^-1. Eleven days later, simultaneous NuSTAR and Swift X-ray observations revealed optically thin thermal plasma shock-heated to kT_shock = 4 keV. The lack of a detectable 6.7 keV Fe K_alpha 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.