The Rise of Nova V1674 Herculis

TL;DR

This paper presents unprecedented early observations of the fast nova V1674 Her, revealing detailed rise behavior, constraining the white dwarf's size, and challenging existing models of gamma-ray production in novae.

Contribution

It provides the first high-cadence, early-phase observations of a nova's rise, offering new insights into the physical processes and challenging previous assumptions about white dwarf overflow and wind interactions.

Findings

Detected an 8 mag rise in 5 hours with two distinct transition phases.

Constrained the white dwarf's photosphere size assuming blackbody emission at Eddington luminosity.

Challenged the idea that gamma-ray emission results from wind interaction with stripped gas.

Abstract

Observational constraints on classical novae are heavily biased to phases near optical peak and later because of the simple fact that novae are not typically discovered until they become bright. The earliest phases of brightening, coming before discovery, are typically missed, but this is changing with the proliferation of wide-field optical monitoring systems including ZTF, ASAS-SN, and Evryscope. Here, we report on unprecedented observations of the fast nova V1674 Her beginning >10 mag below its optical peak and including high-cadence (2 min.) observations that chart a rise of ~8 mag in just 5 hours. Two clear breaks are identified as the light curve transitions first from rising slowly to rising rapidly, followed by a transition to an even faster, nearly linear rate of increasing flux with time. The depths of the observations allow us to place tight constraints on the size of the photosphere under the assumption of blackbody emission from a white dwarf emitting at its Eddington luminosity. We find that the white dwarf was unlikely to have overflowed its Roche lobe prior to the launch of a fast wind, which poses a challenge for explaining the Fermi $\gamma$-ray detections as the interaction of a fast wind with a slow-torus of gas stripped from the inflated white dwarf envelope by the companion. High-cadence observations of novae from Evryscope and the planned Argus Array can record the diversity of rising nova light curves and help resolve how the interplay between thermonuclear fusion, binary interaction, and shocks power their earliest light.