Novae are dramatic outbursts that occur when material builds up on the surface of a white dwarf star in a binary system, leading to a sudden and intense brightening. In their paper, Quimby et al. (2024) provide an in-depth look into the early stages of the outburst of Nova V1674 Herculis (V1674 Her), which has intrigued astronomers due to its extremely fast rise and unique features.

Introduction

Novae are known for their rapid brightening as the material accumulated on the surface of a white dwarf ignites. However, observations of these events often miss the earliest phases, especially for fast novae, which can brighten quickly and unpredictably. This paper presents detailed early observations of Nova V1674 Her, providing new insights into how these events unfold.

Early Detection and Rise to Peak

Nova V1674 Her was first discovered by Seiji Ueda of Japan on July 12, 2021. Remarkably, thanks to advanced observation networks like the Zwicky Transient Facility (ZTF) and Evryscope, the rise of V1674 Her was recorded from a very early stage, 10 magnitudes below its peak brightness. Observations showed that the nova brightened by about 8 magnitudes in just 5 hours, displaying three distinct phases of brightening. Evryscope’s high-cadence observations were particularly valuable, providing minute-by-minute detail of the nova’s rise, something rarely seen before.

Understanding the Light Curve

The light curve of V1674 Her was divided into three phases: the "slow rise," the "fast rise," and the "faster rise." During the slow rise, the nova brightened gradually before accelerating in the fast rise phase. This phase lasted for about an hour, with the brightness increasing nearly linearly. Finally, the faster rise phase showed an even more rapid and sustained increase in brightness, which led to the nova’s peak. These phases suggest that multiple processes might be at play, such as changes in the expansion velocity of the white dwarf's outer layers or the interaction of the nova's wind with surrounding material.

Physical Implications and Models

To explain the observed rise, Quimby and colleagues considered a model in which the white dwarf expands while maintaining a constant luminosity. As the white dwarf grows, its surface temperature decreases, shifting its emission from X-rays to visible light, causing the observed increase in brightness. This model matches the observed data well but cannot explain all details, such as the rapid transitions between the different phases of brightening.

Interestingly, the early detection also placed constraints on the size of the white dwarf’s photosphere and the speed of the winds ejected during the outburst. The authors estimate that the white dwarf likely did not overflow its Roche lobe— the region around the star where material can escape—before the fast wind was launched. This challenges some existing theories about how fast novae produce gamma-ray emission.

Conclusion and Future Observations

V1674 Her’s rapid rise provides a rare glimpse into the early evolution of novae. This nova’s unique features—its extremely fast rise, periodic oscillations, and gamma-ray emissions—highlight the importance of early, high-cadence observations in understanding these explosive events. The authors emphasize that future projects, such as the Argus Array, could capture more of these early stages and help refine our understanding of the physics driving novae.

Source: Quimby