Optical detection of the X-ray flash in the very fast nova V1674 Her: Optical contribution of the irradiated accretion disk

TL;DR

This study reports the first optical detection of the X-ray flash phase in the very fast nova V1674 Her, using a composite light curve model that highlights the roles of the irradiated accretion disk and free-free emission.

Contribution

The paper introduces the first optical observation of the X-ray flash phase in a nova and presents a detailed light curve model incorporating the irradiated accretion disk and ejecta.

Findings

Optical detection of the X-ray flash phase at 0.014 days after thermonuclear runaway.

Optically thick winds emerge at 0.04 days, with flux dominated by free-free emission outside the WD.

The model reproduces the dense early light curve and the change in rise rate without shock evidence.

Abstract

V1674 Her is one of the fastest and brightest novae, characterized by dense optical photometry in the pre-maximum phase, a rise from $g=17$ to 7 mag, in one-fourth of a day. We present a composite theoretical $V$ light curve model of its early rising phase starting from a quiescent brightness of $g=19.2$ mag. Our light curve model consists of a hot and bright white dwarf (WD) and irradiated accretion disk and companion star. We found that the earliest optical detection of ASAS-SN $g$ band brightness of $g=17.0$ at $t=0.014$ day from the onset of thermonuclear runaway can be explained with the irradiated accretion disk and companion star in the X-ray flash phase of a $1.35 ~M_\odot$ WD. This is the first detection in optical of an X-ray flash phase of a nova. Optically thick winds emerge from the WD photosphere at $t=0.04$ day, and optical flux is dominated by free-free emission from optically-thin ejecta just outside the WD photosphere. Our free-free emission model $V$ light curve reasonably reproduces the dense $g$ light curve of Evryscope that spans from $g=14.8$ (at 0.078 day) to $g=7.1$ (at 0.279 day), including a sudden change of slope in the $g$ light curve from slow to rapid rise at $g=14.3$ on day $0.1$. There is no indication of shocking power during the rising phase from $g=14.8$ to 7.1.