Radiative response of nova ejecta to illumination variations in the post-outburst white dwarf: V1405 Cas 2021

¹ Dipartimento di Fisica, Università di Pisa, largo B. Pontecorvo 3, Pisa 56127, Italy
² INAF-OATS, Via G.B. Tiepolo 11, 34143 Trieste, Italy
³ INAF Astronomical Observatory of Padova, 36012 Asiago (VI), Italy
⁴ School of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁵ Astronomical Ring for Amateur Spectroscopy (ARAS Group), Rouen, France

^★ Corresponding authors: r.digiacomo1@studenti.unipi.it; steven.neil.shore@unipi.it

Received: 22 May 2025
Accepted: 24 September 2025

Abstract

Aims. The nebular stage is that interval in the expansion of classical nova ejecta when the transparency of the matter permits a comprehensive view of the structure of the ejecta. We aim to study the radiative response of nova ejecta to illumination variations in the post-outburst white dwarf (WD).

Methods. We used medium-resolution (R ≈ 10⁴) optical spectra, a substantial fraction of which were flux-calibrated. Time series were obtained for profile and flux variations supplemented by Swift X-ray Telescope (XRT) spectrophotometry. The ejecta structure and density stratification were modeled assuming nebular conditions with biconical geometries, and densities were derived using the [O III] lines and scaled for ejecta expansion.

Results. Antiphase variations were observed for the C IV 5808 Å recombination line and coronal transitions of [Fe VII] and [Ca V] coincident with X-ray flaring activity of the central WD. In the earlier transition stage to nebular conditions, similar profile and flux variations were observed for [N II] 5755 Å, although no X-rays were detected during that time. We show that the profile variations map the density structure of the ejecta and are entirely due to changes in the hardness of the incident spectral distribution of the white dwarf. No dynamical explanation is required.

Conclusions. Nebular stage line intensity and profile variations permit a comprehensive view of the development of the central white dwarf without recourse to other wavelength intervals, such as X-rays or the far-UV.