V407 Lup, an intermediate polar nova

TL;DR

This paper reports the detection of a 564.64-second X-ray flux modulation in V407 Lup, confirming its classification as an intermediate polar nova in quiescence, with detailed spectral and optical observations supporting this classification.

Contribution

First detection of stable spin period in quiescent phase of V407 Lup, confirming its intermediate polar nature with comprehensive X-ray and optical spectral analysis.

Findings

Detected stable 564.64 s X-ray flux modulation in quiescence

X-ray spectra fit with power law or thermal models

Optical spectra show typical IP emission lines

Abstract

We present X-ray and optical observations of nova V407 Lup (Nova Lup 2016), previously well monitored in outburst, as it returned to quiescent accretion. The X-ray light curve in 2020 February revealed a clear flux modulation with a stable period of 564.64$\pm$0.64 s, corresponding to the period measured in outburst and attributed to the spin of a magnetized white dwarf in an intermediate polar (IP) system. This detection in quiescence is consistent with the IP classification proposed after the nova eruption. The XMM-Newton EPIC X-ray flux is about 1.3 $\times 10^{-12}$ erg/cm$^2$/s at a distance, most likely, larger than 5 kpc, emitted in the whole 0.2-12 keV range without a significant cut-off energy. The X-ray spectra are complex; they can be fitted including a power law component with a relatively flat slope (a power law index of about 1), although, alternatively, a hard thermal component at kT$\geq$19 keV also yields a good fit. The SALT optical spectra obtained in 2019 March and 2022 May are quite typical of IPs, with strong emission lines, including some due to a high ionization potential, like He II at 4685.7 Angstrom. Nebular lines of O [III] were prominent in 2019 March, but their intensity and equivalent width appeared to be decreasing during that month, and they were no longer detectable in 2022, indicating that the nova ejecta dispersed. Complex profiles of the He II lines of V407 Lup are also characteristic of IPs, giving further evidence for this classification.