Nova V1674 Her as a fast magnetic rotator

Abstract

We report on the results of our XMM-Newton and NuSTAR campaign targeting V1674 Her in October 2023. These observations were timed to investigate the post-nova quiescent state 28 months after the 2021 outburst, although the optical luminosity was still slowly declining. The X-ray spectra are well-modeled by a multi-temperature thermal plasma. While in the best fit temperature we find that the highest plasma temperature is indicative of a massive white dwarf ($M_{\text{WD}} \approx 1.35 M_\odot$), we caution that there is a degeneracy between the plasma temperature and the metallicity in the spectral fit, hindering a definitive constraint. Alternatively, the continuum can be phenomenologically described by a Comptonized component, although this model neglects the thermal bremsstrahlung component. Timing analysis reveals a stable 501.31 $\pm$ 0.19 s modulation in the post-quiescent phase, whose amplitude diminishes with increasing energy; this is quite typical in intermediate polars (IP). The phase alignment between the quiescent and early post-outburst Chandra spin-folded light curves suggests that the soft X-ray emission originates from the same polar regions, which quickly re-established accretion following the eruption. V1674 Her belongs to a growing class of novae that are also IPs, and is one of four in which the modulation with rotation was observed during the outburst supersoft X-ray phase. We propose that the fast rise in outburst was due to a fast or centrifugal magnetic rotator wind, causing rapid mass loss instead of the commonly assumed radiation pressure wind.