A very rapidly rotating white dwarf in nova YZ Reticuli

Abstract

YZ Ret (Nova Reticuli 2020) is the first VY Scl-type nova-like variable observed to have undergone a classical nova eruption.Following the outburst, a timing analysis of 20-s cadence TESS data revealed a periodicity at approximately 42 s, suggesting itsplausible classification as a fast-spinning intermediate polar (IP). To definitively identify the nature of this modulation, we performed amulti-instrument timing analysis using high-speed ground-based photometry Zorro/Gemini South (1 s cadence) and the South AfricanAstronomical Observatory (5 s cadence) alongside TESS Sector 97 observations. Our ground-based data reveal a highly coherentperiod of 37.69131±0.00001 s, which we identified as the true rotation period of the white dwarf. We demonstrate that the apparent42.61 s signal in the TESS data is a Nyquist alias of this fundamental frequency. Furthermore, the signal amplitude in the TESSdata is suppressed by a factor of ≈0.6 relative to the Gemini observations, a result that is consistent with the theoretical sinc-functiondamping expected for a 20-s integration time. The extreme coherence and long-term stability of the 37.69131 s signal rule out transientphenomena such as dwarf nova oscillations or non-radial pulsations. We conclude that YZ Ret hosts a fast-spinning magnetic whitedwarf in an IP configuration. This discovery implies that mass loss during the nova eruption was likely driven by a fast magneticrotator wind and provides a physical explanation for the missing supersoft X-ray phase, suggesting that nearly the entire accretedenvelope was exhausted, promptly quenching the process of nuclear burning.