A TESS View of Post-Eruption Variability in the Novae V1405 Cas,V1716 Sco, and V1674 Her

Abstract

We analyzed TESS archival data of three novae after recent outbursts, searching the orbital and white dwarf (WD) rotation period and possible variations of these periods. In V1405 Cas, we detected a period of $\sim$116.88 seconds, which we identified as due to the WD spin, and measured a rate of increase of 0.001542$\pm0.000009\, {\rm s\, d}^{-1}$, one the fastest spin-down rates ever recorded. The rapid spin-down coupled with an X-ray luminosity several orders of magnitude lower than the available spin-down power, strongly indicates that the system is in a magnetic ``propeller'' state, namely the rotational energy powers the system's X-ray luminosity. We measured a previously unknown orbital period of 1.36 days for V1716 Sco. If the X-ray flux modulation with a period of 77.9 s detected in outburst for this nova is due to the rotation of an strongly magnetized white dwarf as in other novae with similar modulations of the supersoft X-ray source in outburst, the system is in a parameter space that challenges standard models of cataclysmic variable evolution. For V1674 Her, which has already been classified as an intermediate polar (IP), we confirm the known spin period of 501.33$\pm$0.01 s and the orbital period of 0.1529$\pm$0.0001 days, suggesting that the spin modulation was also the root cause of the periodicity in X-rays in outburst, and that the WD atmosphere in the supersoft X-ray phase was not thermally homogeneous. Our results highlight the power of high-cadence, continuous observations in revealing extreme and unexpected characteristics of accreting white dwarfs.

Figures (4)

• Figure 1: Top panel: Lomb-Scargle periodograms extracted from five-days, consecutive and overlapping (50% overlap) slices of the Sectors 77 and 78. The central peak is identified as the spin period with an average of 116.88 s. The peaks at both sides of the central peak represent the beat between the spin and orbital periods ($\omega$-$\Omega$ and $\omega$+$\Omega$). Bottom panel: The spin evolution during the TESS observations reported here, where the red line shows a simple linear fit to the spin-down trend.
• Figure 2: Top: Lomb-Scargle power spectra from the TESS light curves of V1716 Sco observed during Sectors 12 (blue), 39 (black) and 93 (red). The horizontal dashed black line in all panels shows the false alarm probability at the 99.99% level. Inset: Phase-folded light curve of Sectors 12, 39 and 93 at the 1.36 days orbital period. Bottom: ASAS-SN $g$-band magnitude (left Y-axis) light curve of V1716 Sco before, during and after the nova outburst recorded on MJD=60055.178. We plot on top the light curves from the four sectors (12, 39, 66, 93) observed with TESS (right Y-axis).
• Figure 3: Top: Lomb-Scargle power spectra from the TESS light curves of V1674 Her observed during Sectors 40, 53, 54 and 80 centered on the frequency corresponding to the orbital period, P$_{orb}$=0.1529 days. The horizontal dashed blue line shows the false alarm probability at the 99.99% level determined from the periodogram of Sector 80 where the orbital period was barely detected. Inset: Phase-folded light curve of Sectors 40, 53, 54 and 80 folded at the 0.1529 days orbital period. Bottom: Lomb-Scargle power spectra from the TESS light curves of V1674 Her observed during Sectors 40, 53, 54 and 80 centered on the frequency corresponding to the spin period, P$_{spin}$=501.39 seconds. The horizontal dashed black line shows the false alarm probability at the 99.99% level determined from the periodogram of Sector 80. This period can not be detected in the other sectors because its frequency is lower than their Nyquist frequency. Inset: Sector 80 light folded at the spin period with the ephemeris from v1674_ephem.
• Figure 4: The spin period versus orbital period diagram for magnetic cataclysmic variables. The population of known intermediate polars (IPs, blue circles) together with the three novae analyzed in this work being highlighted: V1405 Cas (red circle), V1716 Sco (green circle), and V1674 Her (magenta circle).