The Periodic Signals of Nova V1674 Herculis (2021)

Abstract

We present time-series photometry during the early decline phase of the extremely fast nova V1674 Herculis. The 2021 light curve showed periodic signals at 0.152921(3) days and 501.486(5) s, which we interpret as respectively the orbital and white dwarf spin periods in the underlying binary. We also detected a sideband signal at the difference frequency between these two clocks. During the first 15 days of outburst, the spin period appears to have increased by 0.014(1)%. This increase probably arose from the sudden loss of high-angular-momentum gas (“the nova explosion”) from the rotating, magnetic white dwarf. Both periodic signals appeared remarkably early in the outburst, which we attribute to the extreme speed with which the nova evolved (and became transparent to radiation from the inner binary). After that very fast initial period increase of 71 ms, the period subsequently decreased—at 182(18) ms yr−1 in 2021, and 88(18) ms yr−1 in 2022. These rates are ∼100× faster than typically seen in intermediate polars. This could be due to high accretion torques from very high mass-transfer rates, which might be common when low-mass donor stars are strongly irradiated by a nova outburst.

The multi-wavelength view of shocks in the fastest nova V1674 Her

“…The spin period change may be caused by some combination of magnetic coupling between the rotating white dwarf and the ejecta, non-rigid rotation or substantial radial expansion of the heated white dwarf. Following the spin-down associated with the eruption, a spin-up in the post-eruption phase is reported by (Patterson et al 2022) on the basis of optical photometry, while the presence of changes in the X-ray derived period deserves further investigation (Drake et al 2021;Orio et al 2022a).…”

“…A remarkable feature of V1674 Her is the emergence of orbital (3.67 h = 0.153 d; Schmidt et al 2021) and white dwarf spin periods (8.36 min = 0.00580 d; Patterson et al 2021) shortly after the eruption. The two periods are seen in X-rays in addition to optical data (Maccarone et al 2021;Pei et al 2021;Page et al 2022;Orio et al 2022a;Lin et al 2022).…”

“…We repeated the search restricting the analysis to the lowest-energy 3.0-3.5 keV events with the same null result. Finally, we compute the value for the spin and orbital periods reported by Patterson et al (2022) and find the associated single-trial probabilities to be ≫ 0.05 (no significant periodicity in NuSTAR data).…”

“…This variability is distinct from the overall optical brightness decline and is happening on a time-scale longer than the orbital period. The spin variations were first detected after the NuSTAR epoch (Patterson et al 2022). The physical origin of these irregular brightness variations is uncertain.…”

“…Finally, we tested for the presence of orbital and spin periodicities (Patterson et al 2022) in the -ray photon arrival times. For the test we selected 19 events within 5 • of V1674 Her (recorded during the time it has been detected by Fermi-LAT) and assigned to the nova with a probability of at least 68 per cent.…”