Asynchronous polar V1500 Cyg: orbital, spin and beat periods

TL;DR

This study presents a detailed analysis of the long-term post-eruption behavior of the asynchronous polar V1500 Cyg, revealing its orbital, spin, and beat periods, and tracking accretion and magnetic pole-switching phenomena over 40 years.

Contribution

It provides new measurements of the spin, orbital, and beat periods of V1500 Cyg, and demonstrates how accretion geometry and magnetic pole-switching evolve over decades post-eruption.

Findings

Measured the 9.6-day beat period indicating changing accretion geometry.

Detected a likely spin period 1.73 minutes shorter than the orbital period.

Observed phase jumps in light curves consistent with pole switching in an asynchronous polar.

Abstract

The bright Nova Cygni 1975 is a rare nova on a magnetic white dwarf (WD). Later it was found to be an asynchronous polar, now called V1500 Cyg. Our multisite photometric campaign occurring 40 years post eruption covered 26-nights (2015-2017). The reflection effect from the heated donor has decreased, but still dominates the op- tical radiation with an amplitude ~1^m.5. The 0^m.3 residual reveals cyclotron emission and ellipsoidal variations. Mean brightness modulation from night-to-night is used to measure the 9.6-d spin-orbit beat period that is due to changing accretion geometry including magnetic pole-switching of the flow. By subtracting the orbital and beat frequencies, spin-phase dependent light curves are obtained. The amplitude and profile of the WD spin light curves track the cyclotron emitting accretion regions on the WD and they vary systematically with beat phase. A weak intermittent signal at 0.137613-d is likely the spin period, which is 1.73(1) min shorter than the orbital period. The O-C diagram of light curve maxima displays phase jumps every one-half beat period, a characteristic of asynchronous polars. The first jump we interpret as pole switching between regions separated by 180 deg. Then the spot drifts during ~0.1 beat phase before undergoing a second phase jump between spots separated by less than 180 deg. We trace the cooling of the still hot WD as revealed by the irradiated companion. The post nova evolution and spin-orbit asynchronism of V1500 Cyg continues to be a powerful laboratory for accretion flows onto magnetic white dwarfs.