Periodic Variations in the Residual Eclipse Flux and Eclipse Timings of Asynchronous Polar V1432 Aql: Evidence of a Shifting Threading Region

TL;DR

This study of V1432 Aql over 28 months reveals cyclical variations in eclipse flux and timings linked to a shifting threading region, supporting a model where the accretion stream's threading radius varies and affecting the white dwarf's spin evolution.

Contribution

The paper introduces a new model showing the threading region varies along the accretion stream, explaining observed flux and timing variations in an asynchronous polar.

Findings

Eclipse flux and timings vary with the spin-orbit beat cycle.

The threading radius is variable, contrary to previous assumptions.

A strong photometric maximum correlates with a possible third polecap.

Abstract

We report the results of a twenty-eight-month photometric campaign studying V1432 Aql, the only known eclipsing, asynchronous polar. Our data show that both the residual eclipse flux and eclipse O-C timings vary strongly as a function of the spin-orbit beat period. Relying upon a new model of the system, we show that cyclical changes in the location of the threading region along the ballistic trajectory of the accretion stream could produce both effects. This model predicts that the threading radius is variable, in contrast to previous studies which have assumed a constant threading radius. Additionally, we identify a very strong photometric maximum which is only visible for half of the beat cycle. The exact cause of this maximum is unclear, but we consider the possibility that it is the optical counterpart of the third accreting polecap proposed by Rana et al. (2005). Finally, the rate of change of the white dwarf's spin period is consistent with it being proportional to the difference between the spin and orbital periods, implying that the spin period is approaching the orbital period asymptotically.