High and low states of the system AM Herculis

TL;DR

This study analyzes the historical high and low brightness states of AM Herculis, revealing a 63% high state duty cycle, no preferred timescales for state changes, and suggesting magnetic field variations influence state transitions.

Contribution

It provides the first comprehensive statistical analysis of AM Her's state duty cycles over a century, linking magnetic field dynamics to brightness state regulation.

Findings

High state duty cycle is approximately 63%.

No specific timescale preference for state transitions.

Magnetic field variations likely regulate mass transfer and brightness states.

Abstract

Context: We investigate the distribution of optically high and low states of the system AM Herculis (AM Her). Aims: We determine the state duty cycles, and their relationships with the mass transfer process and binary orbital evolution of the system. Methods: We make use of the photographic plate archive of the Harvard College Observatory between 1890 and 1953 and visual observations collected by the American Association of Variable Star Observers between 1978 and 2005. We determine the statistical probability of the two states, their distribution and recurrence behaviors. Results: We find that the fractional high state duty cycle of the system AM Her is 63%. The data show no preference of timescales on which high or low states occur. However, there appears to be a pattern of long and short duty cycle alternation, suggesting that the state transitions retain memories. We assess models for the high/low states for polars (AM Her type systems). We propose that the white-dwarf magnetic field plays a key role in regulating the mass transfer rate and hence the high/low brightness states, due to variations in the magnetic-field configuration in the system.