X-ray and Optical Analysis of the Prototype Polar AN UMa

TL;DR

This study provides a comprehensive 34-year optical and X-ray analysis of the polar cataclysmic variable AN UMa, revealing accretion state transitions, pole switching, and long-term periodicities, enhancing understanding of magnetic accretion processes.

Contribution

It offers the first long-term multi-wavelength observational analysis of AN UMa, identifying accretion pole switching, a new long-term periodicity, and characterizing its low and high accretion states.

Findings

Detected pole switching between two accretion poles.

Identified a ~437-day periodicity related to accretion states.

Observed transitions between high and low accretion states.

Abstract

We present a long-term optical and X-ray photometric study of AN UMa, one of the prototypical polar-type cataclysmic variables, tracing more than 34 years of its accretion history. Observations from both ground-based and space-based facilities have been analysed to investigate state transitions within the system. Throughout this period, significant changes in the light curve have been observed, corresponding to different mass accretion states. From four years of TESS photometry, we derive a revised photometric period that agrees with the spectroscopic period to within 1.2$\sigma$. These optical observations further suggest switching between two accretion poles. During intervals of high accretion, dips in the X-ray light curve indicate that the primary accretion pole is obscured by an accretion stream elevated above the orbital plane, a feature also evident in the TESS light curves. Additionally, periodogram analysis reveals a periodicity of $\approx$437 days, which may be related to long-term accretion state changes. Following a 16-year high state, AN UMa entered two short-lived low states, lasting 180 and 123 days, during which it faded to a magnitude of 19.2, as recorded by the ZTF and ATLAS surveys. Using the system's low-state brightness and the distance provided by Gaia, we estimate that the system may have a white dwarf with an effective temperature of $\approx$15000~K, and a donor of spectral type M4.7. This work provides a useful reference for future studies of polars with variable accretion geometries and highlights the importance of long-term, multi-wavelength monitoring in the study of magnetic cataclysmic variables.