Transient two pole accretion in the polar V496 UMa

TL;DR

This study uses XMM-Newton and TESS data to analyze the complex, transient accretion behavior onto the magnetic poles of the white dwarf in the polar V496 UMa, revealing erratic secondary pole accretion and stable primary pole emission.

Contribution

It provides the first detailed observation of transient two-pole accretion and its variability in V496 UMa, linking accretion disruptions to mass-transfer variations.

Findings

Secondary pole accretion can nearly cease within less than one orbit.

X-ray spectral modeling shows no change in primary accretion structures during secondary disruption.

TESS data reveals stable double-humped modulation with a slow drift in maximum light timings.

Abstract

We report XMM-Newton and TESS observations of V496 UMa, an AM Herculis-type cataclysmic variable. The XMM-Newton observation reveals that at times, two poles on the white dwarf accrete simultaneously, but accretion onto the secondary magnetic pole is erratic and can nearly cease in less than one binary orbit (1.5 h). Modelling of the X-ray spectrum during the primary maximum reveals no change in the accretion structures onto the primary pole when accretion onto the secondary pole is disrupted, suggesting that the disruption of accretion onto the secondary pole may be caused by mass-transfer variations from the donor star. The TESS observation, which spanned eight weeks at a two-minute cadence, shows a stable, double-humped orbital modulation due to cyclotron emission from the post-shock region, while the observed times of maximum light show a slow systematic drift that does not correlate with the system's overall brightness.