XMM-Newton and TESS observations of the highly variable polar V496 UMa

TL;DR

This study analyzes the spectral and timing behavior of the highly variable polar V496 UMa using XMM-Newton and TESS data, revealing a two-pole accretion geometry, variable accretion states, and detailed X-ray and optical characteristics.

Contribution

It provides the first combined spectral and timing analysis of V496 UMa across optical and X-ray regimes, uncovering its accretion geometry and variability patterns.

Findings

V496 UMa shows a double-humped light curve with a 91-minute period.

The system exhibits high and low accretion states with distinct X-ray and optical features.

Accretion onto the second pole is intermittently interrupted, especially in the high state.

Abstract

We aim to study the temporal and spectral behaviour of \vum from the optical to the X-ray regimes. We used archival \xmmn and \tes observations obtained in 2017 and 2019 to perform a spectral and timing analysis of the highly variable polar. The light curves of both satellites, TESS and XMM-Newton, reveal a double-humped pattern modulated with the periodicity of $91.058467 \pm 0.00001$ minutes. V496 UMa displays a two-pole accretion geometry in the high accretion state. X-ray spectra from both regions are composed of thermal plasma radiation and soft blackbody components with almost identical temperatures and a total accretion rate of $\dot{M}=1.4(8)\times10^{-11}$ M$_{\odot}$ yr$^{-1}$. The X-ray centers of the humps show longitudinal shifts of -18 and 4 degrees and -172 and -186 degrees size around photometric phase zero, for the main hump and second hump, respectively. The long-term ZTF light curves reveal high and low accretions states. Low-state ZTF and SDSS photometric data are consistent with an $0.8$ M$_{\odot}$ white dwarf at 10000 K and a main-sequence donor star with a spectral type of M5.0 at a \gai-determined distance of 758 pc. V496 UMa is a very bright polar in X-rays when it is in the high state. Due to its unusual geometric structure, mass accretion onto the second accretion pole is interrupted occasionally. This discontinuous behavior does not follow a certain pattern in time and is observed so far only in the high state. The X-ray light curves display clear evidence for an accretion stream at the photometric phase of $\phi=$0.81, which does not show up in optical light curves. An accurate period was derived using the combined TESS and XMM-Newton data, which differs by 3.8 $\sigma$ from published results.