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

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.