XMM-Newton observations of the highest field polar AR UMa. An extreme case of soft-blobby accretion

The X-ray spectra of magnetic cataclysmic variables (MCVs) with extreme magnetic fields, the polars, often exhibit a prominent soft X-ray excess attributed to buried shocks. Using the first-ever high-state observations of the highest field polar, AR UMa (B≈230 MG), we established the source's accretion scenario and derived the energy budget at the magnetic extreme of known MCVs. XMM-Newton Using X-ray, ultraviolet, and optical spectral and timing analysis, we disentangled the radiation components in the system and located their origin in the binary. AR UMa showed one active (``southern'') pole during our observation. The spectrum of the active pole was completely dominated by soft, blackbody-like emission with kT_ ̊m BB ≃ 35,eV and showed no evidence for an optically thin hard X-ray emission characteristic of a cooling post-shock accretion column. Instead, we find constant, faint, optically thin plasma emission with a temperature of ≈ 1 keV at all orbital phases, which is uncorrelated with the soft X-ray emission, and we attribute this to shock-heated matter at the stagnation region between the two stars. Pronounced absorption dips punctuate the phase of active X-ray emission and they are associated with puffed-up material in the stagnation region. Using spectro-timing analysis of the absorption dips, the density profile and evolution of the clumpy flow were probed. We find evidence of a cyclotron harmonic from the southern pole (160 MG) near ∼2300Å in the UV Grism spectrum. We also report minute-long quasi-periodic oscillations in the optical, UV, and X-ray light curves. AR UMa represents an extreme case of soft, blobby accretion, with an effectively infinite soft excess, extending the magnetic field range over which magnetic accretion can be studied by a factor of two. It is one of the rare systems in which X-ray emission from a stream–magnetosphere interaction is detected.