XMM-Newton observation of the long-period polar V1309 Orionis: the case for pure blobby accretion

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: rschwarz@aip.de
² Universitätssternwarte Göttingen, Geismarlandstraße 11, 37083 Göttingen, Germany
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85740 Garching, Germany

Received: 2 June 2005
Accepted: 4 July 2005

Abstract


Using XMM-Newton we have obtained the first continuous X-ray observation covering a complete orbit of the longest period polar, V1309 Ori. The X-ray light curve is dominated by a short, bright phase interval with EPIC pn count rates reaching up to 15 cts s^-1 per 30 s resolution bin. The bright phase emission is well described by a single blackbody component with eV. The absence of a bremsstrahlung component at photon energies above 1 keV yields a flux ratio . This represents the most extreme case of a soft X-ray excess yet observed in an AM Herculis star. The bright, soft X-ray emission is subdivided into a series of individual flare events supporting the hypothesis that the soft X-ray excess in V1309 Ori is caused by accretion of dense blobs carrying the energy into sub-photospheric layers. On average, the flares have rise and fall times of 10 s. In addition to the bright phase emission, a faint, hard X-ray component is visible throughout the binary orbit with an almost constant count rate of 0.01 cts s^-1. Spectral modelling indicates that this emission originates from a complex multi-temperature plasma. At least three components of an optically thin plasma with temperatures , 0.7, and 2.9 keV are required to fit the observed flux distribution. The faint phase emission is occulted during the optical eclipse. Eclipse ingress lasts about 15–20 min and is substantially prolonged beyond nominal ingress of the white dwarf. This and the comparatively low plasma temperature provide strong evidence that the faint-phase emission is not thermal bremsstrahlung from a post-shock accretion column above the white dwarf. A large fraction of the faint-phase emission is ascribed to the spectral component with the lowest temperature and could be explained by scattering of photons from the blackbody component in the infalling material above the accretion region. The remaining hard X-ray flux could be produced in the coupling region, so far unseen in other AM Herculis systems.