X-ray emission and optical polarization of V1432 aquilae: an asynchronous polar

Abstract

A detailed analysis of X-ray data obtained with ROSAT, ASCA, XMM-Newton, and the Rossi X-ray Timing Explorer (RXTE) for the asynchronous polar V1432 Aquilae is presented. An analysis of Stokes polarimetry data obtained from the South African Astronomical Observatory (SAAO) is also presented. Power spectra from long-baseline ROSAT data show a spin period of 12,150 s along with several frequency components related to the source. However, the second harmonic of the spin period dominates the power spectrum in the XMM-Newton data. For the optical circular polarization, the dominant period corresponds to half the spin period (or its first harmonic). The ROSAT data can be explained as due to accretion onto two hot spots that are not antipodal. The variations seen in the optical polarization and the ASCA and XMM-Newton X-ray data suggest the presence of at least three accretion footprints on the surface of the white dwarf. Two spectral models, a multitemperature plasma model and a photoionized plasma model, are used to understand the spectral properties of V1432 Aql. The data from the RXTE Proportional Counter Array (PCA) with its extended high-energy response are used to constrain the white dwarf mass to 1.2±0.1 M_⊙ using a multitemperature plasma model. The data from the European Photon Imaging Camera (EPIC) on-board XMM-Newton are well fitted by both models. A strong soft X-ray excess (< 0.8 keV) is well modeled by a blackbody component having a temperature of 80-90 eV. The plasma emission lines seen at 6.7 and 7.0 keV are well fitted using the multitemperature plasma model. However, the fluorescent line at 6.4 keV from cold Fe requires an additional Gaussian component. The multitemperature plasma model requires two absorbers: one that covers the source homogeneously and another partial absorber covering ~65% of the source. The photoionized plasma model, with a range of column densities for the Fe ions, gives a slightly better overall fit and fits all emission line features. The intensity and spectral modulations due to the rotation of the white dwarf at a period of 12,150 s require varying absorber densities and a varying covering fraction of the absorber for the multitemperature plasma model. The presence of a strong blackbody component, a rotation period of 12,150 s, modulation of the Fe fluorescence line flux with 12,150 s period, and a very hard X-ray component suggest that V1432 Aql is an unusual polar with X-ray spectral properties similar to that of a soft intermediate polar.