HP Cet and Swift J0820.6-2805: two candidate intermediate polars observed by XMM-Newton
Abstract
We report on the XMM-Newton observation of HP Cet and Swift J0820.6-2805, two X-ray photon sources that are candidates to be members of the intermediate polar class of cataclysmic variables. If the historical optical light curve of HP Cet shows a periodic feature at 96 minutes, a clear identification of such a signature in the high energy band (apart for a variability on a time scale of 8 minutes as detected by the ROSAT satellite) is lacking. By using XMM-Newton archive data, we clearly identify a feature (at 88 minutes) which is marginally consistent with one of the binary system orbital periods reported in the literature. We also found a signature of a periodic features on the time scale of 5.6 minutes. In the case of Swift J0820.6-2805, the intermediate polar nature was previously unclear and the orbital and the white dwarf spin periods were unknown. Here, the 0.3-10 keV data undoubtedly reveal an orbital period and a white dwarf spin of 87.5 minutes and 27.9 minutes, respectively. The spectral analysis showed that both HP Cet and Swift J0820.6-280 are members of the under-luminous IP subclass since their luminosity in the 0.3-10 keV band is estimated to be 5× 1030 erg s-1 and 3.8× 1029 erg s-1, respectively.