HP Cet and Swift J0820.6-2805: two candidate intermediate polars observed by XMM-Newton

TL;DR

This study uses XMM-Newton data to identify and analyze periodic signals in two candidate intermediate polars, HP Cet and Swift J0820.6-2805, revealing their orbital and white dwarf spin periods and classifying them as under-luminous IPs.

Contribution

First X-ray detection of orbital and spin periods in these candidates, confirming their intermediate polar nature and characterizing their luminosity class.

Findings

Identified ~88-minute orbital period in HP Cet.

Detected ~5.6-minute variability in HP Cet.

Measured orbital and spin periods in Swift J0820.6-2805.

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 $\simeq 96$ minutes, a clear identification of such a signature in the high energy band (apart for a variability on a time scale of $\simeq 8$ minutes as detected by the ROSAT satellite) is lacking. By using XMM-Newton archive data, we clearly identify a feature (at $\simeq 88$ 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 $\simeq 5.6$ 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 $\simeq 87.5$ minutes and $\simeq 27.9$ 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 $\simeq 5\times 10^{30}$ erg s$^{-1}$ and $\simeq 3.8\times 10^{29}$ erg s$^{-1}$, respectively.