SWIFT J0503.7-2819: a nearly synchronous intermediate polar below the period gap?

Abstract

Based on the X-ray observations from XMM-Newton and Swift, and optical observations from Transiting Exoplanet Survey Satellite (TESS) and AAVSO, we present temporal and spectral properties of probable intermediate polar SWIFT J0503.7-2819. The X-ray light curve shows two distinctive features, where possibly the second pole seems to be active during the middle of the XMM-Newton observations. Present analysis confirms and also refines the previously reported orbital period of SWIFT J0503.7-2819 as 81.65 ± 0.04 min. The X-ray and optical variations of this target have been found to occur at the period of ~65 min, which we propose as the spin period of the white dwarf. The energy-dependent modulation at this period, which are due to the photoelectric absorption in the accretion flow, also assures this conjecture. Two-temperature thermal plasma model well explains the X-ray spectra with temperatures of ~150 eV and ~18.5 keV, which is absorbed by a dense material with an average equivalent hydrogen column density of 3.8 × 10²² cm^-2 that partially covers ~27 per cent of the X-ray source. An attempt is made to understand the accretion flow in this system using the present data of SWIFT J0503.7-2819. If the proposed spin period is indeed the actual period, then SWIFT J0503.7-2819 could be the first nearly synchronous intermediate polar below the period gap.