X-ray and gamma-ray periodic analysis of the fast rotating, highly magnetic white dwarf EUVE J0317-85.5

Abstract

We report possible X-ray and 𝛾-ray pulsations from the fast rotating, highly magnetic white dwarf EUVE J0317-85.5. Pulsed modulations were found at periods P=724.648±0.001s (-log(P𝑟 )=5.60) at a significance of ∼4.59𝜎 and P=361.685±0.001s (-log(P𝑟 )=5.19) at ∼4.18𝜎 in the Fermi-LAT𝛾-ray data set. These periods are close to, but significantly different from the spin period and half the spin period of the white dwarf, respectively. Although these 𝛾-ray periods statistically exclude for instance, the optical periods (see Table 1), the folded 𝛾-ray light curve and folded optical light curves are in agreement which could possibly suggest that the optical and 𝛾-rays are related. Possible pulsed X-ray emission was found at a period of P=362.82±0.04s (FAP=3.28×10−5) in the ROSAT X-ray data and at a period P∼725.72s but at a low significance (FAP∼0.60). The P∼362.8s X-ray period is close to, but significantly different from the first harmonic of the spin period which was also observed in recent optical observations (see Table 1). These results may suggest that this white dwarf display pulsar-like properties with emission of radiation from its polar cap regions. The pulsed 𝛾-ray emission is most likely produced by electrons moving along the magnetic field lines emitting curvature radiation, while the X-rays are possibly produced by pair-produced particles flowing back towards and heating the polar cap regions of the white dwarf.