A NICER X-ray view of white dwarf pulsar AR Scorpii

AR Scorpii is one of the most peculiar magnetic CVs in our galaxy. Originally misclassified as delta-Scuti variable star with brightness fluctuations over a period of a few hours, the radio detection of pulsations on a two minute period and pulsed non-thermal emission uniquely identified AR Scorpii as the first white dwarf “pulsar”. AR Scorpii is now understood to be a magnetic white dwarf spinning on a two minute period (which is one of the shortest among CVs) in a close binary with an M-dwarf with an orbital period of 3.56 hours. The pulsed non-thermal emission, detected up to the UV band, has been attributed to synchrotron radiation from magnetic interactions between the white dwarf and its M-dwarf companion. A previous XMM observation detected pulsations up to 2 keV, but was not able to distinguish pulsed non-thermal emission and (more predominant) thermal emission in the X-ray band. In this work, we present the results of our recent NICER observations of AR Scorpii. Our timing analysis clearly detected both the spin and beat frequency signals up to 2 keV. We performed phase-resolved spectral analysis over various beat and orbital phases to elucidate the nature of the pulsed emission and its orbital dependence. We will discuss some theoretical implications based on the multi-wavelength results including NICER data analysis.