A Puzzling Binary with a White Dwarf Pulsar Candidate

ZTF J185139.81+171430.3 is an optical source classified as a candidatewhite dwarf (WD) pulsar spinning with a 742 s period. The only otherknown object of this type is AR Scorpii: an intermediate polar system(magnetic WD + main sequence M star), whose binary interaction producespulsed emission seen from radio to X-rays. From short observations withSwift-XRT, we have found that J1851 is a bright X-ray source, with anX-ray luminosity a factor of 400 higher than that of AR Sco, and with asimilarly hard spectrum. In this proposed programme, we will study thespectral and timing properties of this unique object in order to probethe binary properties, the pulsar's interaction with its companion,and the emission mechanisms of this rare and not-well-understood classof objects, WD pulsars.