NICER observation list: Target ID 559401

We propose to observe a white dwarf (WD) pulsar candidate ZTF J185139.81+171430.3 with NICER and NuSTAR. The first WD pulsar system, AR Sco, is one of the rare astrophysical manifestations on how a magnetically interacting binary system can accelerate particles in the magnetosphere and generate broad-band non-thermal synchrotron radiation from radio to X-ray band. ZTF observations revealed a 12.37-minute highly-modulated, coherent periodicity from our target. This is a unique feature previously detected from AR Sco. Alternatively, the source could be an AM CVn star or an accreting intermediate polar with unusually high optical modulation. The proposed NICER + NuSTAR observations will explore the nature of this peculiar CV by investigating its X-ray spectral and timing properties.