2026ApJ...997..166D

We present X-ray observations of the periodic optical source ZTF J185139.81+171430.3 (hereafter ZTF J1851) by the XMM-Newton, NICER, and NuSTAR telescopes. The source was initially speculated to be a white dwarf (WD) pulsar system, due to its short period (P ∼ 12 minutes) and highly modulated optical lightcurves. Our observations revealed a variable X-ray counterpart extending up to 40 keV with an X-ray luminosity of L_X ∼ 3 × 10³³ erg s^–1 (0.3-40 keV). Utilizing timing data from XMM-Newton and NICER, we detected a periodic signal at P_spin = 12.2640(7) ± 0.0583 minutes with >6σ significance. The pulsed profile displays ∼25% and ∼10% modulation in the 0.3-2 and 2-10 keV bands, respectively. Broadband X-ray spectra are best characterized by an absorbed optically thin thermal plasma model with kT ≈ 25 keV and a Fe K-α fluorescent line at 6.4 keV. The bright and hard X-ray emission rules out the possibility of a WD pulsar or ultra-compact X-ray binary. The high plasma temperature and Fe emission lines suggest that ZTF J1851 is an intermediate polar spinning at 12.264 minutes. We employed an X-ray spectral model composed of the accretion column emission and X-ray reflection to fit the broadband X-ray spectra. Assuming spin equilibrium between the WD and the inner accretion disk, we derived a WD mass range of M_WD = (1.07-1.32)M_⊙, exceeding the mean WD mass of IPs (〈M_WD〉 = 0.8M_⊙). Our findings illustrate that follow-up broadband X-ray observations could provide unique diagnostics to elucidate the nature of periodic optical sources anticipated to be detected in the upcoming Rubin all-sky optical surveys.