Broadband X-ray observations of the periodic optical source ZTF J185139.81+171430.3 and its identification as a massive intermediate polar

Abstract

We present X-ray observations of the periodic optical source ZTF J185139.81+171430.3 (hereafter ZTF J1851) by the XMM, NICER and NuSTAR telescopes. The source was initially speculated to be a white dwarf (WD) pulsar system due to its short period ($P\sim 12$ min) and highly-modulated optical lightcurves. Our observations revealed a variable X-ray counterpart extending up to 40 keV with an X-ray luminosity of $L_X\sim 3\times 10^{33}$ erg s${}^{-1}$ (0.3--40 keV). Utilizing timing data from XMM and NICER, we detected a periodic signal at $P_{\mathrm{spin}}=12.2640(7)\pm 0.0583$ min with $>6\ddot{I}ƒ$ significance. The pulsed profile displays $\sim 25\%$ and $\sim 10\%$ 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 $kT\approx 25$ keV and a Fe K-$\hat{I}\pm$ 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 min. 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 $M_{\mathrm{WD}}=(1.07-1.32){\mathrm{M}}_{\odot }$ exceeding the mean WD mass of IPs ($⟨M_{\mathrm{WD}}⟩=0.8M_{\odot })$. 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.

13 pages, 9 figures, submitted to ApJ