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

Ren Deng, Kaya Mori, Eric Miao, Gabriel Bridges, Charles J. Hailey, David A. H. Buckley, Gavin Ramsay, Dan Jarvis

TL;DR

This study presents broadband X-ray observations of ZTF J1851 with XMM-Newton, NICER, and NuSTAR, establishing the intrinsic WD spin period as $P_{ m spin}=12.2640$ min and a hard, reflection-augmented spectrum with $kT\approx25$ keV and a Fe K-$\alpha$ line, consistent with an intermediate polar. Using a self-consistent accretion-column model (MCVSPEC) under spin equilibrium, the WD mass is constrained to $M_{WD}=(1.07-1.32)M_{\odot}$ with a magnetic field of $B\sim20-40$ MG, indicating ZTF J1851 as a massive IP and among the heavier known CVs. The work emphasizes the diagnostic power of broadband X-ray timing and spectroscopy, including phase-resolved absorption, for identifying the nature of short-period optical variables expected from Rubin/LSST surveys. It also highlights the need for follow-up X-ray observations to exploit forthcoming optical survey discoveries for robust source classification and fundamental parameter measurements in magnetic CVs.

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 ($P\sim12$ 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 $L_X \sim 3\times10^{33}$ erg s$^{-1}$ (0.3--40 keV). Utilizing timing data from XMM and NICER, we detected a periodic signal at $P_{\rm spin}=12.2640(7)\pm0.0583$ min with $>6σ$ significance. The pulsed profile displays $\sim 25\%$ and $\sim10\%$ 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-$α$ 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 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 $M_{\rm WD}=(1.07\rm{-}1.32)M_{\odot}$ exceeding the mean WD mass of IPs ($\langle M_{\rm WD} \rangle = 0.8 M_\odot)$. 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.

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

TL;DR

This study presents broadband X-ray observations of ZTF J1851 with XMM-Newton, NICER, and NuSTAR, establishing the intrinsic WD spin period as min and a hard, reflection-augmented spectrum with keV and a Fe K- line, consistent with an intermediate polar. Using a self-consistent accretion-column model (MCVSPEC) under spin equilibrium, the WD mass is constrained to with a magnetic field of MG, indicating ZTF J1851 as a massive IP and among the heavier known CVs. The work emphasizes the diagnostic power of broadband X-ray timing and spectroscopy, including phase-resolved absorption, for identifying the nature of short-period optical variables expected from Rubin/LSST surveys. It also highlights the need for follow-up X-ray observations to exploit forthcoming optical survey discoveries for robust source classification and fundamental parameter measurements in magnetic CVs.

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 ( 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 erg s (0.3--40 keV). Utilizing timing data from XMM and NICER, we detected a periodic signal at min with significance. The pulsed profile displays and 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 keV and a Fe K- 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 exceeding the mean WD mass of IPs (. 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.

Paper Structure

This paper contains 14 sections, 9 figures.

Table of Contents

  1. Introduction
  2. X-ray Observations and Data Reduction
  3. Timing Analysis
  4. Periodicity Search
  5. Folded X-ray Lightcurves
  6. Spectral Analysis
  7. Phase-Averaged Spectral Analysis
  8. Phase-Resolved Spectral Analysis
  9. Discussion
  10. Source Identification
  11. White Dwarf Mass Determination
  12. X-ray Spectral Fitting Procedure
  13. Conclusion
  14. Acknowledgments

Figures (9)

  • Figure 1: Long-term optical lightcurve of ZTF J1851. Occasional day-long flares have been observed. The X-ray observation dates are marked by vertical lines.
  • Figure 2: PDS periodograms obtained from the combined MOS (left) and PN (right) cameras. Blue vertical line marks the known optical periodicity $P=12.37$ mins. Note that both axes are in log scale. The dashed and solid lines denote the $3\sigma$ and $5\sigma$ significance, respectively.
  • Figure 3: $Z^2_2$ soft X-ray (0.3-2 keV) periodograms. Left: combined MOS cameras. Right: PN camera. The red vertical line marks the most significant peak. The dashed and solid lines denote the $3\sigma$ and $5\sigma$ significance, respectively.
  • Figure 4: Folded lightcurve plots in the soft (0.3--4 keV for XMM-Newton and 1--4 keV for NICER ; red) and hard (4--10 keV; purple) X-ray band, with 1-$\sigma$ error bars shown (left: NICER middle: XMM-MOS; right: XMM-PN). The bottom three figures show the combined folded lightcurves including photon energy up to $10$ keV. All light curves are folded over the $P=12.26$ min period, phase-aligned, and contain 20 bins per phase.
  • Figure 5: The hardness ratios (black) overlaid with the $0.3-10$ keV (left: XMM-MOS) and $1-10$ keV (right: NICER) folded lightcurves, showing an anti-correlation. Both are folded at the $P=12.26$ min period. Hardness ratio is defined as $\frac{\rm{hard}-\rm{soft}}{\rm{hard}+\rm{soft}}$, where the soft and hard energy bands are defined as $E<4$ keV and $E>4$ keV.
  • ...and 4 more figures