An X-ray view of the Cataclysmic Variable V902 Mon: Discovery of an X-ray eclipse

Nazma Islam, Koji Mukai

First listed 2025-02-14 | Last updated 2025-02-14

Abstract

V902 Mon is one of a few eclipsing Intermediate Polars (IPs), and show deep eclipses in the optical lightcurves. The presence of a strong Fe K$α$ fluorescence line in its X-ray spectrum and its low X-ray flux compared to other IPs suggests significant absorption, most likely from an accretion disk. In an observation carried out using the Nuclear Spectroscopic Telescope Array (NuSTAR), we confirm the presence of an X-ray eclipse in the energy resolved lightcurves, coincident with the optical AAVSO/CV-band lightcurves. Broadband X-ray spectral analysis using NuSTAR and XMM-Newton observations confirm a strong absorption N$_{H}$ $\sim 10^{23}$ cm$^{-2}$ local to the source, along with a high equivalent width of about 0.7 keV for a Fe K$α$ fluorescence line. We interpret this using a model similar to an Accretion Disk Corona source, which have a very high inclination and the compact object is heavily obscured by the body of the accretion disk. We propose that the primary X-rays from the accretion column in V902 Mon is hidden from our direct view at all times by the accretion disk. In this scenario, the observed scattered X-rays indicate substantial absorption of direct X-rays by the accretion disk. Additionally, a strong Fe fluorescence line suggests reprocessing of the radiation by a more extended region, such as the pre-shock region, which could be located a few white dwarf radii above the orbital plane.

Short digest

NuSTAR timing of the eclipsing IP V902 Mon reveals a clear X‑ray eclipse in energy‑resolved lightcurves, phase‑coincident with AAVSO/CV optical minima. Joint NuSTAR+XMM‑Newton spectroscopy requires heavy local absorption (N_H ~1e23 cm^-2) and a strong 6.4 keV Fe Kα line with EW ≈0.7 keV. The authors favor an accretion‑disk‑corona–like, very high‑inclination geometry in which the direct accretion‑column emission is blocked at all phases and we see only scattered/reprocessed X‑rays. This positions V902 Mon as a reflection‑dominated IP where eclipse timing can localize the extended pre‑shock region a few WD radii above the plane.

Key figures to inspect

• Figure 1: Inspect the NuSTAR 3–25, 3–10, and 10–25 keV lightcurves (binned at the 2208 s spin period) to see repeated eclipses aligned with the predicted ephemeris and to gauge any energy dependence of eclipse depth and the absence/presence of coherent spin pulses.
• Figure 2: Compare phase‑folded AAVSO/CV, NuSTAR (3–10, 10–25 keV), and XMM/PN (0.3–10 keV) profiles to confirm co‑phasing of the X‑ray and optical minima, measure the eclipse width/asymmetry, and test whether harder photons are less deeply eclipsed (scattering vs absorption).
• Figure 3: From the joint spectral fit, read off the required local N_H ~1e23 cm^-2 and the Fe Kα EW ≈0.7 keV; check residuals around 6–7 keV and the overall faint, reflection/scattering‑dominated continuum compared to typical IPs.
• Figure 4: Use the schematic to internalize the ADC‑like geometry—WD and primary column hidden by the thick disk at all phases, with scattered/reprocessed emission from a pre‑shock region a few WD radii above the orbital plane being eclipsed by the secondary.