V902 Monocerotis: a likely disc-accreting intermediate polar

Abstract

Aims: We aim to confirm whether the eclipsing cataclysmic variable V902 Mon is an Intermediate Polar, to characterise its X-ray spectrum and flux, and to refine its orbital ephemeris and spin period. Methods: We performed spectrographic observations of V902 Mon in 2016 with the 2.2m Calar Alto telescope, and X-ray photometry and spectroscopy with XMM-Newton in October 2017. This data was supplemented by several years of AAVSO visual photometry. Results: We have confirmed V902 Mon as an IP based on detecting the spin period, with a value of 2,208s, at multiple epochs. Spectroscopy of the donor star and Gaia parallax yield a distance of 3.5+1.3-0.9, kpc, suggesting an X-ray luminosity one or two orders of magnitude lower than the 10^33 erg/s typical of previously known IPs. The X-ray to optical flux ratio is also very low. The inclination of the system is more than 79deg, with a most likely value of around 82deg. We have refined the eclipse ephemeris, stable over 14,000 cycles. The Halpha line is present throughout the orbital cycle and is clearly present during eclipse, suggesting an origin distant from the white dwarf, and shows radial velocity variations at the orbital period. The amplitude and overall recessional velocity seem inconsistent with an origin in the disc. The \emph{XMM-Newton} observation reveals a partially absorbed plasma model typical of magnetic CVs, with a fluorescent iron line at 6.4keV showing a large equivalent width of 1.4keV. Conclusions: V902 Mon is an IP, and probably a member of the hypothesized X-ray underluminous class of IPs. It is likely to be a disc accretor, though the radial velocity behaviour of the Halpha line remains puzzling. The large equivalent width of the fluorescent iron line, the small FX/Fopt ratio, and the only marginal detection of X-ray eclipses suggests that the X-ray emission arises from scattering.

arXiv comment10 pages, 12 figures

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