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X-ray observations of shocked gas in novae can provide a useful probe of thedynamics of the ejecta. Here we report on X-ray observations of the nova V959Mon, which was also detected in GeV gamma-rays with the Fermi satellite. Wefind that the X-ray spectra are consistent with a two-temperature plasma modelwith non-solar abundances. We interpret the X-rays as due to shock interactionbetween the slow equatorial torus and the fast polar outflow that were inferredfrom radio observations of V959 Mon. We further propose that the hottercomponent, responsible for most of the flux, is from the reverse shock driveninto the fast outflow. We find a systematic drop in the column density of theabsorber between Days 60 and 140, consistent with the expectations for such apicture. We present intriguing evidence for a delay of around 40 days in theexpulsion of the ejecta from the central binary. Moreover, we infer arelatively small (a few times 10$^{-6}$ Msun) ejecta mass ahead of the shock,considerably lower than the mass of 10$^4$ K gas inferred from radioobservations. Finally, we infer that the dominant X-ray shock was likely notradiative at the time of our observations, and that the shock power wasconsiderably higher than the observed X-ray luminosity. It is unclear why highX-ray luminosity, closer to the inferred shock power, is never seen in novae atearly times, when the shock is expected to have high enough density to beradiative.

X-ray evolution of the nova V959 Mon suggests a delayed ejection and a non-radiative shock