  We conducted an X-ray spectroscopic study of the classical nova V2491 Cygni
using our target-of-opportunity observation data with the Suzaku and XMM-Newton
satellites as well as archived data with the Swift satellite. Medium-resolution
(R~10-50) spectra were obtained using the X-ray CCD spectrometers at several
post-nova epochs on days 9, 29, 40, 50, and 60-150 in addition to a pre-nova
interval between days -322 and -100 all relative to the time when the classical
nova was spotted. We found remarkable changes in the time series of the
spectra: (a) In the pre-nova phase and on day 9, the 6.7 keV emission line from
Fe XXV was significantly detected. (b) On day 29, no such emission line was
found. (c) On day 40, the 6.7 keV emission line emerged again. (d) On days 50
and 60-150, three emission lines at 6.4, 6.7, and 7.0 keV respectively from
quasi-neutral Fe, Fe XXV, and Fe XXVI were found. Statistically significant
changes of the Fe K line intensities were confirmed between day 29 and 50.
Based on these phenomena, we conclude that (1) the post-nova evolution can be
divided into two different phases, (2) ejecta is responsible for the X-ray
emission in the earlier phase, while rekindled accretion is for the later
phase, and (3) the accretion process is considered to be reestablished as early
as day 50 when the quasi-neutral Fe emission line emerged, which is a common
signature of accretion from magnetic cataclysmic variables.
