  We present results of a comprehensive analysis of the X-ray pulsar X Persei
over the period 1996 to 2011, encompassing the quite low state and subsequent
strong outburst activity. Using data from the RXTE and Swift observatories we
detected several consecutive outbursts, during which the source luminosity
increased by a factor of ~5 up to L_x~1.2x10^{35} erg/s. Previously the source
was observed in a high state only once. The source spectrum in a standard
energy band (4-25 keV) is independent of the flux change and can be described
by a model that includes both thermal and non-thermal components. With the help
of the INTEGRAL observatory data we registered the highly significant cyclotron
absorption line in the source spectrum and, for the first time, significantly
detected a hard X-ray emission from the pulsar up to ~160 keV. We also report
drastic changes of the pulse period during the outburst activity: a long
episode of spin-down was changed to spin-up with a rate of Pdot/P =
-(3-5)x10^{-4} yr^{-1}, that is several times higher than previous rates of
spin-up and spin-down. To search for a correlation between the X-ray and
optical lightcurves we took data from the AAVSO International Database. No
significant correlation between optical and X-ray fluxes at any time lag from
dozens of days to years was found.
