  X-rays from massive stars are ubiquitous yet not clearly understood. In an
XMM-Newton observation devoted to observe the first site of star formation in
the $\rho$ Ophiuchi dark cloud, we detect smoothly variable X-ray emission from
the B2IV+B2V system of $\rho$ Ophiuchi. Tentatively we assign the emission to
the primary component. The light curve of the pn camera shows a first phase of
low, almost steady rate, then a rise phase of duration of 10 ks, followed by a
high rate phase. The variability is seen primarily in the band 1.0-8.0 keV
while little variability is detected below 1 keV. The spectral analysis of the
three phases reveals the presence of a hot component at 3.0 keV that adds up to
two relatively cold components at 0.9 keV and 2.2 keV. We explain the smooth
variability with the emergence of an extended active region on the surface of
the primary star due to its fast rotation (v $sin~i \sim315$ km/s). We estimate
that the region has diameter in the range $0.5-0.6$ R$_*$. The hard X-ray
emission and its variability hint a magnetic origin, as suggested for few other
late-O$-$early-B type stars. We also discuss an alternative explanation based
on the emergence from occultation of a young (5-10 Myr) low mass companion
bright and hot in X-rays.
