  We present the first X-ray monitoring observations of the X-ray bright FRI
radio galaxy NGC6251 observed with RXTE for 1 year. The primary goal of this
study is to shed light on the origin of the X-rays, by investigating the
spectral variability with model-independent methods coupled with time-resolved
and flux-selected spectroscopy. The main results can be summarized as follows:
1) Throughout the monitoring campaign, NGC6251 was in relatively high-flux
state. 2) The flux persistently changed with fluctuations of the order of ~2 on
time scales of 20-30 days. 3) When the hardness ratio is plotted against the
average count rate, there is evidence for a spectral hardening as the source
brightens; this finding is confirmed by a flux-selected spectral analysis. 4)
The fractional variability appears to be more pronounced in the hard energy
band (5-12 keV) than in the soft one (2.5-5 keV). 5) 2-month averaged and
flux-limited energy spectra are adequately fitted by a power law. A Fe Kalpha
line is never statistically required, although the presence of a strong iron
line cannot be ruled out, due to the high upper limits on the line equivalent
width. The inconsistency of the spectral variability behavior of NGC6251 with
the typical trend observed in Seyfert galaxies and the similarity with blazars
lead support to a jet-dominated scenario during the RXTE monitoring campaign.
However, a possible contribution from a disk-corona system cannot be ruled out.
