Long-Term X-Ray Monitoring of NGC 6251: Evidence for a Jet-dominated Radio Galaxy

We present the first X-ray monitoring observations of the X-ray-bright FR I radio galaxy NGC 6251 observed with RXTE for 1 yr. 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, NGC 6251 was in relatively high-flux state with an average 2-10 keV absorbed flux of the order of 4.5 × 10^-12 erg cm^-2 s^-1 and a corresponding intrinsic luminosity of 6 × 10⁴² erg s^-1. (2) The flux persistently changed with fluctuations of the order of ~2 on timescales 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) Two month averaged and flux-limited energy spectra are adequately fitted by a power law. A Fe Kα 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 NGC 6251 with the typical trend observed in Seyfert galaxies and the similarity with blazars lend support to a jet-dominated scenario during the RXTE monitoring campaign. However, a possible contribution from a disk-corona system cannot be ruled out.