Long-Term X-Ray monitoring of NGC6251: Evidence for a jet-dominated radio galaxy

Abstract

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.