The hard X-ray view of the low-luminosity blazar in the radio galaxy NGC6251

Abstract

We present results from a BeppoSAX (July 2001) observation of the FRI radio galaxy NGC6251, together with a re-analysis of archival ASCA (October 1994) and Chandra (September 2000) data. The weak detection above 10 keV and the lack of iron fluorescent K-alpha emission lines in the BeppoSAX spectrum rule out that the bulk of the X-ray emission is due to an obscured Seyfert nucleus. The study of the multiwavelength spectral energy distribution suggests instead that X-rays probably originate as inverse-Compton of synchrotron seed photons in a relativistic jet, indicating that NGC6251 hosts a low radio luminosity [L(5 GHz) ~10^40 erg/s] blazar. The BeppoSAX spectrum is flatter than in the earlier ASCA observation. This might be due to the emergence of a different spectral component during phases of lower X-ray flux. In this context, we discuss some possible explanations for the intense and mildly-ionized fluorescent iron line measured by ASCA.