On the X‐Ray Baldwin Effect for Narrow Fe Kα Emission Lines

Abstract

Most active galactic nuclei (AGNs) exhibit a narrow Fe Kα line at ~6.4 keV in the X-ray spectra, due to the fluorescent emission from cold material far from the inner accretion disk. Using XMM-Newton observations, Page et al. found that the equivalent width (EW) of the narrow Fe Kα line decreases with increasing luminosity (EW ∝ L-0.17±0.08), suggesting a decrease in the covering factor of the material emitting the line (presumably the torus). By combining the archival Chandra HETG observations of 34 type 1 AGNs with XMM observations in the literature, we build a much larger sample with 101 AGNs. We find a similar X-ray Baldwin effect in the sample (EW ∝ L-0.2015±0.0426); however, we note that the anticorrelation is dominated by the radio-loud AGNs in the sample, whose X-ray spectra might be contaminated by the relativistic jet. Excluding the radio-loud AGNs, we find a much weaker anticorrelation (EW ∝ L-0.1019±0.0524). We present Monte Carlo simulations showing that such a weak anticorrelation can be attributed to the relative short timescale variations of the X-ray continuum.