On the X-ray Baldwin Effect in Active Galactic Nuclei Observed by the Chandra High Energy Grating

TL;DR

This study uses high-resolution Chandra observations of 32 AGNs to investigate the X-ray Baldwin effect, revealing that rapid continuum variations significantly influence the observed anti-correlation between Fe Kα line EW and X-ray luminosity, and that the line flux likely originates far from the nucleus.

Contribution

It provides the first observational evidence that the Fe Kα line flux does not respond to continuum changes, supporting a distant origin, and demonstrates the impact of rapid variability on the X-ray Baldwin effect.

Findings

Strong anti-correlation between EW and Lx normalized to average values

Fe Kα line flux remains constant despite continuum variability

Continuum variation alone can statistically produce the XBE

Abstract

Using Chandra High Energy Grating (HEG) observations of 32 AGNs, we present a systematic study of the X-ray Baldwin effect (XBE, i.e. the anti-correlation between narrow Fe K{\alpha} line EW and X-ray continuum luminosity for AGN samples) with the highest spectral resolution currently available. We have previously reported an anti-correlation with EW\proptoL(2-10 keV)^-0.22 in a HEG sample, and the correlation is much weaker after averaging multiple observations of individual AGNs (EW\proptoL(2-10 keV)^-0.13). This indicates that rapid variation in X-ray continuum plays an important role in producing the XBE, and such an effect should also be visible in individual AGNs. In this paper, by normalizing the line EWs and continuum luminosities to the time-averaged values for each AGN in our sample with multiple HEG observations, we find a strong anti-correlation between EW and Lx (EW/<EW>\propto (L/<L>)^-0.82+/-0.10), consistent with the XBE expected in an individual AGN if the narrow line flux remains constant while the continuum varies. This is first observational evidence that the Fe K{\alpha} line flux in a large sample of AGNs lacks of a corresponding response to the continuum variation, supporting that the narrow Fe-K line emission originates from a region far from the nucleus. We then performed Monte-Carlo simulations to address whether the global XBE can be produced by X-ray continuum variation solely, and found that such interpretation of XBE cannot be ruled out statistically. One thus should be very cautious before reaching any scientific conclusion based on an observed XBE.