A BLR origin for the iron K$\alpha$ line in NGC7213

TL;DR

This study demonstrates that in NGC7213, the iron Kα line originates from the Broad Line Region, confirmed by high-resolution X-ray and optical observations, marking a unique case among Seyfert 1 galaxies.

Contribution

The paper provides the first clear evidence linking the iron Kα line in NGC7213 to the BLR, supported by simultaneous X-ray and optical data, and explores the nature of ionised iron lines and potential starburst wind signatures.

Findings

Iron Kα line is resolved with FWHM matching broad Hα component.

The iron Kα line originates in the BLR, not the torus or disc.

Ionised iron lines suggest collisionally ionised gas and possible starburst wind.

Abstract

The X-ray spectrum of NGC7213 is known to present no evidence for Compton reflection, a unique result among bright Seyfert 1s. The observed neutral iron K$\alpha$ line, therefore, cannot be associated with a Compton-thick material, like the disc or the torus, but is due to Compton-thin gas, with the Broad Line Region (BLR) as the most likely candidate. To check this hypothesis, a long Chandra HETG observation, together with a quasi-simultaneous optical spectroscopic observation at the ESO NTT EMMI were performed. We found that the iron line is resolved with a FWHM=$2 400^{+1 100}_{-600}$ km/s, in perfect agreement with the value measured for the broad component of the H$\alpha$, $2640^{+110}_{-90}$ km/s. Therefore, NGC7213 is the only Seyfert 1 galaxy whose iron K$\alpha$ line is unambiguously produced in the BLR. We also confirmed the presence of two ionised iron lines and studied them in greater detail than before. The resonant line is the dominant component in the Fe XXV triplet, therefore suggesting an origin in collisionally ionised gas. If this is the case, the blueshift of around 1000 km/s of the two ionised iron lines could be the first measure of the velocity of a starburst wind from its X-ray emission.