XMM-Newton observation of the ULIRG NGC 6240: The physical nature of the complex Fe K line emission

TL;DR

This study uses XMM-Newton observations to analyze the complex Fe K line emission in NGC 6240, revealing multiple plasma components, a neutral Fe K line, and a temperature and column density gradient towards the galaxy center.

Contribution

It provides the first detailed spectral decomposition of the Fe K line complex in NGC 6240, linking plasma components to specific emission lines and comparing physical conditions with a starburst galaxy.

Findings

Three plasma components with distinct temperatures identified.

The Fe K line complex is resolved into three narrow lines.

Temperature and column density increase towards the galaxy center.

Abstract

We report on an XMM-Newton observation of the ultraluminous infrared galaxy NGC 6240. The 0.3-10 keV spectrum can be successfully modelled with: (i) three collisionally ionized plasma components with temperatures of about 0.7, 1.4, and 5.5 keV; (ii) a highly absorbed direct power-law component; and (iii) a neutral Fe K_alpha and K_beta line. We detect a significant neutral column density gradient which is correlated with the temperature of the three plasma components. Combining the XMM-Newton spectral model with the high spatial resolution Chandra image we find that the temperatures and the column densities increase towards the center. With high significance, the Fe K line complex is resolved into three distinct narrow lines: (i) the neutral Fe K_alpha line at 6.4 keV; (ii) an ionized line at about 6.7 keV; and (iii) a higher ionized line at 7.0 keV (a blend of the Fe XXVI and the Fe K_beta line). While the neutral Fe K line is most probably due to reflection from optically thick material, the Fe XXV and Fe XXVI emission arises from the highest temperature ionized plasma component. We have compared the plasma parameters of the ultraluminous infrared galaxy NGC 6240 with those found in the local starburst galaxy NGC 253. We find a striking similarity in the plasma temperatures and column density gradients, suggesting a similar underlying physical process at work in both galaxies.