Discovery of the K-shell Lines of Neutral Ar, Ca, Cr, and Mn Atoms from the Galactic Center Region

Abstract

One of the remarkable features of the X‐ray emission from the Galactic center (GC) region is a Kα line at 6.4 keV from an Fe atom. Although some scenarios, such as photo‐ionization by external X‐rays (XRN) or collisional ionization by low energy cosmic‐ray electrons (LECRe) have been proposed, the origin of the 6.4 keV line is still an open question. We analyzed X‐ray data obtained by the X‐ray satellite Suzaku and discovered K‐shell lines of neutral Ar, Ca, Cr, and Mn atoms in addition to Fe and Ni. To explain the observed equivalent widths of the neutral lines, the elemental abundances are required to be ∼1.6 and ∼4 solar in the XRN and LECRe scenarios. On the other hand, the abundances of the various elements in the GC hot plasma were 1–2 solar. Ourresult suggests that the origin of the neutral K‐shell lines is the external X‐rays.