Resolving the Fe K α Doublet of the Galactic Center Molecular Cloud G0.11-0.11 with XRISM

Abstract Fe K α line emission from Galactic center molecular clouds can be produced either via fluorescence after illumination by an X-ray source or by cosmic-ray ionization. Unparalleled high-resolution X-ray spectroscopy obtained by XRISM/Resolve for the Galactic center molecular cloud G0.11-0.11 resolves its Fe K α line complex for the first time, and points to a new method for discrimination between the X-ray reflection and cosmic-ray ionization models. The Fe K α line complex is resolved into Fe K α 1 at E 1 = 6.4040 keV and Fe K α 2 at E 2 = 6.3910 keV. Both lines have noninstrumental FWHM of ≈3 eV, close to the predicted quantum mechanical width of the lines, suggesting scant other sources of line broadening other than instrumental and quantum effects. We measure a radial velocity of v LSR = 50 ± 12 fit ± 14 scale km s −1 for G0.11-0.11, achieving the same precision reached by radio observations of such clouds. The high-resolution spectrum tests for the presence of secondary Fe K α lines, expected as a signature of cosmic-ray proton/ion ionization. The absence of the secondary lines argues against the cosmic-ray ionization model for G0.11-0.11. In the preferred X-ray reflection model, if the illuminating source is Sgr A ⋆ , the required luminosity for an X-ray outburst about 200 yr ago is L 8 ≈ 10 38 erg s −1 in an 8 keV wide band at 8 keV.