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

Abstract

Fe K$\alpha$ 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$\alpha$ 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$\alpha$ line complex is resolved into Fe K$\alpha_1$ at $E_{1} = 6.4040 \: \rm{keV}$ and Fe K$\alpha_2$ at $E_{2}= 6.3910 \:\rm{keV}$. Both lines have non-instrumental FWHM of $\approx 3 \:\rm{eV}$, 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 $v_{\rm{LSR}} = 50 \pm 12_{fit} \pm 14_{scale} \:\rm{km/s}$ 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$\alpha$ 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$^{\star}$, the required luminosity for an X-ray outburst about 200 years ago is $L_8 \approx 10^{38} \:\rm{erg/s}$ in an $8\:\rm{keV}$-wide band at $8\:\rm{keV}$.