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

TL;DR

High-resolution XRISM spectroscopy of the Galactic center cloud G0.11-0.11 distinguishes between X-ray reflection and cosmic ray ionization models by resolving the Fe Kα line complex and detecting the absence of secondary lines.

Contribution

First high-resolution spectral resolution of the Fe Kα complex in G0.11-0.11, enabling discrimination between ionization mechanisms and precise velocity measurement.

Findings

Fe Kα lines are narrow, close to quantum width, indicating minimal broadening.

No secondary Fe Kα lines detected, arguing against cosmic ray ionization.

X-ray reflection model suggests a past outburst from Sgr A* about 200 years ago.

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 $8\:\rm{keV}$.