[XeTeX] Cyrillic characters in math environment

[LRN]

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I'm using XeLaTeX to typeset some stuff, and i can't figure out how to
get cyrillic chars to appear in formulas.

This is the minimal example of what i'm trying to do:

\documentclass{article}
\usepackage{amsmath}
\usepackage{unicode-math}
\usepackage{fontspec}
\usepackage{polyglossia}
\setdefaultlanguage{russian}
\setmainfont{FreeSerif}


\begin{document}
Значение \(X_ж = 0\)
\end{document}

In the output X doesn't have a subscript.

If i remove unicode-math and enclose "ж" in \mathrm{}, then it does
show up, but it's not italic the way latin characters are. I could use
\it to make it italic, but that just feels wrong, and makes math even
more unreadable.

Maybe i simply don't have a font that is both appropriate for use in
math environment AND features cyrillic glyphs?
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