Let $X$ be a scheme over a field $k$, $G$ a finite abelian group of size invertible on $X$. Suppose $K/k$ is a Galois field extension and let $Y\to X\_K$ be an étale $G$-torsor.



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> For what field extensions $K/k$ does any such $Y$ descend to $k$? I.e.: for what field extensions $K/k$ does there exist $Y\_0\to X$ an étale $G$-torsor, such that $(Y\_0)\_K\cong Y$ and $(Y\_0)\_K\to X\_K$ is the map $Y\to X\_K$?
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> Equivalently, for what field extensions $K/k$ is the map $H^1(X,G)\to H^1(X\_K, G)$ surjective?
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For example, if $k$ is algebraically closed, for any extension $K/k$ of algebraically closed fields the map is an isomorphism, by the smooth base change theorem.


**Example** Let $k$ be the maximal unramified extension of $\mathbf{Q}\_p$. Does any finite field extension $K/k$ satisfy this property?



Descent of étale torsors
Let $L$ be a finite separable extension of $k$. Let $X$ be the Weil restriction from $L$ to $k$ of $\mathbb G\_m$. Then for any field extension $K$ of $k$ (including $k$ itself),if $L \otimes\_k K$ is a product of $n$ distinct fields, then $H^1(X\_K, \mathbb Z/2) / H^1(K, \mathbb Z/2) = (\mathbb Z/2)^n$. This can be calculated by examining the Galois action on $H^1(X\_{\overline{K}}, \mathbb Z/2)$. 


Hence this descent only holds if $L \otimes\_k K$ is a field, so that $n(K)= n(k)=1$. In particular, we must have this for all extensions $L$. 


So the extension $K/k$ must be entirely inseparable and transcendental.


