  Let $K$ be a compact convex body in $\mathbb R^n.$ For any affine line $L,$
denote $\widehat{\chi}_K(L)=\int_{L}\chi_K(x)dl(x),$ where $dl$ is the arc
length measure, the $X$-ray transform of the characteristic function $\chi_K,$
i.e., the length of the chord $K \cap L.$ We prove that if $K$ is bounded by a
$C^{\infty}$ real algebraic hypersurface $\partial K$ and the $X$-ray transform
$\widehat{\chi}_K(L)$ behaves, under small parallel translations of the line
$L$ to the distance $t,$ as the $m$-th root of a polynomial of $t$, for some
fixed $m \in \mathbb N,$ then $\partial K$ is an ellipsoid.
