  Let $C$ be a smooth projective curve of genus 2 over a number field $k$ with
a rational point. We prove that the index and exponent coincide for elements in
the 2-torsion of $\Sha(Br(C))$.
  In the appendix, an isomorphism of the moduli space of rank 2 stable vector
bundles with odd determinant on a smooth projective hyperelliptic curve $C$ of
genus $g$ with a rational point over any field of characteristic not two with
the Grassmannian of $(g-1)$-dimensional linear subspaces in the base locus of a
certain pencil of quadrics is established, making a result of (\cite{De-Ra})
rational. We establish a twisted version of this isomorphism and we derive as a
consequence a weak Hasse principle for the smooth intersection $X$ of two
quadrics in ${\mathbb P}^5$ over a number field: if $X$ contains a line
locally, then $X$ has a $k$-rational point.
