  To a symmetric, relatively ample line bundle on an abelian scheme one can
associate a linear combination of the determinant bundle and the relative
canonical bundle, which is a torsion element in the Picard group of the base.
We improve the bound on the order of this element found by Faltings and Chai.
In particular, we obtain an optimal bound when the degree of the line bundle d
is odd and the set of residue characteristics of the base does not intersect
the set of primes p dividing d, such that $p\equiv -1\mod(4)$ and p<2g, where g
is the relative dimension of the abelian scheme. Also, we show that in some
cases these torsion elements generate the entire torsion subgroup in the Picard
group of the corresponding moduli stack.
