  Dark matter substructure around nearby galaxies provides an interesting
opportunity for confusion-free indirect detection of dark matter. We calculate
the boost over a smooth background distribution of dark matter for gamma-ray
emission from dark matter self-annihilations in tidal structure in M31,
assuming a cross-section inversely proportional to the relative velocities of
the dark matter particles as proposed by the Sommerfeld effect. The low
velocity of the material in the structure results in a significant increase in
gamma-ray emission compared to both the background halo and the predicted
emission for a velocity-independent cross section. We also calculate the
expected signal for Fermi, for reasonable choices of the dark matter
parameters. We find that for a cross section proportional to the inverse-square
of the relative velocity, the enhancement to the annihilation rate is
sufficient to test the velocity dependence of the cross section by spatial
correlation with the stellar component of the stream, given sufficient detector
sensitivity.
