  We have measured an angle-dependent contribution to the equilibrium
magnetization of a YBa_2Cu_3O_{7-d} single crystal with columnar defects
created by 5.8GeV Pb ions. This contribution manifests itself as a jump in the
equilibrium torque signal, when the magnetic field crosses that of the defects.
The magnitude of the signal, which is observed in a narrow temperature interval
less than 2K and for fields up to about twice the irradiation field, is used to
estimate the the energy gained by vortex pinning on the defects. The vanishing
of the effective pinning energy at a temperature below Tc is attributed to its
renormalization by thermal fluctuations.
