  The energy source of the anomalous X-ray pulsars is not well understood,
hence their designation as anomalous. Unlike binary X-ray pulsars, no
companions are seen, so the energy cannot be supplied by accretion of matter
from a companion star. The loss of rotational energy, which powers radio
pulsars, is insufficient to power AXPs. Two models are generally considered:
accretion from a large disk left over from the birth process, or decay of a
very strong magnetic field (10^15 G) associated with a 'magnetar'. The lack of
counterparts at other wavelengths has hampered progress in our understanding of
these objects. Here, we present deep optical observations of the field around
4U 0142+61, which is the brightest AXP in X-rays. We find an object with
peculiar optical colours at the position of the X-ray source, and argue that it
is the optical counterpart. The optical emission is too faint to admit the
presence of a large accretion disk, but may be consistent with magnetospheric
emission from a magnetar.
