  Anomalous X-ray Pulsars (AXPs) belong to a class of neutron stars believed to
harbor the strongest magnetic fields in the universe, as indicated by their
energetic bursts and their rapid spindowns. However, an unambiguous measurement
of their surface field strengths has not been made to date. It is also not
known whether AXP outbursts result from changes in the neutron star magnetic
field or crust properties. Here we report a spectroscopic measurement of the
surface magnetic field strength of an AXP, XTE J1810$-$197, and solidify its
magnetar nature. The field strength obtained from detailed spectral analysis
and modeling, B = (2.72$\pm$0.03)$\times10^{14}$ G, is remarkably close to the
value inferred from the rate of spindown of this source and remains nearly
constant during numerous observations spanning over an order of magnitude in
source flux. The surface temperature, on the other hand, declines steadily and
dramatically following the 2003 outburst of this source. Our findings
demonstrate that heating occurs in the upper neutron star crust during an
outburst and sheds light on the transient behaviour of AXPs.
