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 ) \times 10 ^ { 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 .