  Middle-aged, cooling neutron stars are observed both as relatively rapidly
spinning radio pulsars and as more slowly spinning, strongly magnetized
isolated neutron stars (INSs), which stand out by their thermal X-ray spectra.
The difference between the two classes may be that the INSs initially had much
stronger magnetic fields, which decayed. To test this, we used the Chandra
X-ray Observatory to observe 1RXS J072559.8-261229, a possible X-ray
counterpart to PSR J0726-2612, which, with its 3.44s period and 3e13G inferred
magnetic field strength, is the nearest and least extincted among the possible
slowly-spinning, strong-field INS progenitors (it likely is in the Gould Belt,
at ~1 kpc). We confirm the identification and find that the pulsar has a
spectrum consistent with being purely thermal, with blackbody temperature
kT=87+/-5 eV and radius R=5.7+2.6-1.3 km at a distance of 1 kpc. We detect
sinusoidal pulsations at twice the radio period with a semi-amplitude of
27\pm5%. The properties of PSR J0726-2612 strongly resemble those of the INSs,
except for its much shorter characteristic age of 200 kyr (instead of several
Myr). We conclude that PSR J0726-2612 is indeed an example of a young INS, one
that started with a magnetic field strength on the low end of those inferred
for the INSs, and that, therefore, decayed by a relatively small amount. Our
results suggest that the long-period, strong-field pulsars and the INSs are
members of the same class, and open up new opportunities to understand the
puzzling X-ray and optical emission of the INSs through radio observations of
PSR J0726-2612.
