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We report non-detections of the $\sim 3\times 10^8$ yr old, slow, isolated,rotation-powered pulsar PSR J2144$-$3933 in observations with the Hubble SpaceTelescope in one optical band (F475X) and two far-ultraviolet bands (F125LP andF140LP), yielding upper bounds $F_{\rm F475X}< 22.7$ nJy, $F_{\rm F125LP}< 5.9$nJy, $F_{\rm F140LP}< 19.5$ nJy, at the pivot wavelengths 4940 \AA, 1438 \AA\and 1528 \AA, respectively. Assuming a blackbody spectrum, we deduce aconservative upper bound on the surface (unredshifted) temperature of thepulsar of $T<42,000$ K. This makes PSR~J2144--3933 the coldest known neutronstar, allowing us to study thermal evolution models of old neutron stars. Thistemperature is consistent with models with either direct or modified Urcareactions including rotochemical heating, and, considering frictional heatingfrom the motion of neutron vortex lines, it puts an upper bound on the excessangular momentum in the neutron superfluid, $J<10^{44}\,\mathrm{erg\,s}$.

Hubble Space Telescope non-detection of PSR J2144-3933: the coldest known neutron star