New constraints on neutron star cooling from Chandra observations of 3C 58

3C 58 is a young Crab-like supernova remnant. Historical evidence strongly suggests an association of the remnant with supernova SN 1181, which would make 3C 58 younger than the Crab Nebula. Recent Chandra observations have identified the young 65 ms pulsar J0205+6449 at its center, embedded in a compact nebula that, as we show here, appears to be confined by the pulsar wind termination shock. We present new Chandra observations of this compact nebula and the embedded pulsar that set strong upper limits on the thermal emission originating from the neutron star surface. These limits fall far below predictions of standard neutron star cooling, requiring the presence of exotic cooling processes in the neutron star core.