We present a model of thermal X-ray emission from hot spots on the surface of a rotating compact star with an unmagnetized light-element atmosphere .
An application to ROSAT , Chandra , and XMM-Newton X-ray observations of the nearest known rotation-powered millisecond pulsar ( MSP ) PSR J0437–4715 reveals that the thermal emission from this pulsar is fully consistent with such a model , enabling constraints on important properties of the underlying neutron star .
We confirm that the observed thermal X-ray pulsations from J0437–4715 are incompatible with blackbody emission and require the presence of an optically thick , light element ( most likely hydrogen ) atmosphere on the neutron star surface .
The morphology of the X-ray pulse profile is consistent with a global dipole configuration of the pulsar magnetic field but suggests an off-center magnetic axis , with a displacement of 0.8 - 3 km from the stellar center .
For an assumed mass of 1.4 M _ { \odot } , the model restricts the allowed stellar radii to R = 6.8 - 13.8 km ( 90 % confidence ) and R > 6.7 km ( 99.9 % confidence ) , which is consistent with standard NS equations of state and rules out an ultracompact star smaller than its photon sphere .
Deeper spectroscopic and timing observations of this and other nearby radio MSPs with current and future X-ray facilities ( Constellation-X and XEUS ) can provide further insight into the fundamental properties of neutron stars .