The Nearest Millisecond Pulsar Revisited with XMM-Newton: Improved Mass-Radius Constraints for PSR J0437-4715. (arXiv:1211.6113v1 [astro-ph.HE])

The Nearest Millisecond Pulsar Revisited with XMM-Newton: Improved Mass-Radius Constraints for PSR J0437-4715. (arXiv:1211.6113v1 [astro-ph.HE]):
I present an analysis of the deepest X-ray exposure of a radio millisecond
pulsar (MSP) to date, an X-ray Multi Mirror-Newton European Photon Imaging
Camera spectroscopic and timing observation of the nearest known MSP, PSR
J0437--4715. The timing data clearly reveal a secondary broad X-ray pulse
offset from the main pulse by $\sim$0.55 in rotational phase. In the context of
a model of surface thermal emission from the hot polar caps of the neutron
star, this can be plausibly explained by a magnetic dipole field that is
significantly displaced from the stellar center. Such an offset, if commonplace
in MSPs, has important implications for studies of the pulsar population, high
energy pulsed emission, and the pulsar contribution to cosmic ray positrons.
The continuum emission shows evidence for at least three thermal components,
with the hottest radiation most likely originating from the hot magnetic polar
caps and the cooler emission from the bulk of the surface. I present pulse
phase-resolved X-ray spectroscopy of PSR J0437--4715, which for the first time,
properly accounts for the system geometry of a radio pulsar. Such an approach
is essential for unbiased measurements of the temperatures and emission areas
of polar cap radiation from pulsars. Detailed modelling of the thermal pulses,
including relativistic and atmospheric effects, provides a constraint on the
redshift-corrected neutron star radius of R>11.1 km (at 3 sigma conf.) for the
current radio timing mass measurement of 1.76 M_sun. This limit favors "stiff'"
equations of state.