Abstract
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 XMMNewton 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⊙, 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. Subject headings: pulsars: general — pulsars: individual (PSR J0437–4715) — stars: neutron — X-rays: stars — gravitation — relativity
Highlights
After four decades of considerable observational and theoretical investigation of neutron stars (NSs), very little is known about several key properties of these objects
We have presented a model of thermal emission from rotating NSs covered with an unmagnetized hydrogen atmosphere, applicable to systems for which the radiation is localized in small regions on the surface
The broad modulations and moderate pulsed fractions ( 50%) of the lightcurves produced by our model are in full agreement with the limits obtained for the thermal millisecond pulsar (MSP) in 47 Tuc (Cameron et al 2007)
Summary
After four decades of considerable observational and theoretical investigation of neutron stars (NSs), very little is known about several key properties of these objects. For a NS radiating from the entire surface, the gravitational redshift can be obtained by determining the effective emission radius, Reff This method assumes that the inferred radius is exactly equal to the radius at infinity, R∞ = (1 + zg)R, of the star. A promising approach is to study systems in which the thermal radiation is confined to a very small fraction of the stellar surface (Reff ≪ R) Such emission geometry is observed in rotation- and accretion-powered milisecond pulsars (MSPs) as well as old normal pulsars. In these objects, modeling the spectrum and rotation-induced variations of the observed flux can provide a measure of the compactness of the NS (see Pavlov & Zavlin 1997; Zavlin & Pavlov 1998).
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