MASS-RADIUS AND EQUATION OF STATE CONSTRAINTS FROM X-RAY OBSERVATIONS OF MILLISECOND PULSARS
March 30, 2010
Abstract: Millisecond pulsars are rapidly rotating neutron stars where general relativity plays a strong role in the propagation of light from the neutron star to observer and hence on observed X-ray pulse shapes. Here, calculations are described for the modeling of such pulse shapes. These include General Relativity calculations for the neutron star metric and for geodesics for photons in that metric. They also include time-delays and Doppler effects for photons and oblateness of the neutron star surface, which results from the rapid rotation. Frame dragging is also calculated and shown to be not important for the observed rotation periods. These new calculations enable determination of constraints on neutron star mass and radius from the observed pulse shapes. The modeling methods have been applied to Rossi X-Ray Timing Explorer observations of the millisecond pulsar SAX J1808-3658, including data from the 1998, 2002 and 2005 outbursts of SAX J1808-3658. Since the observed pulse shape varies greatly and it is a challenging test of the model to fit the different observations. A consistent neutron star model can describe all the observations only if the model includes an extra component, which is scattered light. New mass and radius constraints are given for SAX J1808-3658, as well as the consequent nuclear equation of state constraints.