Nordita Astrophysics seminars

Mass and radius constraints for millisecond pulsars

by Tuomo Salmi (University of Turku)

Measurements of neutron star masses and radii from observations can in principle help us to constrain microphysical properties of high-density matter. One way to obtain information on these parameters is to study pulse profiles produced by hotspots on the surface of rapidly spinning neutron stars (NS). We model the shape of the pulses using “oblate Schwarzschild” approximation, which takes into account the deformed shape of the star together with the special and general relativistic corrections to the photon trajectories and angles. I will present the method and the results of the Bayesian analysis obtained for SAX J1808.4−3658. They show that accurate constraints for the radius can be obtained if the mass is a priori known. Further improvements can be obtained when the X-ray polarization data from the Imaging X-ray Polarimeter Explorer (IXPE) will become available. In order to get tight and non-biased constraints for NS parameters, we also need a self-consistent model for the energy and angular spectrum of the emitted photons. I will therefore also discuss a new model for particle heated atmospheres for rotation-powered millisecond pulsars, where the polar caps of the NS are heated by magnetospheric return currents.