A NICER View of the Nearest and Brightest Millisecond Pulsar: PSR J0437−4715.*

Neutron stars provide a unique laboratory for probing the nature of ultra-dense matter in the universe. NICER, the Neutron Star Interior Composition Explorer is an X-ray telescope by NASA aboard the International Space Station designed for Pulse Profile Modeling (PPM) of rotation-powered Millisecond Pulsars (MSPs). Leveraging NICER’s megasecond exposures, large effective collecting area, and high time and energy resolution, PPM enables us to precisely measure the properties of these neutron stars, especially mass and radius, providing new insights into dense matter Equations of State (EoS). NICER has already successfully obtained radius constraints for two MSPs: PSR J0030+0451 and the massive pulsar PSR J0740+6620. In this talk, I will present our latest results on the nearest and brightest MSP, PSR J0437−4715, using the open-source software package X-PSI (X-ray Pulse Simulation and Inference; github.com/xpsi-group/xpsi). In combination with highly informative radio priors and advanced background modeling, for this 1.4 solar mass neutron star, we have inferred the tightest radius constraint obtained by NICER thus far. I will further discuss the implications of our findings for the neutron star EoS and the stellar magnetosphere.