24 - Polarized X-rays from magnetized neutron stars

Summary

We review the polarization properties of X-ray emission from highly magnetized neutron stars, focusing on emission from the stellar surfaces. We discuss how X-ray polarization can be used to constrain neutron star magnetic field and emission geometry, and to probe strong-field quantum electrodynamics and possibly constrain the properties of axions.

Introduction

One of the most important advances in neutron star (NS) astrophysics in the last decade has been the detection and detailed studies of surface (or near-surface) X-ray emission from a variety of isolated NSs. This has been made possible by X-ray telescopes such as Chandra and XMM-Newton. Such studies can potentially provide invaluable information on the physical properties and evolution of NSs (e.g. equation of state at super-nuclear densities, cooling history, surface magnetic fields and compositions, different NS populations). The inventory of isolated NSs with detected surface emission includes: (i) radio pulsars: e.g. the phase-resolved spectroscopic observations of the ‘three musketeers’ revealed the geometry of the NS polar caps; (ii) magnetars (AXPs and SGRs): e.g. the quiescent emission of magnetars consists of a black body at T ∼ 0.5 keV with a power-law component (index 2.7–3.5), plus significant emission up to ∼ 100 keV; (iii) central compact objects (CCOs) in SNRs: these now include six to eight sources, several have P, measurements and two have absorption lines; (iv) thermally-emitting isolated NSs: these are a group of seven nearby (≲1 kpc) NSs with low (∼10³² erg s⁻¹) X-ray luminosities and long (3–10 s) spin periods, and recent observations have revealed absorption features in many of the sources.