Multicomponent X‐Ray Emissions from Regions near or on the Pulsar Surface

Abstract

We present a model of X-ray emission from rotation-powered pulsars, which in general consist of one nonthermal component, two hard thermal components, and one soft thermal component. The nonthermal X-rays come from synchrotron radiation of e ± pairs created in the strong magnetic field near the neutron star surface by curvature photons emitted by charged particles on their way from the outer gap to the neutron star surface. The first hard thermal X-ray component results from polar-cap heating by the return current in the polar gap. The second hard thermal X-ray component results from polar-cap heating by the return particles from the outer gap. Because of cyclotron resonance scattering, most of the hard thermal X-rays will be effectively reflected back to the stellar surface and eventually reemitted as soft thermal X-rays. However, some of the hard thermal X-rays can still escape along the open magnetic field lines, where the e+/e− pair density is low. Furthermore, the characteristic blackbody temperatures of the two hard X-ray components emitted from the polar-cap area inside the polar gap and the polar-cap area defined by the footprints of the outer-gap magnetic field lines are strongly affected by the surface magnetic field, which can be much larger than the dipolar field. In fact, the strong surface magnetic field can explain why the effective blackbody radiation area is nearly 2 orders of magnitude larger than that deduced from the dipolar field for young pulsars (2 orders of magnitude less for old pulsars). Our model indicates how several possible X-ray components may be observed, depending on the magnetic inclination angle and viewing angle. Using the expected X-ray luminosity and spectra, we explain the observed X-ray spectra from pulsars such as Geminga, PSR B1055-52, PSR B0656+14, and PSR B1929+10.