Iron K photons from weakly magnetized neutron stars in X-ray binaries

Hot plasmas that produce continuum X-rays from X-ray binaries are found to produce negligible amounts of iron K photons. The reason is that because of intense X-ray fluxes inside the continuum sources, virtually all iron atoms are fully stripped. In contrast, the atmosphere of the neutron star in an X-ray binary might be an important source of iron K photons (epsilonapprox. =6.5 keV), because it is bombarded by a large number of hard X-rays capable of photo-ejecting K-shell electrons from iron atoms. If the continuum source is located in the 105--106 cm range above the neutron star surface, the equivalent width of the iron line blend due to the neutron star can be approx.300 eV. If the magnetic field at the surface is < or approx. =2 x 1011 gauss, then the energy shift due to the Zeeman effect is small enough so that the observed iron line features of X-ray binaries may be accounted for by fluorescent iron K photons from the neutron star surface. If fluorescent K photons produced by the neutron star surface are dominant over iron K photons produced by surrounding materials, or if the energy shift due to the Zeeman effect and the gravitational redshift is adequate for distinguishing the former from the latter, one can get information about the magnetic field strength and the gravitational potential at the neutron star surface and the direction of the magnetic dipole axis. If the height of the continuum source is less than 105 cm, backscattered continuum X-rays might play an important role in determining the pulse-phase dependences of the flux and spectrum of the X-ray continuum