Effects of the Geometry of the Line‐forming Region on the Properties of Cyclotron Resonant Scattering Lines

Abstract

We use a Monte Carlo radiative transfer code to examine the dependence of the properties of cyclotron resonant scattering lines on the spatial geometry and the optical depth of the line-forming region. We focus most of our attention on a line-forming region that is a plane-parallel slab. We also consider a cylindrical line-forming region. In both cases, the line-forming region contains an electron-proton plasma at the equilibrium Compton temperature, T c , and is threaded with a uniform magnetic field with strength ∼ 10 12 gauss. We consider geometries in which the photon source illuminates the line-forming region from below, and in which the photon source is embedded in the line-forming region.The former may correspond to a line-forming region in the magnetosphere of a neutron star, illuminated from below; the latter to a line-forming region on or near the surface of a neutron star as in an accretion column.In the case of the plane-parallel slab line-forming region, we study the behavior of the resonant Compton temperature and the properties of the cyclotron scattering lines as a function of the column depth of the line-forming region and the orientation of the magnetic field. At small or moderate optical depths (N e ≪ 10 25 cm −2 [electron scattering optical depth τ T o ≪ 10]), the resonant Compton temperature ranges from T e /E B ≈ 0.2 when the magnetic field is perpendicular to the slab to T e /E B ≈ 0.4 when the field is parallel to the slab. At large optical depths (N e ∼ > 1.5 × 10 25 cm −2 [τ T o ∼ > 10]), the resonant Compton temperature is higher.At small or moderate optical depths (τ T o ≪ 10), the equivalent widths, W E , of the spectral lines depend on two effects. First, W E increases as the viewing angle with respect to the field decreases, due to Doppler broadening.Second, W E increases as the viewing angle with respect to the slab normal

Study of the accreting pulsar 4U 0115+63 using a bulk and thermal Comptonization model

“…The relation depends on the optical depth in the absorption lines, the angle that the radiation forms with the magnetic field lines, and the system geometry (e.g. Isenberg et al 1998, and references therein). In a slab geometry and for radiation injected parallel to the magnetic field, the cyclotron energy should be more than two times the plasma temperature.…”

Confirmation of two cyclotron lines in Vela X-1

“…Both radiative transfer and radiation hydrodynamics have to be taken into account at the same time (Isenberg et al 1998). Although this problem has been investigated for many years, there still exists no convincing theoretical model for the continuum of accreting X-ray pulsars (Harding 1994, and references therein).…”

“…Because the emission is highly aniostropic, the spectra observed during different pulse phases vary dramatically. This is especially true for CRSFs whose characteristics depend strongly on the viewing angle with respect to the magnetic field (for an in-depth discussion see Araya-Góchez & Harding 2000;Araya & Harding 1999;Isenberg et al 1998, and references therein). This dependence At lower energies (below 6 keV), the pulse profile is very complex and shows a five peaked structure: two peaks in the main pulse and three in the secondary pulse.…”

Simulation of cyclotron resonant scattering features

“…Monte Carlo simulations of CRSFs provide a straight forward way to obtain the exact line shape of this complex fundamental cyclotron line and a reasonable amount of harmonics with computationally handable effort. Such Monte Carlo simulations have been performed by a variety of authors ( [3], [4], [5]). Here we discuss the influence of the electrons' bulk velocity on the line shape.…”