A Reflection Model for the Cyclotron Lines in the Spectra of X-Ray Pulsars

Abstract

Cyclotron resonance scattering features observed in the spectra of some X-ray pulsars show significant changes of the line energy with the pulsar luminosity. At high luminosities, these variations are often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and of the cyclotron lines. However, this scenario inevitably implies a large gradient of the magnetic field strength within the line-forming region, which makes the formation of the observed line-like features problematic. Moreover, the observed variation of the cyclotron line energy is much smaller than could be anticipated for the corresponding luminosity changes. We argue here that a more physically realistic situation is that the cyclotron line forms when the radiation emitted by the accretion column is reflected from the neutron star surface, where the gradient of the magnetic field strength is significantly smaller. Here we develop a reflection model and apply it to explain the observed variations of the cyclotron line energy in a bright X-ray pulsar V 0332+53 over a wide range of luminosities.

Transient X-ray pulsar V 0332+53: pulse-phase-resolved spectroscopy and the reflection model

“…We now will try to describe qualitatively the observed pulse-phase changes of the cyclotron line parameters using the recently proposed reflection model (Poutanen et al 2013). According to the widely accepted paradigm (Basko & Sunyaev 1976), bright X-ray pulsars have relatively tall (comparable with the neutron star radius) accretion columns at both magnetic poles.…”

“…This behaviour was interpreted as a result of changes in the height of the accretion column where the cyclotron absorption line was assumed to originate (see, e.g., Mihara 1995;Schönherr et al 2007;Nishimura 2008Nishimura , 2014Becker et al 2012, and references therein). However, alternative scenario has recently been proposed by Poutanen et al (2013), where the line forms in the illuminated atmosphere of the neutron star. In our paper, we focus on the latter model when discussing observed spectral variations.…”

On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass–radius relation

“…The radiation of the accretion column is beamed towards the NS surface due to photon scattering by fast electrons at the edge of accretion channel (Kaminker et al 1976;Lyubarskii & Syunyaev 1988). Thus, the NS intercepts a significant fraction of the total luminosity of the accretion column (Poutanen et al 2013). The fraction of the intercepted radiation is even higher if one takes into account effects of GR (light bending).…”

“…For given mass accretion rate, τ ff ∝ 1/l0. Large optical thickness of the free-falling region results in strong beaming of the X-ray radiation from accretion column walls (Kaminker et al 1976;Lyubarskii & Syunyaev 1988;Poutanen et al 2013). …”

“…16b, Lutovinov et al 2015). The cyclotron scattering feature forms at the NS surface due to reflection of the X-ray flux from the stellar atmosphere (Poutanen et al 2013) and varies with accretion luminosity and pulse phase. The maximum value of cyclotron centroid energy is detected when the NS magnetic pole is oriented towards the observer, because in this case the reprocessed flux is dominated by photons originating from regions close to the magnetic poles, where the surface B-field is stronger.…”