Spectral formation in a radiative shock: application to anomalous X-ray pulsars and soft gamma-ray repeaters

Abstract

Context. In the fallback disk model for the persistent emission of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), the hard X-ray emission arises from bulk-and thermal Comptonization of bremsstrahlung photons, which are generated in the accretion column. The relatively low X-ray luminosity of these sources implies a moderate transverse optical depth to electron scattering, with photons executing a small number of shock crossings before escaping sideways. Aims. We explore the range of spectral shapes that can be obtained with this model and characterize the most important parameter dependencies. Methods. We use a Monte Carlo code to study the crisscrossing of photons in a radiative shock in an accretion column and compute the resulting spectrum. Results. As expected, high-energy power-law X-ray spectra are produced in radiative shocks with photon-number spectral index Γ > ∼ 0.5. We find that the required transverse optical depth is 1 < ∼ τ ⊥ < ∼ 7. Such spectra are observed in low-luminosity X-ray pulsars. Conclusions. We demonstrate here with a simple model that Compton upscattering in the radiative shock in the accretion column can produce hard X-ray spectra similar to those seen in the persistent and transient emission of AXPs and SGRs. In particular, one can obtain a high-energy power-law spectrum, with photon-number spectral-index Γ ∼ 1 and a cutoff at 100−200 keV, with a transverse Thomson optical depth of ∼5, which is shown to be typical in AXPs/SGRs.

Magnetar Broadband X-Ray Spectra Correlated with Magnetic Fields: Suzaku Archive of SGRs and AXPs Combined with NuSTAR, Swift, and RXTE

“…In addition, our current observations of the HXC are limited up to ∼60 keV. The HXC cutoff is expected at ∼400 keV by the fossil disk model (Kylafis et al 2014), while the annihilation line is suggested in some magnetar models (Beloborodov 2013b). Thus, soft gamma-ray observation is expected to provide a smoking gun.…”

“…Canonical pulsars (dot symbols), X-ray Dim Isolated Neutron Stars (XDINS; triangles), and Rotating RAdio Transient (RRAT; inverted triangles) are shown for comparison. (Trümper et al 2010(Trümper et al , 2013Kylafis et al 2014).…”

Broad-band X-ray spectra of anomalous X-ray pulsars and soft γ-ray repeaters: pulsars in a weak-accretion regime?

“…The slope of the power law and the energy of the cutoff depend on the energy of the seed photons, the temperature of the electrons, the velocity of the accretion flow, and the optical depth in the accretion column, which determines the number of scatterings that a photon will undergo before emerging (e.g. BW07; Kylafis et al 2014). …”

“…The spectral model required to fit the data corresponds to the following physical picture (first discussed in Trümper et al 2010Trümper et al , 2013Kylafis et al 2014): photons produced in the accretion flow are up-scattered by the thermal as well as the bulk-motion Comptonization mechanisms by the infalling electrons. These upscattered photons comprise the power-law component of the observed spectrum.…”

“…The optical depth in the transverse direction is of the order of a few (e.g. BW07; Trümper et al 2013;Kylafis et al 2014), therefore the photons escaping from the sides of the accretion column have undergone several scatterings and they have increased their energy. This results in a hard power-law spectral component associated with the fan beam.…”