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

Kylafis, N. D. and Trümper, J. E. and Ertan, Ünal (2014) Spectral formation in a radiative shock: application to anomalous X-ray pulsars and soft gamma-ray repeaters. Astronomy and Astrophysics, 562 . ISSN 0004-6361 (Print) 1432-0746 (Online)

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Official URL: http://dx.doi.org/10.1051/0004-6361/201322303


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 Gamma greater than or similar to 0.5. We find that the required transverse optical depth is 1 less than or similar to tau(perpendicular to) less than or similar to 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 Gamma similar to 1 and a cutoff at 100-200 keV, with a transverse Thomson optical depth of similar to 5, which is shown to be typical in AXPs/SGRs.. P

Item Type:Article
Additional Information:Article Number: A62
Uncontrolled Keywords:accretion, accretion disks; stars: magnetars; X-rays: stars; pulsars: general
Subjects:Q Science > QB Astronomy
Q Science > QC Physics
ID Code:25037
Deposited By:Ünal Ertan
Deposited On:19 Dec 2014 21:55
Last Modified:19 Dec 2014 21:55

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