Volume 562, February 2014
|Number of page(s)||8|
|Published online||05 February 2014|
Spectral formation in a radiative shock: application to anomalous X-ray pulsars and soft gamma-ray repeaters
1 University of Crete, Physics Department & Institute of Theoretical & Computational Physics, 71003 Heraklion, Crete, Greece
2 Foundation for Research and Technology-Hellas, 71110 Heraklion, Crete, Greece
3 Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
4 Faculty of Engineering and Natural Sciences, Sabancı University, 34956 Orhanlı, Tuzla, İstanbul, Turkey
Received: 17 July 2013
Accepted: 21 December 2013
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.
Key words: accretion, accretion disks / stars: magnetars / X-rays: stars / pulsars: general
© ESO, 2014
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.