The energy spectrum of anomalous X-ray pulsars and soft gamma-ray repeaters

Trümper, J. E. and Zezas, A. and Ertan, Ünal and Kylafis, N. D. (2010) The energy spectrum of anomalous X-ray pulsars and soft gamma-ray repeaters. Astronomy and Astrophysics, 518 . ISSN 0004-6361

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Official URL: http://dx.doi.org/10.1051/0004-6361/200911834


Context. Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) exhibit characteristic X-ray luminosities (both soft and hard) of around 1035 erg s-1 and characteristic power-law, hard X-ray spectra extending to about 200 keV. Two AXPs also exhibit pulsed radio emission. Aims: Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt to explain both the soft and the hard X-ray emission as the result of the accretion process. We also attempt to explain their radio emission or the lack of it. Methods: We test the hypothesis that the power-law, hard X-ray spectra are produced in the accretion flow mainly by bulk-motion Comptonization of soft photons emitted at the neutron star surface. Fallback disk models invoke surface dipole magnetic fields of 1012 - 1013 G, which is what we assume here. Results: Unlike normal X-ray pulsars, for which the accretion rate is highly super-Eddington, the accretion rate is approximately Eddington in AXPs and SGRs and thus the bulk-motion Comptonization operates efficiently. As an illustrative example we reproduce both the hard and the soft X-ray spectra of AXP 4U 0142+61 well using the XSPEC package compTB. Conclusions: Our model seems to explain both the hard and the soft X-ray spectra of AXPs and SGRs, as well as their radio emission or the lack of it, in a natural way. It might also explain the short bursts observed in these sources. On the other hand, it cannot explain the giant X-ray outbursts observed in SGRs, which may result from the conversion of magnetic energy in local multipole fields.

Item Type:Article
Uncontrolled Keywords:pulsars: individual: 1E 1841–045 / pulsars: individual: 1RXS J1708–4009 / pulsars: individual: 4U 0142+61 / X-rays: stars / stars: magnetic fields
Subjects:Q Science > QB Astronomy > QB460-466 Astrophysics
Q Science > QC Physics
ID Code:14354
Deposited By:Ünal Ertan
Deposited On:15 Sep 2010 09:39
Last Modified:25 Jul 2019 14:06

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