The Astrophysical Journal · 2013 · 54 citations · 64 references
We propose that the quiescent emission of AXPs/SGRs is powered by accretion\nfrom a fallback disk, requiring magnetic dipole fields in the range\n10^{12}-10^{13} G, and that the luminous hard tails of their X-ray spectra are\nproduced by bulk-motion Comptonization in the radiative shock near the bottom\nof the accretion column. This radiation escapes as a fan beam, which is partly\nabsorbed by the polar cap photosphere, heating it up to relatively high\ntemperatures. The scattered component and the thermal emission from the polar\ncap form a polar beam. We test our model on the well-studied AXP 4U 0142+61,\nwhose energy-dependent pulse profiles show double peaks, which we ascribe to\nthe fan and polar beams. The temperature of the photosphere (kT~0.4 keV) is\nexplained by the heating effect. The scattered part forms a hard component in\nthe polar beam. We suggest that the observed high temperatures of the polar\ncaps of AXPs/SGRs, compared with other young neutron stars, are due to the\nheating by the fan beam. Using beaming functions for the fan beam and the polar\nbeam and taking gravitational bending into account, we fit the energy-dependent\npulse profiles and obtain the inclination angle and the angle between the spin\naxis and the magnetic dipole axis, as well as the height of the radiative shock\nabove the stellar surface. We do not explain the high luminosity bursts, which\nmay be produced by the classical magnetar mechanism operating in super-strong\nmultipole fields.\n
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