SGR 0418+5729, Swift J1822.3-1606, and 1E 2259+586 as massive, fast-rotating, highly magnetized white dwarfs

Aims. We describe the so-called low magnetic field magnetars, SGR 0418+5729, Swift J1822.3-1606, and the AXP prototype 1E 2259+586 as massive, fast-rotating, highly magnetized white dwarfs. Methods. We give bounds for the mass, radius, moment of inertia, and magnetic field for these sources by requesting the stability of realistic, general relativistic, uniformly rotating white dwarfs. We also present the theoretical expectation of the infrared, optical, and ultraviolet emission of these objects and show their consistency with the current available observational data. Results. We improve the theoretical prediction of the lower limit of the spindown rate of SGR 0418+5729; for a white dwarf close to its maximum stable mass we obtain the very stringent interval for the spindown rate of 4.1 x 10(-16) < (P) over dot < 6 x 10(-15), where the upper value is the known observational limit. A lower limit has been also set for Swift J1822.3-1606, whose spindown rate is not yet fully confirmed. Our model provides for the source (P) over dot = 2.13x10(-15) if the star is close to its maximum stable mass. We give in addition the frequencies at which absorption features could be present in the spectrum of these sources as the result of the scattering of photons with the quantized electrons by the surface magnetic field.