A white dwarf merger as progenitor of the anomalous X-ray pulsar 4U 0142+61?

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It has been recently proposed that massive fast-rotating highly-magnetized white dwarfs could describe the observational properties of some of Soft Gamma-Ray Repeaters (SGRs) and Anomalous X-Ray Pulsars (AXPs). Moreover, it has also been shown that high-field magnetic (HFMWDs) can be the outcome of white dwarf binary mergers. The products of these mergers consist of a hot central white dwarf surrounded by a rapidly rotating disk. Here we show that the merger of a double degenerate system can explain the characteristics of the peculiar AXP 4U 0142+61. This scenario accounts for the observed infrared excess. We also show that the observed properties of 4U 0142+6 are consistent with an approximately 1.2 M white dwarf, remnant of the coalescence of an original system made of two white dwarfs of masses 0.6 M and 1.0 M. Finally, we infer a post-merging age τ WD≈ 64 kyr, and a magnetic field B≈ 2× 10⁸ G. Evidence for such a magnetic field may come from the possible detection of the electron cyclotron absorption feature observed between the B and V bands at ≈ 10¹⁵ Hz in the spectrum of 4U 0142+61.