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

Abstract

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 and anomalous X-ray pulsars (AXPs). Moreover, it has also been shown that high-field magnetic white dwarfs 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 tWD ˜ 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.