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

TL;DR

This paper proposes that the peculiar AXP 4U 0142+61 can be explained as a white dwarf merger remnant, accounting for its infrared excess, magnetic field, and other observed properties, offering an alternative to neutron star models.

Contribution

It demonstrates that a white dwarf merger scenario can explain the characteristics of AXP 4U 0142+61, including its infrared excess and magnetic field, which is a novel interpretation.

Findings

The AXP 4U 0142+61 can be modeled as a ~1.2 solar mass white dwarf remnant.

The merger scenario explains the infrared excess observed in the source.

The inferred magnetic field is approximately 2×10^8 G, consistent with observations.

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 (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 $1.2 M_{\sun}$ white dwarf, remnant of the coalescence of an original system made of two white dwarfs of masses $0.6\, M_{\sun}$ and $1.0\, M_{\sun}$. Finally, we infer a post-merging age $\tau_{\rm WD}\approx 64$ kyr, and a magnetic field $B\approx 2\times 10^8$ 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 $\approx 10^{15}$ Hz in the spectrum of 4U 0142+61.