Accreting magnetars: a new type of high-mass X-ray binaries?

TL;DR

This paper presents evidence supporting the existence of accreting magnetars in high-mass X-ray binaries, based on XMM-Newton observations of 4U 2206+54, its spin evolution, and spectral analysis indicating a strong magnetic field and weak local matter.

Contribution

It provides the first detailed spectral and timing analysis of 4U 2206+54, supporting its classification as an accreting magnetar with unique properties.

Findings

Spin-down rate consistent with magnetar scenario

No definitive cyclotron line detected at ~30 keV

Detection of weak 6.5 keV iron fluorescence lines

Abstract

The discovery of very slow pulsations (Pspin=5560s) has solved the long-standing question of the nature of the compact object in the high-mass X-ray binary 4U 2206+54 but has posed new ones. According to spin evolutionary models in close binary systems, such slow pulsations require a neutron star magnetic field strength larger that the quantum critical value, suggesting the presence of a magnetar. We present the first XMM-Newton observations of 4U 2206+54 and investigate its spin evolution. We find that the observed spin-down rate agrees with the magnetar scenario. We analyse ISGRI/INTEGRAL observations of 4U 2206+54 to search for the previously suggested cyclotron resonance scattering feature at ~30 keV. We do not find a clear indication of the presence of the line, although certain spectra display shallow dips, not always at 30 keV. The association of these dips with a cyclotron line is very dubious because of its apparent transient nature. We also investigate the energy spectrum of 4U 2206+54 in the energy range 0.3-10 keV with unprecedented detail and report for the first time the detection of very weak 6.5 keV fluorescence iron lines. The photoelectric absorption is consistent with the interstellar value, indicating very small amount of local matter, which would explain the weakness of the florescence lines. The lack of matter locally to the source may be the consequence of the relatively large orbital separation of the two components of the binary. The wind would be too tenuous in the vicinity of the neutron star.