4U 1626-67 as seen by Suzaku before and after the 2008 torque reversal

TL;DR

This study analyzes Suzaku observations of the pulsar 4U 1626-67 before and after its 2008 torque reversal, revealing significant changes in emission lines and supporting a thin disk accretion model.

Contribution

It provides the first detailed spectral comparison of 4U 1626-67 before and after the 2008 torque reversal, highlighting changes in emission lines and accretion behavior.

Findings

Ne Ly-alpha line intensity increased after 2008

No significant change in cyclotron line energy despite luminosity increase

Emission line complex resolved into up to seven lines

Abstract

Aims. The accretion-powered pulsar 4U 1626-67 experienced a new torque reversal at the beginning of 2008, after about 18 years of steadily spinning down. The main goal of the present work is to study this recent torque reversal that occurred in 2008 February. Methods. We present a spectral analysis of this source using two pointed observations performed by Suzaku in 2006 March and in 2010 September. Results. We confirm with Suzaku the presence of a strong emission-line complex centered on 1 keV, with the strongest line being the hydrogen-like Ne Ly-alpha at 1.025(3) keV. We were able to resolve this complex with up to seven emission lines. A dramatic increase of the intensity of the Ne Ly-alpha line after the 2008 torque reversal occurred, with the equivalent width of this line reaching almost the same value measured by ASCA in 1993. We also report on the detection of a cyclotron line feature centered at ~37 keV. In spite of the fact that an increase of the X-ray luminosity (0.5-100 keV) of a factor of ~2.8 occurred between these two observations, no significant change in the energy of the cyclotron line feature was observed. However, the intensity of the ~1 keV line complex increased by an overall factor of ~8. Conclusions. Our results favor a scenario in which the neutron star in 4U 1626-67 accretes material from a geometrically thin disk during both the spin-up and spin-down phases.