Discovery of a New Torque Reversal of the Accreting X-ray Pulsar 4U 1626-67 by Fermi/GBM

TL;DR

This paper reports the discovery of a new torque reversal in the accreting X-ray pulsar 4U 1626-67, observed by Fermi/GBM and Swift/BAT, challenging existing models of neutron star accretion physics.

Contribution

The study identifies a previously unobserved torque reversal in 4U 1626-67 and analyzes its properties, providing new insights into neutron star accretion mechanisms.

Findings

Torque reversal occurred after 18 years of steady spin-down.

The pulsar switched from spin-down to spin-up around Feb 2008.

No correlation found between X-ray flux and torque before the reversal.

Abstract

Recent X-ray observations by Fermi/GBM discovered a new torque reversal of 4U 1626-67 after 18 years of steady spinning down. Using Swift/BAT observations we were able to center this new torque reversal on Feb 4 2008, lasting approximately 150 days. From 2004 up to the end of 2007, the spin-down rate averaged at a mean rate of ~dnu/dt=-4.8e-13 Hz s-1 until the torque reversal reported here. Since then it has been following a steady spin-up at a mean rate of ~dnu/dt= 4e-13 Hz s-1. The properties of this torque reversal, as well as the lack of correlation between the X-ray flux and the torque applied to the neutron star before this transition, challenges our understanding of the physical mechanisms operating in this system.