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The Hot Inner Disk Environment and Torque Reversals in 4U 1626-67

Presentation #116.77 in the session Stellar/Compact Objects.

Published onJul 01, 2023
The Hot Inner Disk Environment and Torque Reversals in 4U 1626-67

We have observed the ultracompact X-ray binary pulsar 4U 1626-67 for a time span over two decades starting before and continuing after the magnetic accretion torque reversal event in 2008 with tha Chandra X-ray Observatory High Energy Transmission Grating Spectrometer. Spectral fits to strong Ne and O emission disk lines with Keplerian profiles are interpreted as arising from the inner edge of the magnetically-truncated accretion disk. The absence of radiative recombination continua in the spectra and the shape of the observed disk lines are consistent with a hot collisionally ionized plasma with temperatures of the order of 10 MK at the inner edge of a truncated accretion disk. Furthermore, by using the inner disk radius from the disk line fits before and after the 2008 torque reversal event as a tracer we observe inner disk radii that are larger than the corotation radius during spin-down but smaller after the torque reversal as the pulsar began to spin faster. Monitoring of these disk ine properties for well over a decade past the torque reversal event in 2008 also shows that temperature of the collisionally ionized plasma region is very stable but also highly clumped allowing for coexistance of hot ionized and cool neutral matter. The data also provide details about system inclination, source distance, and abundance estimates that can be used to constrain the mass and constitution of the degenerate companion. We will provide a detailed picture of what the high energy emissions of this unique system are about. Finally, we extrapolate the current trend to estimate the date of the next torque reversal.

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