Constraints on Torque-Reversing Accretion-Powered X-ray Pulsars

Abstract

The observed abrupt torque reversals in X-ray pulsars, 4U 1626-67, GX 1+4, and OAO 1657-415, can be explained by transition in accretion flow rotation from Keplerian to sub-Keplerian, which takes place at a critical accretion rate, $\sim 10^{16}-10^{17}g/s$. When a pulsar system spins up near equilibrium spin before the transition, the system goes into spin-down after transition to sub-Keplerian. If a system is well into the spin-up regime, the transition can cause a sharp decrease in spin-up rate but not a sudden spin-down. These observable types of abrupt torque change are distinguished from the smooth torque variation caused by change of $\dot{M}$ in the Keplerian flow. The observed abrupt torque reversal is expected when the pulsar magnetic field $B_{\ast }\sim 5\times 10^{11}{b}_p^{-1/2}{L}_{x,36}^{1/2}{P}_{\ast ,10}^{1/2}G$ where the magnetic pitch parameter $b_p\sim$ a few, $L_{x,36}$ is the X-ray luminosity in $10^{36}erg/s$, and $P_{\ast ,10}$ is the pulsar spin period in 10s. Observed quasi-periodic oscillation (QPO) periods tightly constrain the model. For 4U 1626-67, $\dot{M}\approx 2.7\times 10^{16}g/s$ with $b_p^{1/2}{B}_{\ast }\approx 2\times 10^{12}G$. We estimate $\dot{M}\sim 6\times 10^{16}g/s$ and $b_p^{1/2}{B}_{\ast }\sim 5\times 10^{13}G$ for GX 1+4, and $\dot{M}\sim 1\times 10^{17}g/s$ and $b_p^{1/2}{B}_{\ast }\sim 2\times 10^{13}G$ for OAO 1657-415. Reliable detection of QPOs before and after torque reversal could directly test the model.

11 pages, 2 figures, ApJ

References in corpus (2)

• On the Correlation of Torque and Luminosity in GX 1+4
• Torque Reversal in Accretion-Powered X-ray Pulsars

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• Thermal Properties of Two-Dimensional Advection Dominated Accretion Flow
• Spectral Transition and Torque Reversal in X-ray Pulsar 4U 1626-67
• Signatures of Energetic Protons in Hot Accretion Flows: Synchrotron Cooling of Protons in Strongly Magnetized Pulsars