On the Transition from Accretion Powered to Rotation Powered Millisecond Pulsars

The heating associated with the deposition of $\gamma$-rays in an accretion disk is proposed as a mechanism to facilitate the transformation of a low mass X-ray binary to the radio millisecond pulsar phase. The $\gamma$-ray emission produced in the outer gap accelerator in the pulsar magnetosphere likely irradiates the surrounding disk, resulting in its heating and to the possible escape of matter from the system. We apply the model to PSR J1023+0038, which has recently been discovered as a newly born rotation powered millisecond pulsar. The predicted $\gamma$-ray luminosity $\sim 6 \times 10^{34}\mathrm{ergs^{-1}}$ can be sufficient to explain the disappearance of the truncated disk existing during the 8~month$\sim 2$~yr period prior to the 2002 observations of J1023+0038 and the energy input required for the anomalously bright optical emission of its companion star.

💡 Research Summary

The paper proposes a novel mechanism for the transition of low‑mass X‑ray binaries (LMXBs) into rotation‑powered millisecond pulsars (MSPs). The authors argue that γ‑rays generated in the outer‑gap accelerator of a pulsar’s magnetosphere irradiate the surrounding accretion disk, heating it sufficiently to drive mass loss (evaporation) and ultimately cause the disk to disappear. This process would naturally switch the system from an accretion‑dominated state to a radio‑pulsar state.

The work begins with a concise review of the observational landscape. Several transitional systems—most notably PSR J1023+0038, IGR J18245–2452, and XSS J12270–4859—have been caught in the act of switching between an X‑ray bright, disk‑bearing phase and a radio‑bright, disk‑free phase. Existing explanations (e.g., a sudden drop in mass transfer, propeller‑effect ejection, or thermal–viscous instabilities) can account for some aspects but struggle to reproduce the rapid disappearance of the disk and the simultaneous brightening of the companion star observed in J1023+0038.

The authors then describe the outer‑gap model in detail. In a rapidly rotating neutron star, a charge‑starved region near the light cylinder develops a large electric potential. Electrons and positrons accelerated in this gap emit curvature radiation and undergo inverse‑Compton scattering, producing a broad γ‑ray spectrum extending from ∼100 MeV to several GeV. The γ‑ray luminosity is taken to be a fraction ηγ of the spin‑down power Ė; for J1023+0038, Ė ≈ 10³⁵ erg s⁻¹ and ηγ ≈ 0.06 give Lγ ≈ 6 × 10³⁴ erg s⁻¹.

A key part of the analysis is the irradiation geometry. Assuming the disk lies roughly in the pulsar’s equatorial plane, a geometric factor f ≈ 0.1–0.3 of the emitted γ‑rays intercepts the disk surface. The authors adopt an energy‑conversion efficiency ε (the fraction of γ‑ray energy that is deposited as heat) in the range 0.05–0.2, based on pair‑production cascades and Compton heating in dense plasma. Using a simple radiative balance, the temperature increase ΔT of the disk surface is estimated by

ΔT ≈