Pulsar Wind-heated Accretion Disk and the Origin of Modes in Transitional Millisecond Pulsar PSR J1023+0038

Abstract

Transitional millisecond pulsars provide a unique set of observational data for understanding accretion at low rates onto magnetized neutron stars. In particular, PSR.J1023+0038 exhibits a remarkable bimodality of the X-ray luminosity (low and high modes), pulsations extending from the X-ray to the optical band, GeV emission, and occasional X-ray flares. We discuss a scenario for the pulsar interaction with the accretion disk capable of explaining the observed behavior. We suggest that during the high mode the disk is truncated outside the light cylinder, allowing the pulsar wind to develop near the equatorial plane and strike the disk. The dissipative wind-disk collision energizes the disk particles and generates synchrotron emission, which peaks in the X-ray band and extends down to the optical band. The emission is modulated by the pulsar wind rotation, resulting in a pulse profile with two peaks 180 degrees apart. This picture explains the high mode luminosity, spectrum, and pulse profile (X-ray and optical) of PSR.J1023+0038. It may also explain the X-ray flares as events of sudden increase in the effective disk cross section intercepting the wind. In contrast to previously proposed models, we suggest that the disk penetrates the light cylinder only during the low X-ray mode. This penetration suppresses the dissipation caused by the pulsar wind-disk collision, and the system enters the propeller regime. The small duty cycle of the propeller explains the low spin-down rate of the pulsar.