The Luminosity and Energy Dependence of Pulse Phase Lags in the Accretion-powered Millisecond Pulsar SAX J1808.4-3658

TL;DR

This study investigates how pulse phase lags in the X-ray emissions of the millisecond pulsar SAX J1808.4-3658 depend on luminosity and energy, revealing complex behavior linked to accretion regimes.

Contribution

It provides the first detailed analysis of the luminosity and energy dependence of pulse phase lags in SAX J1808.4-3658, connecting lag behavior to accretion states.

Findings

Lag magnitude increases as accretion rate drops during bright stages.

Lag behavior reverses during dimmer flaring-tail stages.

Transition into the propeller regime correlates with lag pattern changes.

Abstract

Soft phase lags, in which X-ray pulses in lower energy bands arrive later than pulses in higher energy bands, have been observed in nearly all accretion-powered millisecond pulsars, but their origin remains an open question. In a study of the 2.5 ms accretion-powered pulsar SAX J1808.4-3658, we report that the magnitude of these lags is strongly dependent on the accretion rate. During the brightest stage of the outbursts from this source, the lags increase in magnitude as the accretion rate drops; when the outbursts enter their dimmer flaring-tail stage, the relationship reverses. We evaluate this complex dependence in the context of two theoretical models for the lags, one relying on the scattering of photons by the accretion disk and the other invoking a two-component model for the photon emission. In both cases, the turnover suggests that we are observing the source transitioning into the "propeller" accretion regime.