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

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.

7 pages, 4 figures; accepted by ApJ

References in corpus (7)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• The long-term evolution of the spin, pulse shape, and orbit of the accretion-powered millisecond pulsar SAX J1808.4-3658
• Helium-rich thermonuclear bursts and the distance to the accretion-powered millisecond pulsar SAX J1808.4-3658
• Orbital Evolution of an Accreting Millisecond Pulsar: Witnessing the Banquet of a Hidden Black Widow?
• Intermittent pulsations in an accretion-powered millisecond pulsar
• Energy dependent ~100 μs time lags as observational evidence of Comptonization effects in the neutron star plasma environment
• The energy dependence of burst oscillations from the accreting millisecond pulsar XTE J1814-338

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• Accreting Millisecond X-Ray Pulsars
• Measurement of neutron star parameters: a review of methods for low-mass X-ray binaries
• 1 Hz flaring in SAX J1808.4--3658: flow instabilities near the propeller stage
• A Double Outburst from IGR J00291+5934: Implications for Accretion Disk Instability Theory
• Type I X-ray bursts and burst oscillations in the accreting millisecond X-ray pulsar IGR J17511-3057
• On the timing properties of SAX J1808.4-3658 during its 2015 outburst
• Simulating X-ray bursts during a transient accretion event
• Classification of X-ray Sources in the XMM-Newton Serendipitous Source Catalog: Objects of Special Interest
• The 2009 outburst of accreting millisecond pulsar IGR J17511-3057 as observed by SWIFT and RXTE
• Effects of scattering in the accretion funnel on the pulse profiles of accreting millisecond pulsars
• Physical modeling of viscous disc evolution around magnetized neutron star. Aql X-1 2013 outburst decay