Scattering in the inner accretion disk and the waveforms and polarization of millisecond flux oscillations in LMXBs

Abstract

The scattering by the inner accretion disk of X-ray radiation generated near the surface of a spinning neutron star in a low-mass X-ray binary (LMXB) has observable effects on the waveforms of millisecond X-ray flux oscillations produced e.g. during type-I bursts or in the millisecond pulsar SAX J1808.4--3658. We study these effects in the framework of a simplified model in which there is a single emitting spot on the stellar surface, which is visible both directly and in X-rays scattered from the disk. The main signature of scattering from a thin disk is that the pulse of scattered flux leads (if the star rotates in the same sense as the disk) or lags (in the contrary case) the primary pulse of direct emission by a quarter of a spin cycle. This is caused by Doppler boosting of radiation in the sub-relativistic Keplerian flow. The disk-scattered flux is revealed better in energy-resolved waveforms and the phase dependence of the polarized flux component. The phenomenon discussed permits direct testing of the presence of standard thin disks near the neutron stars in LMXBs and should be observable with future X-ray timing experiments having a few times better sensitivity than RXTE and also with sensitive X-ray polarimeters.

11 pages, 11 figures, submitted to Astronomy and Astrophysics

References in corpus (8)

• Millisecond Oscillations in X-Ray Binaries
• Broad iron lines in Active Galactic Nuclei
• Accretion Disk Boundary Layers Around Neutron Stars: X-ray Production in Low-Mass X-ray Binaries
• Discovery of nearly coherent oscillations with a frequency of ~567 Hz during type-I X-ray bursts of the X-ray transient and eclipsing binary X 1658-298
• Oscillation Waveforms and Amplitudes from Hot Spots on Neutron Stars
• Energy Release During Disk Accretion onto a Rapidly Rotating Neutron Star
• Discovery of a 270 Hz X-Ray Burst Oscillation in the X-Ray Dipper 4U 1916-053
• Relativistic Effects in the Pulse Profile of the 2.5 msec X-Ray Pulsar SAX J1808.4-3658

Cited by in corpus (10)

• On the Nature of the X-ray Emission from the Accreting Millisecond Pulsar SAX J1808.4-3658
• The Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X-ray Bursts
• The Energy Dependence of Millisecond Oscillations in Thermonuclear X-ray Bursts
• Multi-epoch Analysis of Pulse Shapes from the Neutron Star SAX J1808.4-3658
• Thermonuclear X-ray bursts from LMXB 4U 1636$-$536 observed with AstroSat
• The Luminosity and Energy Dependence of Pulse Phase Lags in the Accretion-powered Millisecond Pulsar SAX J1808.4-3658
• Nuclear-powered X-ray millisecond pulsars
• Instrumentation and Future Missions in the Upcoming Era of X-Ray Polarimetry
• Gamma-Ray Polarimetry
• Hard X-ray Polarimetry -- An overview of the method, science drivers and recent findings