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

¹ Space Research Institute (IKI), Profsouznaya 84/32, 117810 Moscow, Russia
² MPI für Astrophysik, Karl-Schwarzschild-Str. 1, 86740 Garching bei München, Germany

Corresponding author: S. Sazonov, ss@hea.iki.rssi.ru

Received: 20 November 2000
Accepted: 23 April 2001

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.