X-ray power density spectra of black hole binaries : a new deadtime model for the RXTE PCA

Abstract

The power density spectrum is an essential tool for determining the frequency content of X-ray radiation from astronomical sources. For neutron star systems, power density spectra reveal coherent oscillations for those sources that are pulsars, while quasi-periodic oscillations over a wide range of frequencies (0.01 to 1300 Hz) are used to identify subclasses and to probe the details of accretion physics. For black hole binaries, the power density spectrum is useful in many important contexts: distinguishing black hole binaries from neutron star binaries, tracking the evolution of X-ray states, and understanding the dynamics of accretion disks, in particular the high-frequency oscillations that appear to be rooted in general relativity for strong gravitational fields. However, measurements of the power density spectrum are modified by the effects of deadtime in X-ray detectors. In this work, we focus on the Proportional Counter Array (PCA) instrument of the Rossi X-ray Timing Explorer (RXTE), an orbiting observatory that offers fast, microsecond-level time resolution and modest spectral resolution for celestial X-ray sources. We derive a new model for the effect of detector deadtime on measurements of the power density spectrum.

(cont.) The model treats in a unified manner the contributions from self-deadtime among selected events and interference from non-selected events. Using high-frequency power density spectra obtained from observations of X-ray sources, the new model is shown to be more accurate than existing approaches. The comparison between the model and the observations leads to a measurement of 8.83 s for the fundamental instrument deadtime timescale, which is dominated by the analog-to-digital conversion time. We additionally measure 59 jts and 137 /is for the Very Large Event deadtime related to observer-specified settings 1 and 2 respectively. Future refinements to the deadtimle model are discussed, such as corrections for highly variable sources and for individual X-ray energy bands.

(cont.) A preliminary comparison between power density spectra from black hole binaries and neutron star binaries is undertaken using the new deadtime model. While it may be possible to use high-frequency cut-offs in the power continuum to distinguish neutron star binaries from black hole binaries in the thermal and hard X-ray states, the comparison is inconclusive for black hole binaries in the steep power-law state. Since state definitions require considerations of X-ray spectral properties, the comparison results dispute a suggestion in the literature that accreting neutron stars and black holes can be distinguished on the basis of power density spectra alone.