The Time-Delay Spectrum of GX 5-1 in Its Horizontal Branch
Abstract
Using a cross-spectral technique we investigate time delays between intensity variations of GX 5-1 in 10 X-ray spectral channels. The data were taken during a 1989 Ginga observation during which the source was in its horizontal-branch spectral state. We develope a new method to measure 'time-delay spectra' in fixed Fourier frequency ranges and use it to determine the energy and intensity dependence of time delays in the low-frequency noise (nu less than 2 Hz), the horizontal branch quasi-periodic oscillations (QPO), and the QPO second harmonic. These are the first time-delay spectra of a Z-source in its horizontal branch, and the first detection of time delays in the second harmonic. We consider two mechanisms for the production of the time lags: Comptonization and evolving shots. We perform Monte Carlo simulations of Compton scattering in a homogeneous, isotropic, central corona and show that it qualitatively explain the observed energy and time-delay spectra, but that it cannot explain the differences in the QPO first and second harmonic time-delay spectra, nor the observed dependence of the QPO fractional rms variability upon energy. We consider implications of our results for millisecond pulsar searches in low-mass X-ray binaries.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 1994
- DOI:
- 10.1086/173686
- Bibcode:
- 1994ApJ...421..738V
- Keywords:
-
- Astronomical Spectroscopy;
- Horizontal Branch Stars;
- Pulsars;
- Stellar Spectra;
- Time Lag;
- X Ray Astronomy;
- X Ray Binaries;
- X Ray Spectra;
- Compton Effect;
- Computerized Simulation;
- Ginga Satellite;
- Monte Carlo Method;
- Astronomy;
- STARS: PULSARS: GENERAL;
- STARS: INDIVIDUAL ALPHANUMERIC: GX 5-1;
- X-RAYS: STARS