Issue |
A&A
Volume 494, Number 3, February II 2009
|
|
---|---|---|
Page(s) | 1059 - 1071 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20079234 | |
Published online | 14 January 2009 |
A model for the Z-track phenomenon in GX 5-1 and observational evidence for the physical origins of the kHz QPO
1
School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK e-mail: njackson@star.sr.bham.ac.uk
2
Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Cracow, Poland
Received:
4
August
2008
Accepted:
2
December
2008
We present the results of a combined investigation of the spectral and kHz QPO evolution around the Z-track in GX 5-1 based on high-quality Rossi-XTE data. In spectral analysis, we find that the Extended ADC emission model provides very good fits to all of the spectra, and the results point clearly to a model for the nature of the Z-track in this source, in agreement with previous results for the similar source GX 340+0. In this model, at the soft apex of the Z-track, the mass accretion rate is at its minimum and the neutron star has its lowest temperature; but as the source moves along the normal branch, the luminosity of the Comptonized emission increases, indicating that
increases and the neutron star gets hotter. The measured flux f of the neutron star emission increases by a factor of ten becoming super-Eddington, and we propose that this causes disruption of the inner disk and the formation of jets. In flaring, the luminosity of the dominant Comptonized emission from the accretion disk corona is constant, while the neutron star emission increases, and we propose for the first time that flaring consists of unstable nuclear burning on the neutron star, supported by the agreement between the measured mass accretion rate per unit area
at the onset of flaring and the theoretical critical value at which burning becomes unstable. There is a striking correlation between the frequencies of the kHz QPO and the ratio of the flux to the Eddington value:
, suggesting an explanation of the higher frequency QPO and of its variation along the Z-track. It is well known that a Keplerian orbit in the disk at this frequency corresponds to a position some distance from the neutron star; we propose that the oscillation always occurs at the inner disk edge, which moves radially outwards on the upper normal and horizontal branches as the measured increasing radiation pressure increasingly disrupts the inner disk.
Key words: accretion, accretion disks / stars: binaries: close / stars: neutron / X-rays: binaries / X-rays: individuals: GX 5-1
© ESO, 2009
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.