6,945 research outputs found
A model for the Z-track phenomenon in GX 5-1 and observational evidence for the physical origins of the kHz QPO
We present results of a combined investigation of the spectral and kHz QPO
evolution around the Z-track in GX 5-1 based on high-quality RXTE data. The
Extended ADC emission model provides very good fits to the spectra, the results
pointing clearly to a model for the nature of the Z-track, 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 Mdot is 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
Mdot 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 disrupts the inner disk so forming jets. In flaring,
the luminosity of the dominant Comptonized emission from the ADC 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, and the
measured mass accretion rate per unit area mdot at the onset of flaring agrees
well with 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: f/f_Edd, 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.Comment: Astronomy and Astrophysics, in pres
Spectral Investigations of the nature of the Sco X-1 like sources
We present results of spectral investigations of the Sco X-1 like Z-track
sources Sco X-1, GX 349+2 and GX 17+2 based on Rossi-XTE observations using an
extended accretion disk corona model. The results are compared with previous
results for the Cyg X-2 like group: Cyg X-2, GX 340+0 and GX 5-1 and a general
model for the Z-track sources proposed. On the normal branch, the Sco-like and
Cyg-like sources are similar, the results indicating an increase of mass
accretion rate Mdot between soft and hard apex, not as in the standard view
that this increases around the Z. In the Cyg-like sources, increasing Mdot
causes the neutron star temperature kT to increase from ~1 to ~2 keV. At the
lower kT, the radiation pressure is small, but at the higher kT, the emitted
flux of the neutron star is several times super-Eddington and the high
radiation pressure disrupts the inner disk launching the relativistic jets
observed on the upper normal and horizontal branches. In the Sco-like sources,
the main physical difference is the high kT of more than 2 keV on all parts of
the Z-track suggesting that jets are always possible, even on the flaring
branch. The flaring branch in the Cyg-like sources is associated with release
of energy on the neutron star consistent with unstable nuclear burning. The
Sco-like sources are very different as flaring appears to be a combination of
unstable burning and an increase of Mdot which makes flaring much stronger.
Analysis of 15 years or RXTE ASM data on all 6 classic Z-track sources shows
the high rate and strength of flaring in the Sco-like sources suggesting that
continual release of energy heats the neutron star causing the high kT. A Sco
X-1 observation with unusually little flaring supports this. GX 17+2 appears to
be transitional between the Cyg and Sco-like types. Our results do not support
the suggestion that Cyg or Sco-like nature is determined by luminosity.Comment: Astronomy and Astrophysics in press; 21 pages, 13 figure
On the nature of the Cygnus X-2 like Z-track sources
Based on the results of applying the extended ADC emission model for low mass
X-ray binaries to three Z-track sources: GX340+0, GX5-1 and CygX-2, we propose
an explanation of the CygnusX-2 like Z-track sources. The Normal Branch is
dominated by the increasing radiation pressure of the neutron star caused by a
mass accretion rate that increases between the soft apex and the hard apex. The
radiation pressure continues to increase on the Horizontal Branch becoming
several times super-Eddington. We suggest that this disrupts the inner
accretion disk and that part of the accretion flow is diverted vertically
forming jets which are detected by their radio emission on this part of the
Z-track. We thus propose that high radiation pressure is the necessary
condition for the launching of jets. On the Flaring Branch there is a large
increase in the neutron star blackbody luminosity at constant mass accretion
rate indicating an additional energy source on the neutron star. We find that
there is good agreement between the mass accretion rate per unit emitting area
of the neutron star mdot at the onset of flaring and the theoretical critical
value at which burning becomes unstable. We thus propose that flaring in the
CygnusX-2 like sources consists of unstable nuclear burning. Correlation of
measurements of kilohertz QPO frequencies in all three sources with spectral
fitting results leads to the proposal that the upper kHz QPO is an oscillation
always taking place at the inner accretion disk edge, the radius of which
increases due to disruption of the disk by the high radiation pressure of the
neutron star.Comment: Astronomy and Astrophysics, in pres
An empirical study of software design practices
Software engineers have developed a large body of software design theory and folklore, much of which was never validated. The results of an empirical study of software design practices in one specific environment are presented. The practices examined affect module size, module strength, data coupling, descendant span, unreferenced variables, and software reuse. Measures characteristic of these practices were extracted from 887 FORTRAN modules developed for five flight dynamics software projects monitored by the Software Engineering Laboratory (SEL). The relationship of these measures to cost and fault rate was analyzed using a contingency table procedure. The results show that some recommended design practices, despite their intuitive appeal, are ineffective in this environment, whereas others are very effective
Discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 with XMM-Newton
We report the discovery of narrow X-ray absorption features from the dipping
low-mass X-ray binary X 1624-490 during an XMM-Newton observation in 2001
February. The features are identified with the K alpha absorption lines of Fe
xxv and Fe xxvi and have energies of 6.72 +/- 0.03 keV and 7.00 +/- 0.02 keV
and equivalent widths (EWs) of -7.5 +1.7 -6.3 eV and -16.6 +1.9 -5.9 eV,
respectively. The EWs show no obvious dependence on orbital phase, except
during a dip, and correspond to a column of greater than 10^17.3 Fe atom /cm2.
In addition, faint absorption features tentatively identified with Ni xxvii K
alpha and Fe xxvi K beta may be present. A broad emission feature at 6.58 +0.07
-0.04 keV with an EW of 78 +19 -6 eV is also evident. This is probably the 6.4
keV feature reported by earlier missions since fitting a single Gaussian to the
entire Fe-K region gives an energy of 6.39 +0.03 -0.04 keV. A deep absorption
feature is present during the dip with an energy consistent with Fe xxv K
alpha. This is the second dipping LMXRB source from which narrow Fe absorption
features have been observed. Until recently the only X-ray binaries known to
exhibit narrow X-ray absorption lines were two superluminal jet sources and it
had been suggested that these features are related to the jet formation
mechanism. It now appears likely that ionized absorption features may be common
characteristics of accreting systems with accretion disks.Comment: 6 pages. To appear in A&
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