144 research outputs found
Discovery of a red and blue shifted iron disk line in the galactic jet source GRO J1655-40
We report the discovery of emission features in the X-ray spectrum of GRO
J1655-40 obtained with RXTE during the observation of 1997, Feb 26. We have
fitted the features firstly by two Gaussian lines which in four spectra
analysed have average energies of 5.85 +/- 0.08 keV and 7.32 +/- 0.13 keV,
strongly suggestive that these are the red and blue shifted wings of an iron
disk line. These energies imply a velocity of ~0.33 c. The blue wing is less
bright than in the calculated profiles of disk lines near a black hole subject
to Doppler boosting, however known Fe absorption lines in GRO J1655-40 at
energies between ~7 and 8 keV can reduce the apparent brightness of the blue
wing. Secondly, we have fitted the spectra using the disk line model of Laor
based on a full relativistic treatment plus an absorption line, and show that
good fits are obtained. This gives a restframe energy of the disk line between
6.4 and 6.8 keV indicating that the line is iron K_alpha emission probably of
significantly ionized material. The Laor model shows that the line originates
in a region of the accretion disk extending from ~10 Schwarzschild radii from
the black hole outwards. The line is direct evidence for the black hole nature
of the compact object and is the first discovery of a highly red and blue
shifted iron disk line in a Galactic source.Comment: 5 pages, 5 ps figures, latex. MNRAS accepted (submitted 23rd August,
1999
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
Discovery of hard X-ray features around hotspots of Cygnus A
We present results of analysis of a Chandra observation of Cygnus A in which
the X-ray hotspots at the ends of the jets are mapped in detail. A hardness map
reveals previously unknown structure in the form of outer and inner hard arcs
around the hotspots, with hardness significantly enhanced compared with the
hotspot central regions. The outer hard arcs may constitute the first detection
of the bow shock; the inner hard arcs may reveal where the jets impact on the
hotspots. We argue that these features cannot result from electrons radiating
by the synchrotron self-Compton process. Instead we consider two possible
sources of the hard emission: the outer arcs may be due to thermal radiation of
hot intracluster gas compressed at the bow shock. Alternatively, both outer and
inner arcs may be due to synchrotron radiation of electrons accelerated in
turbulent regions highly perturbed by shocks and shear flows. Comparison of
measured hardness ratios with simulations of the hardness ratios resulting from
these processes show that it is more diffcult to explain the observations with
a thermal model. Although we cannot rule out a thermal model, we argue in
favour of the non-thermal explanation. The hard regions in the secondary
hotspots suggest that jet activity is still powering these hotspots.Comment: MNRAS in press; 5 pages, 3 figures (2 figures in colour in jpeg
format should be printed separately
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