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

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

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