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We report on the timing analysis of RXTE observations of the Galactic micro-quasar GRS 1915+105 performed in 2003. Out of a total of six times ∼ 20 ks, we focus here only on the three observations during which GRS 1915+105 is found in a steady C-state (referred to as class χ) resulting in a total of ∼ 50 ks. During these observations, we detect low frequency quasi-periodic oscillations with high (∼ 14%) rms amplitude in the 2-40 keV energy range. Contrary to what is usually observed in GRS 1915+105, in most of our observations the QPO frequency present no correlation with the RXTE/PCA count rate, nor with the RXTE/ASM count rate. We present, for the first time, high resolution (22 spectral channels) 2-40 keV spectral fits of the energy dependence of the QPO amplitude ("QPO spectra"). The QPO spectra are well modeled with a cut-off power law except on one occasion where a single power law gives a satisfactory fit (with no cut-off at least up to ∼ 40 keV). The cut-off energy evolves significantly from one observation to the other, from a value of ∼ 21.8 keV to ∼ 30 keV in the other observations where it is detected. We discuss the possible origin of this behavior and suggest that the compact jet detected in the radio contributes to the hard X-ray (≥ 20 keV) mostly through synchrotron emission, whereas the X-ray emitted below 20 keV would originate through inverse Compton scattering. The dependence of the QPO amplitude on the energy can be understood if the modulation of the X-ray flux is contained in the Comptonized photons and not in the synchrotron ones.

We present 12 CO(J=1-0) observations in the direction of the Soft Gamma Repeater SGR 1806-20 with the SEST telescope. We detected several molecular clouds, and we discuss in this paper the implications of these observations for the distance to the X-ray counterpart AX 1805.7-2025, the supernova remnant G10.0-0.3 and the very luminous O9-B2 star detected in the line of sight. The distance of SGR 1806-20 is estimated to be 14.5 ± 1.4 kpc and this Soft Gamma Repeater is very likely associated with one of the brightest HII regions in the Galaxy, W31. The large size of G10.0-0.3 (25 × 38 pc) for a young supernova remnant possibly powered by a central pulsar (AX 1805.7-2025) indicates that G10.0-0.3 could be expanding in the very low density region produced by the wind of the blue star.

Aims. We studied and compared the long-term average hard X-ray (>20 keV) spectra of a sample of twelve bright low-mass X-ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright ("GX") bulge sources while two are weaker ones in the 2-10 keV range (H 1750-440 and H 1608-55). Methods. For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL. Results. We can describe all the spectral states in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local luminosity. The two low-dim Atoll source spectra are dominated by photons up-scattered presumably due to dynamical and thermal Comptonisation in an optically thin, hot plasma. For the first time, we extend the detection of H 1750-440 up to 150 keV. The Z and bright "GX" Atoll source spectra are very similar and are dominated by Comptonised blackbody radiation of seed photons, presumably coming from the accretion disc and NS surface, in an optically thick cloud with plasma temperature in the range of 2.5-3 keV. Six sources show a hard tail in their average spectrum: Cyg X-2 (Z), GX 340+0 (Z), GX 17+2 (Z), GX 5-1 (Z), Sco X-1 (Z) and GX 13+1 (Atoll). This is the first detection of a hard tail in the X-ray spectrum of the peculiar GX 13+1. Using radio data from the literature we find, in all Z sources and bright "GX" Atolls, a systematic positive correlation between the X-ray hard tail (40-100 keV) and the radio luminosity. This suggests that hard tails and energetic electrons causing the radio emission may have the same origin, most likely the Compton cloud located inside the NS magnetosphere.