High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, to the equation of state of ultradense matter and to black hole masses and spins. A 10 m²-class instrument in combination with good spectral resolution is required to exploit the relevant diagnostics and answer two of the fundamental questions of the European Space Agency (ESA) Cosmic Vision Theme “Matter under extreme conditions”, namely: does matter orbiting close to the event horizon follow the predictions of general relativity? What is the equation of state of matter in neutron stars? The Large Observatory For X-ray Timing (LOFT), selected by ESA as one of the four Cosmic Vision M3 candidate missions to undergo an assessment phase, will revolutionise the study of collapsed objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei. Thanks to an innovative design and the development of large-area monolithic silicon drift detectors, the Large Area Detector (LAD) on board LOFT will achieve an effective area of ~12 m² (more than an order of magnitude larger than any spaceborne predecessor) in the 2–30 keV range (up to 50 keV in expanded mode), yet still fits a conventional platform and small/medium-class launcher. With this large area and a spectral resolution of <260 eV, LOFT will yield unprecedented information on strongly curved spacetimes and matter under extreme conditions of pressure and magnetic field strength.

We report results of a BeppoSAX (0.1-200 keV) observation of the Z-type low-mass X-ray binary GX 17 + 2. The source was on the so-called horizontal and normal branches. Energy spectra were selected based on the source position in the X-ray hardness-intensity diagram. The continuum could be fairly well described by the sum of a similar to0.6 keV blackbody, contributing similar to 10% of the observed 0.1-200 keV flux, and a Comptonized component, resulting from upscattering of similar to1 keV seed photons by an electron cloud with temperature of similar to3 keV and optical depth of similar to 10. Iron K line and edge were also present at energies of similar to6.7 and similar to8.5 keV, respectively. In the spectra of the horizontal branch, a hard tail was clearly detected at energies above similar to 30 keV. It could be fit by a power law of photon index similar to2.7, contributing similar to8% of the source flux. This component gradually faded as the source moved toward the normal branch, where it was no longer detectable. We discuss the possible origin of this component and the similarities with the spectra of atoll sources and black hole X-ray binaries

We present the broadband X-ray spectrum of Cyg X-1 in the low/hard state as observed by the instruments on board BeppoSAX. The spectrum spans from 0.1 to 200 keV, allowing the total accretion luminosity to be observed rather than extrapolated, corresponding to similar to2% of the Eddington limit for a 10 M-. black hole. The broad bandpass allows us to determine the continuum shape with great accuracy. Simple models of Compton upscattering of seed photons from the accretion disk do not adequately match the spectrum. At low energies an additional continuum component is required, giving a complex soft excess which extends up to similar to4 keV, in line with previous results from ASCA. Moreover, we clearly detect a reflected component from the accretion disk that is smeared, probably because of relativistic and Doppler effects. The reflecting material is not strongly ionized and does not subtend a large solid angle as seen from the corona (Omega /2 pi similar to0.1-0.3). The estimated inner radius of the disk, which depends on the inclination of the system, is most probably between 10 and 70 gravitational radii (R-g). An unsmeared reprocessed component, probably originating from the companion star or the outer disk, could also be present. In this case, the inner radius of the disk, as inferred from the smeared reflection, is smaller, between and 6R(g) and 20R(g)

We present the results of a multi-Lorentzian fit to the power spectra of two kilohertz QPO sources; 4U 0614+09 and 4U 1728-34. This work was triggered by recent results of a similar fit to the black-hole candidates (BHCs) GX 339-4 and Cyg X-1 by Nowak in 2000. We find that one to six Lorentzians are needed to fit the power spectra of our two sources. The use of exactly the same fit function reveals that the timing behaviour of 4U 0614+09 and 4U 1728-34 is almost identical at luminosities which are about a factor 5 different. As the characteristic frequency of the Lorentzians we use the frequency, nu_max, at which each component contributes most of its variance per log frequency as proposed by Belloni, Psaltis & van der Klis in 2001. When using nu_max instead of the centroid frequency of the Lorentzian, the recently discovered hectohertz Lorentzian is practically constant in frequency. We use our results to test the suggestions by, respectively, Psaltis Belloni and van der Klis in 1999 and Nowak in 2000 that the two Lorentzians describing the high-frequency end of the broad-band noise in BHCs in the low state can be identified with the kilohertz QPOs in the neutron star low mass X-ray binaries. We find, that when the two kilohertz QPOs are clearly present, the low-frequency part of the power spectrum is too complicated to draw immediate conclusions from the nature of the components detected in any one power spectrum. However, the relations we observe between the characteristic frequencies of the kilohertz QPOs and the band-limited noise, when compared to the corresponding relations in BHCs, hint towards the identification of the second-highest frequency Lorentzian in the BHCs with the lower kilohertz QPO. They do not confirm the identification of the highest-frequency Lorentzian with the upper kilohertz QPO.Comment: 30 pages, 35 figures, ApJ accepted; changed name of BLN QPO into very low-frequency Lorentzian, removed table 4 and figure 8 from previous versio