On 2004 August 15, we observed a fast (shorter than 10 h) state transition in the bright black hole transient GX 339–4 simultaneously with Rossi X‐Ray Timing Explorer (RXTE) and INTEGRAL. This transition was evident both in timing and spectral properties. Combining the data from the Proportional Counter Array (PCA), the High‐Energy X‐ray Timing Experiment (HEXTE) and the Imager on Board the INTEGRAL Satellite (IBIS), we obtained good quality broad‐band (3–200 keV) energy spectra before and after the transition. These spectra indicate that the hard component steepened. Also, the high‐energy cut‐off that was present at ∼70 keV before the transition was not detected after the transition. This is the first time that an accurate determination of the broad‐band spectrum across such a transition has been measured on a short time‐scale. It shows that, although some spectral parameters do not change abruptly through the transition, the high‐energy cut‐off increases/disappears rather fast. These results constitute a benchmark on which to test theoretical models for the production of the hard component in these systems.

Abstract. We investigated the optical properties of 7 clusters of galaxies observed in three colors over the range of absolute magnitudes −24 ≤ M ≤ −12. Our aim is to estimate the Luminosity Function and the total cluster luminosity of our sample in order to have information about the formation and the evolution of galaxies in clusters. In this paper, we present the main points of our analysis and give the formal parameters obtained by fitting the data using the maximum likelihood algorithm. We find consistency between our results and other works in literature confirming the bimodal nature of the luminosity function of cluster galaxies. More important, we find that the relation L opt /L X versus L X is color dependent: Low X ray Luminosity clusters have a bluer galaxy population.

Abstract. We re-analyzed two Rossi X-Ray Timing Explorer archival observations of the atoll source 4U 1820-30 in order to investigate the detailed time-frequency properties of the source during the intervals when a ∼7 Hz QPO was detected by Wijnands et al. (1999, ApJ, 512, L39). We find that in both observations, in addition to a QPO signal lasting a couple of minutes as previously reported, there is a much narrower transient oscillation with a life time of only a few seconds. Within this time, the oscillation is consistent with being coherent. Its integrated fractional rms is around 10% and its frequency 7.3 Hz and 5.7 Hz in the two observations. We discuss the possible association of this QPO with other oscillations known both in Neutron-Star and Black-Hole systems, concentrating on the similarities with the narrow 5-7 Hz oscillations observed at high flux in Black-Hole Candidates.