Person:

Fruscione, Antonella

We present results from a Chandra X-Ray Observatory study of the field X-ray source populations in four different observations: two high-redshift (z ~ 0.5) clusters of galaxies 3C 295 and RX J003033.2+261819; and two noncluster fields with similar exposure time. Surprisingly, the 0.5-2 keV source surface densities (~900-1200 sources deg-2 at a flux limit of 1.5 × 10-15 ergs cm-2 s-1) measured in an ~8' × 8' area surrounding each cluster exceed by a factor of ~2 the value expected on the basis of the ROSAT and Chandra log N- log S, with a significance of ~2 σ each, or ~3.5 σ when the two fields are combined (i.e., a probability to be a statistical fluctuation of <1% and <0.04%, respectively). The same analysis performed on the noncluster fields and on the outer chips of the cluster fields does not show evidence of such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the detected objects is well fitted by a power law with Γ ~ 1.7 similar to active galactic nuclei (AGNs) and shows no sign of intrinsic absorption. The few (~10 of 35) optical identifications available to date confirm that most of them are, as expected, AGNs, but the number of redshifts available is too small to allow conclusions on their nature. We discuss possible interpretations of the overdensity in terms of a statistical variation of cosmic background sources; a concentration of AGNs and/or powerful starburst galaxies associated with the clusters; and gravitational lensing of background QSOs by the galaxy clusters. All explanations, however, are difficult to reconcile with the large number of excess sources detected. Deeper X-ray observations and more redshifts measurements are clearly required to settle the issue.

In this paper we present the analysis of two broadband (0.1-150 keV) BeppoSAX observations of the Seyfert 1 galaxy NGC 3516. The two observations were taken 4 months apart, on 1996 November 8 and 1997 March 12. We report a dramatic change in the degree of obscuration of the central source between the two observations and propose, as possible explanations, transient absorption by either a stationary-state cloud of cold gas crossing the line of sight or a varying-state, initially neutral and dense amount of expanding gas with decreasing density and therefore decreasing opacity. We also report the detection of a second highly ionized absorber/emitter, which causes deep Fe XVII-XXII K edges at ~7.8 keV to appear in both of the BeppoSAX spectra of NGC 3516 and possibly produces the soft X-ray continuum emission in the 1 keV blend of Fe L recombination lines detected during the epoch of heavy nuclear obscuration.