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.

We report monitoring of the 0.3–10 keV spectrum of NGC 4258 with the the XMM-Newton Observatory at five epochs over 1.5 years. We also report reprocessing of an overlapping four-epoch series of archival Chandra observations (0.5– 10 keV). By including earlier ASCA and Beppo-SAX observations, we present a new, nine-year time-series of models fit to the X-ray spectrum of NGC 4258. We model the Chandra and XMM-Newton data self-consistently with partially absorbed, hard power-law, soft thermal plasma, and soft power-law components. Over the nine years, the photoelectric absorbing column (∼ 1023 cm−2) did not vary detectably, except for a ∼ 40% drop between two ASCA epochs separated by 3 years (in 1993 and 1996) and a ∼ 60% rise between two XMM-Newton epochs separated by just 5 months (in 2001 and 2002). In contrast, factor of 2–3 changes are seen in absorbed flux on the timescale of years. These are uncorrelated with changes in absorbing column and indicative of central engine variability. The most rapid change in luminosity (5–10 keV) that we detect (with XMM-Newton and Chandra) is ∼30% over 19 days. The warped disk, a known source of H2O maser emission in NGC 4258, is believed to cross the line of sight to the central engine. We propose that the variations in absorbing column arise from inhomogeneities sweeping across the line of sight in the rotating disk at the radius where the disk crosses the line of sight. We estimate that the inhomogeneities are ∼ 1015 cm in size at the crossing radius of 0.29 pc, slightly smaller than the expected scale height of the disk. This result thus provides strong evidence that the warped accretion disk is the absorber in this (and possibly other) active galactic nuclei (AGNs). This is the first direct confirmation that obscuration in type-2 AGN may, in some cases, arise in thin, warped accretion disks, rather than in geometrically thick tori. Some previous studies report detection of weak Fe Kα emission in NGC 4258. We do not detect this line emission in any of our XMM-Newton spectra with a 90% upper limit to the equivalent width of ∼ 49 eV for one observation. Weak, time-variable Fe line absorption has also been reported for a previous Chandra study. We do not observe evidence of absorption lines in the XMM-Newton or reprocessed Chandra data. The absence of Fe line emission is consistent with the disk being optically thin to hard photons as well as subtending a small solid angle as seen from the central engine because of the known shallowness of the warp.