Roberto Della Ceca

The ROSAT Deep Cluster Survey: The X-Ray Luminosity Function out to z = 0.8

We present the X-ray luminosity function (XLF) of the ROSAT Deep Cluster Survey sample over the redshift range 0.05-0.8. Our results are derived from a complete flux-limited subsample of 70 galaxy clusters, representing the brightest half of the total sample, which have been spectroscopically identified down to the flux limit of 4×10 -->−14 ergs m-2 s-1 (0.5-2.0 keV) and have been selected via a serendipitous search in ROSAT PSPC pointed observations. The redshift baseline is large enough that evolutionary effects can be studied within the sample. The local XLF (z≤0.25) is found to be in excellent agreement with previous determinations using ROSAT All-Sky Survey data. The XLF at higher redshifts, when combined with the deepest number counts constructed to date (f>2×10 -->−14 ergs cm -->−2 s -->−1), reveals no significant evolution at least out to z=0.8, over a luminosity range of 2 × 1042 to 3 × 1044 ergs s -->−1 in the 0.5-2 keV band. These findings extend the study of cluster evolution to the highest redshifts and the faintest fluxes probed so far in X-ray surveys. They complement and do not necessarily conflict with those of the Einstein Extended Medium-Sensitivity Survey, leaving open the possibility of negative evolution of the brightest end of the XLF at high redshifts.

Measuring Ωm with the ROSAT Deep Cluster Survey

We analyze the ROSAT Deep Cluster Survey (RDCS) to derive cosmological constraints from the evolution of the cluster X-ray luminosity distribution. The sample contains 103 galaxy clusters out to z 0.85 and flux limit Flim = 3 × 10-14 ergs s-1 cm-2 (RDCS-3) in the [0.5-2.0] keV energy band, with a high-redshift extension containing four clusters at 0.90 ≤ z ≤ 1.26 and brighter than Flim = 1 × 10-14 ergs s-1 cm-2 (RDCS-1). We assume cosmological models to be specified by the matter density parameter Ωm, the rms fluctuation amplitude at the 8 h-1 Mpc scale σ8, and the shape parameter for the cold dark matter-like power spectrum Γ. Model predictions for the cluster mass function are converted into the X-ray luminosity function in two steps. First, we convert mass into intracluster gas temperature by assuming hydrostatic equilibrium. Then, temperature is converted into X-ray luminosity by using the most recent data on the LX-TX relation for nearby and distant clusters. These include the Chandra data for six distant clusters at 0.57 ≤ z ≤ 1.27. From RDCS-3 we find Ωm = 0.35 and σ8 = 0.66 for a spatially flat universe with a cosmological constant, with no significant constraint on Γ (errors correspond to 1 σ confidence levels for three fitting parameters). Even accounting for both theoretical and observational uncertainties in the mass-X-ray luminosity conversion, an Einstein-de Sitter model is always excluded at far more than the 3 σ level. We also show that the number of X-ray-bright clusters in RDCS-1 at z > 0.9 is expected from the evolution inferred at z < 0.9 data.

The merger fraction of active and inactive galaxies in the local Universe through an improved non-parametric classification

We investigate the possible link between mergers and the enhanced activity of supermassive black holes (SMBHs) at the centre of galaxies, by comparing the merger fraction of a local sample (0.003 ≤ z< 0.03) of active galaxies – 59 active galactic nuclei host galaxies selected from the All-Sky Swift Burst Alert Telescope (BAT) Survey – with an appropriate control sample (247 sources extracted from the HyperLeda catalogue) that has the same redshift distribution as the BAT sample. We detect the interacting systems in the two samples on the basis of non-parametric structural indexes of concentration (C), asymmetry (A), clumpiness (S), Gini coefficient (G) and second-order momentum of light (M20). In particular, we propose a new morphological criterion, based on a combination of all these indexes, that improves the identification of interacting systems. We also present a new software – PyCASS o( PYTHON CAS software) – for the automatic computation of the structural indexes. After correcting for the completeness and reliability of the method, we find that the fraction of interacting galaxies among the active population (20 +7 per cent) exceeds the merger fraction of the control sample (4 +1.7 −1.2 per cent). Choosing a mass-matched control sample leads to equivalent results, although with slightly lower statistical significance. Our findings support the scenario in which mergers trigger the nuclear activity of SMBHs.

The Nature of Composite Seyfert/Star-forming Galaxies Revealed by X-Ray Observations

This paper presents new Chandra and BeppoSAX observations aimed at investigating the optical-X-ray mismatch in the enigmatic class of the composite galaxies discovered by a cross-correlation of IRAS and ROSAT all-sky survey catalogs. These galaxies have been classified as star-forming objects on the basis of their optical spectra, while the detection of weak broad wings in the Hα emission in a few of them and their high X-ray luminosity in the ROSAT band indicated the presence of an active nucleus. The analysis of Chandra observations for four composite galaxies has revealed nuclear pointlike sources, with a typical AGN spectrum (Γ ~ 1.7-1.9) and little intrinsic absorption. A strong flux variability has been observed on different timescales; in particular, most of the sources were brighter at the ROSAT epoch. Although it is of relatively low luminosity for the AGN class (L2-10 keV ~ 3-60 × 1041 ergs s-1), the active nucleus is nevertheless dominant in the X-ray domain. At other wavelengths it appears to be overwhelmed by the starburst and/or host galaxy light, yielding the composite classification for these objects.

The properties of X-ray selected active galactic nuclei. II - A deeper look at the cosmological evolution

A detailed study of the cosmological properties of X-ray selected AGN is presented. The data are analyzed within the framework of a pure luminosity evolution (PLE) model and the two most population evolutionary forms. Evidence is found for luminosity-dependent luminosity evolution if the evolution function has the exponential form. The simpler PLE model is more acceptable if the data are fitted with a power-law evolution function. Similar results are obtained in the optical domain from an analysis of a sample of optically selected QSOs with z less than 2.2 and B less than 20.

The x-ray mirrors for the EXIST/SXI telescope

The Energetic X-ray Imaging Survey Telescope (EXIST) will continuously survey the full sky in scanning mode for 2- years followed by a 3-years pointing phase. The mission includes three instruments: a High Energy coded mask Telescope; a 1.1m aperture optical-IR Telescope; and a Soft X-ray Imager (SXI), sensitive in the 0.1-10 keV band. SXI is proposed as a contribution of ASI-Italy, fully developed by Italian institutes. Here we will present the optical and mechanical design of the SXI mirror module, that includes also a pre-collimator and a magnetic diverter to ensure a low background on the detector. In particular we will describe the mirror module characteristics in term of effective area, imaging capability, thermal requirement and mechanical properties. The current optical design foresees 26 shells providing an effective area comparable to one XMM-Newton mirror module up to 3 keV. The realization of these shells is based on the well-proven Nickel replication-process technology.

Mining the XRT archive to probe the X-ray absorber structure in the AGN population

One of the key ingredients of the Unified Model of Active Galactic Nuclei (AGN) is the presence of a torus-like optically thick medium composed by dust and gas around the putative supermassive black hole. However, the structure, size and composition of this circumnuclear medium are still matter of debate. To this end, the search for column density variations through X-ray monitoring on different timescales (months, weeks and few days) is fundamental to constrain size, kinematics and location of the X-ray absorber(s). Here we describe our project of mining the Swift-XRT archive to assemble a sample of AGN with extreme column density variability and determining the physical properties of the X-ray absorber(s). We also present the results obtained from a daily-weekly Swift-XRT follow-up monitoring recently performed on one of the most interesting new candidates for variability discovered so far, Mrk 915.

X-ray selected Narrow-Line Seyfert 1 Galaxies

We present and discuss the properties of a complete and well defined sample of X-ray selected type 1 AGNs including 26 Narrow-Line Seyfert 1 and 129 Broad-Line Seyfert 1 galaxies derived from the XMM-Newton Bright Serendipitous Survey (XBS). We study the differences between the two classes of AGNs for what concerns the X-ray properties (the photon-index) and the main physical parameters, like the Black-Hole mass and the Eddington ratio. We then consider the two classes together and look for statistical correlations between observed and physical parameters. We find a significant dependence of the photon-index with the Eddington ratio and a weaker (possibly secondary) correlation between the photon index and the Black-Hole Mass.

Design and scientific performance of the soft x-ray imager on board EXIST

The EXIST mission has been recently re-designed prior to being proposed to the ASTRO2010 Decadal Survey. One of the most recent improvements has been the addition of a third instrument consisting of a powerful Soft X-ray Imager (SXI) that will study in detail and help characterizing the high energy sources detected by the High Energy Telescope (HET). The EXIST concept fully exploits the heritage of Swift in the fast follow-up of transients and in particular GRBs, with 10 to 20 times more sensitivity in the high energy band (from 0.2 to 600 keV) and exceptional performance in the near-IR/optical provided by the Infrared Telescope (IRT). SXI has an important role in extending by more than one decade in energy, down to the soft X-rays the coverage of HET. Such combination will be fully exploited when performing pointed observations. Within the EXIST follow-up program, foreseen during the second part of the mission, SXI and HET will be able to collect high quality spectra for thousands of sources covering the energy range 0.1- hundreds keV. Furthermore, while working in survey mode SXI will cover about half the sky in 2 years and will be able to improve the location accuracy of many faint HET sources (reducing the positional uncertainty from 20 arcsec to ~ 1-2 arcsec). In this paper we will address the performance and the main scientific contributions expected from SXI.

Active and passive galaxies in deep ROSAT surveys

ROSAT deep survey X‐ray sources have been identified with active galaxies (Active Galactic Nuclei and Narrow Emission‐Line galaxies) and also with early‐type, ‘‘passive galaxies’’. In this paper we discuss preliminary results of the galaxy content, both early‐type and emission‐line galaxies, of 5 ROSAT fields.