1,721,155 research outputs found
A hard medium survey with ASCA III. a Type 2 AGN revealed from X-ray spectroscopy
No full textIn this paper we report the discovery of a hard Xray selected Type 2 Seyfert galaxy and we present and discuss its X-ray and optical spectrum together with the radio to X-ray energy distribution. The X-ray source - AXJ2254+1146 - is part of the ASCA Hard Serendipitous Survey (HSS). What makes this discovery particularly noteworthy is the fact that the Spe 2 classification of this Seyfert galaxy has resulted directly from the X-ray data and has been confirmed by optical spectroscopy only subsequently. The X-ray spectrum of AXJ2254+1146 is best described by a model consisting of an unresolved Gaussian line at 6.43 +/- 0.1 keV plus the so called "leaky-absorber" continua having an intrinsic power law photon index of Gamma = 2.51(2.17)(2.76) (1 sigma confidence interval). The best fit values of the absorbing column density (N-H = 1.85(1.47)(2.24) x 10(23) cm(-2)), of the line equivalent width (0.6(0.36)(0.84) keV) and of the scattering fraction (0.7(0.1)(1.4)%), lead us to classify it as a Type 2 AGN from an X-ray point of view. Inspection of the POSS II image reveals the presence, within the ASCA X-ray error circle, of the nearby Sbc spiral galaxy UGC 12237 (m(Bo) = 14.26) that, even on positional ground considerations alone, is the most likely optical counterpart of AXJ2254+1146. Subsequent optical spectroscopy of UGC 12237 has confirmed its Seyfert 2 optical nature
The ROSAT deep cluster survey: The X-ray luminosity function out to z = 0.8
No full textWe 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 x 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 less than or equal to 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 x 10(-14) ergs cm(-2) s(-1)), reveals no significant evolution at least out to z = 0.8, over a luminosity range of 2 x 10(42) to 3 x 10(44) 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
The Simbol-X view of the unresolved X-ray background
Get PDFWe will briefly discuss the importance of sensitive X-ray observations
above 10 keV for a better understanding of the physical mechanisms associated
to the Supermassive Black Hole primary emission and to the cosmological
evolution of the most obscured Active Galactic Nuclei
BeppoSAX observations of two high redshift clusters of galaxies: RXJ 0152.7-1357 and MS 2053.7-0449
No full textWe present X-ray observations of two high redshift clusters of galaxies carried out with the BeppoSAX satellite. One cluster, RXJ0152.7-1357 at z similar or equal to 0.83, was selected from the ROSAT Deep Cluster Survey sample, as one of the most X-ray luminous systems known at z > 0.5. The optical and ROSAT-PSPC data show a complex morphology with at least two cores. Our SAX observations yield a gas temperature kT = 6.46(-1.19)(+1.74) keV and a metallicity A = 0.53(-0.24)(+0.29), with a prominent iron K-alpha line. The second cluster, MS 2053.7-0449 at z similar or equal to 0.58, was selected from the EMSS sample. Given the poor statistics no constraints on the metallicity can be derived fi om the present data. Large uncertainties are associated to the gas temperature (kT = 6.7(-2.3)(+6.8) keV), which has been obtained after fixing the abundance to 0.3 solar. Combining these results with those obtained for similarly high redshift (z = 0.5) clusters of galaxies with broad band X-ray spectra, we discuss the high redshift L-bol - T relationship. The data can easily be accommodated with a non-evolving (or mildly evolving) L-bol - T relation, a result which, when combined with the little observed evolution in the bulk of the X-ray cluster population, gives support to low Omega cosmological models
The BeppoSAX program on X-ray weak Seyfert 2 galaxies
No full textWe present a progress report on the Core Program project dealing with X-ray weak Seyfert 2 galaxies. The aim of the project is to increase the sample of Seyfert 2 galaxies with measured hard X-ray spectra, by adding progressively weaker objects within a well-defined optical sample. The hard X-ray information, together with the [OIII] and IR fluxes, the orientation of the host galaxy and the geometry of the ionization cone, should give the distribution of the absorbing column N-H, and assess the reliability of various proposed "isotropic" indicators of the luminosity. We have up to now a 100% detection rate, and find systematically low (X/[OIII]), high N-H, and high Fe Ew. While the first finding is due to our attempt of removing pre-existing biases, the remaining two support strongly the unified scenario, and point to a fraction of X-ray thick sources higher than usually assumed
A three-dimensional diagnostic diagram for Seyfert 2 galaxies: Probing X-ray absorption and Compton thickness
No full textWe present and discuss a three-dimensional diagnostic diagram for Seyfert 2 galaxies obtained using X-ray and CO mi data on a large sample of objects (reported in the Appendix). The diagram shows the Ka iron line equivalent width as a function of both the column density derived from the photoelectric cutoff and the 2-10 keV flux normalized to the CO mi optical-line flux (the latter corrected for extinction and assumed to be a true indicator of the source intrinsic luminosity). We find that the hard X-ray properties of type 2 objects depend on a single parameter, the absorbing column density along the line of sight, in accordance with the unified model. The diagram can be used to identify Compton-thick sources and to isolate and study peculiar objects. From this analysis we have obtained a column density distribution of Seyfert 2 galaxies that is thought to be a good approximation of the real distribution. A large population of heavily absorbed objects is discovered, including many Compton-thick candidates. Our results indicate that the mean log N-H cm(-2) in type 2 Seyfert galaxies is 23.5 and that as many as 23%-30% of sources have N-H greater than or equal to 10(24) cm(-2)
X-ray observation of ULAS J1120+0641, the most distant quasar at z=7.08
No full textWe probe the emission mechanism of the accreting super massive black holes in the high redshift Universe and, to do this, we study the X-ray spectrum of ULAS1120+064, the highest redshift quasar detected so far at z = 7.085, which has been deeply observed (340 ks) by XMM-Newton. Despite the long integration time, the spectral analysis is limited by the poor statistics, with only 150 source counts being detected. We measured the spectrum in the 2-80 keV rest-frame (0.3-10 keV observed) energy band. Assuming a simple power law model, we find a photon index of 2.0 +/- 0.3 and a luminosity of 6.7 +/- 0.3 x 10(44) erg s(-1) in the 2-10 keV band, while the intrinsic absorbing column can only be loosely constrained (N-H 6 quasar has been investigated through a deep X-ray observation. In agreement with previous studies of z similar to 6 AGN samples, we do not find any hint of evolution in the broadband energy distribution. Indeed from our dataset, ULAS 1120+0641 is indistinguishable from the population of optically bright quasar at lower redshift
The XMM-Newton Bright Survey sample of absorbed quasars: X-ray and accretion properties
No full textAlthough absorbed quasars are extremely important for our understanding of the energetics of the Universe, the main physical parameters of their central engines are still poorly known. In this work, we present and study a complete sample of 14 quasars (QSOs) that are absorbed in the X-rays (column density N-H > 4 x 10(21) cm(-2) and X-ray luminosity L2-10 keV > 10(44) ergs(-1); XQSO2) belonging to the XMM-Newton Bright Serendipitous Survey (XBS). From the analysis of their ultraviolet-to-mid-infrared spectral energy distribution, we can separate the nuclear emission from the host galaxy contribution, obtaining a measurement of the fundamental nuclear parameters, like the mass of the central supermassive black hole and the value of Eddington ratio, lambda(Edd). Comparing the properties of XQSO2s with those previously obtained for the X-ray unabsorbed QSOs in the XBS, we do not find any evidence that the two samples are drawn from different populations. In particular, the two samples span the same range in Eddington ratios, up to lambda(Edd) similar to 0.5; this implies that our XQSO2s populate the 'forbidden region' in the so-called 'effective Eddington limit paradigm'. Acombination of low grain abundance, presence of stars inwards of the absorber, and/or anisotropy of the disc emission can explain this result
- …
