1,721,633 research outputs found
THE LINK BETWEEN THE HIDDEN BROAD LINE REGION AND THE ACCRETION RATE IN SEYFERT 2 GALAXIES
No full text""\\"In the past few years, more and more pieces of evidence have been presented for a revision of the widely accepted unified model of active galactic nuclei. A model based solely on orientation cannot explain all the observed phenomenology. In the following, we will present evidence that accretion rate is also a key parameter for the presence of hidden broad line regions (HBLRs) in Seyfert 2 galaxies. Our sample consists of 21 sources with polarized hidden broad lines and 18 sources without hidden broad lines. We use stellar velocity dispersions from several studies on the Ca II and Mg b triplets in Seyfert 2 galaxies to estimate the mass of the central black holes via the M-BH-sigma(*) relation. The ratio between the bolometric luminosity, derived from the intrinsic (i.e., unabsorbed) X-ray luminosity, and the Eddington luminosity is a measure of the rate at which matter accretes onto the central supermassive black hole. A separation between Compton-thin HBLR and non-HBLR sources is clear, both in accretion rate (log L-bol\\\\\\\/L-Edd = -1.9) and in luminosity (log L-bol = 43.90). When properly luminosity-corrected Compton-thick sources are included, the separation between HBLR and non-HBLR is less sharp but no HBLR source falls below the Eddington ratio threshold. We speculate that non-HBLR Compton-thick sources with accretion rate higher than the threshold do possess a BLR, but something, probably related to their heavy absorption, is preventing us from observing it even in polarized light. Our results for Compton-thin sources support theoretical expectations. In a model presented by Nicastro, the presence of broad emission lines is intrinsically connected with disk instabilities occurring in proximity of a transition radius, which is a function of the accretion rate, becoming smaller than the innermost stable orbit for very low accretion rates and therefore luminosities. RI Bianchi, Stefano\\\\\\\/B-4804-2010 OI Bianchi, Stefano\\\\\\\/0000-0002-4622-4240\\""
The Suzaku X-ray spectrum of NGC 3147 Further insights on the best "true" Seyfert 2 galaxy candidate
No full text""Context. NGC 3147 is so far the most convincing case of a "true" Seyfert 2 galaxy, i.e. a source genuinely lacking the broad line regions. Aims. We obtained a Suzaku observation with the double aim to study in more detail the iron line complex, and to check the Compton-thick hypothesis for the lack of observed optical broad lines. Methods. The Suzaku XIS and HXD\\\/PIN spectra of the source were analysed in detail. Results. The line complex is composed of at least two unresolved lines, one at about 6.45 keV and the other one at about 7 keV, most likely identified with Fe XVII\\\/XIX, the former, and Fe XXVI, the latter. The high-ionization line can originate either in a photoionized matter or in an optically thin thermal plasma. In the latter case, an unusually high temperature is implied. In the photoionized model case, the large equivalent width can be explained either by an extreme iron overabundance or by assuming that the source is Compton-thick. In the Compton-thick hypothesis, however, the emission above 2 keV is mostly due to a highly ionized reflector, contrary to what is usually found in Compton-thick Seyfert 2s, where reflection from low ionized matter dominates. Moreover, the source flux varied between the XMM-Newton and the Suzaku observations, taken 3.5 years apart, confirming previous findings and indicating that the size of the emitting region must be smaller than a parsec. The hard X-ray spectrum is also inconclusive on the Compton-thick hypothesis. Weighting the various arguments, a "true" Seyfert 2 nature of NGC 3147 seems to be still the most likely explanation, even if the "highly ionized reflector" Compton-thick hypothesis cannot at present be formally rejected. RI Bianchi, Stefano\\\/B-4804-2010 OI Bianchi, Stefano\\\/0000-0002-4622-4240"
Active galaxy 4U 1344-60: did the relativistic line disappear?
No full text""Context. X-ray bright active galactic nuclei represent a unique astrophysical laboratory for studying accretion physics around supermassive black holes. Aims. 4U 1344-60 is a bright Seyfert galaxy which revealed relativistic reflection features in the archival XMM-Newton observation. Methods. We present the spectroscopic results of new data obtained with the Suzaku satellite and compare them with the previous XMM-Newton observation. Results. The X-ray continuum of 4U 1344-60 can be well described by a power-law component with the photon index approximate to 1.7 modified by a fully and a partially covering local absorbers. We measured a substantial decrease of the fraction of the partially absorbed radiation from around 45% in the XMM-Newton observation to less than 10% in the Suzaku observation while the power-law slope remains constant within uncertainties. The iron line in the Suzaku spectrum is relatively narrow, sigma = (0.08 +\\\/- 0.02) keV, without any suggestion for relativistic broadening. Regarding this, we interpret the iron line in the archival XMM-Newton spectrum as a narrow line of the same width plus an additional red-shifted emission around 6.1 keV. Conclusions. No evidence of the relativistic reflection is present in the Suzaku spectra. The detected red-shifted iron line during the XMM-Newton observation could be a temporary feature either due to locally enhanced emission or decreased ionisation in the innermost accretion flow. RI Bianchi, Stefano\\\/B-4804-2010 OI Bianchi, Stefano\\\/0000-0002-4622-4240"
The X-ray reflector in NGC 4945: a time- and space-resolved portrait
No full text""\\"We present a time, spectral and imaging analysis of the X-ray reflector in NGC 4945, which reveals its geometrical and physical structure with unprecedented detail. NGC 4945 hosts one of the brightest AGN in the sky above 10 keV, but it is only visible through its reflected\\\\\\\/scattered emission below 10 keV, due to absorption by a column density of similar to 4 x 1024 cm-2. A new Suzaku campaign of five observations spanning similar to 6 months, together with past XMMNewton and Chandra observations, shows a remarkable constancy (within <10 per cent) of the reflected component. Instead, Swift-BAT reveals strong intrinsic variability on time-scales longer than 1 yr. Modelling the circumnuclear gas as a thin cylinder with the axis on the plane of the sky, we show that the reflector is at a distance =30-50 pc, well within the imaging capabilities of Chandra at the distance of NGC 49-45 (1 arcsec similar to 18 pc). Accordingly, the Chandra imaging reveals a resolved, flattened, similar to 150 pc long clumpy structure, whose spectrum is fully due to cold reflection of the primary AGN emission. The clumpiness may explain the small covering factor derived from the spectral and variability properties. RI Bianchi, Stefano\\\\\\\/B-4804-2010 OI Bianchi, Stefano\\\\\\\/0000-0002-4622-4240\\""
On the origin of soft X-rays in obscured AGN: answers from high-resolution spectroscopy with XMM-Newton
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30+ anni di aziendalisti in Laguna. Gli studi manageriali a Venezia
Full text linkLa ricorrenza del trentennale di ALEA, la storica associazione dei laureati in economia aziendale di Ca’ Foscari, apre a una riflessione corale sullo sviluppo degli studi manageriali a Venezia. Primo in Italia assieme alla Bocconi, il corso di laurea in economia aziendale dell’Ateneo veneziano ha formato generazioni di manager, imprenditori e specialisti di caratura internazionale. Scritto assieme a diversi docenti e laureati eccellenti che hanno animato Ca’ Bembo, sede storica del dipartimento, il presente volume indaga l’evoluzione delle discipline aziendali in Laguna, oltre a restituire significative esperienze dei professionisti del management usciti da Ca’ Foscari
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