1,721,106 research outputs found
Testing the ionized disc reprocessing model for the soft X-ray emission of quasars
No full textOne of the current explanations for the soft X-ray emission of active galactic nuclei (AGN) is reprocessing of the hard X-rays by partially ionized, optically thick matter. This idea is very appearing because it would explain the shape of the AGN soft X-ray spectrum in terms of atomic physics. While at present the reflection model correctly describes the soft X-ray spectra of a few low-luminosity Seyfert galaxies, it is not clear whether or not it can be applied to higher luminosity quasars. To investigate this issue quantitatively, we have fitted the high signal-to-noise ratio Position Sensitive Proportional Counter (PSPC) spectra of 11 AGN of different luminosities with a model consisting of a direct hard X-ray component, reflection from the ionized surface of an accretion disc and the direct thermal emission of the disc. We find that the AGN with an acceptable fit are a minority, and all have a low optical (and bolometric) luminosity, flat PSPC energy index alpha(PSPC) and the flattest alpha(OX) of the sample, while those with the worst fit all have high optical (and bolometric) luminosity, steep alpha(PSPC) and the steepest alpha(OX) of the sample. We conclude that either the reprocessing model is not correct and the form of the soft X-ray spectrum of AGN cannot be simply explained in terms of characteristic atomic features (i.e. highly ionized oxygen K-edges), or the origin of the soft X-ray emission of AGN is not 'universal'
Resonant absorption in the active galactic nucleus spectra emerging from photoionized gas: Differences between steep and flat ionizing continua
No full textWe present photoionization models accounting for both photoelectric and resonant absorption. Resonance absorption lines from C, O, Ne, Mg, Si, S, and Fe between 0.1 and 10 keV are treated. In particular we consider the complex of almost 60 strong Fe L absorption lines around 1 keV. We calculate profiles, intensities, and equivalent widths of each line, considering both Doppler and natural broadening mechanisms. Doppler broadening includes a term accounting for turbulence of the gas along the line of sight. We computed spectra transmitted by gas illuminated by drastically different ionizing continua and compared them to spectra observed in flat X-ray spectrum, broad optical emission-line type active galactic nuclei (AGNs), and steep X-ray spectrum, narrow optical emission-line type AGNs. We show that the similar to 1 keV absorption feature observed in moderate-resolution X-ray spectra of several narrow-line Seyfert 1 galaxies can be explained by photoionization models, taking into account resonance absorption, without requiring relativistic outflowing velocities of the gas, if the physical properties of these absorbers are close to those found in Bat X-ray spectrum Seyfert 1 galaxies. We finally present simulations of the spectra emerging from gas illuminated by both steep and flat ionizing continua, as seen by the AXAF high-energy transmission gratings and the baseline Constellation-X calorimeter. We discuss briefly the relevant physics that can be investigated with these instruments
The high energy resolution view of warm absorbers
No full textA number of absorption features are expected to be visible in high energy resolution spectra of type 1 AGN with ionized gas along the line of sight (so called "warm absorbers"). These include photoelectric absorption K and L edges along with many strong K alpha, K beta and L resonance absorption lines from the most abundant elements. We present detailed simulations of our "photoelectric + resonant absorption" model with the high energy resolution gratings and calorimeters of AXAF, XMM and Constellation-X, and discuss the relevant physics which can be addressed with the new generation of X-ray spectrometers
Quantitative and qualitative characteristics of lambs slughtered at three different ages.
No full textAccrescimento post-svezzamento di agnelli di razza massese: rilievi in vivo ed alla macellazione.
No full textTHE 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\\""
X-ray spectral analysis of the low-luminosity active galactic nucleus NGC 7213 using long XMM-Newton observations
No full textWe present the X-ray spectral results from the longest X-ray Multi-mirror Mission–Newton observation, 133 ks, of the low-luminosity active galactic nucleus NGC 7213. The hardness-ratio analysis of the X-ray light curves discloses a rather constant X-ray spectral shape, at least for the observed exposure time, enabling us to perform X-ray spectral studies using the total observed spectrum. Apart from a neutral Fe Kα emission line, we also detect narrow emission lines from the ionized iron species, Fe xxv and Fe xxvi. Our analysis suggests that the neutral Fe Kα originates from a Compton-thin reflector, while the gas responsible for the high-ionization lines is collisionally excited. The overall spectrum, in the 0.3–10 keV energy band, registered by the European Photon Imaging Camera can be modelled by a power-law component (with a slope of Γ ≅ 1.9) plus two thermal components at 0.36 and 8.84 keV. The low-energy thermal component is entirely consistent with the X-ray spectral data obtained by the Reflection Grating Spectrometer between 0.35–1.8 keV
Produzione dell'agnello da latte Massese allevato artificialmente: influenza dell'eta' di macellazione sulle rese e sui parametri quanti-qualitativi della carcassa
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