1,721,110 research outputs found
SPECTRAL CONSTRAINTS ON SEYFERT-2 GALAXIES AS MAJOR CONTRIBUTORS TO THE HARD (3-100 KEV) X-RAY-BACKGROUND
No full textIt has been suggested that the flat spectrum of the X-ray background (XRB) above 3 keV and below approximately 15 keV could be explained by the superposition of absorbed sources at different redshifts. To explain the steepening of the XRB spectrum above approximately 15 keV, the intrinsic source spectrum should cut off at approximately 50-100 keV, as observed in NGC 4151 and galactic black hole candidates. Here, assuming that Seyfert 2 galaxies are Seyfert 1 galaxies obscured by intervening matter in the line of sight, as postulated by the unified model, and that they provide the major contribution to the XRB in the 3-100 keV energy range, we derive some constraints on their spectrum and evolution, and indicate, in view of future ASCA measurements, the use of iron spectral features as a test of the model
Iron K fluorescent lines from relativistic, ionized discs
No full textWe present detailed Monte Carlo calculations of the iron K alpha lines from X-ray photoionized, relativistic accretion discs. The vertical ionization structure of the matter has been taken into account, and Compton scattering and resonant absorption have been treated in detail. We find that the ratio of lines emitted by different iron ions depends, apart from the ionization parameter, on the disc inclination angle. The line profiles with relativistic corrections have also been calculated; they can be used to distinguish between different source geometries
IRON K-ALPHA LINES FROM X-RAY PHOTOIONIZED ACCRETION DISKS
No full textThe properties of the iron Kalpha line emitted by an accretion disc illuminated by an external X-ray source are calculated for different values of the disc accretion rate m, and for two different source geometries: a point source located on the disc axis and an extended source above the innermost part of the disc. We find that for large values of rh the matter can be significantly ionized, and the iron line equivalent width can reach values as high as 250 eV for the point source, and up to about 400 eV for the extended source; the line centroid energy, in the emitting rest frame, is significantly higher than 6.4 keV, the value for neutral iron. A further increase of m leads to a strong decrease of the line intensity, because the iron becomes fully stripped in the inner region of the disc. The line profiles in the Schwarzschild metric are also calculated, and for the point source they appear much more complex than those obtained assuming neutral matter
The iron K line complex in NGC 1068: Implications for X-ray reflection in the nucleus
No full textWe report a new analysis of ASCA data on the iron K line complex in NGC 1068. The line complex basically consists of three components, as previously reported. A weak red wing of the 6.3-keV fluorescence iron K line is found. A plausible explanation is Compton scattering in optically thick, cold matter which can be identified with an obscuring torus or cold gas in the host galaxy. We also show that this 'Compton shoulder' should be observable with ASCA using a simulated reflection spectrum. In order to explain the two higher energy Lines as well as the cold 6.4-keV line, we fit the ASCA data with a composite model of cold and warm reflection. This shows that cold reflection dominates the observed X-ray emission above 4 keV; the estimated scattering fraction from the warm medium is found to be similar to 0.08 per cent, an order of magnitude below the standard value of I per cent obtained from previous observations in other wavebands. The two higher energy lines have large equivalent width (similar to 3 keV) with respect to the warm-scattered continuum, suggesting that efficient resonant scattering operates. The line energies are systematically lower than those expected from resonant lines for Fe XXV and Fe XXVI by similar to 100 eV. The redshifts may be due to either the ionized gas of the warm mirror receding at a radial velocity of 4000-5000 km s(-1) or to effects of Compton scattering in a complicated geometry
X-ray spectroscopy of the scattered emission in NGC 1068
No full textWe discuss the iron K line complex in NGC 1068. The line complex basically consists of three components, as previously reported. A new analysis of the ASCA data shows that cold reflection appears to dominate the observed X-ray emission above 4 keV based on the detection of 'Compton shoulder', a weak red wing of the 6.4 keV fluorescence iron K line, and a very flat continuum. The other two weaker lines at higher energies can be identified with FeXXV and FeXXVI and suggest an highly ionized X-ray mirror as well, although the line energies are systematically lower than those expected from the resonant lines, consistent with a redshift by 1.5%
X-RAY PHOTOIONIZED ACCRETION DISKS - UV AND X-RAY CONTINUUM SPECTRA AND POLARIZATION
No full textWe calculate the UV and X-ray continuum spectrum and polarization from an accretion disc centrally illuminated by X-rays. The calculation of the ionization structure of the disc is performed by using the numerical code described by Ross & Fabian, while the inclination effects and the polarization are calculated by solving the transfer equation for polarized radiation. The general relativistic effects in the Schwarzschild metric are also taken into account. The spectra we obtain can explain the X-ray observation for some active galactic nuclei and several Galactic black hole candidates, for which both the iron-line intensity and the shape of the reflection continuum appear to be inconsistent with reprocessing from neutral matter. We find that the degree and angle of polarization vary strongly with the energy; this fact should be testable by future observations. We also suggest, on the basis of earlier works by Loskutov & Sobolev, that the absorption opacity solution of the polarization problem for thin discs can reconcile theory with observations, since even a small absorption opacity can strongly reduce the polarization degree to very low, if observable, values
The iron K alpha line complex in Compton-thick Seyfert 2 galaxies
No full textWe calculate the intensities of the fluorescent and resonantly scattered iron K alpha lines in those active galactic nuclei (AGN) in which the primary radiation at the line energy is completely blocked by intervening matter along the line of sight (the so-called 'Compton-thick' Seyfert 2 galaxies). In the framework of unification models for Seyfert galaxies, both a 'cold' fluorescent line from the obscuring torus and highly ionized lines from the warm material responsible for the scattering and polarization of optical photons are expected. The neutral iron line from the obscuring torus has an equivalent width, with respect to the continuum reflected by the same matter, of order 1-2 keV, provided that the matter is Compton thick. The line emission from the warm material, in the optically thin regime for all relevant processes, is dominated by the resonantly scattered lines. However, for plausible physical conditions of this matter, the resonant lines are likely to be optically thick in their centres. In this case, the resonant line intensity can significantly decrease, and for tau(T) greater than or similar to 0.1 the fluorescent/recombination lines dominate the resonant lines. We present analytical formulae for the equivalent widths of iron lines in the optically thin case, and numerical results (obtained by means of Monte Carlo simulations) for the optically thick case. We apply the theoretical results to the most famous Compton-thick source, the Seyfert 2 galaxy NGC 1068, The large uncertainties in the continuum spectral shape and line equivalent widths prevent any definite conclusions, but the application can be regarded as an example of how the results presented in this paper could be used to estimate physical and geometrical quantities in obscured AGN
Geometrical and chemical dependence of K-shell X-ray features
No full textWe review and discuss the chemical and geometrical dependence of iron K features (emission line and absorption edge), and of fluorescent lines from other elements, emitted by neutral, externally X-ray-illuminated matter. Our aim is to provide simple, practical tools which could be useful for observers in order to estimate parameters easily from measured quantities. The dependence of the iron fluorescent line on the iron and oxygen abundances is explored in detail, and simple analytical formulae are given for both the Morrison & McCammon and Anders & Grevesse abundances. In the latter case, the variation of the depth of the iron K edge with the iron abundance is also shown. Fluorescent line intensities from other elements, from oxygen to nickel, are also given as a function of the inclination angle and chemical abundances. The effects of the presence of a hot corona above the cold matter on the iron line and edge are discussed. We also discuss the spectral signature of resonant transitions occurring in the hot corona itself. The present calculations are relevant to AGN and Galactic X-ray binaries, in particular black hole candidates and cataclysmic variables
A hard X-ray constraint on the presence of an AGN in the ultraluminous infrared galaxy Arp 220
No full textWe present X-ray results on the ultraluminous infrared galaxy Arp 220 obtained with BeppoSAX. X-ray emission up to 10 keV is detected. No significant signal is detected with the PDS detector in the higher energy band. The 2-10 keV emission has a flat spectrum (Gamma similar to 1.7), similar to M82, and a luminosity of similar to1 x 10(41) erg s(-1). A population of X-ray binaries may be a major source of this X-ray emission. The upper limit of an iron K line equivalent width at 6.4 keV is similar or equal to 600 eV. This observation imposes the tightest constraint so far on an active nucleus if present in Arp 220. We find that a column density of X-ray absorption must exceed 10(25) cm(-2) for an obscured active nucleus to be significant in the energetics, and the covering factor of the absorption should be almost unity. The underluminous soft X-ray starburst emission may need a good explanation, if the bolometric luminosity is primarily powered by a starburst
On the transmission-dominated to reprocessing-dominated spectral state transitions in Seyfert 2 galaxies
No full textWe present Chandra and XMM-Newton observations of a small sample (11 objects) of optically selected Seyfert 2 galaxies, for which ASCA and BeppoSAX had suggested Compton-thick obscuration of the active galactic nucleus (AGN). The main goal of this study is to estimate the rate of transitions between 'transmission-dominated' and 'reprocessing-dominated' states. We discover one new transition in NGC 4939, with a possible additional candidate in NGC 5643. This indicates a typical occurrence rate of at least similar to0.02 yr(-1). These transitions could be due to large changes of the obscuring gas column density, or to a transient dimming of the AGN activity, the latter scenario being supported by detailed analysis of the best-studied events. Independently of the ultimate mechanism, comparison of the observed spectral dynamics with Monte Carlo simulations demonstrates that the obscuring gas is largely inhomogeneous, with multiple absorbing components possibly spread through the whole range of distances from the nucleus between a fraction of parsecs up to several hundred parsecs. As a by-product of this study, we report the first measurement ever of the column density covering the AGN in NGC 3393 (N(H)similar or equal to 4.4 x 10(24) cm(-2)), and the discovery of soft X-ray extended emission, apparently aligned along the host galaxy main axis in NGC 5005. The latter object most likely hosts an historically misclassified low-luminosity Compton-thin AGN
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