1,721,015 research outputs found
THE IRON K-ALPHA RESPONSE IN AN X-RAY ILLUMINATED RELATIVISTIC DISK AND A BLACK-HOLE MASS ESTIMATE
No full textA method is proposed for the estimate of the black hole mass in an active galactic nucleus which is based on the response of the intensity, centroid energy and width of the iron Kalpha fluorescence line to variations of the ionizing continuum. The method is illustrated for the case of a geometrically thin and optically thick, X-ray illuminated disc, in Keplerian rotation around a Schwarzschild black hole, using a detailed model of the line emissivity. This method is complementary to the one that makes use of variations in detailed line profiles, and could be applied also to measurements obtained at moderate energy resolution
X-ray obscuration and obscured AGN in the local universe
No full textWe discuss the X-ray properties of 49 local (z = sigma(t)(-1) similar or equal to 1.6 x 10(24) cm(-2)) is similar or equal to 50%, and reaches similar or equal to 80% for log (F2-10) 0.6 keV) are common in our sample ( 6 new detections at a confidence level >= 2 sigma). They are explained as due to reflection off the illuminated side of optically thick material. We confirm a correlation between the presence of a similar to 100-pc scale nuclear dust in the WFC2 images and Compton-thin obscuration. We interpret this correlation as due to the large covering fraction of gas associated with the dust lanes. The X-ray spectra of highly obscured AGN invariably present a prominent soft excess emission above the extrapolation of the hard X-ray component. This soft component can account for a very large fraction of the overall X-ray energy budget. As this component is generally unobscured - and therefore likely produced in extended gas structures - it may lead to a severe underestimation of the nuclear obscuration in z similar to 1 absorbed AGN, if standard X-ray colors are used to classify them. As a by-product of our study, we report the discovery of a soft X-ray, luminous (similar or equal to 7 x 10(40) erg s(-1)) halo embedding the interacting galaxy pair Mkn 266
A model for the X-ray absorption in Compton-thin AGN
No full textThe fraction of AGN with photoelectric absorption in the X-rays ranging from N-H of 10(22) up to about 10(24) cm(-2) (Compton-thin) appears observationally to be anticorrelated to their luminosity L-x. This recently found evidence is used to investigate the location of the absorbing gas. The molecular torus invoked in the unified picture of AGN, while it can be regarded as confirmed on several grounds to explain the Compton-thick objects, do not conform to this new constraint, at least in its physical models as developed so far. In the frame of observationally based evidence that in Compton-thin sources the absorbing gas might be located far away from the X-ray source, it is shown that the gravitational effects of the black hole (BH) on the molecular gas in a disk, within 25 - 450 pc ( depending on the BH mass, from 10(6) to 10(9) M-.), leads naturally to the observed anticorrelation, under the assumption of a statistical correlation between the BH mass and Lx. Its normalization is also reproduced provided that the surface density, Sigma, of this gas is larger than about 150 - 200 M-. pc(-2), and assuming that the bolometric luminosity is one tenth of the Eddington limit. Interestingly, the required values are consistent with the value of the 300 pc molecular disk in our own galaxy, namely 500 M-. pc(-2). In a sample of nearby galaxies from the BIMA SONG survey, it is found that half of the objects have central Sigma larger than 150 M-. pc(-2). Given the simplicity of the proposed model, this finding is very encouraging, waiting for future higher resolution surveys in CO on more distant galaxies
MULTIPLE-PEAKED LINE-PROFILES FROM RELATIVISTIC DISKS AT HIGH INCLINATION ANGLES
No full textThe profiles of lines from a flat. relativistic disk around a Schwarzschild black hole are computed with a rigorous treatment of the energy shifts and photon trajectories. It is found that, unless the disk is optically thin, further structures emerge between the two Doppler horns, at high ( greater than or similar to 80-degrees) inclination angles. These structures are due to a geodesics focusing effect on photons emitted from the ''far'' side of the disk. Their shape and relative intensity are a function of the radial dependence and angular distribution of the line emissivity. Since these structures are due to a purely general relativistic effect, they could provide an unambiguous signature of a relativistic disk in Active Galactic Nuclei and in X-ray binaries
THE IRON LINE AND HIGH-ENERGY BUMP AS X-RAY SIGNATURES OF COLD MATTER IN SEYFERT-1 GALAXIES
No full textGinga observations of several bright Seyfert 1 galaxies have recently revealed the presence in their X-ray spectra of both the iron K fluorescence line and a high energy bump, which together are currently interpreted as due to the reprocessing of the primary radiation by cold matter located near the putative black hole. A number of quantitative model predictions on the reprocessed radiation, when the matter is assumed to be distributed in an accretion disk, are presented in this paper; they were obtained, on the basis of Monte Carlo simulations, for different geometries of the disk surface and of the primary X-ray source. The case of a flat disk and a point source placed above the disk on its symmetry axis is treated in greater detail, taking into account the general relativistic and kinematic effects, which have dramatic consequences on the shape of the K line and edge. These effects are less relevant for the equivalent width (W-alpha) of the line and for the strength of the high energy bump, and were therefore ignored for simplicity in computing these two quantities for other source geometries (an optically thin corona, which turns out to be well approximated by the point source, and an optically thick sphere), and for other configurations of the disk surface (conical and concave). Concerning W-alpha, its maximum value is about 150 eV for a flat, 180 eV for a conical and 230 eV for a concave disk seen face-on. Concerning the strength of the bump, it turns out to be an excellent indicator of strong deviations from the planar configuration, and it is shown that, being practically independent of the iron abundance, it can be used to constrain the abundance when used in conjunction with W-alpha. Several observational implications are discussed, in particular it is noted that the relativistic and kinematic effects on the shape of the edge might lead to a misinterpretation of low resolution measurements in terms of absorption by highly ionized iron
X-ray absorption in Seyfert 2 galaxies
No full textWe have studied the correlation among X-ray absorption, optical reddening and nuclear dust morphology in Seyfert 2 galaxies. Two main conclusions emerge: (i) the Balmer decrement and the amount of X-ray absorption are anticorrelated over a wide range of column density, 10(21) less than or similar to N-H less than or similar to 10(24) cm(-2) - the correlation no longer applies to Compton-thick objects (N-H greater than or similar to 10(24) cm(-2)), although they span a comparable range in Balmer decrement; (ii) Compton-thin Seyfert 2s seem to prefer nuclear environments, which are rich in dust on scales of hundreds of parsecs. On the other hand, Compton-thick Seyferts indifferently exhibit 'dust-poor' and 'dust-rich' environments. These results support an extension of the Seyfert unification scenario (as recently proposed by Matt), where Compton-thick Seyfert 2s are observed through compact 'tori', whereas Compton-thin ones are obscured by dust on much larger scales
IRON K-ALPHA LINE FROM X-RAY ILLUMINATED RELATIVISTIC DISKS
No full textThe intensity and profile of the iron K-alpha fluorescence line from a flat, optically thick accretion disk rotating around a Schwarzschild black hole and illuminated by a central X-ray source are computed using a fully relativistic treatment of the photon intensity and shifts. The X-ray source is modelled as an isotropic point source located on the symmetry axis at a height h in units of the gravitational radius. These calculations represent a refinement and an extension of those presented in Paper I for h = 20, carried out using a weak field approximation and therefore of validity limited to inclination angles less-than-or-similar-to 70-degrees. Here it is shown that at high inclination angles purely relativistic effects lead to the growth of features between the two Doppler horns and that, as a consequence, the line equivalent width maintains a sizeable value, while the centroid energy and the line width go through a broad maximum at about approximately 80-degrees. The statistical implications for the expected distribution of the line parameters in a sample of randomly oriented disks in Seyfert galaxies are briefly discussed
A possible bias on the estimate of L-bol/L-edd in AGN as a function of luminosity and redshift
Full text linkContext. The BH mass (and the related Eddington ratio, l = L-bol/L-edd) in broad line AGN is usually evaluated by combining estimates (often indirect) of the BLR radius and of the FWHM of the broad lines, under the assumption that the BLR clouds are in Keplerian motion around the BH. Such an evaluation depends on the geometry of the BLR. There are two major options for the BLR configuration: spherically symmetric or "flattened". In the latter case the inclination to the line of sight becomes a relevant parameter. Aims. This paper is devoted to evaluate the bias on the estimate of the Eddington ratio when a spherical geometry is assumed (more generally when inclination effects are ignored), while the actual configuration is "flattened", as some evidence suggests. This is done as a function of luminosity and redshift, on the basis of recent results which show the existence of a correlation between the fraction of obscured AGN and these two parameters up to at least z = 2.5 (date at larger redshifts being insufficient). Methods. The assumed BLR velocity field is akin to the "generalized thick disk" proposed by Collin et al. (2006, A&A, 456, 75). Assuming an isotropic orientation in the sky, the mean value of the bias is calculated as a function of luminosity and redshift. Results. It is demonstrated that, on average, the Eddington ratio obtained assuming a spherical geometry is underestimated for high luminosities, and overestimated for low luminosities. This bias converges for all luminosities at z about 2.7, while nothing can be said on this bias at larger redshifts due to the lack of data. The effects of the bias, averaged over the luminosity function of broad line AGN, have been calculated. The results imply that the bias associated with the a-sphericity of the BLR make even worse the discrepancy between the observations and the predictions of evolutionary models
X-ray absorption in Compton-thin AGN: the predictions of a model revisited
No full textContext. The evidence of a decrease with increasing luminosity in the fraction f(abs) of absorbed and Compton-thin among X-ray-selected (2-10 keV) AGN is observationally well supported, while that of an increase in f(abs) with redshift is fairly controversial. In a previous paper, the gravitational effect of the SMBH on the molecular interstellar gas, in the central region of the host galaxy, was shown to predict an anti-correlation between f(abs) and the black-hole mass M(BH). Aims. The most recent findings on the distribution of the Eddington ratio lambda = L(b)/L(E) as a function of M(BH) and z are used to convert that relationship into one between f(abs) and both bolometric (L(b)) and X-ray (L(X)) luminosities at various values of z. Methods. The findings for lambda(M(BH), z) are properly treated to ensure completeness in the prediction of f(abs) above a certain luminosity, at values of z = 0.1, 0.35, 0.7, and >= 1. To verify the consequence of these findings alone, we first adopted a distribution of gas surface density Sigma, observed in a sample of local spiral galaxies, irrespective of the galaxy morphological type and z. Results. Assuming the Eddington limit, lambda = 1, in the lambda(M(BH), z) distribution as a "natural" cut-off, the predictions are consistent with the existence of an anti-correlation between f(abs) and L(X), but they fail to reproduce an increase in f(abs) with z. Because the early type galaxies are on average much poorer in molecular gas than late type ones, a quantitative agreement with the local value of f(abs) requires the existence of a correlation between Sigma and the central activity. An increase in typical values of Sigma with z, correlated with the activity, might explain an increase in f(abs) with z. However, f(abs) could hardly exceed about 0.3 at the highest luminosities
Close and distant reprocessing media in Mkn 509 studied with BeppoSAX
No full textWe present the broad band analysis of two BeppoSAX observations of the Seyfert 1 Mkn 509. In 2000 the source was in a typical flux state, F2-10 keV=2.7x10(-11) erg cm(-2) s(-1), while in 1998 it was found in a high flux state, F2-10 (keV)=5.7x10(-11) erg cm(-2) s(-1). A comparison between the two states shows a energy-dependent flux variation of about a factor of three and a factor of two in the LECS (0.15-3 keV) and MECS (1.5-10 keV), respectively, while in the PDS(13-200 keV) the difference is marginal. A soft excess, a narrow iron line and a Compton reflection hump above 10 keV are clearly apparent in the residuals after fitting the spectra with a simple power law. We tested two alternative models. In the first the iron line and the high energy bump are well reproduced by reprocessing in a cold and Compton thick material. The intensity of the iron line (also consistent with a Chandra measurement) as well as the normalization of the reflection hump are consistent with a constant in the two epochs: this, combined with the fact that the line is narrow as observed by Chandra, suggests a common origin from distant and optically thick matter. This model further requires a component to model the soft excess: the empirical choice of two black bodies accounts well for the excess in both observations; their combined strength was a factor of about three higher in the high than in the low flux state defined above. However, the relative contribution of the soft excess is higher in the low flux state. In the second model we attempted to reproduce all spectral features, except for the narrow cold line, with reflection from an ionized disc. This model is successful only in the high flux state, but it fails in the low flux state, when the soft excess is only partially accounted for. In either model, the slope of the power law is greater in the high than in the low flux state, (DeltaGammasimilar to0.2), in agreement with a behaviour known to be shared by several objects of the same type
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