1,415 research outputs found
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
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
The Phoenix galaxy: UGC 4203 re-birth from its ashes?
No full textWe report on a dramatic transition between a Compton-thick, reflection-dominated state and a Compton-thin state in the Seyfert 2 galaxy UGC 4203, discovered by comparing a recent (May 2001) XMM-Newton observation with ASCA observations performed about six years earlier. This transition can be explained either as a change in the column density of the absorber, maybe due to moving clouds in a clumpy torus, or as the revival of a transient active nucleus, which was in a phase of very low activity when observed by ASCA. If the latter explanation is correct, spectral transitions of this kind provide observational support to the idea that Compton-thick and Compton-thin regions coexist in the same source, the former likely to be identified with the "torus", the latter with dust lanes on much larger scales
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
Spectral ageing in the lobes of FR-II radio galaxies: new methods of analysis for broad-band radio data
No full textThe broad-bandwidth capabilities of next generation telescopes such as the Jansky Very Large Array (JVLA) mean that the spectrum of any given source varies significantly within the bandwidth of any given observation. Detailed spectral analysis taking this variation into account is set to become standard practice when dealing with any new broad-band radio observations; it is therefore vital that methods are developed to handle this new type of data. In this paper, we present the Broadband Radio Astronomy ToolS (brats) software package and, use it to carry out detailed analysis of JVLA observations of three powerful radio galaxies. We compare two of the most widely used models of spectral ageing, the Kardashev–Pacholczyk and Jaffe–Perola models and also results of the more complex, but potentially more realistic, Tribble model. We find that the Tribble model provides both a good fit to observations as well as providing a physically realistic description of the source. We present the first high-resolution spectral maps of our sources and find that the best-fitting injection indices across all models take higher values than have previously been assumed. We present characteristic hotspot advance speeds and make comparison to those derived from dynamical ages, confirming the previously known discrepancy in speed remains present when determined at high spectral resolutions. We show that some previously common assumptions made in determining spectral ages with narrow-band radio telescopes may not always hold and strongly suggest that these are accounted for in future investigations
Sequential double cross-validation for assessment of added predictive ability in high-dimensional omic applications
No full textEnriching existing predictive models with new biomolecular markers is an important task in the new multi-omic era. Clinical studies increasingly include new sets of omic measurements which may prove their added value in terms of predictive performance. We introduce a two-step approach for the assessment of the added predictive ability of omic predictors, based on sequential double cross-validation and regularized regression models. We propose several performance indices to summarize the two-stage prediction procedure and a permutation test to formally assess the added predictive value of a second omic set of predictors over a primary omic source. The performance of the test is investigated through simulations. We illustrate the new method through the systematic assessment and comparison of the performance of transcriptomics and metabolomics sources in the prediction of body mass index (BMI) using longitudinal data from the Dietary, Lifestyle, and Genetic determinants of Obesity and Metabolic syndrome (DILGOM) study, a population-based cohort from Finland
The recent X-ray history of NGC 5506
No full textWe present a detailed discussion of the spectral and spatial components
of NGC 5506, based on XMM–Newton, Chandra and BeppoSAX observations.
The overall picture consists of a nucleus absorbed by cold gas with column
density of ≈
1022 cm-2 and surrounded by a Compton-thick torus,
whose existence is inferred by a cold reflection component and an iron Kα
line. On a much larger scale, a photoionized gas extended on ≈
350 pc
reprocesses the nuclear radiation, producing a soft excess and ionized iron
lines. Noteworthy, we find no evidence for the presence of the accretion disc.
X-ray reprocessing in Seyfert galaxies: Simultaneous XMM-Newton/BeppoSAX observations
No full textWe selected a sample of eight bright unobscured (at least at the iron line energy) Seyfert galaxies observed simultaneously by XMM-Newton and BeppoSAX, taking advantage of the complementary characteristics of the two missions. The main results of our analysis can be summarized as follows: narrow neutral iron lines are confirmed to be an ubiquitous component in Seyfert spectra; none of the analyzed sources shows unambiguously a broad relativistic iron line; all the sources of our sample (with a single exception) show the presence of a Compton reflection component; emission lines from ionized iron are observed in some sources; peculiar weak features around 5–6 keV (possibly arising from rotating spots on the accretion disk) are detected in two sources. The scenario emerging from these results strongly requires some corrections for the classical model of reprocessing from the accretion disk. As for materials farther away from the Black Hole, our results represent a positive test for the Unification Model, suggesting the presence of the torus in (almost) all sources, even if unobscured.
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