1,721,397 research outputs found
Physical interpretation of the NGC 7469 UV/X-ray variability
No full textWe present a re-analysis of the simultaneous ∼30 day IUE/XTE observation of NGC 7469 done in 1996. Our main progress in this paper, in comparison to previous spectral analyses (Nandra et al. 1998, 2000), is to adopt and fit directly to the data a detailed model of the Comptonized spectrum. This firstly allows one to fit simultaneously the data from the UV to the hard X-ray band in a self-consistent way and secondly it gives direct constraints on the physical parameters of the disc-corona system, like the temperature and optical depth of the corona. Our results are completely consistent with a slab geometry where all the observed UV emission is supposed to cross the corona but more photon-starved geometries also give acceptable fits. Whatever the geometry, the UV seed photon emission appears to be dominated by the reprocessing of the X-rays. We also found very interesting correlations between the different model parameters, the most important one being the anticorrelation between the corona temperature kTe and the UV flux FUV. Such an anticorrelation is clearly inconsistent with a fixed disc-corona configuration and suggests a variation of the geometry of the system. We also find a correlation between the corona optical depth and the X-ray flux which may reflect processes linked to the corona formation. During the observations, NGC 7469 appears to accrete near its Eddington limit. This source could then be an example of magnetically dominated disc-corona system as recently proposed by Merloni (2003). Finally, these data strongly support the presence of a pair free corona
Thermodynamic perturbations in the X-ray halo of 33 clusters of galaxies observed with Chandra ACIS (Corrigendum)
No full textThe complex FeK line of the narrow-line seyfert 1 galaxy IRAS 13349+2438
No full textThe observation of the Narrow-Line Seyfert 1 galaxy IRAS 13349+2438 performed by XMM-Newton European Photon Imaging Camera between 0.3-10 keV, is presented here. The broadband spectrum of the source is dominated at low energies (E ≤ 2 keV) by a strong excess of emission and by complex emission/absorption features between ∼5.5-8.0 keV. The soft X-ray spectrum is consistent with ionized absorption found by the Reflection Grating Spectrometer (Sako et al. 2001a). We focus on the 2-10 keV spectrum which shows clear evidence for a broad, complex FeKα line, previously unseen by ASCA, and for an Fe K-shell edge detected at ∼7.3-7.4 keV (rest-frame). The presence of this edge could be explained by either partial covering or ionized/relativistic reflection models, with the latter being preferred and with a resulting power law slope of Γ ∼ 2.2. The line profile is complex, with a broad bump between ∼5.5-6.5 keV and a narrow emission line at ∼7 keV, separated by a sharp drop at ∼6.8 keV. This profile is compatible with two possible scenarios: i) a broad, ionized and gravitationally redshifted Laor diskline plus a narrow and ionized emission line; ii) a broad, ionized and single Schwarzschild (double-peaked) diskline with a superimposed narrow absorption line
The x-ray spectrum of the highly-polarized quasar PKS 1502+106
No full textWe report the detection and spectrum of the distant (z = 1.839), highly polarized quasar PKS 1502+106 in the X-ray band based on data collected over the period 1990-1994 using ROSAT and ASCA. We find the source to exhibit intensity variations of a factor ≳2 at 1 keV on timescales of years, small compared to the variability observed historically in the radio, millimeter, and optical bands. The (energy) spectral index is found in the range 0.4 ≲ α X ≲ 0.8 (at 90% confidence). Its multiwaveband spectral index is broadly consistent with other highly polarized quasars (HPQs). From the application of an inhomogeneous synchrotron self-Compton (SSC) jet we find that a model similar to that proposed for 3C 279 is consistent with the multiwaveband spectrum. We suggest that a change in the energy injection and/or transport mechanisms within such a jet could easily result in PKS 1502+106 being detectable at γ-ray energies
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Wide field imager instrument for the Advanced Telescope for High Energy Astrophysics
No full textThe Advanced Telescope for High Energy Astrophysics (Athena) has been selected for ESA's L2 mission, scheduled for launch in 2028. It will provide the necessary capabilities to achieve the ambitious goals of the science theme "The Hot and Energetic Universe." Athena's X-ray mirrors will be based on silicon pore optics technology with a 12-m focal length. Two complementary focal plane camera systems are foreseen, which can be moved interchangeably to the focus of the mirror system: the actively shielded micro-calorimeter spectrometer X-IFU and the wide field imager (WFI). The WFI camera will provide an unprecedented survey power through its large field of view of 40 arc min with a high count-rate capability (∼1 Crab). It permits a state-of-the-art energy resolution in the energy band of 0.1 to 15 keV during the entire mission lifetime (e.g., full width at half maximum ≤150 eV at 6 keV). This performance is accomplished by a set of depleted P-channel field effect transistor (DEPFET) active pixel sensor matrices with a pixel size well suited to the angular resolution of 5 arc sec (on-axis) of the mirror system. Each DEPFET pixel is a combined detector-amplifier structure with a MOSFET integrated onto a fully depleted 450-μm-thick silicon bulk. This manuscript will summarize the current instrument concept and design, the status of the technology development, and the envisaged baseline performance
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Thermodynamic perturbations in the X-ray halo of 33 clusters of galaxies observed with Chandra ACIS
No full textIn high-resolution X-ray observations of the hot plasma in clusters of galaxies, significant structures caused by AGN feedback, mergers, and turbulence can be detected. Many clusters have been observed by Chandra in great depth and at high resolution.
Aims: With the use of archival data taken with the Chandra ACIS instrument, the aim was to study thermodynamic perturbations of the X-ray emitting plasma and to apply this to better understand the thermodynamic and dynamic state of the intracluster medium (ICM).
We analysed deep observations for a sample of 33 clusters with more than 100 ks of Chandra exposure each at distances between redshift 0.025 and 0.45. The combined exposure of the sample is 8 Ms. Fitting emission models to different regions of the extended X-ray emission, we searched for perturbations in density, temperature, pressure, and entropy of the hot plasma.
For individual clusters, we mapped the thermodynamic properties of the ICM and measured their spread in circular concentric annuli. Comparing the spread of different gas quantities to high-resolution 3D hydrodynamic simulations, we constrained the average Mach number regime of the sample to Mach1D ≈ 0.16 ± 0.07. In addition we found a tight correlation between metallicity, temperature, and redshift with an average metallicity of Z ≈ 0.3 ± 0.1 Z☉.
Conclusions: This study provides detailed perturbation measurements for a large sample of clusters that can be used to study turbulence and make predictions for future X-ray observatories like eROSITA, Astro-H, and Athena
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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