1,720,989 research outputs found
Comprehensive Analysis of RXTE Data from Cygnus X-1: Spectral Index-Quasi-Periodic Oscillation Frequency-Luminosity Correlations
No full textWe present timing and spectral analysis of ~2.3 Ms of Rossi X-Ray Timing Explorer (RXTE) archival data from Cyg X-1. Using a generic Comptonization model we reveal that the spectrum of Cyg X-1 consists of three components: a thermal seed photon spectrum, a Comptonized part of the seed photon spectrum, and the iron line. We find a strong correlation between the 0.1-20 Hz frequencies of quasi-periodic oscillations (QPOs) and the spectral index. The presence of two spectral phases (states) is clearly seen in the data when the spectral indices saturate at low and high values of QPO frequencies. This saturation effect was discovered earlier in a number of black hole candidate (BHC) sources, and now we strongly confirm this phenomenon in Cyg X-1. In the soft state this index-QPO frequency correlation shows a saturation of the photon index Γ~2.1 at high values of the low frequency νL. The saturation level of Γ~2.1 is the lowest value found yet in BHCs. The bolometric luminosity does not show clear correlation with the index. We also show that Fe Kα emission line strength (equivalent width [EW]) correlates with the QPO frequency. The EW increases from 200 eV in the low/hard state to 1.5 keV in the high/soft state. The observational correlations revealed compel us to propose a scenario for the spectral transition and iron line formation that occur in BHC sources. We also present the spectral state (power-law index) evolution for 8 yr of Cyg X-1 observations by RXTE
Determination of Black Hole Masses in Galactic Black Hole Binaries Using Scaling of Spectral and Variability Characteristics
No full textWe present a study of correlations between X-ray spectral and timing properties observed from a number of Galactic black hole (BH) binaries during hard-soft state spectral evolution. We analyze 17 transition episodes from eight BH sources observed with Rossi X-Ray Timing Explorer. Our scaling technique for BH mass determination uses a correlation between the spectral index and quasi-periodic oscillation (QPO) frequency. In addition, we use a correlation between the index and the normalization of the disk "seed" component to cross-check the BH mass determination and estimate the distance to the source. While the index-QPO correlations for two given sources contain information on the ratio of the BH masses in those sources, the index-normalization correlations depend on the ratio of the BH masses and the distance square ratio. In fact, the index-normalization correlation also discloses the index-mass accretion rate saturation effect given that the normalization of disk "seed" photon supply is proportional to the disk mass accretion rate. We present arguments that this observationally established index saturation effect is a signature of the bulk motion (converging) flow onto a BH, which was early predicted by the dynamical Comptonization theory. We use GRO J1655 - 40 as a primary reference source for which the BH mass, distance, and inclination angle are evaluated by dynamical measurements with unprecedented precision among other Galactic BH sources. We apply our scaling technique to determine BH masses and distances for Cygnus X-1, GX 339 - 4, 4U 1543 - 47, XTE J1550 - 564, XTE J1650 - 500, H 1743 - 322, and XTE J1859 - 226. A good agreement of our results for sources with known values of BH masses and distance provides independent verification for our scaling technique
On the Nature of the Flux Variability during an Expansion Stage of a Type I X-Ray Burst: Constraints on Neutron Star Parameters for 4U 1820-30
No full textWe investigate closely the flux profile during the burst expansion stage observed from 4U 1820-30 with the Rossi X- Ray Timing Explorer on 1997 May 2. We are able to uncover the behavior of a photospheric radius and to simulate the evolution of the neutron star (NS) accretion disk system. We argue that although the bolometric luminosity is always the Eddington value L_Edd, the photon flux at the bottom of the expanded envelope can decrease during the expansion stage. In fact, at the initial moment of explosion when the bottom burning temperature is ~2×10^9 K, the bottom flux Lbot is a few times the Eddington limit, because the electron cross section is a few times less than the Thomson cross section at such high temperatures. The surplus of energy flux with respect to the Eddington, L_bot-L_Edd, goes into the potential energy of the expanded envelope. As cooling of the burning zone starts the surplus decreases, and thus the envelope shrinks while the emergent photon flux stays the same, L=L_Edd. At a certain moment the NS low hemisphere, previously screened by the disk, becomes visible to the observer. Consequently, the flux detected by the observer increases. We estimate the anisotropy due to geometry and find that the system should have a high inclination angle. Finally, we apply an analytical model of X-ray spectral formation in the NS atmosphere during the burst decay stage to infer the NS mass-radius relation
Determination of Black Hole Mass in Cyg X-1 by Scaling of Spectral Index-QPO Frequency Correlation
Full text linkIt is well established that timing and spectral properties of Galactic Black Hole (BH) X-ray binaries (XRB) are strongly correlated. In particular, it has been shown that low frequency Quasi-Periodic Oscillation (QPO) nu(sub low) - photon index GAMMA correlation curves have a specific pattern. In a number of the sources studied the shape of the index-low frequency QPO correlations are self-similar with a position offset in the nu(sub low) - GAMMA plane determined by a BH mass M(sub BH). Specifically, Titarchuk & Fiorito (2004) gave strong theoretical and observational arguments that the QPO frequency values in this nu(sub low) - GAMMA correlation should be inversely proportional to M(sub BH). A simple translation of the correlation for a given source along frequency axis leads to the observed correlation for another source. As a result of this translation one can obtain a scaling factor which is simply a BH mass ratio for these particular sources. This property of the correlations offers a fundamentally new method for BH mass determination in XRBs. Here we use the observed QPO-index correlations observed in three BH sources: GRO J1655-40, GRS 1915+105 and Cyg X-1. The BH mass of (6.3 plus or minus 0.5) solar mass in GRO J1655-40 is obtained using optical observations. RXTE observations during the recent 2005 outburst yielded sufficient data to establish the correlation pattern during both rise and decay of the event. We use GRO J1655-40 as a standard reference source to measure the BH mass in Cyg X-1. We also revisit the GRS 1915+105 data as a further test of our scaling method. We obtain the BH mass in Cyg X-1 in the range 7.6-9.9
The Bursting Behavior of 4U 1728-34: Parameters of a Neutron Star and Geometry of a NS-disk system
No full textWe analyze a set of Type I X-ray bursts from the low mass X-ray binary 4U 1728-34, observed with Rossi X-ray Timing Explorer (RXTE). We infer the dependence of the neutron star (NS) mass and radius with respect to the assumed distance to the system using an analytical model of X-ray burst spectral formation. The model behavior clearly indicates that the burster atmosphere is helium-dominated. Our results strongly favor the soft equation of state (EOS) of NS for 4U 1728-34. We find that distance to the source should be within 4.5-5.0 kpc range. We obtain rather narrow constrains for the NS radius in 8.7-9.7 km range and interval 1.2-1.6 Ma for NS mass for this particular distance range. We uncover a temporal behavior of red-shift corrected burst flux for the radial expansion episodes and we put forth a dynamical evolution scenario for the NS accretion disk geometry during which an expanded envelope affects the accretion disk and increases the area of the neutron star exposed to the Earth observer. In the framework of this scenario we provide a new method for the estimation of the inclination angle which leads to the value of approximately 50 degrees for 4U 1728-34
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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