508 research outputs found

    The unusual UV continuum of quasar Ton 34 and the possibility of crystalline dust absorption

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    Luminous quasars are known to display a sharp steepening of the continuum near 1100A. This spectral feature is not well fitted by current accretion disk models, unless comptonization of the disk emission is invoked. Absorption by carbon crystalline dust has been proposed to account for this feature. Ton 34 (z=1.928) exhibits the steepest far-UV decline (F_nu prop nu^{-5.3}) among the 183 quasar HST-FOS spectra analyzed by Telfer et al. It is an ideal object to test the crystalline dust hypothesis as well as alternative interpretations of the UV break. We reconstruct the UV spectral energy distribution of Ton 34 by combining HST, IUE and Palomar spectra. The far-UV continuum shows a very deep continuum trough, which is bounded by a steep far-UV rise. We fit the trough assuming nanodiamond dust grains. Extinction by carbon crystalline dust reproduces the deep absorption trough of Ton 34 reasonably well, but not the observed steep rise in the extreme UV. We also study the possibility of an intrinsic continuum rollover. The dust might be part of a high velocity outflow (13000 km/s), which is observed in absorption in the lines of CIV, OVI, NV and Ly_alpha.Comment: 7 figures, to appear in A&

    The Two-Phase, Two-Velocity Ionized Absorber in the Seyfert 1 Galaxy NGC 5548

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    We present an analysis of X-ray high quality grating spectra of the Seyfert 1 galaxy NGC 5548 using archival Chandra HETGS and LETGS observations for a total exposure time of 800ks. The continuum emission is well represented by a powerlaw plus a black-body component. We find that the well known X-ray warm absorber in this source consists of two different outflow velocity systems. Recognizing the presence of these kinematically distinct components allows each system to be fitted independently, each with two absorption components with different ionization levels. The high velocity system consists of a component with temperature of 2.7X10^6K and another component with temperature of 5.8X10^5K. The low-velocity system required also two absorbing components, one with temperature of 5.8X10^5K; the other with lower temperature (3.5X10^4K). Once these components are considered, the data do not require any further absorbers. In particular, a model consisting of a continuous radial range of ionization structures is not required. The two absorbing components in each velocity system are in pressure equilibrium with each other. This suggests that each velocity system consists of a multi-phase medium. This is the first time that different outflow velocity systems have been modelled independently in the X-ray band for this source. The kinematic components and column densities found from the X-rays are in agreement with the main kinematic components found in the UV absorber. This supports the idea that the UV and X-ray absorbing gas is part of the same phenomenon. NGC 5548 can now be seen to fit in a pattern established for other warm absorbers: 2 or 3 discrete phases in pressure equilibrium. There are no remaining cases of a well studied warm absorber in which a model consisting of a multi-phase medium is not viable.Comment: To appear on The Astrophysical Journal March 1, 201

    Cerebral infarct volume change over time In ischemic stroke

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    Cerebral Infarct Volume Change Over Time In Ischemic Stroke Mark Krongold1,2,4, Armin Eilaghi1,2,3,4, Mohammed Almekhalfi1,2, Andrew Demchuk1,2, Richard Frayne1,2,3,4 1Department of Radiology and Clinical Neuroscience, University of Calgary, 2Hotchkiss Brain Institute, University of Calgary, 3Department of Electrical and Computer Engineering, University of Calgary. 4Seaman Family MR Research Centre [email protected] INTRODUCTION Stroke is the second leading cause of death worldwide[1]. Ischemic strokes account for 80% of stroke events and occur due to blood clots which interrupt the flow of blood into the brain. Interruption of blood flow causes a lack of oxygen and nutrients in the brain which leads to a loss of brain function and the build up of infarct tissue[1]. This build up is a dynamic process in which stroke volume changes over time. Stroke evolution is characterized by two types of edema. Cytotoxic edema (imaged using DWI[2]) occurs acutely and causes the build up of fluid intracellularly [3] while vasogenic edema (imaged using FLAIR[4]) occurs due to the breakdown of the blood brain barrier or CSF barrier and is prolonged [5]. Imaging of subjects in clinical trials of stroke is done over periods of time of up to 90 days. Long time periods not only lead to a decrease of patients who follow up in the studies but allows for events such as trauma, secondary stroke, or even death to confound the data acquired. Evidence has shown that stroke volume 90 days post infarct is not significantly different than 30 days post infarct suggesting that stroke volume plateaus at the 30 day mark[6]. The purpose of this research was to study infarct volume evolution and determine if MR imaging at early time points can be used to predict final infarct volume. This would not only increase the number of patients that can be analyzed in clinical trials but help in earlier stroke management and treatment decision making. METHODS This is a retrospective study of patients who had strokes with DWI done at baseline and 2 or more FLAIR imaging sessions post baseline (either around 12 hours, 24 hours, 5 days, 30 days, or 90 days). Infarct tissue was traced using Cerebra and MIPAV software and confirmed by a neuroradiologist. Statistical analysis was done using one way ANOVA and correlation coefficients. RESULTS It was determined that infarct volumes at the 24 hour and 5-day time points were significantly different than volumes at baseline. Significant difference were also found between the 5-day and final (30+90day) time points. Correlation analysis indicated a strong positive correlation between stroke volume at the 5-day vs final time points in patients (Figure 1).   Figure 1. Regression lines of final lesion volume (mL) plotted against the 5-day volume (mL) in subjects. Analysis determined a correlation coefficient of 0.884 (n=51). DISCUSSION AND CONCLUSIONS The results of this study suggest that vasogenic edema affects patients significantly between the acute and prolonged stage, increasing lesion volume until a peak is reached at around 5 days. A strong correlation between the 5-day and final volumes in patients suggests that more vasogenic edema at 5 days correlates to increased overall neuronal damage in the patient. The research shows that approximations of final outcome can be determined at earlier time points leading to a reduced need of subjects coming back in clinical trials, inclusion of more subjects in trial analysis, and quicker decision making in the stroke management process. REFERENCES Donnan GA, et al. Lancet. 371:1612-1623, 2008.Schaefer PW, et al. Stroke. 28:1082-1085, 1997.Liang D, et al. Neurosurg Focus. 22:E2, 2007.Brant-Zawadzki M, et al. Stroke. 27:1187-1191, 1996.Rosenberg GA & Yang Y. Neurosurg Focus. 22:1-9, 2007. Gaudinski MR, et al. Stroke. 39:2765-2768, 2008
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