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Search Results: 1 - 10 of 87843 matches for " I. Papadakis "
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RXTE observations of Seyfert-2 galaxies: evidence for spectral variability
I. Georgantopoulos,I. Papadakis
Physics , 2000, DOI: 10.1046/j.1365-8711.2001.04059.x
Abstract: We present a series of RXTE observations of the nearby obscured Seyfert galaxies ESO103-G35, IC5063, NGC4507 and NGC7172. The period of monitoring ranges from seven days for NGC7172 up to about seven months for ESO103-G035. The spectra of all galaxies are well fit with a highly obscured (N_H>10^23 cm-2) power-law and an Fe line at 6.4 keV. We find strong evidence for the presence of a reflection component in ESO103-G35 and NGC4507. The observed flux presents strong variability on day timescales in all objects. Spectral variability is also detected in the sense that the spectrum steepens with increasing flux similar to the behaviour witnessed in some Seyfert-1 galaxies.
Temporal characteristics of the X-ray emission of NGC 7469
K. Nandra,I. E. Papadakis
Physics , 2001, DOI: 10.1086/321423
Abstract: We present a study of the time variability of NGC 7469, based on a ~30 d RXTE/PCA observation. Variability is seen across the X-ray band, and spectral changes are observed. The softness ratio is correlated with the ultraviolet flux, but appears to show more rapid variability. The RMS variability parameter calculated on ~1d time scales is significantly variable, and sharp increases are seen close to the periods where the spectrum is hardest. Cross correlation of 2-4 keV, 4-10 keV and 10-15 keV light curves show a peak at zero delay, but the CCFs are skewed towards positive lags (i.e. the soft leading the hard) with an apparent time lag of about 0.5d. The power spectral density (PSD) function in the 2-10 keV band shows no clear features - for example periodicities or breaks - but a power law is a rather poor fit, particularly to the high frequency spectrum. The normalized PSD in the soft X-ray band shows a larger amplitude of variability on long time scales but the hard X-ray PSD is flatter, and shows more power on time scales less than about 1d. Our data broadly support the idea that the X-rays are produced by Compton upscattering of lower-energy seed photons, as had been previously concluded. They are difficult to reconcile with models in which the sole source of variability is the seed photons, and more likely suggest a variability process intrinsic to the X-ray corona. Our interpretation for these results is that the low-frequency variability in NGC 7469 does arise via variations in the Compton seed photons, but that the high frequency variability arises from the coronal heating mechanism (truncated)
The long term X-ray spectral variability of AGN
M. A. Sobolewska,I. E. Papadakis
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15382.x
Abstract: We present the results from the spectral analysis of more than 7,500 RXTE spectra of 10 AGN, which have been observed by RXTE regularly over a long period of time ~ 7-11 years. These observations most probably sample most of the flux and spectral variations that these objects exhibit, thus, they are ideal for the study of their long term X-ray spectral variability. We modelled the 3-10 spectrum of each observation in a uniform way using a simple power-law model (with the addition of Gaussian line and/or edge to model the iron Kalpha emission/absorption features, if necessary) to consistently parametrize the shape of the observed X-ray continuum. We found that the average spectral slope does not correlate with source luminosity or black hole mass, while it correlates positively with the average accretion rate. We have also determined the (positive) "spectral slope-flux" relation for each object, over a larger flux range than before. We found that this correlation is similar in almost all objects. We discuss this global "spectral slope-flux" trend in the light of current models for spectral variability. We consider (i) intrinsic variability, expected e.g. from Comptonization processes, (ii) variability caused by absorption of X-rays by a single absorber whose ionization parameter varies proportionally to the continuum flux variations, (iii) variability resulting from the superposition of a constant reflection component and an intrinsic power-law which is variable in flux but constant in shape, and, (iv) variability resulting from the superposition of a constant reflection component and an intrinsic power-law which is variable both in flux and shape. Our final conclusion is that scenario (iv) describes better our results.
A morphological comparison between the central region in AGN and normal galaxies using HST data
E. M. Xilouris,I. E. Papadakis
Physics , 2002, DOI: 10.1051/0004-6361:20020429
Abstract: We study the morphology of the central region of a sample of Active Galactic Nuclei (AGN) and a ``control'' sample of normal galaxies using archival observations of the WFPC2 instrument onboard the Hubble Space Telescope (HST). We use the ellipse fitting technique in order to get a good description of the inner ``smooth'' light distribution of the galaxy. We then divide the observed galaxy image by the artificial image from the fitted ellipses in order to detect morphological signatures in the central region around the nucleus of the galaxy. We perform quantitative comparisons of different subgroups of our sample of galaxies (according to the Hubble type and the nuclear activity of the galaxies) by calculating the average amplitude of the structures that are revealed with the ellipse fitting technique. Our main conclusions are as follows: 1) All AGNs show significant structure in their inner 100 pc and 1 kpc regions whose amplitude is similar in all of them, independent of the Hubble type of the host galaxy. 2) When considering early-type galaxies, non-AGN galaxies show no structure at all, contrary to what we find for AGN. 3) When considering late-type galaxies, both AGN and non-AGN galaxies show significant structure in their central region. Our results are consistent with the hypothesis that all early-type galaxies host a supermassive black hole, but only those that have enough material in the central regions to fuel it show an active nucleus. The situation is more complicated in late-type galaxies. Either not all of them host a central black hole, or, in some of them, the material inside the innermost 100 pc region is not transported to the scales of the central engine for some reason, or the large amount of gas and dust hides the active nucleus from our sight.
Aperiodic variability of low-mass X-ray binaries at very low frequencies
P. Reig,I. Papadakis,N. D. Kylafis
Physics , 2002, DOI: 10.1051/0004-6361:20021693
Abstract: We have obtained discrete Fourier power spectra of a sample of persistent low-mass neutron-star X-ray binaries using long-term light curves from the All Sky Monitor on board the Rossi X-ray Timing Explorer. Our aim is to investigate their aperiodic variability at frequencies in the range 1 x 10^{-7}-5 x 10^{-6} Hz and compare their properties with those of the black-hole source Cyg X-1. We find that the classification scheme that divides LMXBs into Z and atoll sources blurs at very low frequencies. Based on the long-term (~ years) pattern of variability and the results of power-law fits (P ~ v^{-a}) to the 1 x 10^{-7}-5 x 10^{-6} Hz power density spectra, low-mass neutron-star binaries fall into three categories. Type I includes all Z sources, except Cyg X-2, and the atoll sources GX9+1 and GX13+1. They show relatively flat power spectra (a < 0.9) and low variability (rms < 20%). Type II systems comprise 4U 1636-53, 4U 1735-44 and GX3+1. They are more variable (20% < rms < 30%) and display steeper power spectra (0.9 < a < 1.2) than Type I sources. Type III systems are the most variable (rms > 30%) and exhibit the steepest power spectra (a > 1.2). The sources 4U 1705-44, GX354-0 and 4U 1820-30 belong to this group. GX9+9 and Cyg X-2 appear as intermediate systems in between Type I and II and Type II and III sources, respectively. We speculate that the differences in these systems may be caused by the presence of different types of mass-donor companions. Other factors, like the size of the accretion disc and/or the presence of weak magnetic fields, are also expected to affect their low-frequency X-ray aperiodic varibility.
Frequency-dependent time lags in the X-ray emission of the Seyfert galaxy NGC 7469
I. E. Papadakis,K. Nandra,D. Kazanas
Physics , 2001, DOI: 10.1086/321722
Abstract: We report the discovery of time lags in the cross-spectra of the X-ray light curves of the Seyfert galaxy NGC 7469. This behavior is common in Galactic black hole and neutron star binaries, and is in the sense that harder X-rays are delayed with respect to the soft with a time lag approximately proportional to the Fourier period. At the longest period probed by our observation we find a time lag of approximately 3.5 hours between the 2-4 and 4-10 keV X-rays. A similar lag and period dependence is found comparing the 2-4 and 10-15 keV light curves, albeit with less significance. We find the coherence function of the light curves to be close to 1. The implications of these results for the X-ray production mechanism in active galactic nuclei (AGN) is discussed.
The aperiodic variability of Cyg X-1 and GRS 1915 +105 at very low frequencies
P. Reig,I. Papadakis,N. D. Kylafis
Physics , 2001, DOI: 10.1051/0004-6361:20011689
Abstract: We have carried out a timing analysis of Cyg X-1 and GRS 1915+105 using more than 5 years worth of data from the All Sky Monitor (ASM) on board RXTE. We have obtained for the first time power density spectra, colour-colour and colour-intensity plots, cross-correlation functions and phase-lag diagrams to investigate the variability of the sources at frequencies < 10^{-5} Hz. We find that the PDS are not flat but consistent with a power-law of index ~-1. This is the same slope as that found in the power spectra of Cyg X-1 during the soft state. In fact, the power spectrum of Cyg X-1 in the frequency range 10^{-7}-10^{-5} Hz appears as a continuation of the 10^{-3}-10 Hz power spectrum during the soft state. Significant variability, expressed as the fractional rms in the frequency range 2 x 10^{-7}-1 x 10^{-5} Hz, is detected at a level of 27% and 21% for GRS 1915+105 and Cyg X-1, respectively. Our results confirm previous suggestions that the innermost regions of the accretion disc are affected by variations occurring at larger radii from the black hole, presumably due to changes in the mass accretion rate. We also observe strong spectral changes: in Cyg X--1 the spectrum softens as the flux increases when it is in the low/hard state, and hardens with increasing flux in the high/soft state. GRS 1915+105 follows the same trend as Cyg X-1 in the soft state, but the hardness ratios show a larger amplitude of variation. The cross-correlation shows that the variations occur simultaneously in all energy bands. However, the cross-correlation functions are asymmetric toward negative lags. The phase spectrum also reveals negative lags at periods larger than a few days, suggesting that the long term variations in the soft energy band are delayed with respect to the harder bands.
The long time-scale X-ray variability of the radio-quiet quasar PG 0804+761
I. E. Papadakis,P. Reig,K. Nandra
Physics , 2003, DOI: 10.1046/j.1365-8711.2003.06891.x
Abstract: We present the results from a study of the timing properties and the energy spectrum of the radio-quiet quasar PG0804+761, based on monitoring RXTE PCA observations that lasted for a year. This is a systematic study of the X-ray variations on time scales of weeks/months of the most luminous radio-quiet quasar studied so far. We detect significant variations in the 2-10 keV band of an average amplitude of ~15%. The excess variance of the light curve is smaller than that of Seyfert galaxies, entirely consistent with the relationship between variability amplitude and luminosity defined from the Seyfert data alone. The power spectrum of the source follows a power-like form of slope ~ -1. However, when we extend the power spectrum estimation at higher frequencies using archival ASCA data, we find strong evidence for an intrinsic steepening to a slope of ~ -2 at around ~ 1x10^(-6) Hz. This "break frequency" corresponds to a time scale of ~ 10 days. The time-average energy spectrum is well fitted by a power law model with Gamma ~ 2. We also find evidence for an iron line at \~ 6.4 keV (rest frame) with EW 110 eV, similar to what is observed in Seyfert galaxies. The flux variations are not associated with any spectral variation. This is the only major difference that we find when we compare the variability properties of the source with those of Seyfert galaxies. Our results support the hypothesis that the same X-ray emission and variability mechanism operates in both Seyfert galaxies and quasars.
EUVE-RXTE simultaneous observations of NGC 4051: variability and spectrum
I. Cagnoni,A. Fruscione,I. M. McHardy,I. E. Papadakis
Physics , 1998,
Abstract: We present timing and spectral analysis of the data collected by the Extreme Ultraviolet Explorer Satellite (EUVE) for the Seyfert 1 galaxy NGC 4051 during 1996. NGC 4051 was observed twice in May 1996 and again in December 1996 for a total of more than 200 ks. The observations were always simultaneous with hard X-ray observations conducted with the Rossi X-Ray Timing Explorer (RXTE). The EUVE light curves are extremely variable during each observation, with the maximum variability of more than a factor of 15 from peak to minimum. We detected signal in the EUVE spectrograph in the 75-100 A range which is well fitted by absorbed power law models. We illustrate the results of our spectral and detailed power spectrum analysis performed on EUVE data and the comparison with RXTE lightcurves and discuss the constraint we can place on the mass of the central object and on the size of the emitting region.
Nasal packing aspiration in a patient with Alzheimer’s disease: a rare complication
Koudounarakis E, Chatzakis N, Papadakis I, Panagiotaki I, Velegrakis G
International Journal of General Medicine , 2012, DOI: http://dx.doi.org/10.2147/IJGM.S34676
Abstract: sal packing aspiration in a patient with Alzheimer’s disease: a rare complication Case report (1028) Total Article Views Authors: Koudounarakis E, Chatzakis N, Papadakis I, Panagiotaki I, Velegrakis G Published Date July 2012 Volume 2012:5 Pages 643 - 645 DOI: http://dx.doi.org/10.2147/IJGM.S34676 Received: 06 June 2012 Accepted: 06 July 2012 Published: 27 July 2012 Eleftherios Koudounarakis, Nikolaos Chatzakis, Ioannis Papadakis, Irene Panagiotaki, George Velegrakis Department of Otorhinolaryngology, University Hospital of Heraklion, Crete, Greece Abstract: Nasal bleeding is a frequent problem for patients receiving anticoagulant agents. Most cases are successfully managed with anterior or posterior nasal packing. However, the complications of nasal packing should be always considered. We report the case of a 78-year-old man with Alzheimer’s disease who was treated for anterior epistaxis with anterior nasal packing using three pieces of antibiotic-soaked gauze. Two days later, the patient was admitted to the emergency department in respiratory distress. A chest x-ray demonstrated atelectasis of the right lung. During an examination of the nasal cavities, the nasal packing was removed, and one of the gauze pieces was missing. The patient underwent rigid bronchoscopy, and the missing gauze was found to be obstructing the right main bronchus. The patient’s respiratory function improved considerably after removal of the foreign body. It is assumed that gauze packs should be used with caution in patients with an impaired level of consciousness and neurodegenerative diseases.
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