The long term X-ray spectral variability of AGN (2009)
Sobolewska, M. A., Papadakis, I. E.
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...
A correlation between the spectral and timing properties of AGN (2009)
Papadakis, I. E., Sobolewska, M., Arevalo, P., Markowitz, A., McHardy, I. M., Miller, L., ...
We present the results from a combined study of the average X-ray spectral and timing properties of 14 nearby AGN. For 11 of the sources in the sample, we used all the available data from the RXTE...
Short-term variability and PSD analysis of the radio-loud AGN 3C 390.3 (2009)
Gliozzi, M., Papadakis, I. E., Eracleous, M., Sambruna, R. M., Ballantyne, D. R., Braito, V., ...
We investigate the short-term variability properties and the power spectral density (PSD) of the Broad-Line Radio Galaxy (BLRG) 3C 390.3 using observations made by XMM, RXTE, and Suzaku on several...
Papadakis, I. E., Ioannou, Z., Brinkmann, W., Xilouris, E. M.
We present the results from a study of the X-ray variability and the near-IR to X-ray spectral energy distribution of four low-luminosity, Seyfert 1 galaxies. We compared their variability amplitude...
A jet model for Galactic black-hole X-ray sources: Some constraining correlations (2008)
Kylafis, N. D., Papadakis, I. E., Reig, P., Giannios, D., Pooley, G. G.
Some recent observational results impose significant constraints on all the models that have been proposed to explain the Galactic black-hole X-ray sources in the hard state. In particular, it has...
The long-term X-ray variability properties of AGN in the Lockman Hole region (2008)
Papadakis, I. E., Chatzopoulos, E., Athanasiadis, D., Markowitz, A., Georgantopoulos, I.
We present the results from a detailed X-ray variability analysis of 66 AGN in the Lockman Hole, which have optical spectroscopic identifications. We compare, quantitatively, their variability...
Fourier-resolved energy spectra of the Narrow Line Seyfert 1 Mkn 766 (2008)
Arevalo, P., McHardy, I. M., Markowitz, A., Papadakis, I. E., Turner, T. J., Miller, L., ...
We compute Fourier-resolved X-ray spectra of the Seyfert 1 Markarian 766 to study the shape of the variable components contributing to the 0.3-10 keV energy spectrum and their time-scale dependence....
Long-Term X-Ray monitoring of NGC6251: Evidence for a jet-dominated radio galaxy (2008)
Gliozzi, M., Papadakis, I. E., Sambruna, R. M.
We present the first X-ray monitoring observations of the X-ray bright FRI radio galaxy NGC6251 observed with RXTE for 1 year. The primary goal of this study is to shed light on the origin of the...
A jet model for galactic black-hole X-ray sources: some constraining correlations (2008)
Kylafis, N. D., Papadakis, I. E., Reig, P., Giannios, D., Pooley, G. G.
McHardy, I. M., Arevalo, P., Uttley, P., Papadakis, I. E., Summons, D. P., Brinkmann, W., ...
We present a power spectral analysis of a 100 ksec XMM-Newton observation of the narrow line Seyfert 1 galaxy Ark~564. When combined with earlier RXTE and ASCA observations, these data produce a...
The long-term optical spectral variability of BL Lacertae (2007)
Papadakis, I. E., Villata, M., Raiteri, C. M.
We present the results from a study of the long-term optical spectral variations of BL Lacertae, using the long and well-sampled B and R-band light curves of the Whole Earth Blazar Telescope (WEBT)...
A jet model for black-hole X-ray sources (2007)
Kylafis, N. D., Papadakis, I. E., Reig, P.
A jet model for Galactic black-hole X-ray binaries will be presented that appears to explain several observational characteristics. In particular, it explains the energy spectrum from radio to hard...
Fourier-Resolved Spectroscopy of AGN using XMM-Newton data: I. The 3-10 keV band results (2007)
Papadakis, I. E., Ioannou, Z., Kazanas, D.
We present the results from the Fourier Resolved Spectroscopy of archival XMM-Newton data of five AGN, namely, Mrk 766, NGC 3516, NGC 3783, NGC 4051 and Ark 564. This work supplements the earlier...
Spectral variability analysis of an XMM-Newton observation of Ark 564 (2007)
Brinkmann, W., Papadakis, I. E., Raeth, C.
We present a spectral variability analysis of the X-ray emission of the Narrow Line Seyfert 1 galaxy Ark 564 using the data from a ~100 ks XMM-Newton observation. Taking advantage of the high...
Papadakis, I.E., Brinkmann, W., Page, M.J., McHardy, I., Uttley, P.
Context: .We present the results from the spectral analysis of the time-average spectrum of the Narrow Line Seyfert 1 (NLS1) galaxy Ark 564 from a ~100 ks XMM-Newton observation. Aims: .Our aim is to...
Long-Term X-Ray spectral variability of the radio-loud NLS1 galaxy PKS 0558-504 (2006)
Gliozzi, M., Papadakis, I. E., Brinkmann, W.
We present X-ray observations of the Narrow-Line Seyfert 1 galaxy PKS 0558-504 obtained with RXTE during a 1-year monitoring campaign. This source, which is one of the very few radio-loud NLS1...
The Energy-dependent X-ray Timing Characteristics of the Narrow Line Seyfert 1 Mkn 766 (2006)
Markowitz, A., Papadakis, I. E., Arevalo, P., Turner, T. J., Miller, L., Reeves, J. N.
We present the energy-dependent power spectral density (PSD) and cross-spectral properties of Mkn 766, obtained from combining data obtained during an XMM-Newton observation spanning six revolutions...
Gallo, L. C., Brandt, W. N., Costantini, E., Fabian, A. C., Iwasawa, K., Papadakis, I. E.
We present the second XMM-Newton observation (85 ks) of the narrow-line Seyfert 1 galaxy (NLS1) I Zw 1 and describe its mean spectral and timing characteristics. On average, I Zw 1 is ~35 per cent...
Papadakis, I. E., Brinkmann, W., Page, M. J., McHardy, I., Uttley, P.
We use the data from a recent, 100 ksec XMM-Newton observation of the Narrow Line Seyfert 1 galaxy Ark 564 to obtain its time average, X-ray spectrum. The 3-11 keV spectrum is well fitted by a...
Spectral-timing evidence for a Very High State in the Narrow Line Seyfert 1 Ark 564 (2006)
Arevalo, P., Papadakis, I. E., Uttley, P., McHardy, I. M., Brinkmann, W.
We use a 100 ks long XMM-Newton observation of the Narrow-Line Seyfert 1 galaxy Ark 564 and combine it with the month-long monitoring of the same source produced by ASCA, to calculate the phase lags...
Fourier resolved spectroscopy of 4U 1543-47 during the 2002 outburst (2006)
Reig, P., Papadakis, I. E., Shrader, C. R., Kazanas, D.
We have obtained Fourier-resolved spectra of the black-hole binary 4U 1543-47 in the canonical states (high/soft, very high, intermediate and low/hard) observed in this source during the decay of an...
XMM-Newton and Chandra observations of SHEEP sources (2006)
Georgantopoulos, I., Nandra, K., Brotherton, M., Georgakakis, A., Papadakis, I. E., O'Neill, P.
We present Chandra and XMM observations of 12 bright (f(2-10 keV) > 10^-13 cgs) sources from the ASCA SHEEP (Search for the High Energy Extragalactic Population) survey. Most of these have been...
Spectral-timing evidence for a very high state in the narrow-line Seyfert 1 Ark 564 (2006)
Arévalo, P., Papadakis, I. E., Uttley, P., McHardy, I. M., Brinkmann, W.
Spectral-timing evidence for a very high state in the narrow-line Seyfert 1 Ark 564 (2006)
Arévalo, P., Papadakis, I.E., Uttley, P., McHardy, I.M., Brinkmann, W.
We use a 100ks long XMM-Newton observation of the narrow-line Seyfert 1 galaxy Ark 564 and combine it with the month-long monitoring of the same source produced by ASCA, to calculate the phase lags...
Spectral-timing evidence for a very high state in the narrow-line Seyfert 1 Ark 564 (2006)
Ar¿lo, P., Papadakis, I.E., Uttley, P., McHardy, I.M., Brinkmann, W.
We use a 100ks long XMM-Newton observation of the narrow-line Seyfert 1 galaxy Ark 564 and combine it with the month-long monitoring of the same source produced by ASCA, to calculate the phase lags...
Gliozzi, M., Papadakis, I. E., Raeth, C.
This work presents the results from a systematic search for evidence of temporal changes (i.e., non-stationarity) associated with spectral variations in 3C 390.3, using data from a two-year intensive...
Xilouris, E. M., Papadakis, I. E., Boumis, P., Dapergolas, A., Alikakos, J., Papamastorakis, J., ...
We have observed S5 2007+777 and 3C371 in the B and I bands for 13 and 8 nights, respectively, during various observing runs in 2001, 2002 and 2004. The observations resulted in almost evenly sampled...
XMM-Newton timing mode observations of Mrk 421 (2005)
Brinkmann, W., Papadakis, I. E., Raeth, C., Mimica, P., Haberl, F.
We present the results of a detailed temporal analysis of the bright BL Lac object Mrk 421 using the three available long timing mode observations by the EPIC PN camera. This detector mode is...
Boller, Th., Voges, W., Dennefeld, M., Lehmann, I., Predehl, P., Burwitz, V., ...
We present X-ray and optical observations of the X-ray bright AGN 1ES 1927+654. The X-ray observations obtained with ROSAT and Chandra reveal persistent, rapid and large scale variations, as well as...
Fourier Resolved Spectroscopy of the XMM-Newton Observations of MCG -6-30-15 (2005)
Papadakis, I. E., Kazanas, D., Akylas, A.
We study the Frequency Resolved Spectra of the Seyfert galaxy MCG -6-30-15 obtained during two recent XMM-Newton observations. Splitting the Fourier spectra in soft (2 keV) bands, we find that the...
XMM-Newton timing mode observations of Mrk 421 (2005)
Brinkmann, W., Papadakis, I. E., Raeth, C., Mimica, P., Haberl, F.
XMM-Newton timing mode observations of Mrk 421 (2005)
Brinkmann, W., Papadakis, I. E., Raeth, C., Mimica, P., Haberl, F.
Non-stationary variability in AGN: the case of 3C 390.3 (2004)
Gliozzi, M., Papadakis, I. E., Sambruna, R. M., Eracleous, M.
We use data from a two-year intensive RXTE monitoring campaign of the broad-line radio galaxy 3C 390.3 to investigate its stationarity. In order to exploit the potential information contained in a...
Multi-band optical micro-variability observations of the BL Lac object S4 0954+658 (2004)
Papadakis, I. E., Samaritakis, V., Boumis, P., Papamastorakis, J.
We have observed S4 0954+658 in the BVRI bands in 2001, and in the BI bands in 2002. The observations resulted in almost evenly sampled light curves, 3-8 hours long, with an average sampling interval...
Black hole mass estimation from X-ray variability measurements in AGN (2004)
Nikolajuk, M., Papadakis, I. E., Czerny, B.
We propose a new method of estimation of the black hole masses in AGN based on the normalized excess variance, sigma_{nxs}^2. We derive a relation between sigma_{nxs}^2, the length of the...
Combined long and short time-scale X-ray variability of NGC 4051 with RXTE and XMM-Newton (2004)
McHardy, I.M., Papadakis, I.E., Uttley, P., Page, M.J., Mason, K.O.
We present a comprehensive examination of the X-ray variability of the narrow-line Seyfert 1 (NLS1) galaxy NGC 4051, one of the most variable active galactic nuclei (AGN) in the sky. We combine over...
Misiriotis, A., Papadakis, I. E., Kylafis, N. D., Papamastorakis, J.
We address the problem of modeling the far-infrared (FIR) spectrum and deriving the star-formation rate (SFR) and the dust mass of spiral galaxies. We use the realistic physical model of Popescu et...
Combined Long and Short Timescale X-ray Variability of NGC4051 with RXTE and XMM-Newton (2003)
McHardy, I. M., Papadakis, I. E., Uttley, P., Page, M. J., Mason, K. O.
We present over 6.5 years of frequent X-ray observations of the NLS1 galaxy NGC4051 by RXTE together with a >100ks observation by XMM-Newton and so derive a 2-10 keV PSD covering an unprecedent...
Nandra, K., Georgantopoulos, I., Brotherton, M., Papadakis, I. E.
We present followup observations of five hard X-ray sources from the ASCA 5-10 keV SHEEP survey, which has a limiting flux of $\sim 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. Chandra data have been obtained...
The scaling of the X-ray variability with black hole mass in AGN (2003)
The relation between the 2-10 keV, long term, excess variance and AGN black hole mass is considered in this work. A significant anti-correlation is found between these two quantities in the sense...
Coordinated Multiwavelength Observations of BL Lacertae in 2000 (2003)
Böttcher,M., Marscher,A.P., Ravasio,M., Villata,M., Raiteri,C.M., Aller,H.D., ...
BL Lacertae (BL Lac) was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (University of...
Combined Long and Short Timescale X-ray Variability of NGC4051 with RXTE and XMM-Newton (2003)
McHardy, I. M., Papadakis, I. E., Uttley, P., Mason, K. O., Page, M. J.
We present a comprehensive examination of the X-ray variability of the narrow line Seyfert 1 (NLS1) galaxy NGC4051. We combine over 6.5 years of frequent monitoring observations by RXTE with a >100ks...
The long time-scale X-ray variability of the radio-quiet quasar PG 0804+761 (2003)
Papadakis, I. E., Reig, P., Nandra, K.
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...
Optical and radio behaviour of the BL Lacertae object 0716+714 (2003)
Raiteri, C. M., Villata, M., Tosti, G., Nesci, R., Massaro, E., Aller, M. F., ...
Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while...
Boller, T., Voges, W., Dennefeld, M., Lehmann, I., Predehl, P., Burwitz, V., ...
We present X-ray and optical observations of the X-ray bright AGN IES 1927+654. The X-ray observations obtained with ROSAT and Chandra reveal persistent, rapid and large scale variations, as well as...
Optical and X-ray monitoring of the NLS1 galaxy Ark 564 (2003)
Brinkmann, W., Papadakis, I. E., Negoro, H., Detsis, E., Papamastorakis, I., Gliozzi, M., ...
Coordinated Multiwavelength Observations of BL Lacertae in 2000 (2003)
Böttcher, M., Marscher, A.P., Ravasio, M., Villata, M., Raiteri, C.M., Aller, H.D., ...
BL Lacertae (BL Lac) was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (University of...
Optical and radio behaviour of the BL Lacertae object 0716+714 (2003)
Raiteri, C. M., Villata, M., Tosti, G., Nesci, R., Massaro, E., Aller, M. F., ...
Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while...
Multi-band optical micro-variability observations of BL Lacertae (2002)
Papadakis, I. E., Boumis, P., Samaritakis, V., Papamastorakis, J.
We have observed BL Lacertae in the B, R and I bands for 2 nights in July, 1999, and 3 nights in July, 2001. The observations resulted in almost evenly sampled light curves, with an average sampling...
Boller, Th., Voges, W., Dennefeld, M., Lehmann, I., Predehl, P., Burwitz, V., ...
We present X-ray and optical observations of the X-ray bright AGN 1ES 1927+654. The X-ray observations obtained with ROSAT and Chandra reveal persistent, rapid and large scale variations, as well as...
On the Nature of X-ray Variability in Ark 564 (2002)
Gliozzi, M., Brinkmann, W., Raeth, C., Papadakis, I. E., Negoro, H., Scheingraber, H.
We use data from a recent long ASCA observation of the Narrow Line Seyfert 1 Ark 564 to investigate in detail its timing properties. We show that a thorough analysis of the time series, employing...
Measuring the broad-band power spectra of active galactic nuclei with RXTE (2002)
Uttley, P., McHardy, I., Papadakis, I.E.
We develop a Monte Carlo technique to test models for the true power spectra of intermittently sampled light curves against the noisy, observed power spectra, and produce a reliable estimate of the...
Xilouris, E. M., Papadakis, I. E.
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...
Measuring the broad-band power spectra of active galactic nuclei with RXTE (2002)
Uttley, P., McHardy, I.M., Papadakis, I.E.
We develop a Monte Carlo technique to test models for the true power spectra of intermittently sampled light curves against the noisy, observed power spectra, and produce a reliable estimate of the...
Long Term Spectral Variability of Seyfert Galaxies from RXTE Color-Flux Diagrams (2002)
Papadakis, I. E., Petrucci, P. O., Maraschi, L., McHardy, I. M., Uttley, P., Haardt, F.
We present results from RXTE data obtained during a systematic monitoring program of four Seyfert galaxies (NGC5548, NGC5506, MCG-6-30-15 and NGC4051). We studied the variability of three hardness...
Measuring the broadband power spectra of active galactic nuclei with RXTE (2002)
Uttley, P., McHardy, I. M., Papadakis, I. E.
We have developed a Monte Carlo technique to test models for the true power spectra of intermittently sampled lightcurves against the noisy, observed power spectra, and produce a reliable estimate of...
Boller, Th., Voges, W., Dennefeld, M., Lehmann, I., Predehl, P., Burwitz, V., ...
We present X-ray and optical observations of the X-ray bright AGN 1ES 1927+654. The X-ray observations obtained with ROSAT and Chandra reveal persistent, rapid and large scale variations, as well as...
Boller, Th., Voges, W., Dennefeld, M., Lehmann, I., Predehl, P., Burwitz, V., ...
We present X-ray and optical observations of the X-ray bright AGN 1ES 1927+654. The X-ray observations obtained with ROSAT and Chandra reveal persistent, rapid and large scale variations, as well as...
On the nature of X-ray variability in Ark 564 (2002)
Gliozzi, M., Brinkmann, W., Räth, C., Papadakis, I. E., Negoro, H., Scheingraber, H.
We use data from a recent long ASCA observation of the Narrow Line Seyfert 1 galaxy Ark 564 to investigate in detail its timing properties. We show that a thorough analysis of the time series,...
Detection of a high frequency break in the X-ray power spectrum of Ark 564 (2002)
Papadakis, I. E., Brinkmann, W., Negoro, H., Gliozzi, M.
We present a power spectrum analysis of the long ASCA observation of Ark 564 in June/July 2001. The observed power spectrum covers a frequency range of similar to 3.5 decades. We detect a high...
M. Gliozzi, W. Brinkmann, C. Räth, I. E. Papadakis, H. Negoro, H. Scheingraber
Abstract. We use data from a recent long ASCA observation of the Narrow Line Seyfert 1 galaxy Ark 564 to investigate in detail its timing properties. We show that a thorough analysis of the time...
Detection of a high frequency break in the X-ray power spectrum of Ark 564 (2001)
Papadakis, I. E., Brinkmann, W., Negoro, H., Gliozzi, M.
We present a power spectrum analysis of the long ASCA observation of Ark 564 in June/July 2001. The observed power spectrum covers a frequency range of ~ 3.5 decades. We detect a high frequency break...
Monitoring RXTE Observations of Markarian 348: the origin of the column density variations (2001)
Akylas, A., Georgantopoulos, I., Griffiths, R. G., Papadakis, I. E., Mastichiadis, A., Warwick, R. S., ...
We analyze 37 RXTE observations of the type 2 Seyfert galaxy Mrk348 obtained during a period of 14 months. We confirm the spectral variability previous reported by Smith et al., in the sense that...
Frequency-dependent time lags in the X-ray emission of the Seyfert galaxy NGC 7469 (2001)
Papadakis, I. E., Nandra, K., Kazanas, D.
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...
Temporal characteristics of the X-ray emission of NGC 7469 (2001)
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...
Optical and X-ray monitoring of the NLS1 galaxy Ark 564 (2000)
Papadakis, I. E., Brinkmann, W., Negoro, H., Detsis, E., Papamastorakis, I., Gliozzi, M.
The Narrow-line Seyfert galaxy Ark 564 was monitored between June 04, 2000 and June 24, 2000 from Skinakas Observatory. The B,V,R and I band light curves show night to night variations of 0.1 mag....
Simultaneous EUV and X-ray variability of NGC 4051 (1999)
Uttley, P., McHardy, I. M., Papadakis, I. E., Cagnoni, I., Fruscione, A.
We present a flux variability study of simultaneous RXTE and EUVE observations of the highly variable Seyfert galaxy NGC4051. We find a strong correlation between variability in the EUV and medium...
The swansong in context: long-timescale X-ray variability of NGC 4051 (1999)
Uttley, P., McHardy, I. M., Papadakis, I. E., Guainazzi, M., Fruscione, A.
On 9-11 May 1998, the highly-variable, low luminosity Seyfert 1 galaxy NGC4051 was observed in an unusual low flux state by BeppoSAX (Guainazzi et al. 1998) RXTE and EUVE. We present fits of the 4-15...
EUVE-RXTE simultaneous observations of NGC 4051: variability and spectrum (1998)
Cagnoni, I., Fruscione, A., McHardy, I. M., Papadakis, I. E.
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...