1,720,990 research outputs found
Measuring the broad-band power spectra of active galactic nuclei with RXTE
No full textWe 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 goodness of fit of the given model. We apply this technique to constrain the broad-band power spectra of a sample of four Seyfert galaxies monitored by the Rossi X-ray Timing Explorer (RXTE ) over three years. We show that the power spectra of three of the AGN in our sample (MCG-6-30-15, NGC 5506 and NGC 3516) flatten significantly towards low frequencies, while the power spectrum of NGC 5548 shows no evidence of flattening. We fit two models for the flattening: a 'knee' model, analogous to the low-frequency break seen in the power spectra of BHXRBs in the low state (where the power-spectral slope flattens to α =0), and a 'high-frequency break' model (where the power-spectral slope flattens to α =1), analogous to the high-frequency break seen in the high- and low-state power spectra of the classic BHXRB Cyg X-1. Both models provide good fits to the power spectra of all four AGN. For both models, the characteristic frequency for flattening is significantly higher in MCG-6-30-15 than in NGC 3516 (by a factor of ~10), although both sources have similar X-ray luminosities, suggesting that MCG-6-30-15 has a lower black hole mass and is accreting at a higher rate than NGC 3516. Assuming linear scaling of characteristic frequencies with black hole mass, the high accretion rate implied for MCG-6-30-15 favours the high-frequency break model for this source, and further suggests that MCG-6-30-15, and possibly NGC 5506, may be analogues of Cyg X-1 in the high state. Comparison of our model fits with naive fits, where the model is fitted directly to the observed power spectra (with errors estimated from the data), shows that Monte Carlo fitting is essential for reliably constraining the broad-band power spectra of AGN light curves obtained to date
X-ray spectral analysis of the low-luminosity active galactic nucleus NGC 7213 using long XMM-Newton observations
No full textWe present the X-ray spectral results from the longest X-ray Multi-mirror Mission–Newton observation, 133 ks, of the low-luminosity active galactic nucleus NGC 7213. The hardness-ratio analysis of the X-ray light curves discloses a rather constant X-ray spectral shape, at least for the observed exposure time, enabling us to perform X-ray spectral studies using the total observed spectrum. Apart from a neutral Fe Kα emission line, we also detect narrow emission lines from the ionized iron species, Fe xxv and Fe xxvi. Our analysis suggests that the neutral Fe Kα originates from a Compton-thin reflector, while the gas responsible for the high-ionization lines is collisionally excited. The overall spectrum, in the 0.3–10 keV energy band, registered by the European Photon Imaging Camera can be modelled by a power-law component (with a slope of Γ ≅ 1.9) plus two thermal components at 0.36 and 8.84 keV. The low-energy thermal component is entirely consistent with the X-ray spectral data obtained by the Reflection Grating Spectrometer between 0.35–1.8 keV
The long term X-ray time lags of NGC 4051
No full textWe present the long term, frequency-dependent, X-ray time lags of the Seyfert galaxy NGC 4051. We used Rossi X-ray Timing Explorer(RXTE) light curves in the 2–4, 4–7, and 7–10 keV bands and we measured the time lags at 10−7–10−6 Hz. This is the lowest frequency range that active galactic nuclei time lags have ever been measured. The variations in the higher energy bands are delayed with respect to the variations we observe at 2–4 keV, in agreement with the time lags at high frequencies. When we combine our results with those from the model fitting of the time lags at higher frequencies we find that that the X-ray hard lags in NGC 4051 follow a power law of slope ∼−1 over a broad frequency range, from ∼10−7 to ∼10−3 Hz. This is consistent with the time lags of Cyg X–1, a result which further supports the analogy between active galaxies and Galactic X-ray black hole binaries
General relativistic modelling of the negative reverberation X-ray time delays in AGN
No full textWe present the first systematic physical modelling of the time-lag spectra between the soft (0.3–1 keV) and the hard (1.5–4 keV) X-ray energy bands, as a function of Fourier frequency, in a sample of 12 active galactic nuclei which have been observed by XMM–Newton. We concentrate particularly on the negative X-ray time-lags (typically seen above 10-4 Hz), i.e. soft-band variations lag the hard-band variations, and we assume that they are produced by reprocessing and reflection by the accretion disc within a lamp-post X-ray source geometry. We also assume that the response of the accretion disc, in the soft X-ray bands, is adequately described by the response in the neutral Fe Kα line at 6.4 keV for which we use fully general relativistic ray-tracing simulations to determine its time evolution. These response functions, and thus the corresponding time-lag spectra, yield much more realistic results than the commonly used, but erroneous, top-hat models. Additionally, we parametrize the positive part of the time-lag spectra (typically seen below 10-4 Hz) by a power law. We find that the best-fitting black hole (BH) masses, M, agree quite well with those derived by other methods, thus providing us with a new tool for BH mass determination. We find no evidence for any correlation between M and the BH spin parameter, α, the viewing angle, θ, or the height of the X-ray source above the disc, h. Also on average, the X-ray source lies only around 3.7 gravitational radii above the accretion disc and θ is distributed uniformly between 20° and 60°. Finally, there is a tentative indication that the distribution of α may be bimodal above and below 0.62
Combined long and short time-scale X-ray variability of NGC 4051 with RXTE and XMM-Newton
No full textWe 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 6.5 years of frequent monitoring observations by RXTE with a >100-ks continuous observation by XMM-Newton and so present an overall 2-10 keV power spectral density (PSD) covering an unprecedented frequency range of over 6.5 decades from <10-8 to >10-2 Hz. The combined RXTE and XMM-Newton PSD is a very good match to the PSD of the galactic black-hole binary system (GBH) Cygnus X-1 when in a 'high', rather than 'low', state, providing the first definite confirmation of an AGN in a 'high' state. We also find that a bending power law, rather than a sharply broken power law, besides being more physical, is a much better description of the high-state PSD of Cygnus X-1 and is also a better description of the PSD of NGC 4051. At low frequencies the PSD of NGC 4051 has a slope of -1.1 bending, at a frequency νB= 8+4-3× 10-4 Hz, to a slope of alphaH~-2. Although νB does not depend on photon energy, αH is steeper at lower energies. If νB scales with mass, we imply a black-hole mass of 3+2-1× 105 Msolar in NGC 4051, which is consistent with the recently reported reverberation value of 5+6-3× 105 Msolar. Hence NGC 4051 is emitting at ~30 per cent LEdd.NGC 4051 follows the same rms-flux relationship as GBHs, consistent with higher Fourier frequencies being associated with smaller radii.From the cross-power spectra and cross-correlation functions between XMM-Newton light curves in different energy bands, we note that the higher-energy photons lag the lower-energy ones. We also note that the lag is greater for variations of longer Fourier period and increases with the energy separation of the bands. Variations in different wavebands are very coherent at long Fourier periods but the coherence decreases at shorter periods and as the energy separation between bands increases. This behaviour is again similar to that of GBHs, and of MCG-6-30-15, and suggests a radial distribution of frequencies and photon energies with higher energies and higher frequencies being associated with smaller radii.Combining our observations with observations from the literature we find it is not possible to fit all AGN to the same linear scaling of break time-scale with black-hole mass. However, broad-line AGN are consistent with a linear scaling of break time-scale with mass from Cygnus X-1 in its low state and NLS1 galaxies scale better with Cygnus X-1 in its high state. We suggest that the relationship between black-hole mass and break time-scale is a function of at least one other underlying parameter which may be accretion rate or black-hole spin or both. <br/
A search for X-ray reprocessing echoes in the power spectral density functions of AGN
No full textWe present the results of a detailed study of the X-ray power spectral density (PSD) functions of 12 X-ray bright AGN, using almost all the archival XMM–Newton data. The total net exposure of the EPIC-pn light curves is larger than 350 ks in all cases (and exceeds 1 Ms in the case of 1H 0707?497). In a physical scenario in which X-ray reflection occurs in the inner part of the accretion disc of AGN, the X-ray reflection component should be a filtered echo of the X-ray continuum signal and should be equal to the convolution of the primary emission with the response function of the disc. Our primary objective is to search for these reflection features in the 5–7 keV (iron line) and 0.5–1 keV (soft) bands, where the X-ray reflection fraction is expected to be dominant. We fit to the observed periodograms two models: a simple bending power-law model (BPL) and a BPL model convolved with the transfer function of the accretion disc assuming the lamp-post geometry and X-ray reflection from a homogeneous disc. We do not find any significant features in the best-fitting BPL model residuals either in individual PSDs in the iron band, soft and full band (0.3–10 keV) or in the average PSD residuals of the brightest and more variable sources (with similar black hole mass estimates). The typical amplitude of the soft and full-band residuals is around 3–5 per cent. It is possible that the expected general relativistic effects are not detected because they are intrinsically lower than the uncertainty of the current PSDs, even in the strong relativistic case in which X-ray reflection occurs on a disc around a fast rotating black hole having an X-ray source very close above it. However, we could place strong constrains to the X-ray reflection geometry with the current data sets if we knew in advance the intrinsic shape of the X-ray PSDs, particularly its high-frequency slope
Far-ultraviolet spectroscopy of active galactic nuclei with AstroSat/UVIT
No full textWe study accretion disk emission from eight Seyfert 1−1.5 active galactic nuclei (AGN) using far ultra-violet (1300−1800 Å) slit-less grating spectra acquired with AstroSat/UVIT. We correct for the Galactic and intrinsic extinction, contamination from the host galaxies, narrow and broad-line regions, Fe II emission and Balmer continuum, and derive the intrinsic continua. We use HST COS/FOS spectra to account for the emission/absorption lines in the low-resolution UVIT spectra. We find generally redder power-law (fν∝να) slopes (α∼−1.1−0.3) in the far UV band than predicted by the standard accretion disk model in the optical/UV band. We fit accretion disk models such as the multi-temperature disk blackbody (DISKBB) and relativistic disk (ZKERRBB, OPTXAGNF) models to the observed intrinsic continuum emission. We measure the inner disk temperatures using the DISKBB model for seven AGN. These temperatures in the range ∼3.6−5.8 eV are lower than the peak temperatures predicted for standard disks around maximally spinning super-massive black holes accreting at Eddington rates. The inner disks in two AGN, NGC 7469 and Mrk 352, appear to be truncated at ∼35−125rg and 50−135rg, respectively. While our results show that the intrinsic FUV emission from the AGN are consistent with the standard disks, it is possible that UV continua may be affected by the presence of soft X-ray excess emission, X-ray reprocessing, and thermal Comptonisation in the hot corona. Joint spectral modeling of simultaneously acquired UV/X-ray data may be necessary to further investigate the nature of accretion disks in AGN
The ‘harder when brighter’ X-ray behaviour of the low-luminosity active galactic nucleus NGC 7213
No full textWe present the first robust evidence of an anticorrelation between the X-ray photon index, Γ, and the X-ray luminosity in a single low-luminosity active galactic nucleus (LLAGN), NGC 7213. Until today, such anticorrelation trends have been seen only in large samples of LLAGN that span a wide range of X-ray fluxes, although the opposite behaviour (i.e. a positive correlation between Γ and X-ray luminosity) has been extensively studied for individual X-ray bright active galactic nuclei. For NGC 7213, we use the long-term X-ray monitoring data of the Rossi X-ray Timing Explorer (RXTE), regularly obtained on average every two days from 2006 March to 2009 December. Based on our X-ray data, we derive the Γ versus flux and the hardness ratio versus flux relations, indicating clearly that NGC 7213 follows a ‘harder when brighter’ spectral behaviour. Additionally, by analysing radio and optical data, and combining data from the literature, we form the most complete spectral energy distribution (SED) of the source across the electromagnetic spectrum yielding a bolometric luminosity of 1.7 × 1043 erg s-1. Phenomenologically, the SED of NGC 7213 is similar to that of a low-ionization nuclear emission-line region. The robust anticorrelation trend that we find between Γ and X-ray luminosity together with the low accretion rate of the source, 0.14 per cent that of the Eddington limit, makes NGC 7213 the first LLAGN exhibiting a similar spectral behaviour with that of black hole X-ray binaries in the ‘hard state’
Extensive X-ray variability studies of NGC 7314 using long XMM–Newton observations
No full textWe present a detailed X-ray variability study of the low-mass active galactic nuclei (AGN) NGC 7314 using the two newly obtained XMM–Newton observations (140 and 130 ks), together with two archival data sets of shorter duration (45 and 84 ks). The relationship between the X-ray variability characteristics and other physical source properties (such as the black hole mass) are still relatively poorly defined, especially for low-mass AGN. We perform a new, fully analytical, power spectral density (PSD) model analysis method, which will be described in detail in a forthcoming paper, that takes into consideration the spectral distortions, caused by red-noise leak. We find that the PSD in the 0.5–10 keV energy range, can be represented by a bending power law with a bend around 6.7 × 10?5 Hz, having a slope of 0.51 and 1.99 below and above the bend, respectively. Adding our bend time-scale estimate, to an already published ensemble of estimates from several AGN, supports the idea that the bend time-scale depends linearly only on the black hole mass and not on the bolometric luminosity. Moreover, we find that as the energy range increases, the PSD normalization increases and there is a hint that simultaneously the high-frequency slope becomes steeper. Finally, the X-ray time-lag spectrum of NGC 7314 shows some very weak signatures of relativistic reflection, and the energy resolved time-lag spectrum, for frequencies around 3 × 10?4 Hz, shows no signatures of X-ray reverberation. We show that the previous claim about ks time delays in this source, is simply an artefact induced by the minuscule number of points entering during the time-lag estimation in the low-frequency part of the time-lag spectrum (i.e. below 10?4 Hz)
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