255 research outputs found
Generating artificial light curves: Revisited and updated
The production of artificial light curves with known statistical and
variability properties is of great importance in astrophysics. Consolidating
the confidence levels during cross-correlation studies, understanding the
artefacts induced by sampling irregularities, establishing detection limits for
future observatories are just some of the applications of simulated data sets.
Currently, the widely used methodology of amplitude and phase randomisation is
able to produce artificial light curves which have a given underlying power
spectral density (PSD) but which are strictly Gaussian distributed. This
restriction is a significant limitation, since the majority of the light curves
e.g. active galactic nuclei, X-ray binaries, gamma-ray bursts show strong
deviations from Gaussianity exhibiting `burst-like' events in their light
curves yielding long-tailed probability distribution functions (PDFs). In this
study we propose a simple method which is able to precisely reproduce light
curves which match both the PSD and the PDF of either an observed light curve
or a theoretical model. The PDF can be representative of either the parent
distribution or the actual distribution of the observed data, depending on the
study to be conducted for a given source. The final artificial light curves
contain all of the statistical and variability properties of the observed
source or theoretical model i.e. same PDF and PSD, respectively. Within the
framework of Reproducible Research, the code, together with the illustrative
example used in this manuscript, are both made publicly available in the form
of an interactive Mathematica notebook.Comment: Accepted for publication in MNRAS. The paper is 23 pages long and
contains 21 figures and 2 tables. The Mathematica notebook can be found in
the web as part of this paper (Online Material) or at
http://www.astro.soton.ac.uk/~de1e08/ArtificialLightCurves
General relativistic modelling of the negative reverberation X-ray time delays in AGN
We 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 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 iron
line (Fe k) 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 Hz) by a power-law. We find that the
best-fitting 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, ,
the viewing angle, , or the height of the X-ray source above the disc,
. Also on average, the X-ray source lies only around 3.7 gravitational radii
above the accretion disc and the viewing angles are distributed uniformly
between 20 and 60 degrees. Finally, there is a tentative indication that the
distribution of spin parameters may be bimodal above and below 0.62.Comment: Accepted for publication in MNRAS. The paper is 22 pages long and
contains 19 figures and 2 table
Extensive X-ray variability studies of NGC 7314 using long XMM-Newton observations
We 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
( and ks), together with two archival data sets of shorter duration
( and 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 keV energy range, can be represented by a bending
power-law with a bend around Hz, having a slope of
and 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 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 Hz).Comment: Accepted for publication in MNRAS. The paper is 21 pages long and
contains 15 figures and 3 table
Signatures of X-ray reverberation in the power spectra of AGN
We compute fully relativistic disc response functions in the case of the
"lamp-post" geometry using the full observed reflection spectrum for various
X-ray source heights, disc inclination, and spin values of the central black
hole. Since the observed PSD is equal to the product of the intrinsic power
spectrum with the "transfer function" (i.e. the Fourier transform of the disc
response function), we are able to predict the observed PSDs in the case of
X-ray illumination of the inner disc. The observed PSD should show a prominent
dip at high frequencies and an oscillatory behaviour, with a decreasing
amplitude, at higher frequencies. The reverberation "echo" features should be
more prominent in energy bands where the reflection component is more
pronounced. The frequency of the dip is independent of energy, and it is mainly
determined by the black hole mass and the X-ray source height. The amplitude of
the dip increases with increasing black hole spin and inclination angle, as
long as the height of the "lamp" is smaller than ~10 gravitational radii. The
detection of the X-ray reverberation signals in the PSDs can provide further
evidence for X-ray illumination of the inner disc in AGN. Our results are
largely independent of the assumed geometry of the disc-corona system, as long
as it does not change with time, and the disc response function is
characterized by a sharp rise, a "plateau", and a decline at longer times.
Irrespective of the geometry, the frequency of the main dip should decrease
with increasing "mean time" of the response function, and the amplitude of the
dip should increase with increasing reflection fraction.Comment: Astronomy and Astrophysics accepte
A search for X-ray reprocessing echoes in the power spectral density functions of AGN
We present the results of a detailed study of the X-ray power spectra density
(PSD) functions of twelve 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.Comment: Accepted for publication in MNRAS. The paper is 15 pages long and
contains 7 figures and 6 table
Long-term X-ray spectral variability in AGN from the Palomar sample observed by Swift
We present X-ray spectral variability of 24 local active galactic nuclei (AGN) from the Palomar sample of nearby galaxies, as observed mainly by Swift. From hardness ratio measurements, we find that 18 AGN with low accretion rates show hardening with increasing count rate, converse to the softer-when-brighter behaviour normally observed in AGN with higher accretion rates. Two AGN show softening with increasing count rate, two show more complex behaviour, and two do not show any simple relationship. Sufficient data were available for the spectra of 13 AGN to be summed in flux-bins. In nine of these sources, correlated luminosity-dependent changes in the photon index (?) of a power-law component are found to be the main cause of hardness variability. For six objects, with a low accretion rate as a fraction of the Eddington rate (m?Edd), ? is anticorrelated with m?Edd, i.e. āharder-when-brighterā behaviour is observed. The three higher m?Edd-rate objects show a positive correlation between ? and m?Edd. This transition from harder-when-brighter at low m?Eddto softer-when-brighter at high m?Edd can be explained by a change in the dominant source of seed-photons for X-ray emission from cyclo-synchrotron emission from the Comptonizing corona itself to thermal seed-photons from the accretion disc. This transition is also seen in the āhard stateā of black hole X-ray binaries (BHXRBs). The results support the idea that low-ionization nuclear emission-line regions are analogues of BHXRBs in the hard state and that Seyferts are analogues of BHXRBs in either the high-accretion rate end of the hard state or in the hard-intermediate state
X-ray spectral analysis of the low-luminosity active galactic nucleus NGC 7213 using long XMM-Newton observations
We present the X-ray spectral results from the longest X-ray Multi-mirror MissionāNewton 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 K? 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 K? originates from a Compton-thin reflector, while the gas responsible for the high-ionization lines is collisionally excited. The overall spectrum, in the 0.3ā10?keV energy band, registered by the European Photon Imaging Camera can be modelled by a power-law component (with a slope of ? ? 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 0.35ā1.8?keV
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