40 research outputs found
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
Signatures of X-ray reverberation in the power spectra of AGN
Aims: we study the effects of X-ray reprocessing in the power spectra (PSDs) of active galactic nuclei (AGNs).Methods: we compute fully relativistic disc response functions in the case of 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.Results: 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.Conclusions: 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 fractio
A multi-tier adaptive grid algorithm for the evolutionary multi-objective optimisation of complex problems
The multi-tier Covariance Matrix Adaptation Pareto Archived Evolution Strategy (m-CMA-PAES) is an evolutionary multi-objective optimisation (EMO) algorithm for real-valued optimisation problems. It combines a non-elitist adaptive grid based selection scheme with the efficient strategy parameter adaptation of the elitist Covariance Matrix Adaptation Evolution Strategy (CMA-ES). In the original CMA-PAES, a solution is selected as a parent for the next population using an elitist adaptive grid archiving (AGA) scheme derived from the Pareto Archived Evolution Strategy (PAES). In contrast, a multi-tiered AGA scheme to populate the archive using an adaptive grid for each level of non-dominated solutions in the considered candidate population is proposed. The new selection scheme improves the performance of the CMA-PAES as shown using benchmark functions from the ZDT, CEC09, and DTLZ test suite in a comparison against the (μ+λ) μ λ Multi-Objective Covariance Matrix Adaptation Evolution Strategy (MO-CMA-ES). In comparison with MO-CMA-ES, the experimental results show that the proposed algorithm offers up to a 69 % performance increase according to the Inverse Generational Distance (IGD) metric
Searching for structural bias in particle swarm optimization and differential evolution algorithms
A community effort in SARS-CoV-2 drug discovery.
peer reviewedThe COVID-19 pandemic continues to pose a substantial threat to human lives and is likely to do so for years to come. Despite the availability of vaccines, searching for efficient small-molecule drugs that are widely available, including in low- and middle-income countries, is an ongoing challenge. In this work, we report the results of an open science community effort, the "Billion molecules against Covid-19 challenge", to identify small-molecule inhibitors against SARS-CoV-2 or relevant human receptors. Participating teams used a wide variety of computational methods to screen a minimum of 1 billion virtual molecules against 6 protein targets. Overall, 31 teams participated, and they suggested a total of 639,024 molecules, which were subsequently ranked to find 'consensus compounds'. The organizing team coordinated with various contract research organizations (CROs) and collaborating institutions to synthesize and test 878 compounds for biological activity against proteases (Nsp5, Nsp3, TMPRSS2), nucleocapsid N, RdRP (only the Nsp12 domain), and (alpha) spike protein S. Overall, 27 compounds with weak inhibition/binding were experimentally identified by binding-, cleavage-, and/or viral suppression assays and are presented here. Open science approaches such as the one presented here contribute to the knowledge base of future drug discovery efforts in finding better SARS-CoV-2 treatments.R-AGR-3826 - COVID19-14715687-CovScreen (01/06/2020 - 31/01/2021) - GLAAB Enric
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