This article presents a review of the state of the art of the Wind Farm Design and Optimization (WFDO) problem. The WFDO problem refers to a set of advanced planning actions needed to extremize the ...performance of wind farms, which may be composed of a few individual Wind Turbines (WTs) up to thousands of WTs. The WFDO problem has been investigated in different scenarios, with substantial differences in main objectives, modelling assumptions, constraints, and numerical solution methods. The aim of this paper is: (1) to present an exhaustive survey of the literature covering the full span of the subject, an analysis of the state-of-the-art models describing the performance of wind farms as well as its extensions, and the numerical approaches used to solve the problem; (2) to provide an overview of the available knowledge and recent progress in the application of such strategies to real onshore and offshore wind farms; and (3) to propose a comprehensive agenda for future research.
Recent simulations have shown that asymmetries in the ejecta distribution of supernova remnants can still reflect asymmetries from the initial supernova explosion. Thus, their study provides a great ...means to test and constrain model predictions in relation to the distributions of heavy elements or the neutron star kicks, both of which are key to better understanding the explosion mechanisms in core-collapse supernovae. The use of a novel blind source separation method applied to the megasecond X-ray observations of the well-known Cassiopeia A supernova remnant has revealed maps of the distribution of the ejecta endowed with an unprecedented level of detail and clearly separated from continuum emission. Our method also provides a three-dimensional view of the ejecta by disentangling the red- and blue-shifted spectral components and associated images of the Si, S, Ar, Ca and Fe, providing insights into the morphology of the ejecta distribution in Cassiopeia A. These mappings allow us to thoroughly investigate the asymmetries in the heavy elements distribution and probe simulation predictions about the neutron star kicks and the relative asymmetries between the different elements. We find in our study that most of the ejecta X-ray flux stems from the red-shifted component, suggesting an asymmetry in the explosion. In addition, the red-shifted ejecta can physically be described as a broad, relatively symmetric plume, whereas the blue-shifted ejecta is more similar to a dense knot. The neutron star also moves directly opposite to the red-shifted parts of the ejecta similar to what is seen with
44
Ti. Regarding the morphological asymmetries, it appears that heavier elements have more asymmetrical distributions, which confirms predictions made by simulations. This study is a showcase of the capacities of new analysis methods to revisit archival observations to fully exploit their scientific content.
We report the first high-significance GeV γ-ray detections of supernova remnants HESS J1731−347 and SN 1006, both of which have been previously detected by imaging atmospheric Cherenkov Telescopes ...above 1 TeV. Using 8 years of Fermi-LAT Pass 8 data at energies between 1 GeV and 2 TeV, we detect emission at the position of HESS J1731−347 with a significance of and a spectral index of . The hardness of the index and the good connection with the TeV spectrum of HESS J1731−347 support an association between the two sources. We also confirm the detection of SN 1006 at with a spectral index of . The northeast (NE) and southwest (SW) limbs of SN 1006 were also fit separately, resulting in the detection of the NE region ( ) and the non-detection of the SW region. The significance of different spectral components for the two limbs is , providing first indications of an asymmetry in the GeV γ-ray emission.
Context. The breakthrough developments of Cherenkov telescopes in the past decade have led to angular resolution of 0.1° and an unprecedented sensitivity. This has allowed the current generation of ...Cherenkov telescopes (H.E.S.S., MAGIC, and VERITAS) to discover a population of supernova remnants (SNRs) radiating in very-high-energy (VHE; E > 100 GeV) γ-rays. A number of those VHE SNRs exhibit a shell-type morphology that is spatially coincident with the shock front of the SNR. Aims. The members of this VHE shell SNR club are RX J1713.7−3946, RX J0852.0−4622, RCW 86, SN 1006, and HESS J1731−347. The last two objects have been poorly studied in high-energy (HE; 0.1 < E < 100 GeV) γ-rays and need to be investigated in order to draw the overall picture of this class of SNRs and to constrain the characteristics of the underlying population of accelerated particles. Methods. Using 6 years of Fermi-LAT P7 reprocessed data, we studied the GeV counterpart of the SNRs HESS J1731−347 and SN 1006. The two SNRs are not detected in the data set, and given that there is no hint of detection, we do not expect any detection in coming years from the SNRs. However in both cases, we derived upper limits that significantly constrain the γ-ray emission mechanism and can rule out a standard hadronic scenario with a confidence level >5σ. Results. With this Fermi analysis, we now have a complete view of the HE to VHE γ-ray emission of TeV shell SNRs. All five sources have a hard HE photon index (Γ < 1.8), which suggests a common scenario where the bulk of the emission is produced by accelerated electrons radiating from radio to VHE γ-rays through synchrotron and inverse Compton processes. In addition when correcting for the distance, all SNRs show a surprisingly similar γ-ray luminosity supporting the idea of a common emission mechanism. While the γ-ray emission is likely to be leptonic-dominated at the scale of the whole SNR, this does not rule out efficient hadron acceleration in those objects.
Context.
Some young supernova remnants exhibit thin filaments of X-ray synchrotron radiation coinciding with the forward shock due to accelerated electrons interacting with the local magnetic field. ...The two main models accounting for the radial brightness evolution of these filaments differ in their prediction of the narrowing (or not) of the filaments with increasing photon energy.
Aims.
In this paper, we report our observation of such a narrowing of the synchrotron filaments in Cassiopeia A at X-ray energies, and how this finding could inform our understanding of the mechanisms at stake in their formation.
Methods.
We used a new blind-source-separation method on the 1 Ms
Chandra
observation of Cassiopeia A in order to obtain detailed and unpolluted images of the synchrotron emission in three energy bands. We then extracted the profiles of several filaments at the forward shock and the reverse shock to estimate and compare their widths.
Results.
We find that there is indeed a narrowing with energy of the synchrotron filaments both at the forward and at the reverse shocks in Cassiopeia A. The energy dependency of this narrowing seems stronger at high energy, which is indicative of a damping effect that is also suggested by radio observations.
Context.
The Cygnus Loop (G74.0-8.5) is a very well-known nearby supernova remnant (SNR) in our Galaxy. Thanks to its large size, brightness, and angular offset from the Galactic plane, it has been ...studied in detail from radio to
γ
-ray emission. The
γ
-rays probe the populations of energetic particles and their acceleration mechanisms at low shock speeds.
Aims.
We present an analysis of the
γ
-ray emission detected by the Large Area Telescope on board the
Fermi
Gamma-ray Space Telescope over 11 yr in the region of the Cygnus Loop.
Methods.
We performed detailed morphological and spectral studies of the
γ
-ray emission toward the remnant from 100 MeV to 100 GeV and compared it with X-ray, UV, optical, and radio images. The higher statistics with respect to the previous studies enabled us to decompose the emission from the remnant into two morphological components to model its nonthermal multiwavelength emission.
Results.
The extended
γ
-ray emission is well correlated with the thermal X-ray and UV emission of the SNR. Our morphological analysis reveals that a model considering two contributions from the X-ray and the UV emission regions is the best description of the
γ
-ray data. Both components show a curved spectrum, but the X-ray component is softer and more curved than the UV component, suggesting a different physical origin. The multiwavelength modeling of emission toward the SNR suggests that the nonthermal radio and
γ
-ray emission associated with the UV component is mostly due to the reacceleration of preexisting cosmic rays by radiative shocks in the adjacent clouds, while the nonthermal emission associated with the X-ray component arises from freshly accelerated cosmic rays.
N132D is the brightest gamma-ray supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out 12CO(J = 1-0, 3-2) observations toward the SNR using the Atacama Large ...Millimeter/submillimeter Array (ALMA) and Atacama Submillimeter Telescope Experiment. We find diffuse CO emission not only at the southern edge of the SNR as previously known, but also inside the X-ray shell. We spatially resolved nine molecular clouds using ALMA with an angular resolution of 5″, corresponding to a spatial resolution of ∼1 pc at the distance of the LMC. Typical cloud sizes and masses are ∼2.0 pc and ∼100 M , respectively. High intensity ratios of CO J = 3-2/1-0 > 1.5 are seen toward the molecular clouds, indicating that shock heating has occurred. Spatially resolved X-ray spectroscopy reveals that thermal X-rays in the center of N132D are produced not only behind a molecular cloud but also in front of it. Considering the absence of a thermal component associated with the forward shock toward one molecular cloud located along the line of sight to the center of the remnant, this suggests that this particular cloud is engulfed by shock waves and is positioned on the near side of the remnant. If the hadronic process is the dominant contributor to the gamma-ray emission, the shock-engulfed clouds play a role as targets for cosmic rays. We estimate the total energy of cosmic-ray protons accelerated in N132D to be ∼0.5-3.8 × 1049 erg as a conservative lower limit, which is similar to that observed in Galactic gamma-ray SNRs.
In high-energy astronomy, spectro-imaging instruments such as X-ray detectors allow investigation of the spatial and spectral properties of extended sources including galaxy clusters, galaxies, ...diffuse interstellar medium, supernova remnants, and pulsar wind nebulae. In these sources, each physical component possesses a different spatial and spectral signature, but the components are entangled. Extracting the intrinsic spatial and spectral information of the individual components from this data is a challenging task. Current analysis methods do not fully exploit the 2D-1D (x, y, E) nature of the data, as spatial information is considered separately from spectral information. Here we investigate the application of a blind source separation (BSS) algorithm that jointly exploits the spectral and spatial signatures of each component in order to disentangle them. We explore the capabilities of a new BSS method (the general morphological component analysis; GMCA), initially developed to extract an image of the cosmic microwave background from Planck data, in an X-ray context. The performance of the GMCA on X-ray data is tested using Monte-Carlo simulations of supernova remnant toy models designed to represent typical science cases. We find that the GMCA is able to separate highly entangled components in X-ray data even in high-contrast scenarios, and can extract the spectrum and map of each physical component with high accuracy. A modification of the algorithm is proposed in order to improve the spectral fidelity in the case of strongly overlapping spatial components, and we investigate a resampling method to derive realistic uncertainties associated to the results of the algorithm. Applying the modified algorithm to the deep Chandra observations of Cassiopeia A, we are able to produce detailed maps of the synchrotron emission at low energies (0.6–2.2 keV), and of the red- and blueshifted distributions of a number of elements including Si and Fe K.
The study of the extreme weather space events is important for a technological‐dependent society. Extreme value theory could be decisive to characterize those extreme events in order to have the ...knowledge to make decisions in technological, economic, and social matters, in all fields with possible impacts. In this work, the hourly values of the Dst geomagnetic index have been studied for the period 1957–2014 using the peaks‐over‐threshold technique. The shape parameter obtained from the fit of the generalized Pareto distribution to the extreme values of the |Dst| index leads to a negative value implying an upper bound for this time series. This result is relevant because the estimation of this limit for the extreme values leads to 850 nT as the highest expected value for this geomagnetic index. Thus, from the previous characterization of the Carrington geomagnetic storm and our results, it could be considered the worst‐case scenario.
Plain Language Summary
The study of the space environment that surrounds the Earth is relevant due to the negative effects that unfavorable conditions cause to technological systems. The geomagnetic Dst index is used in this work in order to evaluate the severity of the magnetic storms. The use of statistical tools specifically designed to study rare and scarce extreme events is applied to understand the behavior of this geomagnetic index. The results show that there is a limit value for the geomagnetic Dst index, and therefore, previous registered extreme storms could be considered as the “worst case scenario.”
Key Points
The shape parameter leads to a negative value implying an upper bound for the |Dst| index
The estimation of this limit for the extreme values leads to 850 nT as the highest expected value for this geomagnetic index
The results show that previous registered extreme storms could be considered as the “worst‐case scenario”
We report on the analysis of XMM-Newton observations of the non-thermal shell-type supernova remnant HESS J1731−347 (G353.6-0.7). For the first time the complete remnant shell has been covered in ...X-rays, which allowed direct comparison with radio and TeV observations. We carried out a spatially resolved spectral analysis of XMM-Newton data and confirmed the previously reported non-thermal power-law X-ray spectrum of the source with negligible variations of spectral index across the shell. On the other hand, the X-ray absorption column is strongly variable and correlates with the CO emission thus confirming that the absorbing material must be in the foreground and reinforcing the previously suggested lower limit on distance. Finally, we find that the X-ray emission of the remnant is suppressed towards the Galactic plane, which points to lower shock velocities in this region, likely due to the interaction of the shock with the nearby molecular cloud.
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