| 1. |
- Akiyama, Kazunori, et al.
(författare)
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The persistent shadow of the supermassive black hole of M 87: I. Observations, calibration, imaging, and analysis*
- 2024
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 681
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Tidskriftsartikel (refereegranskat)abstract
- In April 2019, the Event Horizon Telescope (EHT) Collaboration reported the first-ever event-horizon-scale images of a black hole, resolving the central compact radio source in the giant elliptical galaxy M 87. These images reveal a ring with a southerly brightness distribution and a diameter of ∼42 μas, consistent with the predicted size and shape of a shadow produced by the gravitationally lensed emission around a supermassive black hole. These results were obtained as part of the April 2017 EHT observation campaign, using a global very long baseline interferometric radio array operating at a wavelength of 1.3 mm. Here, we present results based on the second EHT observing campaign, taking place in April 2018 with an improved array, wider frequency coverage, and increased bandwidth. In particular, the additional baselines provided by the Greenland telescope improved the coverage of the array. Multiyear EHT observations provide independent snapshots of the horizon-scale emission, allowing us to confirm the persistence, size, and shape of the black hole shadow, and constrain the intrinsic structural variability of the accretion flow. We have confirmed the presence of an asymmetric ring structure, brighter in the southwest, with a median diameter of 43.3-3.1+1.5 μas. The diameter of the 2018 ring is remarkably consistent with the diameter obtained from the previous 2017 observations. On the other hand, the position angle of the brightness asymmetry in 2018 is shifted by about 30 relative to 2017. The perennial persistence of the ring and its diameter robustly support the interpretation that the ring is formed by lensed emission surrounding a Kerr black hole with a mass ∼6.5× 109M. The significant change in the ring brightness asymmetry implies a spin axis that is more consistent with the position angle of the large-scale jet.
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| 2. |
- Kim, Jae-Young, et al.
(författare)
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Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 640
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Tidskriftsartikel (refereegranskat)abstract
- 3C 279 is an archetypal blazar with a prominent radio jet that show broadband flux density variability across the entire electromagnetic spectrum. We use an ultra-high angular resolution technique - global Very Long Baseline Interferometry (VLBI) at 1.3mm (230 GHz) - to resolve the innermost jet of 3C 279 in order to study its fine-scale morphology close to the jet base where highly variable-ray emission is thought to originate, according to various models. The source was observed during four days in April 2017 with the Event Horizon Telescope at 230 GHz, including the phased Atacama Large Millimeter/submillimeter Array, at an angular resolution of ∼20 μas (at a redshift of z = 0:536 this corresponds to ∼0:13 pc ∼ 1700 Schwarzschild radii with a black hole mass MBH = 8 × 108 M⊙). Imaging and model-fitting techniques were applied to the data to parameterize the fine-scale source structure and its variation.We find a multicomponent inner jet morphology with the northernmost component elongated perpendicular to the direction of the jet, as imaged at longer wavelengths. The elongated nuclear structure is consistent on all four observing days and across diffierent imaging methods and model-fitting techniques, and therefore appears robust. Owing to its compactness and brightness, we associate the northern nuclear structure as the VLBI "core". This morphology can be interpreted as either a broad resolved jet base or a spatially bent jet.We also find significant day-to-day variations in the closure phases, which appear most pronounced on the triangles with the longest baselines. Our analysis shows that this variation is related to a systematic change of the source structure. Two inner jet components move non-radially at apparent speeds of ∼15 c and ∼20 c (∼1:3 and ∼1:7 μas day-1, respectively), which more strongly supports the scenario of traveling shocks or instabilities in a bent, possibly rotating jet. The observed apparent speeds are also coincident with the 3C 279 large-scale jet kinematics observed at longer (cm) wavelengths, suggesting no significant jet acceleration between the 1.3mm core and the outer jet. The intrinsic brightness temperature of the jet components are ≤1010 K, a magnitude or more lower than typical values seen at ≥7mm wavelengths. The low brightness temperature and morphological complexity suggest that the core region of 3C 279 becomes optically thin at short (mm) wavelengths.
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| 3. |
- Li, Jun, et al.
(författare)
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An efficient and reliable approach for quality-of-service-aware service composition
- 2014
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Ingår i: Information Sciences. - : Elsevier. - 0020-0255 .- 1872-6291. ; 269, s. 238-254
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Tidskriftsartikel (refereegranskat)abstract
- With the rapidly increasing number of independently developed Web services that provide similar functionalities with varied quality of service (QoS), service composition is considered as a problem in the selection of component services that are in accordance with users' QoS requirements; a practice known as the QoS-aware service composition problem. However, current solutions are unsuitable for most real-time decision-making service composition applications required to obtain a relatively optimal result within a reasonable amount of time. These services are also unreliable (or even risky) given the open service-oriented environment. In this paper, we address these problems and propose a novel heuristic algorithm for an efficient and reliable selection of trustworthy services in a service composition. The proposed algorithm consists of three steps. First, a trust-based selection method is used to filter untrustworthy component services. Second, convex hulls are constructed to reduce the search space in the process of service composition. Finally, a heuristic global optimization approach is used to obtain the near-optimal solution. The results demonstrate that our approach obtains a close-to-optimal and reliable solution within a reasonable computation time.
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| 4. |
- Wang, Jing, et al.
(författare)
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A dynamic firefly algorithm based on two-way guidance and dimensional mutation
- 2022
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Ingår i: International Journal of Bio-Inspired Computation (IJBIC). - : Inderscience Publishers. - 1758-0366 .- 1758-0374. ; 20:2, s. 126-126
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Tidskriftsartikel (refereegranskat)abstract
- As a stochastic optimiser, the firefly algorithm (FA) has been successfully and widely used in the solutions to various optimisation problems. Recent related research shows that the standard FA does not sufficiently balance between exploration and exploitation. Especially in high-dimensional problems, it is easy for the standard FA to fall into the local optimum and lead to premature convergence. To overcome the problems as mentioned above, DMTgFA uses three strategies: dynamic step length setting strategy (DS), non-elite two-way guidance model (TG) and elites dimensional mutation strategy (DM). The dynamic step length setting strategy makes the algorithm convergence speed faster. The non-elite two-way guidance model and the elite dimensional mutation strategy cooperate to solve the balance problem between global search and local search. Experimental results show that DMTgFA has stronger optimisation ability and faster convergence speed than other state-of-the-art FA variants.
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| 5. |
- Barakat, Serena, et al.
(författare)
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A Three-Stage Periodic Model : An Initial Analysis of Government Preventive Measures against COVID-19 Epidemics
- 2020
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Ingår i: ACM International Conference Proceeding Series. - New York, NY, USA : ACM Digital Library. - 9781450387859 ; , s. 26-31
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Konferensbidrag (refereegranskat)abstract
- In early spring 2020, Covid-19 was categorized as a pandemic and has since infected several millions of people in many countries and claimed hundreds of thousands of lives. Various strict strategies and prevention measures, such as curfews and lockdowns of cities or entire countries, have been enforced by governments to mitigate the spread of the virus. While the results of the aforementioned enforced measures deemed promising for some countries, the same could not be said about others. This paper serves as an initial analysis of the effect of government enforced strategies and safety measures on the transmission of Covid-19. We propose a three-stage periodic model: The rise stage, plateau stage, and decline stage, to describe the changes of the spread of Covid-19. The results show a positive and constructive answer to our proposed three-stage model. © 2020 ACM.
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| 6. |
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| 7. |
- Du, Xiaofeng, et al.
(författare)
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Conceptual graph : an approach to improve quality of business services modeling
- 2014
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Ingår i: Web Information Systems Engineering - WISE 2013 workshops. - Berlin : Springer Berlin/Heidelberg. - 9783642543708 - 9783642543692 ; , s. 239-251
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Konferensbidrag (refereegranskat)abstract
- Business development or renovation is to introduce newer, more efficient routines and processes through redesign or re-engineering of businesses, which form a set of business patterns. Business patterns encapsulate the best solutions for business practices and tasks confirming business strategies of the enterprise. Nowadays, services with SOA (Service oriented-Architecture) become more and more important in implementing and supporting business routines and processes. An enterprise that can encapsulate their SOA solutions into patterns will make the business more agile and effective. However, with the SOA solutions to automation of locating relevant instance services for its business patterns with minimum human intervention one has to look into the semantic and operational difference between the description of a business pattern and that of an instance service-a gap between the two levels of semantic descriptions. In this paper, we propose a conceptual modeling method to address how to bridge the gap, by a semantic service description for usage contextual approach formalized with the conceptual graphs formalism.
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| 8. |
- Du, Xiaofeng, et al.
(författare)
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Quality improvement framework for business oriented geo-spatial data
- 2015
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Ingår i: Web Information Systems Engineering - WISE 2014 workshops. - Berlin : Springer Berlin/Heidelberg. - 9783319203706 - 9783319203690 ; , s. 239-249
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Konferensbidrag (refereegranskat)abstract
- In the past few years, Geo-spatial data quality has received increasing attention and concerns. As more and more business decisions are made based on data analytic result from geo-spatial related data, low quality data means wrong or inappropriate decisions, which could have substantial effects on a business's future. In this paper, we propose a framework that can systematically ensure and improve geo-spatial data quality throughout the whole life cycle of data.
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| 9. |
- Li, Jun, et al.
(författare)
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Trust based service selection in service oriented environment
- 2012
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Ingår i: International Journal of Web Services Research. - : IGI Global. - 1545-7362 .- 1546-5004. ; 9:3, s. 23-42
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Tidskriftsartikel (refereegranskat)abstract
- In a service-oriented environment, it is inevitable and indeed quite common to deal with web services, whose reliability is unknown to the users. The reputation system is a popular technique currently used for providing a global quality score of a service provider to requesters. However, such global information is far from sufficient for service requesters to choose the most qualified services. In order to tackle this problem, the authors present a trust based architecture containing a computational trust model for quantifying and comparing the trustworthiness of services. In this trust model, they firstly construct a network based on the direct trust relations between participants and rating similarity in service oriented environments, then propose an algorithm for propagating trust in the social network based environment which can produce personalized trust information for a specific service requester, and finally implement the trust model and simulate various malicious behaviors in not only dense but also sparse networks which can verify the attack-resistant and robustness of the proposed approach. The experiment results also demonstrate the feasibility and benefit of the approach.
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| 10. |
- Lou, Qing, et al.
(författare)
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Combination of evaluation methods for assessing the quality of service for express delivery industry
- 2015
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Ingår i: Web Information Systems Engineering – WISE 2015. - Cham : Springer. - 9783319261867 - 9783319261867 ; , s. 414-425
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Konferensbidrag (refereegranskat)abstract
- In the view of the current service development of the express delivery industry and the data quality problem experienced thereof, we consider to construct an index-based evaluation system for the service quality for the express delivery industry through the analysis of market investigation and data analysis. This system applies analytic hierarchy process (AHP), to survey expert’s options and obtain the index weights. The analytical evaluation of service quality for the specific express delivery company is conducted with the fuzzy comprehensive evaluation method. A service satisfaction degree for the express delivery company is generated to improve the overall service performance. Through evaluating results of the solution to the problems in the quality of service, this paper aims at establishing a guideline to improve their service quality in express delivery enterprises. This research aims at the development of a novel method for service quality evaluation in the area of the fast growing businesses of express delivery enterprises.
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