| 1. |
- Algaba, Juan-Carlos, et al.
(författare)
-
Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign
- 2021
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 911:1
-
Forskningsöversikt (refereegranskat)abstract
- In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M o˙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
|
|
| 2. |
- Fischer, Andreas, et al.
(författare)
-
A pragmatic approach for assessment of laser-induced compressive residual stress profiles
- 2021
-
Ingår i: Journal of Manufacturing Processes. - : Elsevier. - 1526-6125 .- 2212-4616. ; 68, part A, s. 778-787
-
Tidskriftsartikel (refereegranskat)abstract
- Laser hardening is a very efficient technique for local surface treatment, however, in case of complex and large components robust processing is highly challenging due to limitations in terms of the absolute size of the overall heat-affected zone. As is shown in the present work, an increased in-depth effect can be achieved by tailoring the laser parameters without melting the surface layer. Optimization of process parameters leads to an elaborate test design demanding numerous verification measurements to determine essential material properties. In this context, the evaluation of compressive residual stress values in the surface layer is very important, e.g. in case of fatigue loaded components. However, residual stress profile measurements obtained by X-ray diffraction are very time-consuming and, thus, can significantly impair the laser parameter development cycle. For this reason, the present study introduces a novel pragmatic approach allowing for qualitative evaluation of laser-induced compressive residual stress states, in particular for multiple laser pass processes based on a Gaussian-like intensity profile. Based on straightforward analytical evaluation, several characteristic features of the affected surface layer, e.g. the position of the residual stress transition zone, can be correlated to a change of the local energy input. A novel parameter referred to as modified area energy is established in present work for this purpose. This novel energy approach provides for an essential contribution to the field of laser hardening to considerably shorten the experimental effort within the laser parameter search.
|
|
