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- 2019
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Tidskriftsartikel (refereegranskat)
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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. |
- Dong, Yu, et al.
(författare)
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Observation of a Ubiquitous (π, π)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe2–xNixAs2 Single Crystals
- 2021
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Ingår i: Chinese Physics Letters. - : Institute of Physics Publishing (IOPP). - 0256-307X .- 1741-3540. ; 38:9
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Tidskriftsartikel (refereegranskat)abstract
- In iron-based superconductors, the (0, pi) or (pi, 0) nematicity, which describes an electronic anisotropy with a four-fold symmetry breaking, is well established and believed to be important for understanding the superconducting mechanism. However, how exactly such a nematic order observed in the normal state can be related to the superconducting pairing is still elusive. Here, by performing angular-dependent in-plane magnetoresistivity using ultra-thin flakes in the steep superconducting transition region, we unveil a nematic superconducting order along the (pi, pi) direction in electron-doped BaFe2 - x Ni x As2 from under-doped to heavily overdoped regimes with x = 0.065-0.18. It shows superconducting gap maxima along the (pi, pi) direction rotated by 45 degrees from the nematicity along (0, pi) or (pi, 0) direction observed in the normal state. A similar (pi, pi)-type nematicity is also observed in the under-doped and optimally doped hole-type Ba1 - y K y Fe2As2, with y = 0.2-0.5. These results suggest that the (pi, pi) nematic superconducting order is a universal feature that needs to be taken into account in the superconducting pairing mechanism in iron-based superconductors.
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| 4. |
- Zhu, Shun-Wei
(författare)
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Brain neurotrophin levels and mouse behavior : relationship to environmental influences
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- All normal functions over the lifetime of an animal are based on a dynamic balance of at least three fundamental elements: brain, environment and behaviour. This thesis investigated different aspects of these three elements and revealed novel interactions of brain neurotrophins, environmental influences and emotional behaviour in mice. In the first two studies, we used C57BL6 mice housed in standard or enriched conditions to investigate anxiety- and fear-related behaviour and neurotrophin levels in different brain regions. We discovered: (a) The normal (steady state) levels of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) protein are differently distributed in dorsal and ventral parts of hippocampus in both male and female mice, with the dorsal hippocampal levels being consistently higher than those in ventral hippocampus. (b) Exposure to a series of behavioural tests induced complex changes on neurotrophin levels in selected brain regions. (c) Differential housing exerted significant influences on anxiety-related behaviour and brain neurotrophins; these changes were gender and brain region dependent. (d) Significant correlations between behavioural measures and postmortem brain regional neurotrophic factors contents were related: whereby the magnitude of anxiety-like behaviour in the elevated plus maze was positively related to dorsal hippocampal BDNF levels, but negatively related to NGF levels in dorsal hippocampus and in the amygdala, while the expression of conditioned fear is positively related to amygdala BDNF and NGF levels, and to dorsal hippocampus NGF levels. The third and fourth studies further explored the interrelationship of environmental conditions, behaviour and brain neurotrophins, using BALB/c mice and BDNF knockout mice. The studies involved exposure to intermittent individual housing with or without access to physical exercise in anxious BALB/c mice and environmental enrichment (EE) in BDNF mutant mice. The results showed that: (a) Intermittent exposure to individual housing induced anxietylike behaviours with significantly enhanced motor activity, (b) Alternate social isolation caused down regulation of NGF and BDNF levels in frontal cortex, while up regulation of BDNF protein content in the amygdala and BDNF protein and mRNA levels in the hippocampus. (c) Access to running wheels for intermittently isolated mice normalized motor activity. Besides increased cerebellar BDNF and hippocampal NGF and BDNF protein levels, physical exercise did not attenuate down regulation of cortical NGF and BDNF protein levels induced by intermittent social isolation. These results demonstrated for the first time that alternate housing had significant impact on behaviour and neurotrophin levels in selected brain regions in mice, which can be partially altered by voluntary physical exercise. It also suggested that substantial changes induced by intermittent social isolation are different from previous findings caused by sustained social isolation on behaviour and brain neurotrophin. (d) EE increased numbers of dendritic spines in the hippocampal dentate gyrus neurons in wild-type (WT) mice, whereas less impact was found in this brain region in the BDNF mutant mice. (e) Behavioural results showed that in enriched WT mice, there was increased exploration and faster habituation, while this effect was abolished or attenuated in BDNF mutant mice. These findings provided evidence for the involvement of BDNF in regulating emotional behaviour and neural plasticity associated with EE.
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| 5. |
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| 6. |
- Zhu, Shun-Wei, et al.
(författare)
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Influence of environmental manipulation on exploratory behaviour in male BDNF knockout mice
- 2009
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Ingår i: Behavioural Brain Research. - : Elsevier BV. - 0166-4328 .- 1872-7549. ; 197:2, s. 339-346
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Tidskriftsartikel (refereegranskat)abstract
- It is widely accepted that brain derived neurotrophic factor (BDNF) plays a crucial role in mediating changes in learning and memory performance induced by environmental conditions. In order to ascertain whether BDNF modulates environmentally induced changes in exploratory behaviour, we examined mice carrying a deletion in one copy of the BDNF gene. Young heterozygous male BDNF knockout mice (BDNF+/−) and their wild-type (WT) controls were exposed to the enriched environment condition (EC) or the standard condition (SC) for 8 weeks. Exploratory behaviour was assessed in the open-field (OF) and hole-board (HB) test. Brains from EC and SC reared animals were processed for Golgi-Cox staining and the dendritic spine density in the dentate gyrus (DG) and CA1 hippocampal regions were examined. We found behavioural differences both due to the genetic modification and the environmental manipulation, with the BDNF+/− mice being more active in the OF whereas the EC mice had increased exploratory behaviour in the HB test. Environmental enrichment also led to an increase in dendritic spines in the hippocampal CA1 region and DG of the wild-type mice. This effect was also found in the enriched BDNF+/− mice, but was less pronounced. Our findings support the critical role of BDNF in behavioural and neural plasticity associated with environmental enrichment and suggest that besides maze learning performance, BDNF dependent mechanisms are also involved in other aspects of behaviour. Here we provide additional evidence that exploratory activity is influenced by BDNF.
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