作者:未玖 来源:科学网微信公众号 发布时间:2023/7/8 20:47:39


Nature, 6 July 2023, VOL 619, ISSUE 7968




X-ray polarization evidence for a 200-year-old flare of Sgr A*

Sgr A* 200年耀斑的X射线偏振证据

▲ 作者:Frédéric Marin, Eugene Churazov, Ildar Khabibullin, Riccardo Ferrazzoli, Laura Di Gesu, Thibault Barnouin, et al.

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▲ 摘要:

银河系中心有一个质量约为太阳400万倍的黑洞(人马座A* ,Sgr A),目前非常沉寂,其亮度比活跃星系核低几个数量级。星系中心区域致密气体对SgrA* X射线的反射为研究其过去数百年和数千年的耀斑活动提供了一种方法。

从Sgr A*附近巨大分子云观测到的X射线连续体的形状和强荧光铁线与反射情景一致。如果这种解释正确,反射的连续辐射应该是极化的。

研究组报道了使用成像X射线偏振探测器在银河系中心分子云方向上的偏振X射线发射的观测结果。经测量偏振度为31%±11%,偏振角为- 48°±11°。

所测偏振角与Sgr A*为主要发射源的偏振角相一致,偏振度表明大约200年前,Sgr A*的X射线光度与塞弗特星系的光度曾短暂相当。

▲ Abstract:

The centre of the Milky Way Galaxy hosts a black hole with a solar mass of about 4 million (Sagittarius A* (Sgr A)) that is very quiescent at present with a luminosity many orders of magnitude below those of active galactic nuclei. Reflection of X-rays from Sgr A* by dense gas in the Galactic Centre region offers a means to study its past flaring activity on timescales of hundreds and thousands of years. The shape of the X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A* are consistent with the reflection scenario. If this interpretation is correct, the reflected continuum emission should be polarized. Here we report observations of polarized X-ray emission in the direction of the molecular clouds in the Galactic Centre using the Imaging X-ray Polarimetry Explorer. We measure a polarization degree of 31%±11%, and a polarization angle of 48°±11°. The polarization angle is consistent with Sgr?A* being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sgr?A* was briefly comparable to that of a Seyfert galaxy.

材料科学Materials Science

Liquid metal for high-entropy alloy nanoparticles synthesis


▲ 作者:Guanghui Cao, Jingjing Liang, Zenglong Guo, Kena Yang, Gang Wang, Huiliu Wang, et al.

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研究组发现,赋予其他元素负混合焓的液态金属可提供稳定的热力学条件,并作为理想的动态混合储层,从而在温和反应条件下合成多种金属元素的HEA-NPs。所涉及的元素具有广泛的原子半径(1.24-1.97 ?)和熔点(303-3683 K)。


▲ Abstract:

High-entropy alloy nanoparticles (HEA-NPs) show great potential as functional materials. However, thus far, the realized high-entropy alloys have been restricted to palettes of similar elements, which greatly hinders the material design, property optimization and mechanistic exploration for different applications. Herein, we discovered that liquid metal endowing negative mixing enthalpy with other elements could provide a stable thermodynamic condition and act as a desirable dynamic mixing reservoir, thus realizing the synthesis of HEA-NPs with a diverse range of metal elements in mild reaction conditions. The involved elements have a wide range of atomic radii (1.24–1.97 ) and melting points (303–3,683 K). We also realized the precisely fabricated structures of nanoparticles via mixing enthalpy tuning. Moreover, the real-time conversion process (that is, from liquid metal to crystalline HEA-NPs) is captured in situ, which confirmed a dynamic fission–fusion behaviour during the alloying process.

机械工程Mechanical Engineering

Harnessing a paper-folding mechanism for reconfigurable DNA origami


▲ 作者:Myoungseok Kim, Chanseok Lee, Kyounghwa Jeon, Jae Young Lee, Young-Joo Kim, Jae Gyung Lee, et al.

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▲ Abstract:

The paper-folding mechanism has been widely adopted in building of reconfigurable macroscale systems because of its unique capabilities and advantages in programming variable shapes and stiffness into a structure. However, it has barely been exploited in the construction of molecular-level systems owing to the lack of a suitable design principle, even though various dynamic structures based on DNA self-assembly have been developed. Here we propose a method to harness the paper-folding mechanism to create reconfigurable DNA origami structures. The main idea is to build a reference, planar wireframe structure whose edges follow a crease pattern in paper folding so that it can be folded into various target shapes. We realized several paper-like folding and unfolding patterns using DNA strand displacement with high yield. Orthogonal folding, repeatable folding and unfolding, folding-based microRNA detection and fluorescence signal control were demonstrated. Stimuli-responsive folding and unfolding triggered by pH or light-source change were also possible. Moreover, by employing hierarchical assembly we could expand the design space and complexity of the paper-folding mechanism in a highly programmable manner. Because of its high programmability and scalability, we expect that the proposed paper-folding-based reconfiguration method will advance the development of complex molecular systems.


General cross-coupling reactions with adaptive dynamic homogeneous catalysis


▲ 作者:Indrajit Ghosh, Nikita Shlapakov, Tobias A. Karl, Jonas Düker, Maksim Nikitin, Julia V. Burykina, et al.

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▲ Abstract:

Cross-coupling reactions are among the most important transformations in modern organic synthesis. Although the range of reported (het)aryl halides and nucleophile coupling partners is very large considering various protocols, the reaction conditions vary considerably between compound classes, necessitating renewed case-by-case optimization of the reaction conditions. Here we introduce adaptive dynamic homogeneous catalysis (AD-HoC) with nickel under visible-light-driven redox reaction conditions for general C(sp2)–(hetero)atom coupling reactions. The self-adjustive nature of the catalytic system allowed the simple classification of dozens of various classes of nucleophiles in cross-coupling reactions. This is synthetically demonstrated in nine different bond-forming reactions (in this case, C(sp2)–S, Se, N, P, B, O, C(sp3, sp2, sp), Si, Cl) with hundreds of synthetic examples under predictable reaction conditions. The catalytic reaction centre(s) and conditions differ from one another by the added nucleophile, or if required, a commercially available inexpensive amine base.

地球科学Earth Science

Medieval demise of a Himalayan giant summit induced by mega-landslide


▲ 作者:Jér?me Lavé, Cyrielle Guérin, Pierre G. Valla, Valery Guillou, Thomas Rigaudier, Lucilla Benedetti, et al.

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研究组报道了公元1190年左右安纳普尔纳地块(尼泊尔中部)发生的一次巨大岩石滑坡的地质证据,涉及岩石总体积约为23 km3。这一事件使一座古山顶坍塌,其高峰可能在海拔8000米以上。



▲ Abstract:

Despite numerous studies on Himalayan erosion, it is not known how the very high Himalayan peaks erode. Although valley floors are efficiently eroded by glaciers, the intensity of periglacial processes, which erode the headwalls extending from glacial cirques to crest lines, seems to decrease sharply with altitude. This contrast suggests that erosion is muted and much lower than regional rock uplift rates for the highest Himalayan peaks, raising questions about their long-term evolution. Here we report geological evidence for a giant rockslide that occurred around 1190 AD in the Annapurna massif (central Nepal), involving a total rock volume of about 23 km3. This event collapsed a palaeo-summit, probably culminating above 8,000?m in altitude. Our data suggest that a mode of high-altitude erosion could be mega-rockslides, leading to the sudden reduction of ridge-crest elevation by several hundred metres and ultimately preventing the disproportionate growth of the Himalayan peaks. This erosion mode, associated with steep slopes and high relief, arises from a greater mechanical strength of the peak substratum, probably because of the presence of permafrost at high altitude. Giant rockslides also have implications for landscape evolution and natural hazards: the massive supply of finely crushed sediments can fill valleys more than 150 km farther downstream and overwhelm the sediment load in Himalayan rivers for a century or more.

Safe and just Earth system boundaries


▲ 作者:Johan Rockstr?m, Joyeeta Gupta, Dahe Qin, Steven J. Lade, Jesse F. Abrams, Lauren S. Andersen, et al.

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▲ Abstract:

The stability and resilience of the Earth system and human well-being are inseparably linked, yet their interdependencies are generally under-recognized; consequently, they are often treated independently. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice). The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future.

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