Spanish National Research Council

About: Spanish National Research Council is a government organization based out in Madrid, Spain. It is known for research contribution in the topics: Population & Galaxy. The organization has 79563 authors who have published 220470 publications receiving 7698991 citations. The organization is also known as: CSIC & Consejo Superior de Investigaciones Científicas.

Broad line emission from iron K- and L-shell transitions in the active galaxy 1H 0707-495.

Abstract: The emission line arising from a transition of an electron from the iron K shell to the ground state (the K line) is prominent in the reflection spectrum of the hard X-ray continuum irradiating dense accreting matter around a black hole. The corresponding iron L-line emission should be detectable when iron abundance is high. That's the theory, and now broad iron L-line emission has been observed, together with the broad K line in the narrow-line Seyfert galaxy 1H0707. There is a reverberation lag of about 30 s between the direct X-ray continuum and its reflection from matter falling into the hole, a timescale comparable to the light-crossing time of the innermost radii around a supermassive black hole. This discovery opens a window on events close to the black hole event horizon in these objects. Emission arising from a transition of an electron from the iron K shell to the ground state (the K line) is prominent in the reflection spectrum created by the hard X-ray continuum irradiating the dense accreting matter around a black hole. Here the presence of both iron K and L emission is reported in the spectrum of the active galaxy 1H 0707-495. There is a 'reverberation lag' with a timescale comparable to the light-crossing time of the innermost radii around a supermassive black hole. Since the 1995 discovery of the broad iron K-line emission from the Seyfert galaxy MCG–6-30-15 (ref. 1), broad iron K lines have been found in emission from several other Seyfert galaxies2, from accreting stellar-mass black holes3 and even from accreting neutron stars4. The iron K line is prominent in the reflection spectrum5,6 created by the hard-X-ray continuum irradiating dense accreting matter. Relativistic distortion7 of the line makes it sensitive to the strong gravity and spin of the black hole8. The accompanying iron L-line emission should be detectable when the iron abundance is high. Here we report the presence of both iron K and iron L emission in the spectrum of the narrow-line Seyfert 1 galaxy9 1H 0707-495. The bright iron L emission has enabled us to detect a reverberation lag of about 30 s between the direct X-ray continuum and its reflection from matter falling into the black hole. The observed reverberation timescale is comparable to the light-crossing time of the innermost radii around a supermassive black hole. The combination of spectral and timing data on 1H 0707-495 provides strong evidence that we are witnessing emission from matter within a gravitational radius, or a fraction of a light minute, from the event horizon of a rapidly spinning, massive black hole.

Attractive and repulsive tip-sample interaction regimes in tapping-mode atomic force microscopy

Abstract: Attractive and repulsive tip-sample interaction regimes of a force microscope operated with an amplitude modulation feedback were investigated as a function of tip-sample separation, free amplitude, and sample properties. In the attractive regime, a net attractive force dominates the amplitude reduction while in the repulsive regime the amplitude reduction is dominated by a net repulsive force. The transition between both regimes may be smooth or steplike, depending on free amplitude and sample properties. A steplike discontinuity is always a consequence of the existence of two oscillation states for the same conditions. Stiff materials and small free amplitudes give rise to steplike transitions while the use of large free amplitudes produce smooth transitions. Simulations performed on compliant samples showed cases where the cantilever dynamics is fully controlled by a net attractive force. Phase-shift measurements provide a practical method to determine the operating regime. Finally, we discuss the influence of those regimes in data acquisition and image interpretation.

Do biotic interactions shape both sides of the humped-back model of species richness in plant communities?

Abstract: A humped-back relationship between species richness and community biomass has frequently been observed in plant communities, at both local and regional scales, although often improperly called a productivity-diversity relationship. Explanations for this relationship have emphasized the role of competitive exclusion, probably because at the time when the relationship was first examined, competition was considered to be the significant biotic filter structuring plant communities. However, over the last 15 years there has been a renewed interest in facilitation and this research has shown a clear link between the role of facilitation in structuring communities and both community biomass and the severity of the environment. Although facilitation may enlarge the realized niche of species and increase community richness in stressful environments, there has only been one previous attempt to revisit the humped-back model of species richness and to include facilitative processes. However, to date, no model has explored whether biotic interactions can potentially shape both sides of the humped-back model for species richness commonly detected in plant communities. Here, we propose a revision of Grime's original model that incorporates a new understanding of the role of facilitative interactions in plant communities. In this revised model, facilitation promotes diversity at medium to high environmental severity levels, by expanding the realized niche of stress-intolerant competitive species into harsh physical conditions. However, when environmental conditions become extremely severe the positive effects of the benefactors wane (as supported by recent research on facilitative interactions in extremely severe environments) and diversity is reduced. Conversely, with decreasing stress along the biomass gradient, facilitation decreases because stress-intolerant species become able to exist away from the canopy of the stress-tolerant species (as proposed by facilitation theory). At the same time competition increases for stress-tolerant species, reducing diversity in the most benign conditions (as proposed by models of competition theory). In this way our inclusion of facilitation into the classic model of plant species diversity and community biomass generates a more powerful and richer predictive framework for understanding the role of plant interactions in changing diversity. We then use our revised model to explain both the observed discrepancies between natural patterns of species richness and community biomass and the results of experimental studies of the impact of biodiversity on the productivity of herbaceous communities. It is clear that explicit consideration of concurrent changes in stress-tolerant and competitive species enhances our capacity to explain and interpret patterns in plant community diversity with respect to environmental severity.