A survey of stellar X-ray flares from the XMM-Newton serendipitous source catalogue: Hipparcos-Tycho cool stars

The X-ray emission from flares on cool (i.e. spectral-type F-M) stars is indicative of very energetic, transient phenomena, associated with energy release via magnetic reconnection. We present a uniform, large-scale survey of X-ray flare emission. The XMM-Newton Serendipitous Source Catalogue and its associated data products provide an excellent basis for a comprehensive and sensitive survey of stellar flares - both from targeted active stars and from those observed serendipitously in the half-degree diameter field-of-view of each observation. The 2XMM Catalogue and the associated time-series (`light-curve') data products have been used as the basis for a survey of X-ray flares from cool stars in the Hipparcos Tycho-2 catalogue. In addition, we have generated and analysed spectrally-resolved (i.e. hardness-ratio), X-ray light-curves. Where available, we have compared XMM OM UV/optical data with the X-ray light-curves. Our sample contains ~130 flares with well-observed profiles; they originate from ~70 stars. The flares range in duration from ~1e3 to ~1e4 s, have peak X-ray fluxes from ~1e-13 to ~1e-11 erg/cm2/s, peak X-ray luminosities from ~1e29 to ~1e32 erg/s, and X-ray energy output from ~1e32 to ~1e35 erg. Most of the ~30 serendipitously-observed stars have little previously reported information. The hardness-ratio plots clearly illustrate the spectral (and hence inferred temperature) variations characteristic of many flares, and provide an easily accessible overview of the data. We present flare frequency distributions from both target and serendipitous observations. The latter provide an unbiased (with respect to stellar activity) study of flare energetics; in addition, they allow us to predict numbers of stellar flares that may be detected in future X-ray wide-field surveys. The serendipitous sample demonstrates the need for care when calculating flaring rates.Comment: 26 pages, 24 figures. Additional tables and figures available as 4 ancillary files. To be published in Astronomy and Astrophysic

Unextractable fossil fuels in a 1.5 °C world

Parties to the 2015 Paris Agreement pledged to limit global warming to well below 2 °C and to pursue efforts to limit the temperature increase to 1.5 °C relative to pre-industrial times1. However, fossil fuels continue to dominate the global energy system and a sharp decline in their use must be realized to keep the temperature increase below 1.5 °C (refs. 2,3,4,5,6,7). Here we use a global energy systems model8 to assess the amount of fossil fuels that would need to be left in the ground, regionally and globally, to allow for a 50 per cent probability of limiting warming to 1.5 °C. By 2050, we find that nearly 60 per cent of oil and fossil methane gas, and 90 per cent of coal must remain unextracted to keep within a 1.5 °C carbon budget. This is a large increase in the unextractable estimates for a 2 °C carbon budget9, particularly for oil, for which an additional 25 per cent of reserves must remain unextracted. Furthermore, we estimate that oil and gas production must decline globally by 3 per cent each year until 2050. This implies that most regions must reach peak production now or during the next decade, rendering many operational and planned fossil fuel projects unviable. We probably present an underestimate of the production changes required, because a greater than 50 per cent probability of limiting warming to 1.5 °C requires more carbon to stay in the ground and because of uncertainties around the timely deployment of negative emission technologies at scale

Particle dynamics of a cartoon dune

The spatio-temporal evolution of a downsized model for a desert dune is observed experimentally in a narrow water flow channel. A particle tracking method reveals that the migration speed of the model dune is one order of magnitude smaller than that of individual grains. In particular, the erosion rate consists of comparable contributions from creeping (low energy) and saltating (high energy) particles. The saltation flow rate is slightly larger, whereas the number of saltating particles is one order of magnitude lower than that of the creeping ones. The velocity field of the saltating particles is comparable to the velocity field of the driving fluid. It can be observed that the spatial profile of the shear stress reaches its maximum value upstream of the crest, while its minimum lies at the downstream foot of the dune. The particle tracking method reveals that the deposition of entrained particles occurs primarily in the region between these two extrema of the shear stress. Moreover, it is demonstrated that the initial triangular heap evolves to a steady state with constant mass, shape, velocity, and packing fraction after one turnover time has elapsed. Within that time the mean distance between particles initially in contact reaches a value of approximately one quarter of the dune basis length

Broadband Acoustic Measurement of the Agar-based Tissue Mimicking Material: a Longitudinal Study

Commercially available ultrasound quality assurance test phantoms rely on the long-term acoustic stability of the tissue-mimicking-material (TMM). Measurement of the acoustic properties of the TMM can be technically challenging, and it is important to ensure its stability. The standard technique is to film-wrap samples of TMM and to measure the acoustic properties in a water bath. In this study, a modified technique was proposed whereby the samples of TMM are measured in a preserving fluid that is intended to maintain their characteristics. The acoustic properties were evaluated using a broadband pulse-echo substitution technique over the frequency range 4.5–50 MHz at 0, 6 and 12 months using both techniques. For both techniques, the measured mean values for the speed of sound and attenuation were very similar and within the International Electrotechnical Commission-recommended value. However, the results obtained using the proposed modified technique exhibited greater stability over the 1-y period compared with the results acquired using the standard technique

Lost generation: Reflections on resilience and flexibility from an energy system architecture perspective

Whole energy system modelling is a valuable tool to support the development of policy to decarbonise energy systems, and has been used extensively in the UK for this purpose. However, quantitative insights produced by such models necessarily omit potentially important features of physical and engineering reality. The authors argue that important socio-technical insights can be gained by studying critical events such as the loss of 2.1 GW generation from the electricity system of Great Britain on 9th August 2019, in conjunction with literature on the behaviour of complex systems. Among these insights is the idea that models of the operation and evolution of energy systems can never be complete. Both system behaviour (operation) and the emergence and evolution of structure in such systems are formally uncomputable. This provides a starting point for a discussion of the need for additional tools, drawn from the System Architecture literature, to support the design and realisation of future, fully-decarbonised systems with high penetrations of renewable energy. Desirable properties of System Architectures, including current and future Energy System Architectures, are discussed. These include resilience and flexibility, for which there is an extensive literature. They also include the properties of comprehensibility, which helps to make complex systems easier to operate, and of evolvability, for which a working definition is offered

Secondary organic aerosol (SOA) formation from reaction of isoprene with nitrate radicals (NO_3)

Secondary organic aerosol (SOA) formation from the reaction of isoprene with nitrate radicals (NO3) is investigated in the Caltech indoor chambers. Experiments are performed in the dark and under dry conditions (RH<10%) using N2O5 as a source of NO3 radicals. For an initial isoprene concentration of 18.4 to 101.6 ppb, the SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) ranges from 4.3% to 23.8%. By examining the time evolutions of gas-phase intermediate products and aerosol volume in real time, we are able to constrain the chemistry that leads to the formation of low-volatility products. Although the formation of ROOR from the reaction of two peroxy radicals (RO2) has generally been considered as a minor channel, based on the gas-phase and aerosol-phase data it appears that RO2+RO2 reaction (self reaction or cross-reaction) in the gas phase yielding ROOR products is a dominant SOA formation pathway. A wide array of organic nitrates and peroxides are identified in the aerosol formed and mechanisms for SOA formation are proposed. Using a uniform SOA yield of 10% (corresponding to Mo≅10 μg m−3), it is estimated that ~2 to 3 Tg yr−1 of SOA results from isoprene + NO3. The extent to which the results from this study can be applied to conditions in the atmosphere depends on the fate of peroxy radicals (i.e. the relative importance of RO2+RO2 versus RO2+NO3 reactions) in the nighttime troposphere

Islet autoantibody status in a multi-ethnic UK clinic cohort of children presenting with diabetes.

OBJECTIVE: We prospectively determined islet autoantibody status in children presenting with diabetes to a single UK region in relation to ethnicity. DESIGN: 316 (68.0% non-white) children presenting with diabetes between 2006 and 2013 were tested centrally for islet cell autoantibodies (ICA) and glutamic acid decarboxylase autoantibodies (GAD-65) at diagnosis, and if negative for both, tested for insulin autoantibodies (IAA). The assay used to measure GAD-65 autoantibodies changed from an in-house to a standardised ELISA method during the study. RESULTS: Even with use of the standardised ELISA method, 25.8% of children assigned a diagnosis of type 1 diabetes still tested negative for all three autoantibodies. 30% of children assigned a diagnosis of type 2 diabetes were autoantibody positive, and these had the highest glycated haemoglobin (HbA1c) levels at 12 months follow-up compared with other groups (p value for analysis of variance <0.001), although the sample size was small. Autoantibody positivity was similar between non-white and white children regardless of assay used (60.0% (n=129) vs 56.4% (n=57), χ2=0.9, p=0.35), as was mean GAD-65 autoantibody levels, but fewer non-white children had two or more autoantibodies detectable (13% (n=28) vs 27.7% (n=28), χ2=12.1, p=0.001). CONCLUSION: Islet autoantibody positivity was associated with a more severe phenotype, as demonstrated by poorer glycaemic control, regardless of assigned diabetes subtype. Positivity did not differ by ethnic group

Lost Generation: System Resilience and Flexibility

Whole energy system modelling is a valuable tool to support the development of policy to decarbonise energy systems, and has been used extensively in the UK for this purpose. However, quantitative insights produced by such models methods necessarily omit potentially important features of physical and engineering reality. The authors argue that important socio-technical insights can be gained by studying critical events such as the loss of 2.1 GW generation from the electricity system of Great Britain in August, 2019. The present paper uses this event as a starting point for a discussion of the need for additional tools, drawn from the System Architecture literature, to support the design and realisation of future fully decarbonised systems with high penetrations of renewable energy, capable of providing high levels of resilience and flexibility