Flash Events Research Articles

A new photometric study of M14 (NGC 6402): an interpretation of the horizontal branch and beyond

ABSTRACT We present a CCD VI photometric study of the globular cluster M14. Particular attention is given to the variable stars. This allowed new classifications and cluster membership considerations. New variables are reported: 3 RRc, 18 SR, and 1 SX Phe. The Fourier decomposition of RR Lyrae light curves lead to the mean cluster metallicity of [Fe/H]ZW = −1.3 ± 0.2. Several independent methods yield a mean distance of 9.36 ± 0.16 kpc. A colour–magnitude diagram outlined by the cluster members enabled a matching with theoretical predictions of isochrones and zero-age horizontal branches (HBs), whose fitting to the observations is in good agreement with the above distance and metallicity. The Oosterhoff type of M14 is confirmed as Oo-int, and the pulsating mode distribution of RR Lyrae stars on the HB shows that the bimodal region of the instability strip is shared by RRab and RRc stars. By modelling the mass-loss at the red giant branch after the He flash events, we were able to represent the blue tail of the HB, using a core mass of 0.48 M⊙ and total masses of 0.52–0.55 M⊙. A progenitor star on the main sequence (MS) of 0.84 M⊙ reaches the HB in about 12.5 Gyr, consonant with previous age determinations of the cluster. Type II Cepheids of M14 may be interpreted as products of post-HB evolution, driven by the complex processes involving the burning of the very thin low mass hydrogen and helium shells of these stars and their minuscule envelopes. No pieces of evidence were found in favour of M14 being of extragalactic origin.

Electrical resistance flash sintering of tungsten carbide

This work explores the possibilities for the ultrafast sintering of binderless tungsten carbide by electric/pressure assisted sintering. A limited voltage (3–4 V) in AC condition was applied to WC powder compact in combination with uniaxial pressure. The thermal insulating ceramic die allows the ultrafast heating (104 °C/min) of the powder compact which undergoes a rapid transition of its electrical properties, from negative to positive dependence of resistivity on temperature, i.e. from NTC to PTC behaviour. Such effect is fundamental for inducing a thermal runaway phenomenon associated with ultrarapid temperature increase and massive electric power dissipation, thus inducing very rapid sintering. The relationship between electrical properties of tungsten carbide and the possibility to achieve “flash sintering” conditions to complete densification in a couple of seconds was investigated. At the optimal conditions of 3.5 V and 4 MPa pressure, pure WC sinters up to 95% in less than 10 s. Longer sintering time after the flash improves only slightly the density, despite a significant energetic consumption. It is also shown that if larger pressure is applied, the flash event duration and final density decrease.

Improving Discriminating Accuracy Rate of DDoS Attacks and Flash Events

Internet security has become a big issue with the passage of time. Among many threats, the distributed denial-of-service (DDoS) attack is the most frequent threat in the networks. The purpose of the DDoS attacks is to interrupt service availability provided by different web servers. This results in legitimate users not being able to access the servers and hence facing denial of services. On the other hand, flash events are a high amount of legitimate users visiting a website due to a specific event. Consequences of these attacks are more powerful when launched during flash events, which are legitimate traffic and cause a denial of service. The purpose of this study is to build an intelligent network traffic classification model to improve the discrimination accuracy rate of DDoS attacks from flash events traffic. Weka is adopted as the platform for evaluating the performance of a random forest algorithm.

Microcrack healing in zirconia ceramics under a DC electric field/current

Flash event caused by a DC electric field/current was applied to the crack healing in 8 mol % Y2O3 stabilized cubic ZrO2 polycrystals (8Y-CSZ). The flash event, which occurred by applying the DC power higher than a critical value of 100 mW/mm3, successfully healed the microcrack within several minutes without any healing agents at a furnace temperature of 800 °C. As compared to the healing treatment under static annealing, the healing phenomena were accelerated about 2 times under the flash treatment even at the same temperatures, suggesting that the enhanced healing phenomena cannot be explained only by the temperature effect. Since the rate of grain growth was accelerated under the flash treatment, the flash healing would be accelerated through the current-enhanced diffusional processes. This study shows for the first time that the flash event has a potential to apply to the crack healing process in the ceramic materials and composites.

On the use of information theory metrics for detecting DDoS attacks and flash events: an empirical analysis, comparison, and future directions

A Distributed Denial of Service (DDoS) attack is one of the lethal threats that can cripple down the computing and communication resources of a web server hosting Internet-based services and applications. It has motivated the researchers over the years to find diversified and robust solutions to combat against DDoS attacks and characterization of flash events (a sudden surge in the legitimate traffic) from HR-DDoS (High-Rate DDoS) attacks. In recent times, the volume of legitimate traffic has also magnified manifolds. It results in behavioral similarities of attack traffic and legitimate traffic that make it very difficult and crucial to differentiate between the two. Predominantly, Netflow-based techniques are in use for detecting and differentiating legitimate and attack traffic flows. Over the last decade, fellow researchers have extensively used distinct information theory metrics for Netflow-based DDoS defense solutions. However, a comprehensive analysis and comparison of these diversified information theory metrics used for particularly DDoS attack detection are needed for a better understanding of the defense systems based on information theory. This paper elucidates the efficacy and effectiveness of information theory-based various entropy and divergence measures in the field of DDoS attack detection. As part of the work, a generalized NetFlow-based methodology has been proposed. The proposed detection methodology has been validated using the traffic traces of various real benchmarked datasets on a set of detection system evaluation metrics such as Detection rate (Recall), Precision, F-Measure, FPR, Classification rate, and Receiver-Operating Characteristics (ROC) curves. It has concluded that generalized divergence-based information theory metrics produce more accuracy in detecting different types of attack flows in contrast to entropy-based information theory metrics.

Thunderstorms Producing Sferic-Geolocated Gamma-Ray Flashes Detected by TETRA-II.

The terrestrial gamma‐ray flash (TGF) and Energetic Thunderstorm Rooftop Array (TETRA‐II) detected 22 X‐ray/gamma‐ray flash events associated with lightning between October 2015 and March 2019 across three ground‐based detector locations in subtropical and tropical climates in Louisiana, Puerto Rico, and Panama. Each detector array consists of a set of bismuth germanate scintillators that record X‐ray and gamma‐ray bursts over the energy range 50 keV–6 MeV (million electron volts). TETRA‐II events have characteristics similar to both X‐ray bursts associated with lightning leaders and TGFs: sub‐millisecond duration, photons up to MeV energies, and association with nearby lightning (typically within 3 km). About 20 of the 22 events are geolocated to individual lightning strokes via spatiotemporally coincident sferics. An examination of radar reflectivity and derived products related to events located within the Next Generation Weather Radar (NEXRAD) monitoring region indicates that events occur within mature cells of severe and non‐severe multicellular and squall line thunderstorms, with core echo tops which are at or nearing peak altitude. Events occur in both high lightning frequency thunderstorm cells and low lightning frequency cells. Events associated with high frequency cells occur within 5 min of significant lightning jumps. Among NEXRAD‐monitored events, hail is present within 8 km and 5 min of all except a single low‐altitude cold weather thunderstorm. An association is seen with maximum thunderstorm development, lightning jumps, and hail cells, indicating that the TETRA‐II X‐ray/gamma‐ray events are associated with the peak storm electrification and development of electric fields necessary for the acceleration of electrons to high energies.

DC electric field‐assisted hot pressing of zirconia: Methodology, phenomenology, and sintering mechanism

AbstractFlash sintering (FS) is an important technique in the field of ceramic sintering. Nevertheless, conventional FS is less attractive for practical applications because of the complex shapes and small sizes of the specimens. In this study, using the novel electric field‐assisted hot pressing (FAHP) technique, we successfully achieved FS during the net‐shape hot pressing (HP) process for the first time. It was found that the 3 mol% yttria‐stabilized zirconia (3YSZ) can be flash sintered at 909°C using a fairly low DC field of 33 V/cm under 30 MPa pressure. The grain sizes of the FAHP‐sintered samples were 20% smaller than that of the HP‐sintered sample. When the current density limit is ≥240 mA/mm2, 3YSZ can be fully densified during the flash events. Careful analysis of the sintering curves suggests that although the carrier type or concentration is changed during flash events, it cannot explain the ultrafast densification. Additionally, we devised a qualitative method to analyze the densification mechanism. The results indicated that the ultrafast densification observed during flash events resulted from the synergistic effects of the rapid heating rate and peak sample temperature. Finally, the atomic force microscopy confirmed the lower grain boundary energy for the FAHP‐sintered samples, which accounts for the smaller grain sizes than the HP‐sintered sample. We believe that the FAHP technique could create new possibilities for theoretical and applied research on field‐assisted sintering techniques.

Ultrafast synthesis and densification of ZrO2 doped KNN ceramics by reactive flash sintering

AbstractFabrication of dense KNN‐based lead‐free piezoelectric ceramics at low temperatures in short time through a cost‐effective way remains a challenge. Herein, this challenge could be addressed by using reactive flash sintering. It is demonstrated that the phase transformation of KNbO3‐NaNbO3 into (K,Na)NbO3 and densification occur simultaneously during the flash event. Most importantly, ZrO2 doping can greatly decrease the onset flash temperature, which is ascribed to the increased conductivity of sample. In addition, the current limit has a significant effect on the phase transformation and densification. The flash‐sintered KNN ceramics exhibit the good ferroelectric and piezoelectric properties. Furthermore, the ZrO2 doped and undoped KNN ceramics show a comparable coercive field Ec, which may be related to the residual point defects after the flash. Besides the Joule heating, the avalanche generation of point defects is suggested to be responsible for the ultrafast solid‐state reaction and densification rates.

On the Arrhenius-like behavior of conductivity during flash sintering of 3 mol% yttria stabilized zirconia ceramics

We discuss the Arrhenius behavior of electrical conductivity σ during flash sintering of 3 mol% yttria stabilized zirconia (3YSZ). In situ x-ray diffraction is used to determine sample temperature. Sintering contribution to σ is excluded by investigating the flash event on a dense ceramic. We show that total conductivity follows an Arrhenius-like equation in both dense and green samples, even during Stage II of flash. The non-linearity often verified during flash sintering of 3YSZ is therefore related to the furnace temperature instead of the sample temperature when building the ln(σ) vs. 1/T plots. Furthermore, we verified a change in the activation energy for conduction prior to the ignition of the flash event and discussed the possible mechanisms. For instance, the decrease in activation energy from Stage I to II in the dense sample is attributed to a contribution from electronic carriers.

Flash sintering of alumina/reduced graphene oxide composites

The flash sintering behavior of Al2O3/reduced graphene oxide (rGO) composites was investigated. rGO was used as a composite component and a conductive additive. Under the electric fields of 250–400 V cm−1, the flash event occurred at extremely low temperatures of 236–249 °C. The current density limit played a significant role in the degree of densification. A larger current density resulted in a higher density of the sample. However, current densities larger than 33.33 A cm−2 resulted in broken samples because of the localization of high current density coupled with the formation of hot spots. Flash sintering at a furnace temperature of 800 °C, electric field of 300 V cm−1 and current density limit of 33.33 A cm−2 produced nearly completely dense Al2O3/rGO composites. In addition to the current limit, the furnace temperature is also a key parameter that controls the degree of densification to achieve “safe” flash sintering.

Electric Field Healing of Micro Crack Damages in Zirconia Ceramics

Flash event caused by DC electric field/current was applied to the healing of micro crack damages in 8mol% Y2O3 stabilized cubic ZrO2 polycrystals (8Y-CSZ). Even in the bulk ZrO2 polycrystal, the flash event occurred by applying the DC power higher than a critical value of Wc ≈ 100 mW/mm3 at 800 °C. For above the Wc value, the micro crack damages can successfully be healed under the flash condition within several minutes without any healing agents. The flash healing phenomena cannot be explained only by the temperature effect and were accelerated about 2 times through the current-enhanced diffusional processes.

A Large Fraction of Hydrogen-rich Supernova Progenitors Experience Elevated Mass Loss Shortly Prior to Explosion

Abstract Spectroscopic detection of narrow emission lines traces the presence of circumstellar mass distributions around massive stars exploding as core-collapse supernovae. Transient emission lines disappearing shortly after the supernova explosion suggest that the material spatial extent is compact and implies an increased mass loss shortly prior to explosion. Here, we present a systematic survey for such transient emission lines (Flash Spectroscopy) among Type II supernovae detected in the first year of the Zwicky Transient Facility survey. We find that at least six out of ten events for which a spectrum was obtained within two days of the estimated explosion time show evidence for such transient flash lines. Our measured flash event fraction (>30% at 95% confidence level) indicates that elevated mass loss is a common process occurring in massive stars that are about to explode as supernovae.

The Treasury Market Flash Event of February 25, 2021

<ns3:p>The Treasury market flash event of February 25, 2021 underscores the pivotal role of high-speed liquidity provision in the most liquid electronic parts of the Treasury market. We find evidence that the sharp drop in prices that day was accompanied by a sudden drop in market depth and a brief deterioration in high-speed liquidity provision amid elevated transaction volumes, albeit to a much lesser extent than during the episode of severe illiquidity in March 2020. Similar to some previous episodes accompanied by moderately elevated economic and financial market uncertainty, market depth has recovered steadily since February 25 at a pace comparable to that observed following other such episodes, while high-speed liquidity provision appears to have rebounded fairly quickly. That said, market depth has taken over a month to partially recover, which suggests that Treasury market liquidity has been more heavily reliant on high-speed replenishment to meet trading demand and may remain fragile.</ns3:p>

Flash‐Sintering Mechanism Studied Through Interrupted Experiments

Herein, the mass‐transfer mechanism of flash sintering during the transient stage is examined using an in‐house‐made flash and quench (FQ) system. Visual findings of samples during and after FQ experiments and high‐resolution electron microscopy are given. Many new observations regarding the flash‐sintering nature are presented and discussed. Samples that underwent FQ experiments either show no sign of sintering or local sintering and grain growth due to a hotspot. These findings aid in untying of the two phenomena. Electron microscopy imaging of flash and quenched samples shows atypical microstructures. Such microstructural anomalies include sintering, massive grain growth, and visual findings on the surface. These findings establish flash sintering as a set of phenomena, caused by an abrupt and local increase in temperature (a “flash event”), where only one of which is sintering.

Flash sintering of cationic conductive ceramics: A way to build multilayer systems

AbstractFlash Sintering (FS), that allows densifying ceramics at low furnace temperature in a few seconds, requires a reversible electrochemical reaction to enable the current flow through the electrode / material interface. For Li+ pure ionic conductors such as Li1.3Al0.3Ti1.7(PO4)3 (LATP), the best material to ensure this fast charge transfer reaction is a mixed Li+/e− conductor, eg, LiCoO2 (LCO). This paper demonstrates the feasibility of FS on LCO between two Pt electrodes and on multi‐layers systems using LCO or LATP+LCO composite as electrodes. It is shown that a composite electrode both allows the flash event to occur and prevents the delamination possibly observed with pure LCO by lowering interfacial stresses.

Near complete densification of flash sintered 8YSZ: controlled shrinkage rate effects

Rate-controlled sintering of 8 mol%Y2O3-doped ZrO2 (8YSZ) polycrystals was performed by rating the current limit of alternating current (AC) electric field during a flash event; this method promotes constant linear shrinkage of the green compact with respect to the sintering time. A power dissipation peak associated with the flash event was suppressed by setting the initial current limit at 100 mA; after AC current hits 100 mA, the current limit was rated to a maximum value of 1100−2000 mA to keep shrinkage rate constant. The power dissipation behavior during the current-rating process exhibited a characteristic inverted S-shaped curve as a function of time. 8YSZ polycrystal with a near-theoretical density of 5.97 g/cm3 and a grain size of approximately 3.4 μm was produced in AC electric field (150 V/cm at 1000 Hz) with a final maximum current limit of 1150 mA as a rectangular bulk style.

Constant shrinkage rate control during a flash event for 8 mol %Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub> polycrystals

Constant shrinkage rate control during a flash event for 8 mol %Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub> polycrystals

Discriminating flash crowds from DDoS attacks using efficient thresholding algorithm

Distributed Denial-of-Service attacks have been a challenge to cyberspace, as the attackers send a large number of attack packets similar to the normal traffic, to throttle legitimate flows. These attacks intentionally disrupt the services offered by the systems resulting in heavy cost. A flash crowd or flash event is an unexpected surge in the number of visitors to a particular website resulting in a sudden increase in server load. Flash crowds, which are legitimate flows, are difficult to be discriminated from Distributed Denial-of-Service attacks that are illicit flows. Effective and accurate detection of Distributed Denial of Service attacks still remains a challenge due to the difficulty in its detection and the false alerts generated in the case of flash crowds. There is a trade off between detection rate and false positive rate. This work deals with an efficient and early detection of distributed denial of service attacks and discriminates flash crowd by considering two network traffic parameters such as packet size and destination IP address. Using these traffic features two attributes are computed and its generalized entropies are calculated. The threshold is computed using the mean value of network attributes to detect the attacks. Threshold updater can automatically adjust the threshold values according to the changes in the channel conditions. The data sets used to evaluate the performance of the proposed approach are the MIT Lincoln Laboratory DARPA data set and a data set generated in a University network. Experimental results show this research approach achieves higher detection rate and lower false positives in a much reduced processing time as compared to the existing methods.

Insights into photoemission origins of flash sintering of ceramics

Electric field applied above a critical temperature leads to ultrafast densification of green ceramic compacts and simultaneous photoemission, termed flash sintering. A similar flash photoemission is also observed in single crystals with no need for sintering. Assuming the formation of a few atomic layer liquid-films at the particle/crystal surfaces due to local Joule heating or surface breakdown, one expects the exponential increase in the electric conductivity by current percolation through the liquid-film. At temperatures above the glass transition temperature of the otherwise crystalline film, high flux of excitons are formed by recombination of the injected electrons with electron holes. The corresponding momentum conservation for the indirect gap transition is assisted by the broad Boson peak of phonons at the low energy vibrational density of states (LEVDOS) of the amorphous/liquid-film. The glass transition temperature of the amorphous film sets the lower bound temperature for the flash event.

Detection of denial of service using a cascaded multi-classifier

The paper proposes a cascaded multi-classifier two-phase intrusion detection (TP-ID) approach that can be trained to monitor incoming traffic for any suspicious data. It addresses the issue of efficient detection of intrusion in traffic and further classifies the suspicious traffic as a DDoS attack or flash event. Features portraying the behaviour of normal, DDoS attack, and flash event are extracted from historical data obtained after merging CAIDA'07, SlowDoS2016, CIC-IDS-2017, and WorldCup 1998 benchmark datasets available online along with the commercial dataset for e-shopping assistant website. Information gain is applied to rank and select the most relevant features. TP-ID applies supervised learning algorithms in the two phases. Each phase tests the set of classifiers, the best of which is chosen for building a model. The performance of the system is evaluated using the detection rate, false-positive rate, mean absolute percentage error, and classification rate. The proposed approach classifies the traffic anomalies with a 99% detection rate, 0.43% FPR, and 99.51% classification rate.