Bursts Of Radiation Research Articles

Astronomical masers and Dicke’s superradiance

ABSTRACT We consider the radiation properties and processes of a gas with a population inversion using the formalism based on the Maxwell–Bloch equations. We focus on the maser action and Dicke’s superradiance to establish their relationship in the overall radiation process during the temporal evolution of the system as a function of position. We show that the maser action and superradiance are not competing phenomena but are rather complementary and define two distinct limits for the intensity of radiation. Masers characterize the quasi-steady state limit, when the population inversion density and the polarization amplitude vary on time-scales longer than those of non-coherent processes affecting their evolution (e.g. collisions), while superradiance defines the fast transient regime taking place when these conditions are reversed. We show how a transition from a maser regime to superradiance will take place whenever a critical threshold for the column density of the population inversion is reached, at which point a strong level of coherence is established in the system and a powerful burst of radiation can ensue during the transient regime. This critical level also determines the spatial region where a transition from the unsaturated to the saturated maser regimes will take place; superradiance can thus be seen as the intermediary between the two. We also quantify the gain in radiation intensity attained during the superradiance phase relative to the two maser regimes and show how the strong coherence level during superradiance is well suited to explain observations that reveal intense and fast radiation flares in maser-hosting regions.

Constraining the local burst rate density of primordial black holes with HAWC

Primordial Black Holes (PBHs) may have been created by density fluctuations in the early Universe and could be as massive as > 109 solar masses or as small as the Planck mass. It has been postulated that a black hole has a temperature inversely-proportional to its mass and will thermally emit all species of fundamental particles via Hawking Radiation. PBHs with initial masses of ∼ 5 × 1014 g (approximately one gigaton) should be expiring today with bursts of high-energy gamma radiation in the GeV-TeV energy range. The High Altitude Water Cherenkov (HAWC) Observatory is sensitive to gamma rays with energies of ∼300 GeV to past 100 TeV, which corresponds to the high end of the PBH burst spectrum. With its large instantaneous field-of-view of ∼ 2 sr and a duty cycle over 95%, the HAWC Observatory is well suited to perform an all-sky search for PBH bursts. We conducted a search using 959 days of HAWC data and exclude the local PBH burst rate density above 3400 pc−3 yr−1 at 99% confidence, the strongest limit on the local PBH burst rate density from any existing electromagnetic measurement.

Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

Analytic waveforms for eccentric gravitational wave bursts

We here present the first analytic effective fly-by (EFB) waveforms designed to accurately capture the burst of gravitational radiation from the closest approach of highly eccentric compact binaries. The waveforms are constructed by performing a re-summation procedure on the well-known Fourier series representation of the two-body problem at leading post-Newtonian order. This procedure results in two models: one in the time-domain, and one in the Fourier domain, which makes use of the stationary phase approximation. We discuss the computational efficiency of these models, and find that the time-domain model is roughly twice as fast as a numerical quadrupole waveform. We compare the time-domain model to both numerical, leading post-Newtonian order, quadrupole waveforms and numerical relativity (NR) fly-by waveforms using the match statistic. While the match is typically >0.97 when compared to the quadrupole waveforms, it is much lower when comparing to the NR fly-by waveforms, due to neglecting relativistic effects within the model. We further show how to use these individual waveforms to detect a repeated burst source.

Solar Radio Bursts Associated with Standing Shock Waves

A number of phenomena with radio bursts in the decimeter and centimeter wavelength ranges similar to type-II bursts in the meter range have been considered. In all phenomena, the radio bursts are characterized by the cessation of frequency drift and its reversal. Analysis of all of the available data on relevant bursts provides evidence of termination shocks with particle accelerations in their fronts. This is confirmed by the generation of new sources of hard X-ray radiation and the radio emission of fast bursts (spikes), fibers, and zebra structures. The sources of drift stripes with a drift turn are located either between the burst loop and the lower shock wave or between the lower and upper shock waves. Estimates of the critical Mach number for common parameters of the burst plasma indicated that the values (Mcr = 1.1–1.3) can easily be observed in the given phenomena and that the radiation can be associated with the Buneman instability. The conditions necessary for the generation of observed electromagnetic radiation bursts can be provided in helmet-shaped magnetic structures in the solar corona.

Predicted response of an atom to a short burst of electro-magnetic radiation

In this work, we present a hypothesis that spectral response of atoms or molecules to a pulse of electro-magnetic radiation with fast rising or falling fronts would contain a unique emission line that is located approximately near the frequency of the natural oscillations of optical electrons. The emission of this “pinging” spectral line would exist during the time that is determined by the time of radiative energy loss by the optical electron. The amplitude of the “pinging” spectral line would be higher for the pulses with faster rising or falling fronts. The simulations using our previously developed model confirmed existence of the “pinging” spectral response. If experimentally confirmed, this work could lead to a new high sensitivity, high signal to noise ratio stand-off detection techniques, and to other yet unknown applications.

The Explosion at the Center of Our Galaxy

At the center of the Milky Way, our black hole may have suddenly changed from supermassive to intermediate-mass status. In doing so, it would have emitted an enormous burst of electromagnetic radiation. Here, the total energy of that burst is calculated and compared with the Fermi bubble data.

February 25, 2014 solar flare data analysis in SunPy

Solar flares are strong radiation bursts, whereas large clouds of solar material and magnetic fields thaterupt at high speeds from the Sun are coronal mass ejections. Harmful radiation from a flare does notpass through the atmosphere of the Earth to physically impact humans on the ground, but can disrupt theatmosphere in the layer where GPS and communication signals travel. Flares generate results across theentire electromagnetic spectrum. They emit x-rays and ultraviolet radiation, which means extremely hightemperatures during a flash. Radio waves mean that tiny fractions of particles are accelerated to high levelsof energy. Most of the radiation is synchrotron radiation produced along magnetic field lines by electronstraveling along spiral paths. In this paper was monitored solar flare registered on February 25, 2015. Thisflare, which peaked at 00:49 am EDT from a sunspot called Active Region 1990 (AR1990), is classifiedas an X4.9-class flare. We have performed solar data analysis using the Python/SunPy tool. SunPy waschosen as the principle data analysis environment since it provides easy to use interfaces to the Virtual SolarObservatory (VSO).

New Approaches to the Identification of Sources and the Onset of Stealth Coronal Mass Ejections

New approaches to the identification of sources and the onset of stealth coronal mass ejections (CMEs) are discussed based on the stealth CME of July 7, 2012. It is shown that the generation of this stealth CME is accompanied by various manifestations of short-term and small-scale activity in the region of its formation in the form of bursts of UV radiation (a microflare), as well as the activation and movement of loop-like structures. The features of the behavior of the photospheric magnetic field in the region of observation of the microflare are studied. The formation of a frontal CME structure there can be considered evidence that the stealth CME is generated in a solar atmospheric region with detected manifestations of solar activity.

Electron correlation in channel-resolved strong-field molecular double ionization

Strong-field ionization plays a fundamental role in attosecond science as the source of electron wave packets which ultimately provide attosecond bursts of radiation as they are driven back to the atom or molecule of origin. Double ionization is an important probe of electron correlation (correlated electron dynamics), which lies at the core of attosecond science. While double ionization of atoms has been studied extensively, the double ionization of molecules is relatively unexplored---particularly for the case of polyatomic systems. We present coincidence (three and four particle) velocity map imaging measurements of molecular double ionization in 1,3-cyclohexadiene and 1,4-cyclohexadiene using few cycle ($\ensuremath{\sim}10$ fs) laser pulses. Our measurements allow us to distinguish between dissociative and nondissociative double ionization, and we find a difference in the correlation of electrons that come from the different channels.

Termination of thunderstorm-related bursts of energetic radiation and particles by inverted intracloud and hybrid lightning discharges

Abstract Recent studies of thunderstorm-related enhancements of fluxes of energetic radiation and particles at ground level suggest that removal of mid-level negative charge from the cloud by negative cloud-to-ground (-CG) lightning flashes or normal intracloud (IC) flashes serves to abruptly terminate those enhancements. However, it was not clear if the electron-accelerating electric field responsible for flux enhancements at ground was primarily between the main negative charge region and ground (produced due to the dominant effect of negative cloud charge) or between the mid-level negative and lower positive charge regions inside the thundercloud. Here, we report that these flux enhancements can be also abruptly terminated by inverted intracloud flashes and hybrid lightning flashes (inverted IC followed by negative CG). Based on the analysis of 13 events of these two types, we provide first evidence that the electric field between the mid-level negative and lower positive charge regions in the thundercloud can be responsible for the flux enhancements at ground level.

Anisotropy in hard bremsstrahlung from a high-voltage laboratory simulation of an atmospheric discharge

The results of the experiments on recording hard gamma radiation and measurements of its angular distribution at the initial stage of a laboratory high-voltage atmospheric discharge are presented. The experiments were performed on an installation at a voltage of ∼1 mV, an atmospheric discharge current of up to 12 kA, and a gap of 0.55 m. The duration of the voltage pulse was about 1 μs with a pulse rise time of 150–200 ns. The radiation was recorded by an assembly of 10 identical scintillation detectors installed each 10◦ around the circumference of a quarter of a circle with a curvature of 1 m. In order to separate the radiation with energies from 20 keV to 1.5 meV, Al and Pb filters of different thicknesses were used. The obtained results show that, as a rule, a multi-beam radiation pattern and several bursts of radiation (each with a directional pattern) are recorded in each shot. In a number of ‘shots’, hard radiation with photon energies comparable to or exceeding the maximum electron energy corresponding to the applied voltage is recorded. In these cases, a needle-like radiation pattern is observed, including at large angles to the axis of the discharge. This may indicate the acceleration of electrons in different plasma channels.

Interpretation of suprathermal emission at deuteron cyclotron harmonics from deuterium plasmas heated by neutral beam injection in the KSTAR tokamak

Intense bursts of suprathermal radiation, with spectral peaks at frequencies corresponding to the deuteron cyclotron frequency in the outer midplane edge region, are often detected from deuterium plasmas in the KSTAR tokamak that are heated by tangential neutral beam injection (NBI) of deuterons at 100 keV. Identifying the physical process by which this deuterium ion cyclotron emission (ICE) is generated, typically during the crash of edge localised modes, assists the understanding of collective energetic ion behaviour in tokamak plasmas. In the context of KSTAR deuterium plasmas, it is also important to distinguish deuterium ICE from the ICE at cyclotron harmonics of fusion-born protons examined by Chapman et al (2017 Nucl. Fusion 57 124004; 2018 Nucl. Fusion 58 096027). We use particle orbit studies in KSTAR-relevant magnetic field geometry, combined with a linear analytical treatment of the magnetoacoustic cyclotron instability (MCI), to identify the sub-population of freshly ionised NBI deuterons that is likely to excite deuterium ICE. These deuterons are then represented as an energetic minority, together with the majority thermal deuteron population and electrons, in first principles kinetic particle-in-cell (PIC) computational studies. By solving the Maxwell–Lorentz equations directly for hundreds of millions of interacting particles with resolved gyro-orbits, together with the self-consistent electric and magnetic fields, the PIC approach enables us to study the collective relaxation of the energetic deuterons through the linear phase and deep into the saturated regime. The Fourier transform of the excited fields displays strong spectral peaks at multiple successive deuteron cyclotron harmonics, mapping well to the observed KSTAR deuterium ICE spectra. This outcome, combined with the time-evolution of the energy densities of the different particle populations and electric and magnetic field components seen in the PIC computations, supports our identification of the driving sub-population of NBI deuterons, and the hypothesis that its relaxation through the MCI generates the observed deuterium ICE signal. We conclude that the physical origin of this signal in KSTAR is indeed distinct from that of KSTAR proton ICE, and is in the same category as the NBI-driven ICE seen notably in the TFTR tokamak and LHD heliotron–stellarator plasmas. ICE has been proposed as a potential passive diagnostic of energetic particle populations in ITER plasmas; this is assisted by clarifying and extending the physics basis of ICE in contemporary magnetically confined plasmas.

Intense intermittent radiation at the plasma frequency on EAST

Intense intermittent radiation has been observed regularly in EAST plasmas. The duration of radiation bursts is with a characterization time of a few microseconds. The radiation is very intense, with an equivalent radiation temperature higher than the electron cyclotron emission by a few orders of magnitude. An electron density threshold exists to terminate the bursts, and it is strongly dependent on the toroidal magnetic field. The radiation frequency f is at the plasma frequency, and the frequency bandwidth is very narrow (∼1.5 MHz FWHM, ). The fine structure of the spectrum in the frequency domain has been observed, and the frequency spacing is around 3 MHz. The cavity modes model is capable of explaining the radiation frequency and the frequency spacing. However, a quantitative and complete description of this phenomenon is still lacking.

On the High‐Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes

AbstractTerrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highest‐energy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens of megaelectronvolts are expected, but the actual upper limit and high‐energy spectral shape are still open questions. Results published in 2011 by the AGILE team proposed a high‐energy component in TGF spectra extended up to ≈100 MeV, which is difficult to reconcile with the predictions from the Relativistic Runaway Electron Avalanche (RREA) mechanism at the basis of many TGF production models. Here we present a new set of TGFs detected by the AGILE satellite and associated to lightning measurements capable to solve this controversy. Detailed end‐to‐end Monte Carlo simulations and an improved understanding of the instrument performance under high‐flux conditions show that it is possible to explain the observed high‐energy counts by a standard RREA spectrum at the source, provided that the TGF is sufficiently bright and short. We investigate the possibility that single high‐energy counts may be the signature of a fine‐pulsed time structure of TGFs on time scales ≈4 μs, but we find no clear evidence for this. The presented data set and modeling results allow also for explaining the observed TGF distribution in the (Fluence × duration) parameter space and suggest that the AGILE TGF detection rate can almost be doubled.

From self-organization in relativistic electron bunches to coherent synchrotron light: observation using a photonic time-stretch digitizer

In recent and future synchrotron radiation facilities, relativistic electron bunches with increasingly high charge density are needed for producing brilliant light at various wavelengths, from X-rays to terahertz. In such conditions, interaction of electron bunches with their own emitted electromagnetic fields leads to instabilities and spontaneous formation of complex spatial structures. Understanding these instabilities is therefore key in most electron accelerators. However, investigations suffer from the lack of non-destructive recording tools for electron bunch shapes. In storage rings, most studies thus focus on the resulting emitted radiation. Here, we present measurements of the electric field in the immediate vicinity of the electron bunch in a storage ring, over many turns. For recording the ultrafast electric field, we designed a photonic time-stretch analog-to-digital converter with terasamples/second acquisition rate. We could thus observe the predicted link between spontaneous pattern formation and giant bursts of coherent synchrotron radiation in a storage ring.

Changes in soil taxonomic and functional diversity resulting from gamma irradiation

Little is known of the effects of ionizing radiation exposure on soil biota. We exposed soil microcosms to weekly bursts of 60Co gamma radiation over six weeks, at three levels of exposure (0.1 kGy/hr/wk [low], 1 kGy/hr/wk [medium] and 3 kGy/hr/wk [high]). Soil DNA was extracted, and shotgun metagenomes were sequenced and characterised using MG-RAST. We hypothesized that with increasing radiation exposure there would be a decrease in both taxonomic and functional diversity. While bacterial diversity decreased, diversity of fungi and algae unexpectedly increased, perhaps because of release from competition. Despite the decrease in diversity of bacteria and of biota overall, functional gene diversity of algae, bacteria, fungi and total biota increased. Cycles of radiation exposure may increase the range of gene functional strategies viable in soil, a novel ecological example of the effects of stressors or disturbance events promoting some aspects of diversity. Moreover, repeated density-independent population crashes followed by population expansion may allow lottery effects, promoting coexistence. Radiation exposure produced large overall changes in community composition. Our study suggests several potential novel radiation-tolerant groups: in addition to Deinococcus-Thermus, which reached up to 20% relative abundance in the metagenome, the phyla Chloroflexi (bacteria), Chytridiomycota (fungi) and Nanoarcheota (archaea) may be considered as radiation-tolerant.

Physics of the laser-plasma interface in the relativistic regime of interaction

The reflection of intense laser radiation from solids appears as a result of relativistic dynamics of the electrons driven by both incoming and self-generated electromagnetic fields at the periphery of the emerging dense plasma. In the case of highly relativistic motion, electrons tend to form a thin oscillating layer, which makes it possible to model the interaction and obtain the temporal structure of the reflected radiation. The modeling reveals the possibility and conditions for producing singularly intense and short extreme ultraviolet (XUV) bursts of radiation, which are interesting for many applications. However, the intensity and duration of the XUV bursts, as well as the high-energy end of the harmonic spectrum, depend on the thickness of the layer and its internal structure which are not assessed by such macroscopic modeling. Here, we analyze the microscopic physics of this layer and clarify how its parameters are bound and how this controls the outlined properties of XUV bursts.

Color centers in NaCl single crystals induced by pulsed intense relativistic electron beams to simulate radiation bursts in Europa

To simulate the burst irradiation environment of Europa, single crystals of NaCl were irradiated by pulsed intense relativistic electron beams (PIREBs) with a peak energy of 6 MeV, a current of −800 A, and a pulse width of 70 ns. After irradiation, the optical absorption of the samples was measured, and results indicated that the irradiation induced F- and F2-centers. The density of F-centers was estimated to be 8.9 × 1016 cm−3 from 1 shot of PIREB irradiation with 6 MeV. The absorbed energy to form F-centers by PIREB was comparable but slightly higher than those induced by conventional direct current accelerators. The effect of pulsed heating, which should be taken into account for the detection of NaCl on Europa, is discussed.

Particle creation in gravitational collapse to a horizonless compact object

Black holes (BHs) play a central role in physics. However, gathering observational evidence for their existence is a notoriously difficult task. Current strategies to quantify the evidence for BHs all boil down to looking for signs of highly compact, horizonless bodies. Here, we study particle creation by objects which collapse to form ultra-compact configurations, with surface at an areal radius $R=R_{f}$ satisfying $1-(2M/R_{f})= \epsilon^{2}\ll 1$ with $M$ the object mass. We assume that gravitational collapse proceeds in a `standard' manner until $R=R_{f}+2M \epsilon^{2\beta}$, where $\beta>0$, and then slows down to form a static object of radius $R_{f}$. In the standard collapsing phase, Hawking-like thermal radiation is emitted, which is as strong as the Hawking radiation of a BH with the same mass but lasts only for $\sim 40~(M/M_{\odot})[44+\ln (10^{-19}/\epsilon)]~\mu \mbox{s}$. Thereafter, in a very large class of models, there exist two bursts of radiation separated by a very long dormant stage. The first burst occurs at the end of the transient Hawking radiation, and is followed by a quiescent stage which lasts for $\sim 6\times 10^{6}~(\epsilon/10^{-19})^{-1}(M/M_{\odot})~\mbox{yr}$. Afterwards, the second burst is triggered, after which there is no more particle production and the star is forever dark. In a model with $\beta=1$, both the first and second bursts outpower the transient Hawking radiation by a factor $\sim 10^{38}(\epsilon/10^{-19})^{-2}$.