High Cutoff Energy Research Articles

The effect of continuous geomagnetic storms on enhancements of ultrarelativistic electrons in the Earth’s outer radiation belt

The effect of continuous geomagnetic storms on enhancements of ultrarelativistic electrons in the Earth’s outer radiation belt

Timing and spectral studies of the X-ray pulsar 2S 1417–624 during the outburst in 2021

We study the timing and spectral properties of the X-ray pulsar 2S 1417--624 during the recent outburst in January 2021 based on the Neutron Star Interior Composition Explorer (NICER) observation. We also used some early data from the 2018 outburst to compare different temporal and spectral properties. The evolution of the spin period and pulsed flux is studied with Fermi/GBM during the outburst and the spin-up rate is found to be varied between $\simeq(0.8-1.8)\times 10^{-11}$ Hz s$^{-1}$. The pulse profile shows energy dependence and variability. The pulse profile shows multiple peaks and dips which evolve with energy. The evolution of the spectral state of this source is also studied using the hardness intensity diagram (HID). The HID shows a transition from the horizontal to the diagonal branch, which implies the source went through a state transition from the subcritical to supercritical accretion regime. The NICER energy spectrum is well described by a composite model of a power-law with a higher cut-off energy and blackbody components along with a photo-electric absorption component. An iron emission line is detected near 6.4 keV in the NICER spectrum with an equivalent width of $\sim$0.05 keV. The photon index shows an anti-correlation with flux below the critical flux. The mass accretion rate is estimated to be $\simeq1.3\times10^{17}$ g s$^{-1}$ near the peak of the outburst. We have found a positive correlation between the pulse frequency derivatives and luminosity. The Ghosh and Lamb model is applied to estimate the magnetic field at different spin-up rates, which is compared to the earlier estimated magnetic field at a relatively high mass accretion rate. The magnetic field is estimated to be $\simeq10^{14}$ G from the torque-luminosity model using the distance estimated by Gaia, which is comparatively higher than most of the other Be/XBPs.

Optimization of surface high-order harmonic generation by tailoring nanostructures

Many applications in attosecond science, lensless imaging, and industrial metrology need coherent extreme-ultraviolet pulses from high-order harmonic generation (HHG) in solids. However, the efficiency of the HHG plateau and the higher cutoff energy are nontrivial due to the limitation of the lower damage threshold of solids irradiated by laser fields. Here, we propose nanoengineered solid bowtie samples to overcome this limitation, which enables one to tailor the characteristic spacing between two nanotips. The near-field effect combined with recombined emissions with multiple scatterings can robustly increase conversion efficiencies and extend the cutoff energies of the HHG plateau. Our result paves the way to the manipulation of harmonic emissions in nanostructures and introduces imaging of the sub-cycle resolution of ultrafast electronic dynamics.

High‐Harmonic Generation from 2D Monolayer Electrides

AbstractThis work reports high‐harmonic generation (HHG) from 2D monolayer electrides investigated by using ab‐initio approach based on time‐dependent density functional theory, mainly focusing on monolayer Mg2N. Due to low ionization energy and wide potential well, they exhibit distinctively high HHG efficiency and high cutoff energy for out‐of‐plane pumping, although they also show atomic‐like behavior. For out‐of‐plane pumping with a peak intensity of 40 TW cm−2 at 1550 nm, high harmonics up to 120th order are generated with efficiency 4 orders‐of‐magnitude higher than monolayer h‐BN. For pump intensity below the direct above‐threshold ionization (ATI) regime, the tunneling from the ground state and ATI from one‐ or two‐photon resonant state simultaneously contribute to HHG. In the other subnitride monolayers of alkaline earth with larger atomic sizes (Ca2N and Sr2N), further higher harmonic generation efficiencies are obtained due to the lower work functions and wider potential wells. For in‐plane pumping, the intraband transition is the major source of the HHG due to the metallic in‐plane responses. The presented results will attract great attention due to the peculiar HHG signature from fundamental aspects and pave the way of obtaining coherent extreme ultraviolet (EUV)/soft‐X‐ray with high efficiency by using monolayer electrides.

Fast Radio Bursts and Their High-energy Counterparts from Magnetar Magnetospheres

Abstract The recent discovery of a Galactic fast radio burst (FRB) occurring simultaneously with an X-ray burst (XRB) from the Galactic magnetar SGR J1935+2154 implies that at least some FRBs arise from magnetar activities. We propose that FRBs are triggered by crust fracturing of magnetars, with the burst event rate depending on the magnetic field strength in the crust. Since the crust-fracturing rate is relatively higher in polar regions, FRBs are more likely to be triggered near the directions of multipolar magnetic poles. Crust fracturing produces Alfvén waves, forming a charge-starved region in the magnetosphere and leading to nonstationary pair plasma discharges. An FRB is produced by coherent plasma radiation due to nonuniform pair production across magnetic field lines. Meanwhile, the FRB-associated XRB is produced by the rapid relaxation of the external magnetic field lines. In this picture, the sharp-peak hard X-ray component in association with FRB 200428 is from a region between adjacent trapped fireballs, and its spectrum with a high cutoff energy is attributed to resonant Compton scattering. The persistent X-ray emission is from a hot spot heated by the magnetospheric activities, and its temperature evolution is dominated by magnetar surface cooling. Within this picture, magnetars with stronger fields tend to produce brighter and more frequent repeated bursts.

Investigation of high harmonic generation from laser ablated plumes of silver

Silver (Ag) is one of the important targets in the studies of high harmonic generation (HHG) in the laser-induced plasma due to the relatively high conversion generation efficiencies and cut-off energy. In this work, we study the dependence of HHG in plasma plumes of Ag as a function of various laser parameters, including intensity of ablation pulse, intensity and ellipticity of driving pulse, and the delay between the two pulses. We identify the type of ion that dominates the measured HHG spectra by comparing the experimental data with strong-field approximation simulations and classical calculations for Ag atoms, Ag+, and Ag2+ ions. We also perform a comparative HHG study between the plasmas of bulk Ag target and the plasmas of a nano-powders target. It is found that the harmonic yields in the latter case are higher over a wide range of the aforementioned laser parameters. The results also indicate that the number of nanoparticles in the plasma generated from a nano-powder sample is indeed significantly greater than that in the plasma from the bulk sample.

Weibel instability mediated laser hole boring and ion acceleration in an electrostatic shock

We show that generation of strong magnetic field filaments due to Weibel instability accompanies the hole-boring ion acceleration by a circularly polarised laser pulse. The magnetic field confines energetic protons accelerated from hole boring in high density collimated jets. A part of energetic protons are reaccelerated by the electric field and focused by the magnetic field generated from the jets. As a result, highly collimated proton jets with high cutoff energy can be produced.

Two Complete Spectral Transitions of Swift J0243.6+6124 Observed by Insight-HXMT

Abstract We performed the broadband (1–100 keV) spectral analysis of the first Galactic Be ultraluminous X-ray pulsar (BeULX) Swift J0243.6+6124 observed by Insight-HXMT during the 2017−2018 outburst. The results show spectral transitions at two typical luminosities, roughly consistently with those reported previously via pure timing analysis. We find that the spectrum evolves and becomes softer and has higher cutoff energies until the luminosity reaches L 1 (∼1.5 × 1038 erg s−1). Afterwards the spectrum becomes harder with lower cutoff energies until the luminosity increases to L 2 (∼4.4 × 1038 erg s−1), around which the second spectral transition occurs. Beyond L 2, the spectrum softens again and has larger cutoff energies. Similar behaviors were observed previously in other high-mass X-ray binary systems (HMXBs), especially for the second transition at higher luminosities, which is believed to have a correlation with the magnetic field of the harbored neutron star. Accordingly, we speculate that Swift J0243.6+6124 owns a neutron star with magnetic field strength >1013 G. The spectral transition at around L 1 of Swift J0243.6+6124 is first observed thoroughly for any HMXB outburst characterized by strong evolution of the thermal component: the temperature of the blackbody drops sharply accompanied by a sudden increase of the blackbody radius. These spectral transitions can in principle be understood in a general scenario of balancing the emission patterns between the pencil and the fan beams at the magnetic pole, for which the extreme brightness of Swift J0243.6+6124 may provide an almost unique lab to probe the details.

The pseudoatomic orbital basis: electronic accuracy and soft-mode distortions in ABO3 perovskites

The perovskite oxides are known to be susceptible to structural distortions over a long wavelength when compared to their parent cubic structures. From an ab initio simulation perspective, this requires accurate calculations including many thousands of atoms; a task well beyond the remit of traditional plane wave-based density functional theory (DFT). We suggest that this void can be filled using the methodology implemented in the large-scale DFT code, CONQUEST, using a local pseudoatomic orbital (PAO) basis. Whilst this basis has been tested before for some structural and energetic properties, none have treated the most fundamental quantity to the theory, the charge density n(r) itself. An accurate description of n(r) is vital to the perovskite oxides due to the crucial role played by short-range restoring forces (characterised by bond covalency) and long range Coulomb forces as suggested by the soft-mode theory of Cochran and Anderson. We find that modestly sized basis sets of PAOs can reproduce the plane-wave charge density to a total integrated error of better than 0.5% and provide Bader partitioned ionic charges, volumes and average charge densities to similar degree of accuracy. Further, the multi-mode antiferroelectric distortion of PbZrO3 and its associated energetics are reproduced by better than 99% when compared to plane-waves. This work suggests that electronic structure calculations using efficient and compact basis sets of pseudoatomic orbitals can achieve the same accuracy as high cutoff energy plane-wave calculations. When paired with the CONQUEST code, calculations with high electronic and structural accuracy can now be performed on many thousands of atoms, even on systems as delicate as the perovskite oxides.

Double-plateau structure and the effect of the carrier-envelope phase on high-order harmonic generation from a Rydberg atom in a few-cycle laser pulse

Recent studies have demonstrated that in a few-cycle laser pulse, a coherent Rydberg atom—an atom in a superposition of the ground and highly excited states—can generate high-order harmonic generation (HHG) spectra with high conversion efficiency and high cutoff energy, making it a potential procedure for producing attosecond pulses. In this study, we theoretically report two interesting findings that can be realized experimentally: the nontrivial dependence of HHG cutoff on the laser carrier-envelope phase (CEP) and the double-plateau structure in the spectrum when the CEP ranges from 75° to 120°. The second effect has not been reported for a Rydberg atom in the previous studies focusing only on CEP of 0°. Finally, using classical simulation and time-frequency analysis, we explain the influence of the CEP on the cutoff energy and, especially, the origin of the double-plateau structure. Unlike the first plateau generated by the recombination of an electron escaping from the Rydberg state but returning to the ground state, the second one arises from the ionization from the ground state. Consequently, by controlling the laser CEP, the electron dynamics can be embedded in the structure of plateaus in the HHG spectra.

X-Ray Measurements of the Particle Acceleration Properties at Inward Shocks in Cassiopeia A

Abstract We present new evidence that the bright nonthermal X-ray emission features in the interior of the Cassiopeia A supernova remnant are caused by inward-moving shocks, based on Chandra and NuSTAR observations. Several bright inward-moving filaments were identified using monitoring data taken by Chandra in 2000–2014. These inward-moving shock locations are nearly coincident with hard X-ray (15–40 keV) hot spots seen by NuSTAR. From proper-motion measurements, the transverse velocities were estimated to be in the range of ∼2100–3800 km s−1 for a distance of 3.4 kpc. The shock velocities in the frame of the expanding ejecta reach values of ∼5100–8700 km s−1, which is slightly higher than the typical speed of the forward shock. Additionally, we find flux variations (both increasing and decreasing) on timescales of a few years in some of the inward-moving shock filaments. The rapid variability timescales are consistent with an amplified magnetic field of B ∼ 0.5–1 mG. The high speed and low photon cut-off energy of the inward-moving shocks are shown to imply a particle diffusion coefficient that departs from the Bohm regime (k 0 = D 0/D 0,Bohm ∼ 3–8) for the few simple physical configurations we consider in this study. The maximum electron energy at these shocks is estimated to be ∼8–11 TeV, which is smaller than the values of ∼15–34 TeV that were inferred for the forward shock. Cassiopeia A is dynamically too young for its reverse shock to appear to be moving inward in the observer frame. We propose instead that the inward-moving shocks are a consequence of the forward shock encountering a density jump of ≳5–8 in the surrounding material.

Implementation of a low-activation Au-In-Cd decoupler into the J-PARC 1 MW short pulsed spallation neutron source

A silver-indium-cadmium (Ag-In-Cd, or AIC) alloy with a 1 eV high neutron cut-off energy was originaly developed as a decoupler to provide a narrow neutron pulse width with a short tail for the J-PARC 1-MW short-pulsed spallation neutron source. As a result of later studies, gold (Au) was chosen as a substitute for Ag for the production of spare decoupled moderators and reflector in order to reduce residual radioactivity thus easing handling and disposal of the spent moderator and reflector. The decoupler material Au-In-Cd (AuIC) was therefore investigated and developed. To implement it into an actual moderator-reflector assembly many critical engineering issues needed to be resolved with regard to large-sized bonding between AuIC and aluminum alloy A5083 by the hot isostatic pressing process (HIP). The HIP process for AuIC and aluminum was investigated in terms of surface condition, size, and heat capacity. Implementation of an AuIC decoupler into a spare reflector assembly was successfuly achieved and will result in a remarkable reduction of radioactivity compared to AIC without sacrificing neutronic performance.

Enhanced stability performance of nickel nanowire with 3D conducting network for planar sodium-nickel chloride batteries

High temperature sodium batteries with inorganic electrolytes are attracting increasing attention due to their high thermal stability, reliability, long-cycle life and safety. Despite the intensive investigation of Na-NiCl2 batteries during last decades, designing a stable conducting network in the cathode is still challenging but desirable. In this work, a new cathode structure composed of Ni nanowires with an effective electron conducting network is proposed for planar sodium–nickel chloride batteries. During the first stage of charge, Ni nanowires reacted with Cl ion to form NiCl2 on the surface and the excessive Ni nanowires inside can serve as conducting framework to facilitate the fast electron transport. As expected, the metal nanowires show a high specific capacity of 130 mA h g−1 (∼14 mAh cm−2) at 0.05 C after 100 cycles. Meanwhile, the batteries show stable cycling performance at 0.05 C with a high cut-off energy density of 360 W h kg−1, much higher than the traditional tubular sodium-nickel chloride batteries (∼100 Wh kg−1). Scanning electron microscope (SEM) images of the cathode after long cycling reveal the ultra-slow growth of Ni particles, confirming the excellent stability of the prepared nanowires cathode.

Shock Acceleration Model for the Toothbrush Radio Relic

Abstract Although many of the observed properties of giant radio relics detected in the outskirts of galaxy clusters can be explained by relativistic electrons accelerated at merger-driven shocks, significant puzzles remain. In the case of the so-called Toothbrush relic, the shock Mach number estimated from X-ray observations ( ) is substantially weaker than that inferred from the radio spectral index ( ). Toward understanding such a discrepancy, we here consider the following diffusive shock acceleration (DSA) models: (1) weak-shock models with and a preexisting population of cosmic-ray electrons (CRe) with a flat energy spectrum, and (2) strong-shock models with and either shock-generated suprathermal electrons or preexisting fossil CRe. We calculate the synchrotron emission from the accelerated CRe, following the time evolution of the electron DSA, and the subsequent radiative cooling and postshock turbulent acceleration (TA). We find that both models could reproduce reasonably well the observed integrated radio spectrum of the Toothbrush relic, but the observed broad transverse profile requires the stochastic acceleration by downstream turbulence, which we label “turbulent acceleration” or TA to distinguish it from DSA. Moreover, to account for the almost uniform radio spectral index profile along the length of the relic, the weak-shock models require a preshock region over 400 kpc with a uniform population of preexisting CRe with a high cutoff energy ( ). Due to the short cooling time, it is challenging to explain the origin of such energetic electrons. Therefore, we suggest the strong-shock models with low-energy seed CRe ( ) are preferred for the radio observations of this relic.

High Harmonics with Controllable Polarization by a Burst of Linearly-Polarized Driver Pulses

We theoretically explore a scheme for generation of bright circularly and elliptically polarized high-order harmonics by bursts of linearly polarized pulses with a rotating polarization axis. Circularly polarized harmonics are formed if the bursts are comprised of N pulses that uphold an N-fold rotational symmetry, for N > 2. Rotating the polarization axes of the comprising pulses can generate elliptical harmonics with a collectively tunable ellipticity, from circular through elliptic to linear. The method preserves the single-cycle, single-atom and macroscopic physics of ‘standard’ linearly polarized high harmonic generation, with a high yield and cutoff energy. We investigate the method from a time-domain perspective, as well as a photonic perspective, and formulate the energy and spin-angular momentum conservation laws for this scheme. We find that the case of N = 4 is optimal for this method, resulting with the highest conversion efficiency of elliptical photons. The new features of this source offer new applications to helical ultrafast spectroscopy and ellipsometry.

Prospects of target nanostructuring for laser proton acceleration

In laser-based proton acceleration, nanostructured targets hold the promise to allow for significantly boosted proton energies due to strong increase of laser absorption. We used laser-induced periodic surface structures generated in-situ as a very fast and economic way to produce nanostructured targets capable of high-repetition rate applications. Both in experiment and theory, we investigate the impact of nanostructuring on the proton spectrum for different laser–plasma conditions. Our experimental data show that the nanostructures lead to a significant enhancement of absorption over the entire range of laser plasma conditions investigated. At conditions that do not allow for efficient laser absorption by plane targets, i.e. too steep plasma gradients, nanostructuring is found to significantly enhance the proton cutoff energy and conversion efficiency. In contrast, if the plasma gradient is optimized for laser absorption of the plane target, the nanostructure-induced absorption increase is not reflected in higher cutoff energies. Both, simulation and experiment point towards the energy transfer from the laser to the hot electrons as bottleneck.

High-order harmonic and attosecond pulse generations from Rydberg state driven by the spatially inhomogeneous field

High-order harmonic spectra and attosecond pulse generation from Rydberg atom (He[Formula: see text]) driven by the spatially inhomogeneous field have been theoretically investigated. (i) Firstly, with an electron initially in a single excited Rydberg state (nth), the harmonic yield can be enhanced due to the decreased ionization potential, and a maximum enhancement can be obtained when the initial state is prepared as the third excited state (n = 3). However, the low cutoff energy from the excited state is unbeneficial to the generation of the higher photon pulse. Thus, with the further introduction of the laser chirp, not only the harmonic cutoff is extended, but also the harmonic modulation is reduced. As a result, five super-bandwidths from 63 eV to 267 eV can be found. (ii) Secondly, by preparing the initial state as a coherent superposition of excited state, the harmonic yield can be further enhanced, especially for the coherent superposition of the first and the third (n = 1 + 3) and the second and the fourth (n = 2 + 4) excited states, the harmonic yield is enhanced by 4–8 orders of magnitude compared with the case of the single ground initial state. Furthermore, by properly adding the laser pulse into the spatially inhomogeneous region (gap center [Formula: see text] a.u.) from left [Formula: see text] to right [Formula: see text], much higher cutoff energies can be obtained in the left region. As a consequence, two super-bandwidths of 248 eV and 496 eV can be obtained. Finally, by properly superposing the harmonics, a series of sub-25-as pulses with intensity enhancement of 5–8 orders of magnitude can be produced.

Angular correlation between IceCube high-energy starting events and starburst sources

Starburst galaxies and star-forming regions in the Milkyway, with high rate of supernova activities, are candidate sources of high-energy neutrinos. Using a gamma-ray selected sample of these sources we perform statistical analysis of their angular correlation with the four-year sample of high-energy starting events (HESE), detected by the IceCube Neutrino Observatory. We find that the two samples (starburst galaxies and local star-forming regions) are correlated with cosmic neutrinos at ∼ (2–3)σ (pre-trial) significance level, when the full HESE sample with deposited energy ≳ 20 TeV is considered. However when we consider the HESE sample with deposited energy ≳ 60 TeV, which is almost free of atmospheric neutrino and muon backgrounds, the significance of correlation decreased drastically. We perform a similar study for Galactic sources in the 2nd Catalog of Hard Fermi-LAT Sources (2FHL, >50 GeV) catalog as well, obtaining ∼ (2–3)σ (pre-trial) correlation, however the significance of correlation increases with higher cutoff energy in the HESE sample for this case. We also fit available gamma-ray data from these sources using a pp interaction model and calculate expected neutrino fluxes. We find that the expected neutrino fluxes for most of the sources are at least an order of magnitude lower than the fluxes required to produce the HESE neutrinos from these sources. This puts the starburst sources being the origin of the IceCube HESE neutrinos in question.

Effects of variance reduction techniques on the efficiency and accuracy of Monte Carlo simulation for the single source channel of Gamma Knife

Objective To evaluate the effects of four types of variance reduction techniques (ring counter grid, high electron cutoff energy, termination of electron tracking in some structures, and emission direction-biased sampling of source) on the efficiency and accuracy of Monte Carlo simulation for the single source channel of the Leksell Gamma Knife. Methods The single source channel of the Leksell Gamma Knife was modeled using Monte Carlo software MCNP. Four types of variance reduction techniques were used to simulate the dose distribution in the water-like phantom. The computation efficiency and simulation result were compared between the four techniques. Results All techniques substantially improved the computation efficiency and had little effect on the accuracy of the simulation (relative error less than 2.5%). However, if the electron cutoff energy was above 50 keV, the simulation became quite inaccurate due to neglect of the scattering of high-energy electrons and their dosimetric contribution to the penumbra. When the scattering of high-energy electrons and their dosimetric contribution to the penumbra were ignored, the dose to the Profile platform was overestimated and the dose to the penumbra was underestimated. Conclusions Rational use of variance reduction techniques can substantially improve the efficiency of Monte Carlo simulation for the single source channel of the Leksell Gamma Knife. However, the impact of variance reduction techniques on the accuracy of the simulation should be carefully evaluated. Key words: Gamma knife; Monte Carlo; Computation efficiency; Variance reduction

On the analytic estimation of radioactive contamination from degraded alphas

The high energy spectrum of alpha particles emitted from a single isotope uniformly contaminating a bulk solid has a flat energy spectrum with a high end cutoff energy equal to the maximal alpha kinetic energy (Tα) of the decay. In this flat region of the spectrum, we show the surface rate rb (Bq keV−1cm−2) arising from a bulk alpha contamination ρb (Bq cm−3) from a single isotope is given by rb =ρb Δ R/ 4 Δ E , where Δ E = E1−E2>0 is the energy interval considered (keV) in the flat region of the spectrum and Δ R = R2−R1, where R2 (R1) is the amount of the bulk material (cm) necessary to degrade the energy of the alpha from Tα to E2 (E1). We compare our calculation to a rate measurement of alphas from 147Sm, (15.32 ± 0.03% of Sm(nat) and half life of (1.06 ± 0.01)× 1011 yr [1]), and find good agreement, with the ratio between prediction to measurement of 100.2%± 1.6% (stat)± 2.1% (sys). We derive the condition for the flat spectrum, and also calculate the relationship between the decay rate measured at the surface for a [near] surface contamination with an exponential dependence on depth and a second case of an alpha source with a thin overcoat. While there is excellent agreement between our implementation of the sophisticated Monte Carlo program SRIM [2] and our intuitive model in all cases, both fail to describe the measured energy distribution of a 148Gd alpha source with a thin (∼200μg cm−2) Au overcoat. We discuss possible origins of the disagreement and suggest avenues for future study.