Cascade Emission Research Articles

Hadronic Origin of Prompt High-energy Emission of Gamma-ray Bursts Revisited: In the Case of a Limited Maximum Proton Energy

Abstract The high-energy (>100 MeV) emission observed by the Fermi Large Area Telescope during the prompt phase of some luminous gamma-ray bursts (GRBs) could arise from the cascade induced by interactions between accelerated protons and the radiation field of GRBs. The photomeson process, which is usually suggested to operate in such a hadronic explanation, requires a rather high proton energy (>1017 eV) for an efficient interaction. However, whether GRBs can accelerate protons to such a high energy is far from guaranteed, although they have been suggested as the candidate source for ultrahigh-energy cosmic rays. In this work, we revisit the hadronic model for the prompt high-energy emission of GRBs with a smaller maximum proton energy than the usually adopted value estimated from the Bohm condition. In this case, the Bethe–Heitler pair production process becomes comparably important or even dominates over the photomeson process. We show that with a relatively low maximum proton energy with a Lorentz factor of 105 in the comoving frame, the cascade emission can still reproduce various types of high-energy spectra of GRBs. For most GRBs without high-energy emission detected, the maximum proton energy could be even lower and relax the constraints on the parameters of the GRB jet resulting from the nondetection of GRB neutrinos by IceCube.

Biexciton cascade emission in multilayered organic nanofibers

The optical performance of multilayered organic nanofibers results from the dynamics of excited states in the system. Here, we show that the presence of biexcitons is crucial to correctly describe such dynamics. This may be the case even if the intensity of the light source is not high. The cascade emission mediated by biexcitons is mainly responsible for the behavior of the photoluminescence profile in the initial steps after light absorption. By using a combination of Kinetic Monte Carlo model and Genetic Algorithm, we simulate Time-Resolved Photoluminescence measurements of multilayered nanofibers. These simulations are compared with experimental results, thus revealing that the usual singlet exciton recombination is insufficient to reproduce the complete physical picture. Our results also include predictions for the behavior of the biexciton signal. These findings are observed to be valid for a wide temperature range, showing the importance of the biexciton cascade emission in several regimes for organic nanofibers in general.

Intra- and inter-configurational luminescence spectroscopy of Pr3+ -doped YPO4 nanophosphors

Nanopowders of YPO4 phosphors with different Pr3+doping were successfully prepared by a sol gel method under different synthesis conditions. The crystallite size and strain show a strong dependence on the Pr3+ doping concentration and on the annealing temperature. By annealing at 300 °C one can obtain the xenotime structure of the pure YPO4. The crystallite size can be controlled by controlling the annealing temperature and it increases with increasing the annealing temperature. The room temperature inter-configurational 4f2 ↔ 4f5d and intra-configurational 4f2↔ 4f2 emission-excitation transitions spectra are measured and investigated. Upon 4f2 → 4f5d excitation transition, all the samples present broad intense emission bands attributed to 4f5d → 4f2 emission transitions and peaks in red region assigned to 1D2→3H4 transition as photon cascade emission phenomena (PCE). The presence of only 1D2→3H4 transition is discussed. In addition, the 1D2 energy level lifetimes as well as the refractive indexes were determined and discussed.

Luminous and high-frequency peaked blazars: the origin of the γ-ray emission from PKS 1424+240

Context. The current generation of ground-based Cherenkov telescopes, together with the LAT instrument on-board the Fermi satellite, have greatly increased our knowledge of γ-ray blazars. Among them, the high-frequency-peaked BL Lacertae object (HBL) PKS 1424+240 (z ≃ 0.6) is the farthest persistent emitter of very-high-energy (VHE; E ≥ 100 GeV) γ-ray photons. Current emission models can satisfactorily reproduce typical blazar emission assuming that the dominant emission process is synchrotron-self-Compton (SSC) in HBLs; and external-inverse-Compton (EIC) in low-frequency-peaked BL Lacertae objects and flat-spectrum-radio-quasars. Alternatively, hadronic models are also able to correctly reproduce the γ-ray emission from blazars, although they are in general disfavored for bright quasars and rapid flares. Aims. The blazar PKS 1424+240 is a rare example of a luminous HBL, and we aim to determine which is the emission process most likely responsible for its γ-ray emission. This will impact more generally our comprehension of blazar emission models, and how they are related to the luminosity of the source and the peak frequency of the spectral energy distribution. Methods. We have investigated different blazar emission models applied to the spectral energy distribution of PKS 1424+240. Among leptonic models, we study a one-zone SSC model (including a systematic study of the parameter space), a two-zone SSC model, and an EIC model. We then investigated a blazar hadronic model, and finally a scenario in which the γ-ray emission is associated with cascades in the line-of-sight produced by cosmic rays from the source. Results. After a systematic study of the parameter space of the one-zone SSC model, we conclude that this scenario is not compatible with γ-ray observations of PKS 1424+240. A two-zone SSC scenario can alleviate this issue, as well as an EIC solution. For the latter, the external photon field is assumed to be the infra-red radiation from the dusty torus, otherwise the VHE γ-ray emission would have been significantly absorbed. Alternatively, hadronic models can satisfactorily reproduce the γ-ray emission from PKS 1424+240, both as in-source emission and as cascade emission.

Constraining Balmer Alpha Fine Structure Excitation Measured in Geocoronal Hydrogen Observations

AbstractCascade contributions to geocoronal Balmer α airglow line profiles are directly proportional to the Balmer β/α line ratio and can therefore be determined with near simultaneous Balmer β observations. Due to scattering differences for solar Lyman β and Lyman γ (responsible for the terrestrial Balmer α and Balmer β fluorescence, respectively), there is an expected trend for the cascade emission to become a smaller fraction of the Balmer α intensity at larger shadow altitudes. Near‐coincident Balmer α and Balmer β data sets, obtained from the Wisconsin H alpha Mapper Fabry‐Perot, are used to determine the cascade contribution to the Balmer α line profile and to show, for the first time, the Balmer β/α line ratio, as a function of shadow altitude. We show that this result is in agreement with direct cascade determinations from Balmer α line profile fits obtained independently by high‐resolution Fabry‐Perot at Pine Bluff, WI. We also demonstrate with radiative transport forward modeling that a solar cycle influence on cascade is expected, and that the Balmer β/α line ratio poses a tight constraint on retrieved aeronomical parameters (such as hydrogen's evaporative escape rate and exobase density).

Simultaneously measured photoluminescence lifetime and quantum yield of two-photon cascade emission on single CdSe/ZnS nanocrystals

The photoluminescence (PL) lifetime and quantum yield (QY) of the two-photon cascade emission (TPCE) provide critical information for assessing the quality of optoelectronic devices. The QY of TPCE has been conventionally evaluated by second-order photon correlation, ${g}^{(2)}$, measurements under weak-excitation conditions in single nanocrystals (NCs). Here, we report that by measuring simultaneously both the ${g}^{(2)}$ and PL-decay curves, the QY of the TPCE can be determined even under strong-excitation conditions. With this technique, the effect of ionization on the QY of the TPCEs in single CdSe/ZnS NCs, which appears under strong-excitation condition, can be revealed. The obtained QYs were compared with the PL lifetime of the first and second photon emissions. We demonstrate that even for ionized NCs, the radiative recombination rates during TPCE are almost proportional to the product of the numbers of electrons and holes.

Electron impact study of the 100 eV emission cross section and lifetime of the Lyman‐Birge‐Hopfield band system of N2: Direct excitation and cascade

AbstractWe have measured the 100 eV emission cross section of the optically forbidden Lyman‐Birge‐Hopfield (LBH) band system (a1Πg → X1Σ+g) of N2 by electron‐impact‐induced fluorescence. Using a large (1.5 m diameter) vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN mission Imaging Ultraviolet Spectrograph optical engineering model, we have obtained calibrated spectral measurements of the LBH band system from 115 to 175 nm over a range of lines of sight to capture all of the optical emissions. These measurements represent the first experiment to directly isolate in the laboratory single‐scattering electron‐impact‐induced fluorescence from both direct excitation of the a1Πg state and cascading contributions to the a1Πg state (a′1Σ−u and w1Δu → a1Πg → X1Σ+g). The determination of the total LBH emission cross section is accomplished by measuring the entire cylindrical glow pattern of the metastable emission from electron impact by imaging lines of sight that measure the glow intensity from zero to ~400 mm radial distance and calculating the ratio of the integrated intensity from the LBH glow pattern to that of a simultaneously observed optically allowed transition with a well‐established cross section: Ni 120.0 nm. The “direct” emission cross section of the a1Πg state at 100 eV was determined to be σemdir = (6.41 ± 1.3) × 10−18 cm2. An important observation from the glow pattern behavior is that the total (direct + cascading) emission cross section is pressure dependent due to collision‐induced cascade transitions between close‐lying electronic states.

Dynamic emission and population control in a Λ-type excitation scheme of atomic potassium

The nonlinear interaction of a four-level atomic configuration in potassium, with ns pump and coupling fields, is numerically investigated. The resonant pump acts on the two-photon transition causing internally generated emissions in two atomic paths: path-1, and path-2, respectively. The coupling laser resonantly excites the single-photon transition in a Λ-type scheme. A controlled temporal shift and enhancement of the partially coherent cascade emissions of path-2 is obtained. The amplification without inversion mechanism is proposed to explain the lower path-2 emission. It is shown that when the coupling field is delayed from the pump, then an efficient dynamic effect is induced, based on an additional population interplay of states and Finally, a novel coherent method for temporally controlling the quasi-stationary population dynamics in a saturated system is proposed, when the driving fields are also delayed. The importance of the present work is related to the fact that the induced coherence (polarisation or energy injection) of the coupling field can be temporally controlled in nonlinear gaseous media.

Cascaded cold atomic ensembles in a diamond configuration as a spectrally entangled multiphoton source

We theoretically investigate the spectral entanglement of a multiphoton source generated from the cascade emissions in the cascaded cold atomic ensembles.\ This photon source is highly directional, guaranteed under the four-wave mixing condition, and is also highly frequency-correlated due to finite driving pulse durations and superradiant decay constants.\ We utilize Schmidt decomposition to study the bipartite entanglement of the biphoton states projected from the multiphoton ones.\ This entropy of entanglement can be manipulated by controlling the driving parameters and superradiant decay rates.\ Moreover the projected biphoton states in the cascaded scheme can have larger entanglement than the one produced from only one atomic ensemble, which results from larger capacity in multipartite entanglement.\ This cascaded scheme enables a multiphoton source useful in quantum information processing.\ It also allows for potential applications in multimode quantum communication and spectral shaping of high-dimensional continuous frequency entanglement.

Search for Magnetically Broadened Cascade Emission from Blazars with VERITAS

We present a search for magnetically broadened gamma-ray emission around active galactic nuclei (AGN), using VERITAS observations of seven hard-spectrum blazars. A cascade process occurs when multi-TeV gamma rays from AGN interact with extragalactic background light (EBL) photons to produce electron-positron pairs, which then interact with cosmic microwave background (CMB) photons via inverse-Compton scattering to produce gamma rays. Due to the deflection of the electron-positron pairs, a non-zero intergalactic magnetic field (IGMF) would potentially produce detectable effects on the angular distribution of the cascade emission. In particular, an angular broadening compared to the unscattered emission could occur. Through non-detection of angularly broadened emission from 1ES 1218+304, the source with the largest predicted cascade fraction, we exclude a range of IGMF strengths around $10^{-14}$G at the 95% confidence level. The extent of the exclusion range varies with the assumptions made about the intrinsic spectrum of 1ES 1218+304 and the EBL model used in the simulation of the cascade process. All of the sources are used to set limits on the flux due to extended emission.

Predicting photon cascade emission in Pr3+ doped fluorides.

We present a theoretical approach, based on a density functional theoretic parametrization of a model Hamiltonian, that allows for the determination of the complete electronic 4fn and 4fn-15d valence manifolds of a lanthanide ion impurity in different host lattices in an easy, fast and reliable way. The model accounts for the electrostatic interactions, ligand field splitting due to the surroundings of the impurity and spin-orbit coupling effects, which means that the parametrization is transferable to different ligand environments. The model is applied to several Pr3+ doped fluorides, which are of great interest for designing warm white lighting devices. We show that the model correctly predicts which of them presents photon-cascade emission as experimentally observed. We also determine the absorption (emission) spectra from the dipolar transition probabilities from the lowest 4f2 (4f5d) state to the states of the 4f5d (4f2) manifold of the Pr3+ impurity, in agreement with the available experimental data. The present approach can be applied to other impurity-doped systems where the accurate determination of the valence impurity states is crucial for characterizing the relevant properties of the system.

Magnetic and Photon Cascade Emission of Gd3+ of NaGd(PO3)4 Monocrystal Under Appropriate Synthesis Conditions

A signal crystal of NaGd(PO3)4 (NGP) was successfully grown with the solid-state reaction method under appropriate synthesis conditions. Structure refinements and the purity of the samples were determined by XRD patterns. It reveals that NaGd(PO3)4 compound crystallizes in the monoclinic system with P 21/n space group and with cell parameters a=7.174(1) A, b=13.033(2) A, c=9.781(1) A, β=90.65(2)°, V=914.47(20) A3 and Z=4. The IR and Raman spectra also indicated that the phosphoric polyhedra of NaGd(PO3)4 has a acyclic symmetry. Differential thermal analysis is given. This compound is thermally stable until 852°C. The magnetic susceptibilities of gadolinium polyphosphate as a function of temperature are reported along with corresponding magnetization measurements. This polyphosphate displays a paramagnetic response, without magnetic phase transitions, and with effective magnetic moments close to those of the corresponding free Gd3+ ions. The luminescence proprieties of Gd3+ was investigated. The emission spectra NGP exhibit intense band at 315 nm due to 6P7/2 →8S7/2 transition of Gd3+ ions when excited at 273 nm. The photon cascade emission (PCE) of Gd3+ has been proved at 164 and 254 nm excitation.

Optical phonon cascade emission by photoelectrons at a p-GaN (Cs,O)–vacuum interface

It has been experimentally established that the transfer of photoelectrons from the bulk of a p-GaN (Cs,O) photocathode to vacuum is accompanied by the emission of a cascade of optical phonons. In the quantum efficiency spectrum of the p-GaN (Cs,O) photocathode, an exciton peak has been identified, indicating a significant contribution of the electron–hole interaction to the generation of free electrons in heavily doped p-GaN.

Biexciton cascade emission reveals absolute absorption cross section of single semiconductor nanocrystals

The sequential two-photon emission process known as biexciton cascade emission is a characteristic phenomenon that occurs in photoexcited semiconductor nanocrystals (NCs). This process occurs when a biexciton state is created in the NCs; thus, the occurrence of the process is related to the photoabsorption properties of the NCs. This paper presents a simple equation that connects the photoabsorption of single NCs and the biexciton cascade emission. The equation is found to be independent of the quantum yields of photoluminescence (PL). With this equation and using an analysis of second-order photon correlation, the absolute absorption cross section $\ensuremath{\sigma}$ of the single NCs can be evaluated, obtaining values on the order of ${10}^{\ensuremath{-}14}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{2}$. This analysis shows that ionization during PL blinking does not affect the validity of the relation, indicating that the evaluation of $\ensuremath{\sigma}$, based on the equation, is applicable for various NCs with unique structures.

Direct optical access to the triplet manifold of states inH2

A number of unassigned lines in the absorption spectrum of diatomic hydrogen are attributed to nominally forbidden transitions from the ground state to the $nf$ manifold of states (Rydberg electron with $\ensuremath{\ell}=3$ orbital momentum). They appear via weak $\ensuremath{\ell}\ensuremath{-}$ mixing interactions leading to local level perturbations. Our analysis is based on multichannel quantum defect theory and uses known theoretical information from the literature. The upper levels of most of these transitions are known to give rise to molecular fluorescence, and they are shown to be singlet-triplet mixed. We conclude that the well-known metastable $c{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Pi}}}_{u}^{\ensuremath{-}}$ state can be populated via one-photon absorption of uv photons followed by cascade emission $4f\ensuremath{\rightarrow}3d\ensuremath{\rightarrow}2p$.

Diagrams and parastatistical factors for cascade emission of a pair of paraparticles

The empirical absence to date of particles obeying parastatistics in high energy collider experiments might be due to their large masses, weak scale couplings, and lack of gauge couplings. Paraparticles of order p=2 must be pair produced, so the lightest such particles are absolutely stable and so are excellent candidates to be associated with dark matter and/or dark energy. If there is a portal to such particles, from a new scalar A1 boson they might be cascade emitted as a pair of para-Majorana neutrinos as in A1 ---> A2 para-nu para-nu or as a pair of neutral spin-zero paraparticles such as in A1 ---> A2 para-A para-B, where para-B is the anti-paraparticle to para-A. In this paper, for an assumed supersymmetric-like "statistics portal" Lagrangian, the associated connected tree diagrams and their parastatistical factors are obtained for the case of order p=2 parastatistics. These factors are compared with the corresponding statistical factors for the analogous emission of a non-degenerate or a 2-fold degenerate pair which obey normal statistics. This shows that diagrams, and diagrammatic thinking, can be use in perturbatively analyzing paraparticle processes. The parastatistical factor associated with each diagram does require explicit calculation.

Controlling the Two-Photon-Induced Photon Cascade Emission in a Gd(3+)/Tb(3+)-Codoped Glass for Multicolor Display.

We reported the first observation of the two-photon-induced quantum cutting phenomenon in a Gd3+/Tb3+-codoped glass in which two photons at ~400 nm are simultaneously absorbed, leading to the cascade emission of three photons in the visible spectral region. The two-photon absorption induced by femtosecond laser pulses allows the excitation of the energy states in Gd3+ which are inactive for single-photon excitation and enables the observation of many new electric transitions which are invisible in the single-photon-induced luminescence. The competition between the two-photon-induced photon cascade emission and the single-photon-induced emission was manipulated to control the luminescence color of the glass. We demonstrated the change of the luminescence color from red to yellow and eventually to green by varying either the excitation wavelength or the excitation power density.

Spectral shaping of cascade emissions from multiplexed cold atomic ensembles

We investigate the spectral properties of the biphoton state from the cascade emissions of cold atomic ensembles, which composes of a telecommunication photon (signal) followed by an infrared one (idler) via four-wave mixing.\ With adiabatic conditions for Gaussian driving pulses of width $\tau$, the spectrum of the biphoton state has the form of a Gaussian that conserves signal and idler photon energies within $\hbar/\tau$ modulated by a Lorentzian with a superradiant linewidth.\ Multiplexing the atomic ensembles with frequency-shifted cascade emissions, we may manipulate and shape the spectrum of the biphoton state.\ The entropy of entanglement is derived from Schmidt decomposition, which can be larger if we multiplex the atomic ensembles in a way that conserves signal and idler photon central energies.\ The eigenvalues in Schmidt bases are degenerate in pairs for symmetric spectral shaping in which the mode probability densities show interference patterns.\ We also demonstrate the excess entropy of entanglement that comes from continuous frequency space, which scales up the total entropy.\ The scheme of multiplexed cascade-emitted biphoton state provides multimode structures that are useful in long-distance quantum communication and multimode quantum information processing.

Probing gamma-ray emissions ofFermi-LAT pulsars with a non-stationary outer gap model

We explore a non-stationary outer gap scenario for gamma-ray emission process in pulsar magnetosphere. Electrons/positrons that migrate along the magnetic field line and enter the outer gap from the outer/inner boundaries activate the pair-creation cascade and high-energy emission process. In our model, the rate of the particle injection at the gap boundaries is key physical quantity to control the gap structure and properties of the gamma-ray spectrum. Our model assumes that the injection rate is time variable and the observed gamma-ray spectrum are superposition of the emissions from different gap structures with different injection rates at the gap boundaries. The calculated spectrum superposed by assuming power law distribution of the particle injection rate can reproduce sub-exponential cut-off feature in the gamma-ray spectrum observed by Fermi-LAT. We fit the phase-averaged spectra for 43 young/middle-age pulsars and 14 millisecond pulsars with the model. Our results imply that (1) a larger particle injection at the gap boundaries is more frequent for the pulsar with a larger spin down power and (2) outer gap with an injection rate much smaller than the Goldreich-Julian value produces observe $>10$GeV emissions. Fermi-LAT gamma-ray pulsars show that (i) the observed gamma-ray spectrum below cut-off energy tends to be softer for the pulsar with a higher spin down rate and (ii) the second peak is more prominent in higher energy bands. Based on the results of the fitting, we describe possible theoretical interpretations for these observational properties. We also briefly discuss Crab-like millisecond pulsars that show phase-aligned radio and gamma-ray pulses.

Bethe-Heitler cascades as a plausible origin of hard spectra in distant TeV blazars

Context. Very high-energy (VHE) $\gamma$-ray measurements of distant TeV blazars can be nicely explained by TeV spectra induced by ultra high-energy cosmic rays. Aims. We develop a model for a plausible origin of hard spectra in distant TeV blazars. Methods. In the model, the TeV emission in distant TeV blazars is dominated by two mixed components. The first is the internal component with the photon energy around 1 TeV produced by inverse Compton scattering of the relativistic electrons on the synchrotron photons (SSC) with a correction for extragalactic background light absorbtion and the other is the external component with the photon energy more than 1 TeV produced by the cascade emission from high-energy protons propagating through intergalactic space. Results. Assuming suitable model parameters, we apply the model to observed spectra of distant TeV blazars of 1ES 0229+200. Our results show that 1) the observed spectrum properties of 1ES 0229+200, especially the TeV $\gamma$-ray tail of the observed spectra, could be reproduced in our model and 2) an expected TeV $\gamma$-ray spectrum with photon energy $>$1 TeV of 1ES 0229+200 should be comparable with the 50-hour sensitivity goal of the Cherenkov Telescope Array (CTA) and the differential sensitivity curve for the one-year observation with the Large High Altitude Air Shower Observatory (LHAASO). Conclusions. We argue that strong evidence for the Bethe-Heitler cascades along the line of sight as a plausible origin of hard spectra in distant TeV blazars could be obtained from VHE observations with CTA, LHAASO, HAWC, and HiSCORE.