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When a positron annihilates, two gamma photons are created with orthogonal polarizations. It is possible to use coincidence measurements where both photons undergo Compton scattering to estimate their initial relative polarization orientation. This information is of great interest in gamma imaging systems, such as Positron Emission Tomography, where it may be used as an additional tool to distinguish true coincidence events from scatter and random background. The successful utilization of this principle critically depends on the detector’s angular and energy resolution, which determine its polarimetric performance. In this study, we use Monte Carlo simulations based on the Geant4 toolkit to model two multi-pixel detector configurations identified as prospective for the measurement of gamma-ray polarization in PET. One is based on 2 mm × 2 mm × 20 mm LYSO scintillators and the other is based on 3 mm × 3 mm × 20 mm GAGG scintillators. Each configuration has a pair of modules, each consisting of 64 crystals set up in a single 8 × 8 matrix, where both the recoil electron and the Compton-scattered photon are absorbed. We simulate positron annihilation by generating two back-to-back gamma photons of 511 keV with orthogonal polarizations. The Compton scattering is successfully identified and the modulation of the azimuthal angle difference is clearly observed. The configuration based on GAGG crystals demonstrates slightly better polarimetric performance than the one based on LYSO crystals, reflected in the more pronounced azimuthal modulation. [ABSTRACT FROM AUTHOR]

Around 17:00 on January 12, 2020 (UTC), radiation detectors installed at two locations with a 1.35 km separation in Kanazawa City, Japan, captured a total of four gamma‐ray enhancements. The first pair was simultaneously observed at the two locations at 17:03 and were abruptly terminated by a lightning discharge. The remaining two enhancements were also nearly simultaneously observed ∼3 min later, and one of them was also terminated by another lightning discharge. At the last termination, a downward terrestrial gamma‐ray flash and a negative energetic in‐cloud pulse were observed. Both pairs were associated with thundercloud cells. In the first pair, simultaneous detection in two locations 1.35 km apart suggests either a gamma‐ray glow emerged in‐between and time variability of its intensity were directly observed or there were two (or more) gamma‐ray glows in the cell which reached the two detectors coincidentally. In the latter pair, the peak time in the downwind detector was ∼40 s later than that of the upwind detector. If the irradiation region moved with the cell, it would have taken ∼110 s. The discrepancy suggests either the glow moved 2.5 times faster than the cell or there were two (or more) glows in the cell. Also, the fact that the thunderstorm cell hosting the latter glows experienced the lightning discharge ∼3 min before suggests that the strong electric field in the cell can develop within a few minutes. Plain Language Summary: Thunderclouds sometime emit gamma rays with a duration of minutes, which are called "gamma‐ray glow". It is bremsstrahlung emission from high‐energy electrons accelerated in the cloud. At around 17:00 (UTC) January 12, 2020, two radiation detectors installed 1.35 km apart in Kanazawa City, Japan, captured a total of four count rate enhancements. The first two disappeared with a lightning discharge, and the last one with another lightning, in this case accompanied by an intense gamma ray flash called a "downward terrestrial gamma‐ray flash" (TGF). The overall behavior is similar to another gamma‐ray glow observed in the same location on January 2018 (e.g. Wada et al. 2019, https://doi.org/10.1038/s42005-019-0168-y). This is also the third report of gamma‐ray glow termination associated with both lightning discharges and TGFs. However, while the 2018 event can be explained well by assuming that the electron acceleration region moves with the thundercloud cell (monitored by rainfall map) and then terminated by a lightning discharge, the time profiles of the four current events cannot. To explain the behavior of the 2020 data, new models such as electron acceleration region(s) rapidly emerging in a thundercloud cell, and/or multiple electron acceleration regions co‐existing in a single cell, are discussed. Key Points: Four gamma‐ray enhancements were observed at two locations 1.35 km apart along the wind direction, and their time profiles were analyzedThe termination of three of them was also recorded. The last one was associated with a downward terrestrial gamma‐ray flash and a negative energetic in‐cloud pulseThe acceleration region of gamma‐ray glows in a thundercloud can develop within a few minutes after experiencing a discharge [ABSTRACT FROM AUTHOR]

The Terrestrial Gamma‐ray Flash (TGF) to lightning ratio, computed over the three tropical chimneys, presents a paradox: African thunderstorms produce the most lightning but yield the lowest fraction of TGFs when compared to American and Southeast Asian thunderstorms. To understand the physical insights into this asymmetry, Tropical Rainfall Measuring Mission Precipitation Radar measurements are used to depict the vertical precipitation structure and infer the vertical electrical charge fraction distribution of the observed thunderstorms in the three regions and the thunderstorms during TGF occurrences detected by the AGILE, Fermi‐GBM, and RHESSI sensors. Regional differences show that African thunderstorms are taller, smaller, and have a higher concentration of denser ice particles (graupel and or hail) above the freezing level in addition to having more lightning flashes per thunderstorm. The overall TGF‐related thunderstorms are taller, more intense (0.5–1.5 dBZ) and present similar radar reflectivity decay with height independent of the region. The two dimensional precipitation vertical distribution diagrams indicate that TGF thunderstorms develop to a mature stage. Independent of the region, thunderstorms show a midlevel negative charge center varying from 4.6 to 8.1 km in height and an upper level positive charge center ranging from 7.4 to 14.6 km. TGF thunderstorms have thicker positive inferred charge layer and present larger vertical distances between the opposite charging layers in comparison with the overall thunderstorm population, here defined as Climatology. African TGF thunderstorms have higher fraction of positive charges near the negative layer, helping to produce more and shorter lightning discharges. Plain Language Summary: Terrestrial Gamma ray Flashes (TGFs) have been detected in space above thunderstorms worldwide, but their main triggering mechanism has not yet been fully understood. In the last two decades, several measurements from ground and satellite instruments have provided information to help our understanding. For instance, TGF emissions are concentrated in the tropics and one of the possible theories suggests to be related to intense intracloud lightning processes produced by thunderstorms that have considerable vertical charge center separations. Although African thunderstorms have the highest lightning activity in the world, they produce fewer TGFs per number of lightning flashes than thunderstorms in America and the Southeast Asia tropical regions, posing a paradox. To explore this asymmetry, this study employed 3D measurements from the Precipitation Radar on board the Tropical Rainfall Measuring Mission (TRMM) satellite to describe the main precipitation vertical structure observed in the three most important tropical thunderstorm regions in the world (Climatology) and in the thunderstorms that produced TGFs. African thunderstorms are taller, smaller, having more ice aloft and lightning discharges. TGF thunderstorms show similar precipitation variation decay with height, are taller and more intense, in addition to showing different vertical cloud charge structure and larger distance between the opposite charge layers. Key Points: Terrestrial Gamma‐ray Flash (TGF) thunderstorms have greater distance between the inferred electrical opposite charging layers in relation to the ClimatologyAfrican TGF thunderstorms have more positive charges close to the negative charging layerVertical profiles of TGF‐related thunderstorms are associated with developing to mature stage [ABSTRACT FROM AUTHOR]

We develop a new method of extracting simultaneous measurements of weak lensing shear and a local rotation of the plane of polarization using observations of resolved radio sources. We show that the direction of polarization is statistically linked with that of the gradient of the total intensity field, and this provides the basis of our method. Using a number of sources spread over the sky, this method allows constraints to be placed on cosmic shear and birefringence, and it can be applied to any resolved radio sources for which such a correlation exists. Assuming that the rotation and shear are constant across the source, we use this relationship to construct a quadratic estimator and investigate its properties using simulated observations. We develop a calibration scheme using simulations based on the observed images to mitigate a bias that occurs in the presence of measurement errors and an astrophysical scatter on the polarization. The method is applied directly to archival data of radio galaxies where we measure a mean rotation signal of 〈ω〉 = -2. 03 ± 0:75 and an average shear compatible with zero using 30 reliable sources. This level of constraint on an overall rotation is comparable with current leading constraints from cosmic microwave background experiments and is expected to increase by at least an order of magnitude with future high precision radio surveys, such as those performed by the Square Kilometer Array (SKA). We also measure the shear and rotation two-point correlation functions and estimate the number of sources required to detect shear and rotation correlations in future surveys. [ABSTRACT FROM AUTHOR]

We analyse the performance of a gas time projection chamber (TPC) as a high-performance gamma-ray telescope and polarimeter in the e + e − pair-creation regime. We use data collected at a gamma-ray beam of known polarisation. The TPC provides two orthogonal projections ( x, z ) and ( y, z ) of the tracks induced by each conversion in the gas volume. We use a simple vertex finder in which vertices and pseudo-tracks exiting from them are identified. We study the various contributions to the single-photon angular resolution using Monte Carlo simulations, compare them with the experimental data and find that they are in excellent agreement. The distribution of the azimuthal angle of pair conversions shows a bias due to the non-cylindrical-symmetric structure of the detector. This bias would average out for a long duration exposure on a space mission, but for this pencil-beam characterisation we have ensured its accurate simulation by a double systematics-control scheme, data taking with the detector rotated at several angles with respect to the beam polarisation direction and systematics control with a non-polarised beam. We measure, for the first time, the polarisation asymmetry of a linearly polarised gamma-ray beam in the low energy pair-creation regime. This sub-GeV energy range is critical for cosmic sources as their spectra are power laws which fall quickly as a function of increasing energy. This work could pave the way to extending polarised gamma-ray astronomy beyond the MeV energy regime. [ABSTRACT FROM AUTHOR]

The excitation functions for the reactions 14 N(p,p′γ) 14 N, 28 Si(p,p′γ) 28 Si and 29 Si(p,p′γ) 29 Si were measured at an angle of 55° by bombarding a thin Si 3 N 4 target with protons in the energy range of 3.6–6.9 MeV. The deduced γ-ray production cross section data is compared with available literature data relevant for ion beam analytical work. Thick-target γ-ray yields for boron, nitrogen and silicon were measured at 4.0, 4.5, 5.0, 5.5, 6.0 and 6.5 MeV proton energies utilizing thick BN and Si 3 N 4 targets. The measured yield values are put together with available yield data found in the literature. The experimental yield data has been used to cross-check the γ-ray production cross section values by comparing them with calculated thick-target yields deduced from the present and literature experimental excitation curves. All values were found to be in reasonable agreement taking into account the experimental uncertainties. [ABSTRACT FROM AUTHOR]

We study a Dirac dark matter particle interacting with ordinary matter via the exchange of a light pseudoscalar, and analyze its impact on both direct and indirect detection experiments. We show that this candidate can accommodate the long-standing DAMA modulated signal and yet be compatible with all exclusion limits at 99s% C.L. This result holds for natural choices of the pseudoscalar-quark couplings (e.g., flavor universal), which give rise to a significant enhancement of the dark matter-proton coupling with respect to the coupling to neutrons. We alsofind that this candidate can accommodate the observed 1-3 GeV gamma-ray excess at the Galactic center and at the same time have the correct relic density today. The model could be tested with measurements of rare meson decays, flavor changing processes, and searches for axionlike particles with mass in the MeV range. [ABSTRACT FROM AUTHOR]

The article review the experimental setup which is used to measure Compton scattering cross sections at high intensity gamma-ray source and cross sections for Lithium isotopes to nucleon isoscalar polarizabilities. Topics discussed include data collection using a quasimonoenergetic 86-mega electron volt (MeV) photon beam, production of gamma-ray beams through Compton backscattering using free-electron laser by high intensity gamma-ray source, and analysis of the setup by phenomenological model.

The differential cross section of electron inelastic scattering by nuclei followed by γ radiation is calculated using the multipole decomposition of the hadronic currents and by taking into account the longitudinal polarization of the initial electron and the circular polarization of the γ radiation.We performed the analysis of the angular and energy dependence of the degree of electron and photon polarization which can yield information on values of weak neutral currents parameters. [ABSTRACT FROM AUTHOR]

The polarization measurement is an important tool to probe the prompt emission mechanism in gamma-ray bursts (GRBs). The synchrotron photons can be scattered by cold electrons in the outflow via Compton scattering (CS) processes. The observed polarization depends on both the photon energy and the viewing angle. With the typicalbulk Lorentz factor Γ ∼ 200, photons with energy E > 10 MeV tend to have smaller polarization than photons with energy E < 1 MeV. At the right viewing angle, i.e., θ ∼ Γ−1, the polarization achieves its maximal value, and the polarization angle changes 90◦ relative to the initial polarization direction. Thus, the synchrotron radiation plus CS model can naturally explain the 90◦ change of the polarization angle in GRB 100826A. [ABSTRACT FROM AUTHOR]

The results of dual-frequency polarization observations of 12 gamma-ray quasars are presented (as a continuation of a study of six blazars carried out earlier). The observations were obtained with the American Very Long Baseline Array. The distributions of intensity and polarization were obtained at 5 and 15 GHz. The degrees of polarization in the cores and jets of the 18 gamma-ray quasars do not stand out from those of other quasars. The brightness temperatures of the core components do not strongly exceed 10 K. [ABSTRACT FROM AUTHOR]

To date in astrophysics, X- and gamma-ray source emissions have been studied almost exclusively through imaging techniques, spectral- and timing-variability analysis. However, often two or more distinct models potentially describe the same observations of a particular celestial-radiation source. By measuring the angle and degree of linear polarization of the detected radiation, it is increased by two the number of observational parameters thereby allowing a better discrimination between different models. Therefore, future \gamma-ray-telescope instrumentation should include the capability to perform highly sensitive polarimetric measurements. This idea has recently become even more important from the perspective of new very sensitive hard X- and soft gamma-ray focusing missions based on Laue lenses such as the Gamma-Ray Imager and DUAL missions, submitted to ESA Cosmic Vision calls. In order to optimize the design of a gamma-ray telescope focal plane, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under a \sim100\\% polarized gamma-ray beam. The tested detector was a 5 mm thick CdZnTe array with segmented anodes into 16\,\times\,16 pixels of 2.5\,\times\,2.5 mm^2. The aim of these tests was to study the polarimetric performance of this prototype as a function of the polarization state of the synchrotron beam. In particular, we determined the sensitivity of the detector to the level of the beam polarization as well as its potential to determine the polarization angle orientation by performing measurements at different polarization direction angles. In both tests the CdZnTe prototype showed a good linearity in its response and good sensitivity to polarization levels down to 50% and a polarization angular resolution better than 5^\circ. [ABSTRACT FROM AUTHOR]

Abstract: Measurement of circular polarization of gamma-rays with position sensitive solid state detectors is proposed. It is based on the transfer of the photon spin to the recoiled electron in the process of Compton scattering and subsequent detection of the electron spin polarization. Bremsstrahlung polarization correlations, in particular the left–right asymmetry of the photon emission from transversally polarized electrons, are used for this purpose. Compton scattering and electron bremsstrahlung events as well as the scattered and the bremsstrahlung photon absorptions take place inside the active volume of the position sensitive detector. These events are identified and their complete kinematics is reconstructed by means of gamma-ray tracking. No spin polarized targets and no inactive materials are needed for this technique. The proposed method is naturally integrated into the concept of a Compton telescope, allowing for the first time to build an imaging gamma-ray circular polarimeter. It should work in the energy region of several 100keV up to several 10MeV. [Copyright &y& Elsevier]

We presented the results of last years GRB observations obtained on the MASTER robotic telescope, which is the only telescope of its kind in Russia. These results include 5 prompt observations of GRB in 2008 and 2009, follow-up observations of 15 other GRBs in 2008-2009, the first observations in different polarization angles of optical emission from the gamma-ray bursts GRB091020, and observations in different polarization angles for GRB091127 and GRB090820. [ABSTRACT FROM AUTHOR]

We investigate the potential of γ γ collisions to probe scalar unparticle couplings via top–antitop quark pair production. We find 95% confidence level limits on the unparticle couplings with an integrated luminosity of 500 fb−1 and an energy of $\sqrt{s}=1$ TeV. We investigate the effect of the top quark spin polarization on the unparticle couplings. It is shown that spin polarization of the top quark leads to a significant improvement in the sensitivity limits. [ABSTRACT FROM AUTHOR]

A possibility to measure the energy and polarization distributions of high intensity γ-beams is considered. This possibility is based on measurements of the number of e±-pairs in such media, as laser waves and single crystals. The method may be useful for future γγ and eγ-colliders. [Copyright &y& Elsevier]

This paper reports the results of the first measurements of the differential cross section of the 80Se(γ,n)79Se reaction with a linearly polarized gamma-ray (γ-ray) beam. The cross section was measured at three incident γ-ray beam energies: 15.6, 15.8, and 16.0 MeV, with a beam energy spread of 3.0% full width at half-maximum (FWHM). The differential cross section for the excitation spectrum in 79Se was measured at two scattering angles in the plane of the beam polarization: θ=90° and 135°, and at one angle in the plane perpendicular to the plane of polarization: θ=90°. The total photoneutron cross sections determined from these data are between 0.8 and 1.3 standard deviations smaller than previously published results. The excitation spectra measured in this work were fit with a Hauser-Feshbach model. Better fits to the data were obtained with a constant-temperature formulation of the nuclear level density (NLD) than with a Fermi-gas NLD model. The parameters for the constant-temperature NLD model obtained in this work are consistent with those obtained for medium-mass nuclei in previous studies. [ABSTRACT FROM AUTHOR]

We present optical photometric and spectroscopic observations of the likely stellar counterpart to the unassociated Fermi Large Area Telescope (LAT) γ-ray source 2FGL J0846.0+2820, selected for study based on positional coincidences of optical variables with unassociated LAT sources. Using optical spectroscopy from the SOAR telescope, we have identified a late-G giant in an eccentric (e = 0.06) 8.133-day orbit with an invisible primary. Modeling the spectroscopy and photometry together led us to infer a heavy neutron star primary of and a partially stripped giant secondary of . Hα emission is observed in some of the spectra, perhaps consistent with the presence of a faint accretion disk. We find that the γ-ray flux of 2FGL J0846.0+2820 dropped substantially in mid-2009, accompanied by an increased variation in the optical brightness, and since then, it has not been detected by Fermi. The long period and giant secondary are reminiscent of the γ-ray bright binary 1FGL J1417.7–4407, which hosts a millisecond pulsar (MSP) apparently in the final stages of the pulsar recycling process. The discovery of 2FGL J0846.0+2820 suggests the identification of a new subclass of MSP binaries that are the likely progenitors of typical field MSPs. [ABSTRACT FROM AUTHOR]

We investigate the electrostatic acceleration of electrons and positrons in the vicinity of the event horizon, applying the pulsar outer-gap model to black hole (BH) magnetospheres. During a low accretion phase, the radiatively inefficient accretion flow (RIAF) cannot emit enough MeV photons that are needed to sustain the force-free magnetosphere via two-photon collisions. In such a charge-starved region (or a gap), an electric field arises along the magnetic field lines to accelerate electrons and positrons into ultra-relativistic energies. These relativistic leptons emit copious gamma rays via curvature and inverse-Compton (IC) processes. Some of such gamma rays collide with the submillimeter-IR photons emitted from the RIAF to materialize as pairs, which polarize to partially screen the original acceleration electric field. It is found that the gap gamma-ray luminosity increases with decreasing accretion rate. However, if the accretion rate decreases too much, the diminished RIAF soft photon field can no longer sustain a stationary pair production within the gap. As long as a stationary gap is formed, the magnetosphere becomes force-free outside the gap by the cascaded pairs, irrespective of the BH mass. If a nearby stellar-mass BH is in quiescence, or if a galactic intermediate-mass BH is in a very low accretion state, its curvature and IC emissions are found to be detectable with Fermi/LAT and imaging atmospheric Cherenkov telescopes (IACT). If a low-luminosity active galactic nucleus is located within about 30 Mpc, the IC emission from its supermassive BH is marginally detectable with IACT. [ABSTRACT FROM AUTHOR]

Reports that the detection of polarization of the γ-rays provides insight into the mechanism driving these powerful explosions. Creation of energy released by the explosions by gravitational collapse of the star to form black hole; Synchrotron emission responsible for gamma ray production; Origination of a strong field near the collapsing object; Occurrence of polarization in randomly oriented magnetic field.

The high polarization observed in the prompt phase of some gamma-ray bursts invites extensive study of the emission mechanism. In this paper, we investigate the polarization properties of the synchrotron self-Compton (SSC) process from a highly relativistic jet. A magnetic-dominated, baryon-loaded jet ejected from the central engine travels with a large Lorentz factor. Shells with slightly different velocities collide with each other and produce shocks. The shocks accelerate electrons to a power-law distribution and, at the same time, magnify the magnetic field. Electrons move in the magnetic field and produce synchrotron photons. Synchrotron photons suffer from the Compton scattering (CS) process and then are detected by an observer located slightly off-axis. We analytically derive the formulae of photon polarization in the SSC process in two magnetic configurations: a magnetic field in the shock plane and perpendicular to the shock plane. We show that photons induced by the SSC process can be highly polarized, with the maximum polarization Π ∼ 24% in the energy band [0.5, 5] MeV. The polarization depends on the viewing angles, peaking in the plane perpendicular to the magnetic field. In the energy band [0.05, 0.5] MeV, in which most γ-ray polarimeters are active, the polarization is about twice that in the Thomson limit, reaching Π ∼ 20%. This implies that the Klein-Nishina effect, which is often neglected in the literature, should be carefully considered. [ABSTRACT FROM AUTHOR]

Gamma rays from the ?-ray burst (GRB) 021206 have been reported to be strongly linearly polarized, with the estimated degree of polarization (80 ± 20%) being close to the absolute maximum of 100% - affording us the opportunity to constrain models of quantum gravity, which has had 1010 years to act on the photons as they travelled towards us. Here I show that if the effects of quantum gravity are linearly proportional to the ratio of the photon energy to the characteristic scale energy of quantum gravity, then the polarization of photons with energies of about 0.1 MeV should be completely random, contrary to what is observed. I conclude that, should the polarization measurement be confirmed, quantum gravity effects act with a power that is greater than linearity, or that loop quantum gravity is not viable. Compared with previous methods and results (see ref. 2, for example), testing of the linear polarization of cosmic ?-ray bursts may substantially extend the observational window on the theory of quantum gravity. [ABSTRACT FROM AUTHOR]

Discusses ways on how gamma rays are produced from a free-electron lasers (FEL). Expected intensity of the gamma beam; Usage of the beam; Other facilities that use Compton backscattering in free-electron lasers (FELs) to produce gammas.