Gamma-ray Continuum Research Articles

Sommerfeld effect for continuum gamma-ray spectra from Dark Matter annihilation

We present a calculation of the continuum part of the gamma-ray spectra resulting from Dark Matter annihilation in the framework of the MSSM taking into account Sommerfeld effects. Concentrating on pure wino and pure higgsino scenarios we compare our calculation to existing work and explore the numerical impact of the features not captured by previous approximative descriptions. We find that, in particular for large neutralino masses, when the Sommerfeld enhancement is very large, chargino-antichargino annihilation processes, which have not been considered before, lead to sizable differences with respect to existing calculations. In scenarios with neutralinos in the intermediate-mass range, we find that the role of the charginos is crucial in the endpoint regime. Our calculation provides the currently most accurate prediction for the continuum gamma-ray spectra.

Gamma Rays as Probes of Cosmic-Ray Propagation and Interactions in Galaxies

Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in unprecedented detail the properties of galactic cosmic rays. A variety of scales and environments are now accessible to us, from the local interstellar medium near the Sun and the vicinity of cosmic-ray accelerators, out to the Milky Way at large and beyond, with a growing number of gamma-ray emitting star-forming galaxies. Gamma-ray observations have been pushing forward our understanding of the life cycle of cosmic rays in galaxies and, combined with advances in related domains, they have been challenging standard assumptions in the field and have spurred new developments in modelling approaches and data analysis methods. We provide a review of the status of the subject and discuss perspectives on future progress.

FLUKA Simulations of Pion Decay Gamma-Radiation from Energetic Flare Ions

Gamma-ray continuum at > 10 MeV photon energy yields information on gtrsim 0.2 – 0.3 GeV/nucleon ions at the Sun. We use the general-purpose Monte Carlo code FLUktuierenden KAskade (FLUKA) to model the transport of ions injected into thick and thin target sources, the nuclear processes that give rise to pions and other secondaries and the escape of the resulting photons from the atmosphere. We give examples of photon spectra calculated with a range of different assumptions about the primary ion velocity distribution and the source region. We show that FLUKA gives results for pion decay photon emissivity in agreement with previous treatments. Through the directionality of secondary products, as well as Compton scattering and pair production of photons prior to escaping the Sun, the predicted spectrum depends significantly on the viewing angle. Details of the photon spectrum in the approx 100 MeV range may constrain the angular distribution of primary ions and the depths at which they interact. We display a set of thick-target spectra produced making various assumptions about the incident ion energy and angular distribution and the viewing angle. If ions are very strongly beamed downward, or ion energies do not extend much above 1 GeV/nucleon, the photon spectrum is highly insensitive to details of the ion distribution. Under the simplest assumptions, flares observed near disc centre should not display significant radiation above 1 GeV photon energy. We give an example application to Fermi Large Area Telescope data from the flare of 12 June 2010.

Anomaly-free dark matter models

We investigate the predictions of anomaly-free dark matter models for direct and indirect detection experiments. We focus on gauge theories where the existence of a fermionic dark matter candidate is predicted by anomaly cancellation, its mass is defined by the new symmetry breaking scale, and its stability is guaranteed by a remnant symmetry after the breaking of the gauge symmetry. We find an upper bound on the symmetry breaking scale by applying the relic density and perturbative constraints. The anomaly-free property of the theories allows us to perform a full study of the gamma lines from dark matter annihilation. We investigate the correlation between predictions for final radiation processes and gamma lines. Furthermore, we demonstrate that the latter can be distinguished from the continuum gamma ray spectrum.

Effect of Gd2O3 addition on mechanical, thermal and shielding properties of Al2O3 ceramics

Gd2O3/Al2O3 ceramic composites with different weight percent of Gd2O3 were prepared by adopting a pressureless sintering approach. The effect of Gd2O3 addition on the phase composition, sintering property, microstructure, bending strength, thermal conductivity and shielding property of the composites was investigated. The results show that the properties of ceramic can be improved greatly by introducing a small amount of Gd2O3, owing to the promotion of reaction between MgO, SiO2 and Al2O3 to produce spinel and forsterite. Compared with ordinary Al2O3 ceramic, the bending strength and thermal conductivity coefficient of Gd2O3/Al2O3 composite with 5 wt% Gd2O3 have increased by 38.03 MPa and 8.95 W/(m K), respectively. When the amount of Gd2O3 surpasses 15 wt%, however, the performances of the composite would become worse for excessive new phases formed in the composite hindering its densification. In addition, screening ratio of the composites for Co-60 gamma rays radiation increases with the addition of Gd2O3. And the Gd2O3/Al2O3 composites also exhibit good mechanical stability when exposed to continuous gamma rays irradiation.

SUSY under siege from direct and indirect WIMP detection experiments

We examine updated prospects for detecting WIMPs in supersymmetric models via direct and indirect dark matter search experiments. We examine several historical and also still viable scenarios: projections for well-tempered neutralinos (WTN), projections from the MasterCode (MC), BayesFits (BF) and Fittino (FO) collaborations, non-thermal wino dark matter (NThW) and finally mixed axion-higgsino dark matter from SUSY with radiatively-driven naturalness (RNS). The WTN is ruled out by recent limits from XENON and LUX collaborations. The NThW scenario, previously on tenuous ground due to gamma-line searches, appears also ruled out by recent combined Fermi-LAT/MAGIC limits combined with new HESS results from continuum gamma rays. Substantial portions of MC parameter space and 1 TeV higgsino parameter space from BF group are ruled out. The 100-300 GeV higgsino-like WIMP from RNS survives due to its possible depleted local abundance (where the axion may make up the bulk of dark matter). Projections from ton-scale noble liquid detectors should discover or rule out WIMPs from the remaining parameter space of these surviving models.

Enhanced lines and box-shaped features in the gamma-ray spectrum from annihilating dark matter in the NMSSM

We study spectral features in the gamma-ray emission from dark matter (DM) annihilation in the Next-to-Minimal Supersymmetric Standard Model (NMSSM), with either neutralino or right-handed (RH) sneutrino DM . We perform a series of scans over the NMSSM parameter space, compute the DM annihilation cross section into two photons and the contribution of box-shaped features, and compare them with the limits derived from the Fermi-LAT search for gamma-ray lines using the latest Pass 8 data. We implement the LHC bounds on the Higgs sector and on the masses of supersymmetric particles as well as the constraints on low-energy observables. We also consider the recent upper limits from the Fermi-LAT satellite on the continuum gamma-ray emission from dwarf spheroidal galaxies (dSphs). We show that in the case of the RH sneutrino the constraint on gamma-ray spectral features can be more stringent than the dSph bounds. This is due to the Breit-Wigner enhancement near the ubiquitous resonances with a CP even Higgs and the contribution of scalar and pseudoscalar Higgs final states to box-shaped features. By contrast, for neutralino DM, the di-photon final state is only enhanced in the resonance with a Z boson and box-shaped features are even more suppressed. Therefore, the observation of spectral features could constitute a discriminating factor between both models. In addition, we compare our results with direct DM searches, including the SuperCDMS and LUX limits on the elastic DM-nucleus scattering cross section and show that some of these scenarios would be accessible to next generation experiments. Thus, our findings strengthen the idea of complementarity among distinct DM search strategies.

Cobalt-56 γ-ray emission lines from the typeIa supernova 2014J.

A typeIa supernova is thought to be a thermonuclear explosion of either a single carbon-oxygen white dwarf or a pair of merging white dwarfs. The explosion fuses a large amount of radioactive (56)Ni (refs 1-3). After the explosion, the decay chain from (56)Ni to (56)Co to (56)Fe generates γ-ray photons, which are reprocessed in the expanding ejecta and give rise to powerful optical emission. Here we report the detection of (56)Co lines at energies of 847 and 1,238kiloelectronvolts and a γ-ray continuum in the 200-400kiloelectronvolt band from the typeIa supernova 2014J in the nearby galaxy M82. The line fluxes suggest that about 0.6±0.1solar masses of radioactive (56)Ni were synthesized during the explosion. The line broadening gives a characteristic mass-weighted ejecta expansion velocity of 10,000±3,000kilometres per second. The observed γ-ray properties are in broad agreement with the canonical model of an explosion of a white dwarf just massive enough to be unstable to gravitational collapse, but do not exclude merger scenarios that fuse comparable amounts of (56)Ni.

A TIME-RESOLVED STUDY OF THE BROAD-LINE REGION IN BLAZAR 3C 454.3

We present multi-epoch optical observations of the blazar 3C 454.3 (z = 0.859) from 2008 August through 2011 December, using the SMARTS Consortium 1.5m+RCSpectrograph and 1.3m+ANDICAM in Cerro Tololo, Chile. The spectra reveal that the broad optical emission lines Mg II, H-beta and H-gamma are far less variable than the optical or gamma-ray continuum. Although, the gamma-rays varied by a factor of 100 above the EGRET era flux, the lines generally vary by a factor of 2 or less. Smaller variations in the gamma-ray flux did not produce significant variation in any of the observed emission lines. Therefore, to first order, the ionizing flux from the disk changes only slowly during large variations of the jet. However, two exceptions in the response of the broad emission lines are reported during the largest gamma-ray flares in 2009 December and 2010 November, when significant deviations from the mean line flux in H-gamma and Mg II were observed. H-gamma showed a maximum 3-sigma and 4-sigma deviation in each flare, respectively, corresponding to a factor of 1.7 and 2.5 increase in flux. Mg II showed a 2-sigma deviation in both flares; no variation was detected in H-beta during either flare. These significant deviations from the mean line flux also coincide with 7mm core ejections reported previously (Jorstad et al. 2012). The correlation of the increased emission line flux with mm core ejections, and gamma-ray, optical and UV flares suggests that the broad line region extends beyond the gamma-emitting region during the 2009 and 2010 flares.

Fermi LARGE AREA TELESCOPE DETECTION OF TWO VERY-HIGH-ENERGY ( E > 100 GeV) γ-RAY PHOTONS FROM THE z = 1.1 BLAZAR PKS 0426–380

We report the Fermi Large Area Telescope (LAT) detection of two very-high-energy (VHE, E>100 GeV) gamma-ray photons from the directional vicinity of the distant (redshift, z = 1.1) blazar PKS 0426-380. The null hypothesis that both the 134 and 122 GeV photons originate from unrelated sources can be rejected at the 5.5 sigma confidence level. We therefore claim that at least one of the two VHE photons is securely associated with the blazar, making PKS 0426-380 the most distant VHE emitter known to date. The results are in agreement with the most recent Fermi-LAT constraints on the Extragalactic Background Light (EBL) intensity, which imply a $z \simeq 1$ horizon for $\simeq$ 100 GeV photons. The LAT detection of the two VHE gamma-rays coincided roughly with flaring states of the source, although we did not find an exact correspondence between the VHE photon arrival times and the flux maxima at lower gamma-ray energies. Modeling the gamma-ray continuum of PKS 0426-380 with daily bins revealed a significant spectral hardening around the time of detection of the first VHE event (LAT photon index \Gamma\ $\simeq$ 1.4) but on the other hand no pronounced spectral changes near the detection time of the second one. This combination implies a rather complex variability pattern of the source in gamma rays during the flaring epochs. An additional flat component is possibly present above several tens of GeV in the EBL-corrected Fermi-LAT spectrum accumulated over the ~8-month high state.

Gamma rays from top-mediated dark matter annihilations

Lines in the energy spectrum of gamma rays are a fascinating experimental signal,which are often considered ''smoking gun" evidence of dark matter annihilation.The current generation of gamma ray observatories are currently closing in onparameter space of great interest in the context of dark matter which is a thermalrelic. We consider theories in which the dark matter's primary connectionto the Standard Model is via the top quark, realizing strong gamma ray linesconsistent with a thermal relic through the forbidden channel mechanismproposed in the Higgs in Space Model.We consider realistic UV-completions of the Higgs in Space and relatedtheories, and show that a rich structure of observable gamma ray linesis consistent with a thermal relic as well as constraints from dark mattersearches and the LHC. Particular attention is paid to theone loop contributions to the continuum gamma rays, which can easilyswamp the line signals in some cases, and have been largely overlookedin previous literature.

Gamma-ray line from radiative decay of gravitino dark matter

We study radiative decay of gravitino dark matter with trilinear R-parity violations. We show that the branching ratio of the decay of gravitino into monochromatic photon can be large enough to explain the observed gamma-ray line from the Galactic centre in the Fermi-LAT data without producing too much continuum gamma-ray and anti-proton flux. This scenario is realized when the mass of sfermions and the trilinear R-parity violating coupling are O(1–10) TeV and O(10−7–10−6) respectively.

Two-loop radiative seesaw mechanism with multicomponent dark matter explaining the possibleγexcess in the Higgs boson decay and at the Fermi LAT

A non-supersymmetric model of a two-loop radiative seesaw is proposed. The model contains, in addition to the standard model (SM) Higgs boson, an inert SU(2)_L doublet scalar eta and two inert singlet scalars phi and chi. The lepton number is softly broken by a dimension-two operator, and the tree-level Dirac mass is forbidden by Z_2 x Z_2' (or D_{2N}), which predicts the existence of two or three dark matter particles. The scalar sector is minimal; none of the scalar fields can be suppressed for the radiative seesaw mechanism to work. There are by-products: The SM Higgs boson decay into two gamma's is slightly enhanced by eta^+ (the charged component of eta) circulating in one-loop diagrams for h to gamma gamma. The 135 GeV gamma-ray line observed at the Fermi LAT can be also explained by the annihilation of chi dark matter. We employ a mechanism of temperature-dependent annihilation cross section to suppress the continuum gamma rays and the production of antiprotons. The explanation can survive even down to the XENON1T sensitivity limit.

130 GeV gamma-ray line and generic dark matter model building constraints from continuum gamma rays, radio, and antiproton data

An analysis of the Fermi gamma-ray space telescope data has recently revealed a resolved gamma-ray feature close to the galactic center which is consistent with monochromatic photons at an energy of about 130 GeV. If interpreted in terms of dark matter (DM) annihilating into $\ensuremath{\gamma}\ensuremath{\gamma}$ ($\ensuremath{\gamma}Z$, $\ensuremath{\gamma}h$), this would correspond to a DM particle mass of roughly 130 GeV (145 GeV, 155 GeV). The rate for these loop-suppressed processes, however, is larger than typically expected for thermally produced DM. Correspondingly, one would generically expect even larger tree-level production rates of standard model fermions or gauge bosons. Here, we quantify this expectation in a rather model-independent way by relating the tree level and loop amplitudes with the help of the optical theorem. As an application, we consider bounds from continuum gamma rays, radio, and antiproton data on the tree-level amplitudes and translate them into constraints on the loop amplitudes. We find that, independently of the DM production mechanism, any DM model aiming at explaining the line signal in terms of charged standard model particles running in the loop is in rather strong tension with at least one of these constraints, with the exception of loops dominated by top quarks. We stress that attempts to explain the 130 GeV feature with internal bremsstrahlung do not suffer from such difficulties.

Axino dark matter with R-parity violation and 130 GeV gamma-ray line

We show that decaying axino dark matter with R-parity violation can explain the observed excess of the 130 GeV gamma-ray line from the Galactic center in the Fermi data. The branching fraction of the axino decay into monochromatic photons can be O(1), and constraints from continuum gamma-rays and the anti-proton flux are ameliorated. The Pecce–Quinn scale of O(1013–1014) GeV and the R-parity violation parameter of O(10−12–10−11) are cosmologically favored.

Importance of thermal reactivity for hexamethylenetetramine formation from simulated interstellar ices

Recent observations of high ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray component. Supposing that this component is made of nuclei, two models for the origin of such particles are explored and low-energy cosmic-ray spectra are calculated which, added to the standard cosmic ray spectra, produce the observed ionization rates. The clearest evidence of the presence of such low-energy nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the interstellar medium would be a detection of nuclear \gamma-ray line emission in the range E_ 0.1 - 10 MeV, which is strongly produced in their collisions with the interstellar gas and dust. Using a recent \gamma-ray cross section compilation for nuclear collisions, \gamma-ray line emission spectra are calculated alongside with the high-energy \gamma-ray emission due to {\pi} 0 decay, the latter providing normalization of the absolute fluxes by comparison with Fermi-LAT observations of the diffuse emission above E \gamma = 0.1 GeV. Our predicted fluxes of strong nuclear \gamma-ray lines from the inner Galaxy are well below the detection sensitivies of INTEGRAL, but a detection, especially of the 4.4-MeV line, seems possible with new-generation \gamma-ray telescopes based on available technology. We predict also strong \gamma-ray continuum emission in the 1-8 MeV range, which in a large part of our model space for low-energy cosmic rays exceeds considerably estimated instrument sensitivities of future telescopes.

From gamma ray line signals of dark matter to the LHC

We explore the relationship between astrophysical gamma-ray signals and LHC signatures for a class of phenomenologically successful secluded dark matter models, motivated by recent evidence for a ~130 GeV gamma-ray line. We consider in detail scenarios in which interactions between the dark sector and the standard model are mediated by a vev-less scalar field \phi, transforming as an N-plet (N > 3) under SU(2)_L. Since some of the component fields of \phi carry large electric charges, loop induced dark matter annihilation to \gamma \gamma and \gamma Z can be enhanced without the need for non-perturbatively large couplings, and without overproduction of continuum gamma-rays from other final states. We discuss prospects for other experimental tests, including dark matter-nucleon scattering and production of \phi at the LHC, where future searches for anomalous charged tracks may be sensitive. The first LHC hints could come from the Higgs sector, where loop corrections involving \phi lead to significantly modified h to \gamma \gamma and h to \gamma Z branching ratios.

Development of a quasi-monoenergetic 6 MeV Gamma Facility at NASA Goddard Space Flight Center

The 6MeV Gamma Facility has been developed at NASA Goddard Space Flight Center (GSFC) to allow in-house characterization and testing of a wide range of gamma-ray instruments such as pixelated CdZnTe detectors for planetary science and Compton and pair-production imaging telescopes for astrophysics. The 6MeV Gamma Facility utilizes a circulating flow of water irradiated by 14MeV neutrons to produce gamma rays via neutron capture on oxygen (16O(n,p)16N→16O⁎→16O+γ). The facility provides a low cost, in-house source of 2.742, 6.129 and 7.117MeV gamma rays, near the lower energy range of most accelerators and well above the 2.614 MeV line from the 228Th decay chain, the highest energy gamma ray available from a natural radionuclide. The 7.13s half-life of the 16N decay allows the water to be irradiated on one side of a large granite block and pumped to the opposite side to decay. Separating the irradiation and decay regions allows for shielding material, the granite block, to be placed between them, thus reducing the low-energy gamma-ray continuum. Comparison between high purity germanium (HPGe) spectra from the facility and a manufactured source, 238Pu/13C, shows that the low-energy continuum from the facility is reduced by a factor ∼30 and the gamma-ray rate is ∼100 times higher at 6.129MeV.

Constraints on the dark matter annihilation scenario ofFermi 130 GeV gamma-ray line emission by continuous gamma-rays,Milky Way halo, galaxy clusters and dwarf galaxies observations

It was recently reported that there may exist monochromatic γ-ray emission at ∼ 130 GeV from the Galactic center in the Fermi Large Area Telescope data, which might be related with dark matter (DM) annihilation. In this work we carry out a comprehensive check of consistency of the results with the DM annihilation scenario, using the 3.7 yrs Fermi observation of the inner Galaxy, Galactic halo, clusters of galaxies and dwarf galaxies. The results found are as follows. 1) Very strong constraints on the DM annihilation into continuous γ-rays from the Galactic center are set, which are as stringent as the “natural” scale assuming thermal freeze-out of DM. Such limit sets strong constraint on the DM models to explain the line emission. 2) No line emission from the Galactic halo is found in the Fermi data, {and the constraints on line emission is marginally consistent with} the DM annihilation interpretation of the ∼ 130 GeV line emission from the inner Galaxy. 3) No line emission from galaxy clusters and dwarf galaxies is detected, although possible concentration of photons from clusters in 120–140 GeV is revealed. The constraints from clusters and dwarf galaxies are weak and consistent with the DM annihilation scenario to explain the ∼ 130 GeV line emission.

Decaying vs. annihilating dark matter in light of a tentative gamma-ray line

Recently reported tentative evidence for a gamma-ray line in the Fermi-LATdata is of great potential interest for identifying the nature of dark matter.We compare the implications for decaying and annihilating dark matter taking the constraints from continuum gamma-rays, antiprotonflux and morphology of the excess into account. We find that higgsinoand wino dark matter are excluded, also for nonthermal production.Generically, the continuum gamma-ray flux severely constrains annihilatingdark matter. Consistency of decaying dark matter with thespatial distribution of the Fermi-LAT excess would require an enhancementof the dark matter density near the Galactic center.