Albedo in the ATIC experiment: Measurements and simulations

We analyze the characteristics of the albedo, or the backscatter current, which constitutes a background for charge measurements in calorimetric experiments in high-energy cosmic rays. We compare the experimental data obtained in the flights of the ATIC spectrometer with the simulations performed using the GEANT 3.21 code. We discuss the influence of the backscatter on the charge resolution in the ATIC experiment.     

Resolving Fermi, PAMELA and ATIC anomalies in split supersymmetry without R-parity

A long-lived decaying dark matter as a resolution to Fermi, PAMELA and ATIC anomalies is investigated in the framework of split supersymmetry (SUSY) without R-parity, where the neutralino is regarded as the dark matter and the extreme fine-tuned couplings for the long-lived neutralino are naturally evaded in the usual approach. The energy spectra of electron and positron are from not only the direct neutralino decays denoted by χ→e ⁺ e ^? ν, but also the decaying chains such as $\chi\to e^{+}\nu\mu(\to \nu_{\mu}e\bar{\nu}_{e})$ . We find that with a proper lifetime of the neutralino, slepton-mediated effects could explain the ATIC and PAMELA data well, but an inconsistence occurs to the Fermi and PAMELA data without considering the ATIC one. However, by a suitable combination of χ→e ⁺ e ^? ν and $\chi\to e^{+}\nu \mu(\to\nu_{\mu}e\bar{\nu}_{e})$ , the sneutrino-mediated effects could simultaneously account for the Fermi and PAMELA data.     

Dissecting cosmic-ray electron-positron data with Occam’s razor: the role of known pulsars

We argue that both the positron fraction measured by PAMELA and the peculiar spectral features reported in the total electron-positron flux measured by ATIC have a very natural explanation in electron-positron pairs produced by nearby pulsars. While this possibility was pointed out a long time ago, the greatly improved quality of current data potentially allow to reverse-engineer the problem: given the regions of pulsar parameter space favored by PAMELA and by ATIC, are there known pulsars that explain the data with reasonable assumptions on the injected electron-positron pairs? In the context of simple benchmark models for estimating the electron-positron output, we consider all known pulsars, as listed in the most complete available catalogue. We find that it is unlikely that a single pulsar be responsible for both the PAMELA positron fraction anomaly and for the ATIC excess, although two single sources are in principle enough to explain both experimental results. The PAMELA excess positrons likely come from a set of mature pulsars (age ～ × 10⁶ yr), with a distance of 0.8–1 kpc, or from a single, younger and closer source like Geminga. The ATIC data require a larger (and less plausible) energy output, and favor an origin associated to powerful, more distant (1–2 kpc) and younger (age ～ × 5 × 10⁵ yr) pulsars. We list several candidate pulsars that can individually or coherently contribute to explain the PAMELA and ATIC data. Although generally suppressed, we find that the contribution of pulsars more distant than 1–2 kpc could contribute for the ATIC excess. Finally, we stress the multi-faceted and decisive role that Fermi-LAT will play in the very near future by (1) providing us with an exquisite measurement of the electron-positron flux, (2) unveiling the existence of as yet undetected gamma-ray pulsars, and (3) searching for anisotropies in the arrival direction of high-energy electrons and positrons.     

Twenty years of Galactic observations in searching for bursts of collapse neutrinos with the Baksan underground scintillation telescope

The results of twenty-year-long Galactic observations in neutrino radiation are summarized. Except for the recording of a neutrino signal from the supernova SN 1987A, no Galactic bursts of collapse neutrinos have been detected. An upper bound on the mean frequency of gravitational collapses in our Galaxy was obtained, f _collapse (at 90% confidence) <0.13 yr^?1.     

Upturn in the heavy nuclei to iron ratio in the ATIC experiment above 100 GeV/nucleon

It is argued that the upturn observed in heavy nuclei to iron ratios as measured in the ATIC experiment can be understood within a model of a closed galaxy with embedded local regions containing sources of cosmic rays. This model predicts a universal upturn near energies of 200–300 GeV/nucleon in the spectra of abundant primary nuclei. It also predicts the source spectral index to lie near 2.5.     

The anomalous PAMELA effect and its explanation

The anomalous effect discovered in the PAMELA experiment includes the unusual behavior of the ratio r of fluxes of Galactic positrons to the total flux of Galactic positrons and electrons. According to theory, the value of r should decrease with increasing energy E of these particles. The experiment has shown, however, that in the energy range of 0.1 GeV < E < 5 GeV, the value of r declines; then, beginning with E > 5 GeV and up to E ≈ 150 GeV (measurements have been conducted only up to this energy level), we observe its growth. An explanation for this phenomenon is given.     

The supersymmetric Standard Models with decaying and stable dark matters

We propose two supersymmetric Standard Models (SMs) with decaying and stable dark matter (DM) particles. To explain the SM fermion masses and mixings and have a heavy decay DM particle S, we consider the Froggatt–Nielsen mechanism by introducing an anomalous U(1) _X gauge symmetry. Around the string scale, the U(1) _X gauge symmetry is broken down to a Z ₂ symmetry under which S is odd while all the SM particles are even. S obtains a vacuum expectation value around the TeV scale, and then it can three-body decay dominantly to the second/third family of the SM leptons in Model I and to the first family of the SM leptons in Model II. Choosing a benchmark point in the constrained minimal supersymmetric SM with exact R parity, we show that the lightest neutralino DM is consistent with the CDMS II experiment. Considering S three-body decay and choosing suitable parameters, we show that the PAMELA and Fermi-LAT experiments and the PAMELA and ATIC experiments can be explained in Model I and Model II, respectively.     

A comment on the emission from the Galactic Center as seen by the Fermi telescope

In the recent paper of Hooper and Goodenough (2010) [10] it was reported that γ-ray emission from the Galactic Center region contains an excess compared to the contributions from the large-scale diffuse emission and known point sources. This excess was argued to be consistent with a signal from annihilation of Dark Matter with a power law density profile. We reanalyze the Fermi data and find instead that it is consistent with the “standard model” of diffuse emission and of known point sources. The main reason for the discrepancy with the interpretation of Hooper and Goodenough (2010) [10] is different (as compared to the previous works) spectrum of the point source at the Galactic Center assumed by Hooper and Goodenough (2010) [10]. We discuss possible reasons for such an interpretation.     

Tachyonic γ -ray wideband of an ultra-relativistic electron plasma: Spectral fitting with Fermi power-law densities

Fermionic power-law distributions are derived by the second quantization of classical power-law ensembles, and applied to ultra-relativistic electron populations in the Galactic center. The γ-ray flux from the direction of the compact central source Sagittarius A* is fitted with a superluminal cascade spectrum. In this way, estimates of the radiating electron plasma in the Galactic center region are obtained, such as the power-law index, temperature, particle number, and internal energy. The spectral averaging of the tachyonic radiation densities with Fermi power-laws is explained. Fugacity expansions of the thermodynamic variables (thermal equation of state, entropy, isochoric heat capacity, and isothermal compressibility) are obtained in the quasiclassical high-temperature/low-density regime, where the spectral fit is carried out. The leading quantum correction to these variables is calculated, and its dependence on the electronic power-law index and the thermal wavelength is discussed. Excess counts of cosmic rays from the Galactic center region are related to the plasma temperature inferred from the cascade fit.     

Galactic cosmic ray propagation: sub-PeV diffuse gamma-ray and neutrino emission

The Tibet ASγ experiment just reported their measurement of sub-PeV diffuse gamma-ray emission from the Galactic disk, with the highest energy up to 957 TeV. These diffuse gamma rays are most likely the hadronic origin by cosmic ray (CR) interaction with interstellar gas in the galaxy. This measurement provides direct evidence to the hypothesis that the Galactic Cosmic Rays (GCRs) can be accelerated beyond PeV energies. In this work, we try to explain the sub-PeV diffuse gamma-ray spectrum with different CR propagation models. We find that there is a tension between the sub-PeV diffuse gamma-ray and the local CR spectrum. To describe the sub-PeV diffuse gamma-ray flux, it generally requires larger local CR flux than measurement in the knee region. We further calculate the PeV neutrino flux from the CR propagation model. Even all of these sub-PeV diffuse gamma rays originate from the propagation, the Galactic Neutrinos (GNs) only account for less than ~15% of observed flux, most of which are still from extragalactic sources.     

The High-Altitude Water Cherenkov Observatory

The High-Altitude Water Cherenkov (HAWC) observatory is a large field of view, continuously operated, TeV γ-ray experiment under construction at 4,100 m a.s.l. in Mexico. The HAWC observatory will have an order of magnitude better sensitivity, angular resolution, and background rejection than its predecessor, the Milagro experiment. The improved performance will allow us to detect both the transient and steady emissions, to study the Galactic diffuse emission at TeV energies, and to measure or constrain the TeV spectra of GeV γ-ray sources. In addition, HAWC will be the only ground-based instrument capable of detecting prompt emission from γ-ray bursts above 50 GeV. The HAWC observatory will consist of an array of 300 water Cherenkov detectors (WCDs), each with four photomultiplier tubes. This array is currently under construction on the flanks of the Sierra Negra volcano near the city of Puebla, Mexico. The first 30 WCDs (forming an array approximately the size of Milagro) were deployed in Summer 2012, and 100 WCDs will be taking data by May, 2013. We present in this paper the motivation for constructing the HAWC observatory, the status of the deployment, and the first results from the constantly growing array.     

Search for the Galactic Disk and Halo Components in the Arrival Directions of High-Energy Astrophysical Neutrinos

The arrival directions of 40 neutrino events with energies ?100 TeV, observed by the IceCube experiment, are studied. Their distribution in the Galactic latitude and in the angular distance to the Galactic Center allow searching for the Milky-Way disk and halo-related components, respectively. No statistically significant evidence for the disk component is found, though even 100% disk origin of the flux is allowed at the 90% confidence level. Contrary, the Galactic Center–Anticenter dipole anisotropy, specific for dark-matter decays (annihilation) or for interactions of cosmic rays with the extended halo of the circumgalactic gas, is clearly favored over the isotropic distribution (the probability of fluctuation of the isotropic signal is ~2%).     

Tachyonic γ-ray spectra of active galactic nuclei: Evidence for intrinsic spectral curvature

Tachyonic spectral densities of ultra-relativistic electron populations are fitted to the γ-ray spectra of two TeV blazars, the BL Lacertae objects 1ES 0229+200 and 1ES 0347-121. The spectral maps are compared to Galactic TeV sources, the γ-ray binary LS 5039 and the supernova remnant W28. In contrast to TeV photons, the extragalactic tachyon flux is not attenuated by interaction with the cosmic background light; there is no absorption of tachyonic γ-rays via pair creation, as tachyons do not interact with infrared background photons. The curvature of the observed γ-ray spectra is intrinsic, caused by the Boltzmann factor of the electron densities, and reproduced by a tachyonic cascade fit. In particular, the curvature in the spectral map of the Galactic microquasar is more pronounced than of the two extragalactic γ-ray sources. Estimates of the thermodynamic parameters of the thermal or, in the case of supernova remnant W28, shock-heated nonthermal electron plasma generating the tachyon flux are obtained from the spectral fits.     

Fraction of cosmic-ray positrons in the galaxy: Results from using the fractional diffusion approach

The fraction of positrons in the electron-positron component of cosmic rays is calculated in a scenario where electrons and positrons are generated by Galactic sources with the same spectral index. It is shown that the proposed scenario allows us to reproduce the existing experimental data on the fraction of positrons if the relative yield of positrons in a source is e ⁺/e ^? ≈ 0.3.     

Double-action dark matter,PAMELA and ATIC

Motivated by a two-bump (or 1-peak plus 1-hump) structure in the ATIC data, we perform a statistical analysis fitting the PAMELA and ATIC data to a dark matter model, in which the dark matter particle can undergo both annihilation and decay. Using a chi-square analysis we show that both data can be simultaneously fitted better with such a double-action dark matter particle. We use an existing neutrino mass model in literature to illustrate the idea.     

The diffuse Galactic γ-rays from dark matter annihilation

The diffuse Galactic γ-rays from EGRET observation shows excesses above 1 GeV in comparison with the expectations from conventional Galactic cosmic ray (CR) propagation model. In the work we try to solve the “GeV excess” problem by dark matter (DM) annihilation in the frame of supersymmetry (SUSY). Compared with previous works, there are three aspects improved in this work: first, the direction-independent “boost factor” for diffuse γ-rays from dark matter annihilation (DMA) is naturally reproduced by taking the DM substructures into account; second, there is no need for renormalization of the diffuse γ-ray background produced by CRs; last but not the least, in this work our new propagation model can give consistent results of both diffuse γ-rays and antiprotons, by directly adding the signals from DMA to the diffuse γ-ray background. This is a self-consistent model among several possible scenarios at present, and can be tested or optimized by the forthcoming experiments such as GLAST, PAMELA and AMS02.     

Galactic cosmic ray propagation: sub-PeV diffuse gamma-ray and neutrino emission

The Tibet ASγ experiment just reported their measurement of sub-PeV diffuse gamma-ray emission from the Galactic disk, with the highest energy up to 957 TeV. These diffuse gamma rays are most likely the hadronic origin by cosmic ray (CR) interaction with interstellar gas in the galaxy. This measurement provides direct evidence to the hypothesis that the Galactic Cosmic Rays (GCRs) can be accelerated beyond PeV energies. In this work, we try to explain the sub-PeV diffuse gamma-ray spectrum with different CR propagation models. We find that there is a tension between the sub-PeV diffuse gamma-ray and the local CR spectrum. To describe the sub-PeV diffuse gamma-ray flux, it generally requires larger local CR flux than measurement in the knee region. We further calculate the PeV neutrino flux from the CR propagation model. Even all of these sub-PeV diffuse gamma rays originate from the propagation, the Galactic Neutrinos (GNs) only account for less than ~15% of observed flux, most of which are still from extragalactic sources.     

On prospects for dark matter indirect detection in the Constrained MSSM

We apply a rigorous statistical analysis to the Constrained MSSM to derive the most probable ranges of the diffuse gamma radiation flux from the direction of the Galactic center and of the positron flux from the Galactic halo due to neutralino dark matter annihilation, for several different choices of the halo model and propagation model parameters. We find that, for a specified halo profile, and assuming flat priors, the 68% probability range of the integrated γ-ray flux spans about one order of magnitude, while the 95% probability range can be much larger and extend over four orders of magnitude (even exceeding five for a tiny region at small neutralino mass). The detectability of the signal by GLAST depending primarily on the cuspiness of the halo profile. The positron flux, on the other hand, appears to be too small to be detectable by PAMELA, unless the boost factor is at least of order ten and/or the halo profile is extremely cuspy. We also briefly discuss the sensitivity of our results to the choice of priors.     

Dark Matter annihilation in small-scale clumps in the Galactic halo

The enhancement of the annihilation signal due to Dark Matter (DM) clumpiness in the Galactic halo, valid for arbitrary DM particles, is described. The mass spectrum of small-scale DM clumps with M≤10³ M _⊙ is calculated with tidal destruction of the clumps taken into account within the hierarchical model of clump structure. The mass distribution of the clumps has a cutoff at M _min due to diffusion of DM particles out of a fluctuation and free streaming. In the case of neutralino (considered as a pure bino) being a DM particle, M _min～10⁸ M _⊙. The evolution of the density profile in a DM clump does not result in singularity, because of formation of the core under the influence of tidal interaction. The number density of clumps as a function of their mass, radius, and distance to the Galactic center is presented.     

Pulsars as the most probable sources of ultrahigh-energy cosmic rays

The arrival directions of ultrahigh-energy extensive air showers detected at the Yakutsk extensive air shower array are considered. The maxima revealed in the distribution of the arrival directions of showers and doublets are found to correlate with the coordinates of pulsars located in the Galactic plane. It is shown that the three showers detected at the Yakutsk extensive air shower with the highest energies E > 10²⁰ eV correlate with the nearest pulsars.