Diffuse Gamma-ray Spectrum Research Articles

Effect of extragalactic magnetic field on cascade gamma-ray emission

The intensity of diffuse cascade gamma-ray emission produced in electromagnetic cascades when ultra-high energy cosmic rays propagate in extragalactic space is discussed. It is obtained that in the range of ~107—109 eV diffuse cascade gamma-ray spectra weakly depends on extragalactic magnetic field value. Thus, there is no need to refine extragalactic magnetic field models to estimate the intensity of cascade emission in this energy range.

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.

Galactic gamma-ray and neutrino emission from interacting cosmic-ray nuclei

We present a study of the expectations for very-high-energy (VHE) to ultra-high-energy (UHE) gamma-ray and neutrino emission from interacting cosmic rays in our Galaxy as well as a comparison to the latest results for the Galactic UHE diffuse emission. We demonstrate the importance of properly accounting for both the mixed cosmic-ray composition and the gamma-ray absorption. We adopt the wounded-nucleon model of nucleus interactions and provide parameterisations of the resulting gamma-ray and neutrino production. Nucleon shielding due to clustering inside nuclei is shown to have a measurable effect on the production of gamma rays and is particularly evident close to breaks and cutoffs in mixed-composition particle spectra. The change in composition around the ‘knee’ in the cosmic ray spectrum has a noticeable impact on the diffuse neutrino and gamma-ray emission spectra. We show that current and near-future detectors can probe these differences in the key energy range from 10 TeV to 1 PeV, testing the paradigm of the universality of the cosmic ray spectrum and composition throughout the Galaxy.

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.

Sharp spectral features from light dark matter decay via gravity portals

We investigate the phenomenology of dark matter decay assuming that it is induced by non-minimal coupling to gravity, when the dark matter mass is in the sub-GeV range, i.e. below the QCD confinement scale. We show that the decay of the singlet scalar dark matter candidate produces sharp features in the photon spectrum, in the form of lines, boxes, and also in the form of a novel spectral feature, characterized by the decay into e+e− γ through a contact interaction, with decay branching fractions depending only on a single parameter, namely the dark matter mass. We also derive upper limits on the strength of the gravity portal from the non-observation of sharp features in the isotropic diffuse gamma-ray spectra measured by COMPTEL, EGRET and Fermi-LAT, and the X-ray spectrum measured by INTEGRAL. Finally, we briefly comment on the impact of dark matter decay via non-minimal coupling to gravity on the reionization history of the Universe.

Revisiting the hardening of the cosmic ray energy spectrum at TeV energies

Measurements of cosmic rays by experiments such as ATIC, CREAM, and PAMELA indicate a hardening of the cosmic-ray energy spectrum at TeV energies. In our recent work (Thoudam & H\"orandel 2012a), we showed that the hardening can be due to the effect of nearby supernova remnants. We showed it for the case of proton and helium. In this paper, we present an improved and detailed version of our previous work, and extend our study to heavier cosmic-ray species such as boron, carbon, oxygen, and iron nuclei. Unlike our previous study, the present work involves a detailed calculation of the background cosmic rays and follows a consistent treatment of cosmic-ray source parameters between the background and the nearby components. Moreover, we also present a detailed comparison of our results on the secondary-to-primary ratios, secondary spectra, and the diffuse gamma-ray spectrum with the results expected from other existing models, which can be checked by future measurements at high energies.

Gamma-ray lines from radiative dark matter decay

The decay of dark matter particles which are coupled predominantly to charged leptons has been proposed as a possible origin of excess high-energy positrons and electrons observed by cosmic-ray telescopes PAMELA and Fermi LAT. Even though the dark matter itself is electrically neutral, the tree-level decay of dark matter into charged lepton pairs will generically induce radiative two-body decays of dark matter at the quantum level. Using an effective theory of leptophilic dark matter decay, we calculate the rates of radiative two-body decays for scalar and fermionic dark matter particles. Due to the absence of astrophysical sources of monochromatic gamma rays, the observation of a line in the diffuse gamma-ray spectrum would constitute a strong indication of a particle physics origin of these photons. We estimate the intensity of the gamma-ray line that may be present in the energy range of a few TeV if the dark matter decay interpretation of the leptonic cosmic-ray anomalies is correct and comment on observational prospects of present and future Imaging Cherenkov Telescopes, in particular the CTA.

GALACTIC DIFFUSE GAMMA RAYS—RECALCULATION BASED ON NEW MEASUREMENTS OF THE COSMIC ELECTRON SPECTRUM

In this work, we revisit the all-sky Galactic diffuse $\gamma$-ray emission taking into account the new measurements of cosmic ray electron/positron spectrum by PAMELA, ATIC and Fermi, which show excesses of cosmic electrons/positrons beyond the expected fluxes in the conventional model. Since the origins of the extra electrons/positrons are not clear, we consider three different scenarios to account for the excesses: the astrophysical sources such as the Galactic pulsars, dark matter decay and annihilation. Further, new results from Fermi-LAT of the (extra-)Galactic diffuse $\gamma$-ray are adopted. The background cosmic rays without the new sources give lower diffuse $\gamma$ rays compared to Fermi-LAT observation, which is consistent with previous analysis. The scenario with astrophysical sources predicts diffuse $\gamma$-rays with little difference with the background. The dark matter annihilation models with $\tau^{\pm}$ final state are disfavored by the Fermi diffuse $\gamma$-ray data, while there are only few constraints on the decaying dark matter scenario. Furthermore, these is always a bump at higher energies ($\sim$ TeV) of the diffuse $\gamma$-ray spectra for the dark matter scenarios due to final state radiation. Finally we find that the Fermi-LAT diffuse $\gamma$-ray data can be explained by simply enlarging the normalization of the electron spectrum without introduce any new sources, which may indicate that the current constraints on the dark matter models can be much stronger given a precise background estimate.

A new approach to searching for dark matter signals in Fermi-LAT gamma rays

Several cosmic ray experiments have measured excesses in electrons and positrons, relative to standard backgrounds, for energies from ∼ 10 GeV–1 TeV. These excesses could be due to new astrophysical sources, but an explanation in which the electrons and positrons are dark matter annihilation or decay products is also consistent. Fortunately, the Fermi-LAT diffuse gamma ray measurements can further test these models, since the electrons and positrons produce gamma rays in their interactions in the interstellar medium. Although the dark matter gamma ray signal consistent with the local electron and positron measurements should be quite large, as we review, there are substantial uncertainties in the modeling of diffuse backgrounds and, additionally, experimental uncertainties that make it difficult to claim a dark matter discovery. In this paper, we introduce an alternative method for understanding the diffuse gamma ray spectrum in which we take the intensity ratio in each energy bin of two different regions of the sky, thereby canceling common systematic uncertainties. For many spectra, this ratio fits well to a power law with a single break in energy. The two measured exponent indices are a robust discriminant between candidate models, and we demonstrate that dark matter annihilation scenarios can predict index values that require ``extreme'' parameters for background-only explanations.

Decaying dark matter in supersymmetric SU(5) models

Motivated by recent observations from PAMELA, Fermi and H.E.S.S., we consider dark matter decays in the framework of supersymmetric SU(5) grand unification theories. An SU(5) singlet S is assumed to be the main component of dark matters, which decays into visible particles through dimension six operators suppressed by the grand unification scale. Under certain conditions, S decays dominantly into a pair of sleptons with universal coupling for all generations. Subsequently, electrons and positrons are produced from cascade decays of these sleptons. These cascade decay chains smooth the e++e− spectrum, which permit naturally a good fit to the Fermi-LAT data. The observed positron fraction upturn by PAMELA can be reproduced simultaneously. We have also calculated diffuse gamma-ray spectra due to the e± excesses and compared them with the preliminary Fermi-LAT data from 0.1 GeV to 10 GeV in the region 0°⩽l⩽360°, 10°⩽|b|⩽20°. The photon spectrum of energy above 100 GeV, mainly from final state radiations, may be checked in the near future.

Diffuse gamma-ray background and cosmic-ray positrons from annihilating dark matter

We study the annihilating dark matter contribution to the extragalactic diffuse gamma-ray background spectrum, motivated by the recent observations of cosmic-ray positron/electron anomalies. The observed diffuse gamma-ray flux provides stringent constraint on dark matter models and we present upper bounds on the annihilation cross section of the dark matter. It is found that for the case of cored dark matter halo profile, the diffuse gamma-rays give more stringent bound compared with gamma-rays from the Galactic center. The Fermi satellite will make the bound stronger.

Dark Matter Interpretation of Recent Electron and Positron Data

We analyze the recently released Fermi-LAT data on the sum of electrons and positrons. Compared to a conventional, pre-Fermi, background model, a surprising excess in the several hundred GeV range is found and here we analyze it in terms of dark matter models. We also compare with newly published results from PAMELA and H.E.S.S., and find models giving very good fits to these data sets as well. If this dark matter interpretation is correct, we also predict the possibility of a sharp break in the diffuse gamma-ray spectrum coming from final state radiation.

High energy diffuse gamma-ray emission of the galactic disk and galactic cosmic-ray spectra

Observations of diffuse Galactic γ-ray spectrum by the EGRET instrument reveal an excess above ∼1 GeV over the expected γ-ray spectrum calculated under the assumption that the locally observed cosmic-ray (CR) spectra represent the galactic CR spectra. Assuming that Galactic CRs of energy below ∼100 TeV are accelerated by supernova remnant (SNR) shock waves and that the shock compression ratio is SNR age dependent, the average source injection spectra from an ensemble of SNRs is calculated both in the inner (330° < l < 30°) and outer (30° < l < 330°) regions of the galaxy. The calculation considers the SNR age distribution in the galaxy. Injecting these spectra in the galaxy and using a 3-D convection–diffusion equation, the CR electrons and protons spectra in the two galactic regions are obtained and their spectra in the galactic disk are found to be flatter than the observed CR spectra. The diffuse gamma-ray spectrum produced by the interaction of these galactic CRs with the ISM and ISRFs is compared with the experimental data in both the galactic regions. Furthermore, the steepening of the observed local CR spectra from the galactic disk CR spectra are discussed by propagating local CRs having a source spectrum derived using local SNR age distribution (SNRs located within 1.5 kpc from the Sun), for a diffusion coefficient D 0 ∼ 0.3 × 10 27 cm 2 s −1 in the local region which is much less than the typical value in the galaxy D 0 ∼ (1–10) × 10 28 cm 2 s −1. The results obtained in this paper support the SNR origin of galactic CRs.

Contribution to diffuse gamma-rays in the Galactic Centre region from unresolved millisecond pulsars

Diffuse gamma-rays in the Galactic Centre region have been studied. We propose that there exists a population of millisecond pulsars in the Galactic Centre, which emit GeV gamma-rays through synchrotron-curvature radiation as predicted by outer gap models. These GeV gamma-rays from unresolved millisecond pulsars probably contribute to the diffuse gamma-ray spectrum detected by EGRET which displays a break at a few GeV. We have used a Monte Carlo method to obtain simulated samples of millisecond pulsars in the Galactic Centre region covered by EGRET according to the different period and magnetic field distributions from observed millisecond pulsars in the Galactic field and globular clusters, and superposed their synchrotron-curvature spectra to derive the total GeV flux. Our simulated results suggest that there probably exist about 6000 unresolved millisecond pulsars in the region of angular resolution of EGRET, the emissions of which could contribute significantly to the observed diffuse gamma-rays in the Galactic Centre.

Gamma‐Ray Spectra Due to Cosmic‐Ray Interactions with Dense Gas Clouds

Gamma-ray spectra from cosmic-ray proton and electron interactions with dense gas clouds have been calculated using a Monte Carlo event simulation code, GEANT4. Such clouds are postulated as a possible form of baryonic dark matter in the universe. The simulation fully tracks the cascade and transport processes that are important in a dense medium, and the resulting gamma-ray spectra are computed as a function of cloud column density. These calculations are used for predicting the Galactic diffuse gamma-ray spectrum that may be contributed by baryonic dark matter; the results are compared with data from the EGRET instrument and used to constrain the fraction of Galactic dark matter that may be in the form of dense gas clouds. In agreement with previous authors, we find useful constraints on the fraction of Galactic dark matter that may be in the form of low column density clouds (Σ 10 g cm-2). However, this fraction rises steeply in the region of Σ ~ 102 g cm-2, and for Σ 200 g cm-2 we find that baryonic dark matter models are virtually unconstrained by the existing gamma-ray data.

High temperature matter and gamma ray spectra from microscopic black holes

The relativistic viscous fluid equations describing the outflow of high temperature matter created via Hawking radiation from microscopic black holes are solved numerically for a realistic equation of state. We focus on black holes with initial temperatures greater than 100 GeV and lifetimes less than 6 days. The spectra of direct photons and photons from $\pi^0$ decay are calculated for energies greater than 1 GeV. We calculate the diffuse gamma ray spectrum from black holes distributed in our galactic halo. However, the most promising route for their observation is to search for point sources emitting gamma rays of ever-increasing energy.

Bounds from primordial black holes with a near critical collapse initial mass function

Recent numerical evidence suggests that a mass spectrum of primordial black holes (PBHs) is produced as a consequence of near critical gravitational collapse. Assuming that these holes formed from the initial density perturbations seeded by inflation, we calculate model independent upper bounds on the mass variance at the reheating temperature by requiring that the mass density does not exceed the critical density and the photon emission does not exceed current diffuse gamma-ray measurements. We then translate these results into bounds on the spectral index n by utilizing the COBE data to normalize the mass variance at large scales, assuming a constant power law, and scale this result to the reheating temperature. We find that our bounds on n differ substantially $(\ensuremath{\delta}n&gt;0.05)$ from those calculated using initial mass functions derived under the assumption that the black hole mass is proportional to the horizon mass at the collapse epoch. We also find a change in the shape of the diffuse gamma-ray spectrum which results from the Hawking radiation. Finally, we study the impact of a nonzero cosmological constant and find that the bounds on n are strengthened considerably if the universe is indeed vacuum-energy dominated today.

The contribution to the Galactic diffuse gamma-ray spectrum from unresolved rotation-powered pulsars

We consider the contribution to the Galactic diffuse γ-ray emission from unresolved γ-ray pulsars. Based on the thick outer gap model, Monte Carlo methods are used to simulate the properties (period, distance, magnetic field, etc.) of the Galactic population of rotation-powered pulsars the gamma-ray flux of which is lower than the threshold sensitivity of the EGRET detector on the Compton Gamma-Ray Observatory. Furthermore, the contribution to the Galactic diffuse γ-ray spectrum from the unresolved γ-ray pulsars is calculated. Our results indicate that the unresolved γ-ray pulsars contribute ∼5 to ∼10 per cent to the measured Galactic diffuse γ-ray emission if the birth rate of neutron stars in the Galaxy is 1 to 2 per century, and that these pulsars contribute significantly to the observed Galactic diffuse γ-ray emission above 1 GeV. Comparing the model spectrum with the observed spectrum, we show that the unresolved γ-ray pulsars contribute very little to the diffuse emission at lower energies but can account for ∼50 per cent of the observed spectrum above 1 GeV if the product of the birth rate of neutron stars and the γ-ray beaming fraction is about unity. Such a large pulsar contribution can explain the difference (∼60 per cent) between the intensity of the Galactic diffuse emission as measured by EGRET above 1 GeV and model predictions based on cosmic ray–matter interaction only.

The contribution to the Galactic diffuse gamma-ray spectrum from unresolved rotation-powered pulsars

We consider the contribution to the Galactic diffuse γ-ray emission from unresolved γ-ray pulsars. Based on the thick outer gap model, Monte Carlo methods are used to simulate the properties (period, distance, magnetic field, etc.) of the Galactic population of rotation-powered pulsars the gamma-ray flux of which is lower than the threshold sensitivity of the EGRET detector on the Compton Gamma-Ray Observatory. Furthermore, the contribution to the Galactic diffuse γ-ray spectrum from the unresolved γ-ray pulsars is calculated. Our results indicate that the unresolved γ-ray pulsars contribute ∼5 to ∼10 per cent to the measured Galactic diffuse γ-ray emission if the birth rate of neutron stars in the Galaxy is 1 to 2 per century, and that these pulsars contribute significantly to the observed Galactic diffuse γ-ray emission above 1 GeV. Comparing the model spectrum with the observed spectrum, we show that the unresolved γ-ray pulsars contribute very little to the diffuse emission at lower energies but can account for ∼50 per cent of the observed spectrum above 1 GeV if the product of the birth rate of neutron stars and the γ-ray beaming fraction is about unity. Such a large pulsar contribution can explain the difference (∼60 per cent) between the intensity of the Galactic diffuse emission as measured by EGRET above 1 GeV and model predictions based on cosmic ray–matter interaction only.

Black holes and sub-millimeter dimensions

Recently, a new framework for solving the hierarchy problem was proposed which does not rely on low energy supersymmetry or technicolor. The fundamental Planck mass is at a TeV and the observed weakness of gravity at long distances is due the existence of new sub-millimeter spatial dimensions. In this letter, we study how the properties of black holes are altered in these theories. Small black holes—with Schwarzschild radii smaller than the size of the new spatial dimensions—are quite different. They are bigger, colder, and longer-lived than a usual (3+1)-dimensional black hole of the same mass. Furthermore, they primarily decay into harmless bulk graviton modes rather than standard-model degrees of freedom. We discuss the interplay of our scenario with the holographic principle. Our results also have implications for the bounds on the spectrum of primordial black holes (PBHs) derived from the photo-dissociation of primordial nucleosynthesis products, distortion of the diffuse gamma-ray spectrum, overclosure of the universe, gravitational lensing, as well as the phenomenology of black hole production. For example, the bound on the spectral index of the primordial spectrum of density perturbations is relaxed from 1.25 to 1.45–1.60 depending on the epoch of the PBH formation. In these scenarios PBHs provide interesting dark matter candidates; for 6 extra dimensions MACHO candidates with mass ∼0.1 M ⊙ can arise. For 2 or 3 extra dimensions PBHs with mass ∼10 4 M ⊙ can occur and may act as both dark matter and seeds for early galaxy and QSO formation.