Measurement of the cosmic diffuse gamma-ray spectrum from 800 keV to 30 MeV

The Cosmic Diffuse Gamma-Ray (CDG) spectrum between 800 keV and 30 MeV has been measured with the Imaging Compton telescope COMPTEL, aboard the Compton Gamma Ray Observatory. COMPTEL is well suited to measure the CDG flux because of its large detection area, wide field-of-view ($\sim$1.5 sr), low background and long exposure times. The major difficulty in measuring the CDG radiation at MeV energies is the intense instrumental background. The instrumental background in COMPTEL is created mainly in the surrounding material. The striking feature of the pre-COMPTEL CDG spectrum was an apparent flattening between 1 and 10 MeV. A simple power law extrapolation from the X-ray regime showed the presence of an excess, referred to as the MeV bump, in the 1 to 10 MeV range. These CDG flux measurements in the 1 to 10 MeV range are about 5 to 10 times lower than the pre-COMPTEL estimates. They show no evidence of a MeV bump in the 1 to 10 MeV range. The measured CDG emission between 0.8 and 30 MeV is well described by a power-law photon spectrum with an index of $-$2.4 $\pm$ 0.2 and a flux normalization of (1.05 $\pm$ 0.2) $\times$ 10\sp{-4} photons/cm\sp2-s-sr-MeV at 5 MeV. No statistically significant deviations from isotropy is observed in the 4.2 to 30 MeV CDG emission when comparing the spectrum from the Virgo and the South Galactic Pole directions. The CDG spectrum was measured using COMPTEL data by first measuring the count rate of gamma rays from high galactic latitudes, during periods when the Earth was outside the COMPTEL field-of-view. Special data selections were applied to suppress the prompt and delayed background components. Above 4.2 MeV, in the absence of long-lived background, the count rates were extrapolated to zero cosmic-ray intensity to eliminate the prompt background and arrive at the CDG count rates. The delayed emission from long-lived radioactivity, present only below 4.2 MeV, was determined by fitting the energy spectrum. Below 4.2 MeV, their contributions were subtracted prior to the extrapolation to zero cosmic-ray flux to determine the CDG count spectrum. The CDG flux was determined by deconvolving the resultant count spectrum with the computed instrument response for an isotropic diffuse source having a power-law distribution in energy. Possible contributions from all processes that explain the MeV bump, such as primordial matter-antimatter annihilations, are significantly lower than previously believed. Our measurements below 3 MeV within errors are consistent with the level predicted by the model where the CDG emission arise from the decay of elements produced in supernovae. A blazar origin for the CDG emission seems likely above $\sim$100 MeV. The situation is less certain in the 1 to 30 MeV range because the blazar emission at these energies is not well known. A simple continuation of the blazar contribution from higher energies is allowed by the COMPTEL measurements. (Abstract shortened by UMI.)

Phenomenological Constraints on Axion Models of Dynamical Dark Matter

In two recent papers (arXiv:1106.4546, arXiv:1107.0721), we introduced "dynamical dark matter" (DDM), a new framework for dark-matter physics in which the requirement of stability is replaced by a delicate balancing between lifetimes and cosmological abundances across a vast ensemble of individual dark-matter components whose collective behavior transcends that normally associated with traditional dark-matter candidates. We also presented an explicit model involving axions in large extra spacetime dimensions, and demonstrated that this model has all of the features necessary to constitute a viable realization of the general DDM framework. In this paper, we complete our study by performing a general analysis of all phenomenological constraints which are relevant to this bulk-axion DDM model. Although the analysis in this paper is primarily aimed at our specific DDM model, many of our findings have important implications for bulk axion theories in general. Our analysis can also serve as a prototype for phenomenological studies of theories in which there exist large numbers of interacting and decaying particles.Comment: 48 pages, LaTeX, 13 figures, 1 tabl

The COMPTEL instrumental line background

The instrumental line background of the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory is due to the activation and/or decay of many isotopes. The major components of this background can be attributed to eight individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl. The identification of instrumental lines with specific isotopes is based on the line energies as well as on the variation of the event rate with time, cosmic-ray intensity, and deposited radiation dose during passages through the South-Atlantic Anomaly. The characteristic variation of the event rate due to a specific isotope depends on its life-time, orbital parameters such as the altitude of the satellite above Earth, and the solar cycle. A detailed understanding of the background contributions from instrumental lines is crucial at MeV energies for measuring the cosmic diffuse gamma-ray background and for observing gamma-ray line emission in the interstellar medium or from supernovae and their remnants. Procedures to determine the event rate from each background isotope are described, and their average activity in spacecraft materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure

Cosmological Constraints on Neutrino Injection

We derive general constraints on the relic abundances of a long-lived particle which mainly decays into a neutrino (and something else) at cosmological time scales. Such an exotic particle may show up in various particle-physics models based on physics beyond the standard model. The constraints are obtained from big-bang nucleosynthesis, cosmic microwave background and diffuse neutrino and photon fluxes, depending on the lifetime and the electromagnetic and hadronic branching ratios.Comment: 33 pages, 23 figure

Particle Physics Probes Of Extra Spacetime Dimensions

The possibility that spacetime is extended beyond the familiar 3+1-dimensions has intrigued physicists for a century. Indeed, the consequences of a dimensionally richer spacetime would be profound. Recently, new theories with higher dimensional spacetimes have been developed to resolve the hierarchy problem in particle physics. These scenarios make distinct predictions which allow for experiment to probe the existence of extra dimensions in new ways. We review the conceptual framework of these scenarios, their implications in collider and short-range gravity experiments, their astrophysical and cosmological effects, as well as the constraints placed on these models from present data.Comment: Submitted to Annual Review of Nuclear and Particle Science, 29 page

Compton Heating of the Intergalactic Medium by the Hard X-ray Background

High-resolution hydrodynamics simulations of the Ly-alpha forest in cold dark matter dominated cosmologies appear to predict line widths that are substantially narrower than those observed. Here we point out that Compton heating of the intergalactic gas by the hard X-ray background (XRB), an effect neglected in all previous investigations, may help to resolve this discrepancy. The rate of gain in thermal energy by Compton scattering will dominate over the energy input from hydrogen photoionization if the XRB energy density is 0.2x/ times higher than the energy density of the UV background at a given epoch, where x is the hydrogen neutral fraction in units of 1e-6 and is the mean X-ray photon energy in units of m_ec^2. The numerical integration of the time-dependent rate equations shows that the intergalactic medium approaches a temperature of about 1.5e4 K at z>3 in popular models for the redshift evolution of the extragalactic background radiation. The importance of Compton heating can be tested experimentally by measuring the Ly-alpha line-width distribution as a function of redshift, thus the Lyman-alpha forest may provide a useful probe of the evolution of the XRB at high redshifts.Comment: LaTeX, 10 pages, 2 figures, final version to be published in the Ap

The Contribution of Blazars to the Extragalactic Diffuse Gamma-ray Background and Their Future Spatial Resolution

We examine the constraints on the luminosity-dependent density evolution model for the evolution of blazars given the observed spectrum of the diffuse gamma-ray background (DGRB), blazar source-count distribution, and the blazar spectral energy distribution sequence model, which relates the observed the blazar spectrum to its luminosity. We show that the DGRB observed by the Large Area Telescope (LAT) aboard the Fermi Gamma Ray Space Telescope can be produced entirely by gamma-ray emission from blazars and nonblazar active galactic nuclei, and that our blazar evolution model is consistent with and constrained by the spectrum of the DGRB and flux source-count distribution function of blazars observed by Fermi-LAT. Our results are consistent with previous work that used EGRET spectral data to forecast the Fermi-LAT DGRB. The model includes only three free parameters, and forecasts that >~ 95% of the flux from blazars will be resolved into point sources by Fermi-LAT with 5 years of observation, with a corresponding reduction of the flux in the DGRB by a factor of ~2 to 3 (95% confidence level), which has implications for the Fermi-LAT's sensitivity to dark matter annihilation photons.Comment: 13 pages, 7 figures; v3: minor changes, matches version to appear in Phys. Rev.

Observed intercamera variability in clinically relevantperformance characteristics for Siemens Symbia gammacameras

This is the peer reviewed version of the following article: Kappadath SC, Erwin WD, Wendt RE 3rd. Observed inter-camera variability of clinically relevant performance characteristics for Siemens Symbia gamma cameras. J Appl Clin Med Phys. 2006;7(4):74-80. Published 2006 Nov 28., which has been published in final form at doi:10.1120/jacmp.v7i4.2376. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.https://openworks.mdanderson.org/mdacc_imgphys_pubs/1009/thumbnail.jp

A novel method to evaluate gamma camera rotational uniformity and sensitivity variation

Kappadath, S.C., Erwin, W.D. and Wendt, R.E., III (2009), A novel method to evaluate gamma camera rotational uniformity and sensitivity variation. Med. Phys., 36: 1947-1955. https://doi.org/10.1118/1.3125642https://openworks.mdanderson.org/mdacc_imgphys_pubs/1007/thumbnail.jp

Constraint on Cosmic Density of the String Moduli Field in Gauge-Mediated Supersymmetry-Breaking Theories

We derive a constraint on the cosmic density of string moduli fields in gauge-mediated supersymmetry-breaking theories by requiring that photons emitted from the unstable moduli fields should not exceed the observed X-ray backgrounds. Since mass of the moduli field lies in the range between $O(0.1)$keV and $O(1)$MeV and the decay occurs through a gravitational interaction, the lifetime of the moduli field is much longer than the age of the present universe. The obtained upperbound on their cosmic density becomes more stringent than that from the unclosure condition for the present universe for the mass greater than about 100keV.Comment: 7 pages, a LaTeX2e file and two postscript figure