Diffuse Cosmic Background Research Articles

Contribution of Neutrino-dominated Accretion Flows to the Cosmic MeV Neutrino Background

Neutrino-dominated accretion flows (NDAFs) are one of the important MeV neutrino sources and significantly contribute to the cosmic diffuse neutrino background. In this paper, we investigate the spectrum of the diffuse NDAF neutrino background (DNNB) by fully considering the effects of the progenitor properties and initial explosion energies based on core-collapse supernova (CCSN) simulations, and estimate the detectable event rate by the Super-Kamiokande detector. We find that the predicted background neutrino flux is mainly determined by the typical CCSN initial explosion energy and progenitor metallicity. For the optimistic cases, in which the typical initial explosion energy is low, the diffuse flux of the DNNB is comparable to the diffuse supernova neutrino background, which might be detected by upcoming larger neutrino detectors, such as Hyper-Kamiokande, the Jiangmen Underground Neutrino Observatory, and the Deep Underground Neutrino Experiment. Moreover, the strong outflows from NDAFs could dramatically decrease their contribution to the neutrino background.

Black hole accretion disk diffuse neutrino background

We study the cosmic MeV neutrino background from accretion disks formed during collapsars and the coalescence of compact-object mergers. We provide updated estimates, including detection rates, of relic neutrinos from collapsars, as well as estimates for neutrinos that are produced in mergers. Our results show that diffuse neutrinos detected at HyperK would likely include some that were emitted from binary neutron-star mergers. The collapsar rate is uncertain, but at its upper limit relic neutrinos from these sources would provide a significant contribution to the cosmic diffuse neutrino background.

THE MYSTERY OF THE COSMIC DIFFUSE ULTRAVIOLET BACKGROUND RADIATION

The diffuse cosmic background radiation in the GALEX far ultraviolet (FUV, 1300 \AA\ - 1700 \AA) is deduced to originate only partially in the dust-scattered radiation of FUV-emitting stars: the source of a substantial fraction of the FUV background radiation remains a mystery. The radiation is remarkably uniform at both far northern and far southern Galactic latitudes, and it increases toward lower Galactic latitudes at all Galactic longitudes. We examine speculation that it might be due to interaction of the dark matter with the nuclei of the interstellar medium but we are unable to point to a plausible mechanism for an effective interaction. We also explore the possibility that we are seeing radiation from bright FUV-emitting stars scattering from a "second population" of interstellar grains---grains that are small compared with FUV wavelengths. Such grains are known to exist (Draine 2011) and they scatter with very high albedo, with an isotropic scattering pattern. However, comparison with the observed distribution (deduced from their $100\ \mu$m emission) of grains at high Galactic latitudes shows no correlation between the grains' location and the observed FUV emission. Our modeling of the FUV scattering by small grains also shows that there must be remarkably few such "smaller" grains at high Galactic latitudes, both North and South; this likely means simply that there is very little interstellar dust of any kind at the Galactic poles, in agreement with Perry & Johnston (1982). We also review our limited knowledge of the cosmic diffuse background at ultraviolet wavelengths shortward of Lyman $\alpha$---it could be that our "second component" of the diffuse far-ultraviolet background persists shortward of the Lyman limit, and is the cause of the re-ionization of the Universe (Kollmeier et al. 2014).

RECONSTRUCTING THE γ-RAY PHOTON OPTICAL DEPTH OF THE UNIVERSE TO z ∼ 4 FROM MULTIWAVELENGTH GALAXY SURVEY DATA

We reconstruct \gamma-ray opacity of the Universe out to z<3-4 using an extensive library of 342 observed galaxy luminosity function surveys extending to high redshifts. We cover the whole range from UV to mid-IR (0.15-25mic) providing for the first time a robust empirical calculation of the \gamma\gamma-optical depth out to several TeV. Here, we use the same database as Helgason et al. 2012 where the EBL was reconstructed from luminosity functions out to 4.5mic and was shown to recover observed galaxy counts to high accuracy. We extend our earlier library of LFs to 25mic such that it covers the energy range of pair production with \gamma-rays 1) in the entire Fermi/LAT energy range, and 2) at higher TeV energies probed by ground-based Cherenkov telescopes. In the absence of significant contributions to the cosmic diffuse background from unknown populations, such as the putative Population III era sources, the Universe appears to be largely transparent to \gamma-rays at all Fermi/LAT energies out to z~2 whereas becoming opaque to TeV photons already at z<0.2 and reaching \tau~10 at z=1. Comparing with the currently available Fermi/LAT GRB and blazar data shows that there is room for significant emissions originating in the first stars era.

Validation of the radiation pattern of the Middle Atmosphere Alomar Radar System (MAARSY)

Abstract. In 2009/2010 the Leibniz-Institute of Atmospheric Physics (IAP) installed a new powerful VHF radar on the island Andøya in Northern Norway (69.30° N, 16.04° E). The Middle Atmosphere Alomar Radar System (MAARSY) allows studies with high spatial and temporal resolution in the troposphere/lower stratosphere and in the mesosphere/lower thermosphere of the Arctic atmosphere. The monostatic radar is operated at 53.5 MHz with an active phased array antenna consisting of 433 Yagi antennas. Each individual antenna is connected to its own transceiver with independent phase control and a scalable power output of up to 2 kW, which implies high flexibility of beam forming and beam steering. During the design phase of MAARSY several model studies have been carried out in order to estimate the radiation pattern for various combinations of beam forming and steering. However, parameters like mutual coupling, active impedance and ground parameters have an impact on the radiation pattern, but can hardly be measured. Hence, experiments need to be designed to verify the model results. For this purpose, the radar has occasionally been used in passive mode, monitoring the noise power received from both distinct cosmic noise sources like e.g. Cassiopeia A and Cygnus A, and the diffuse cosmic background noise. The analysis of the collected dataset enables us to verify beam forming and steering attempts. These results document the current status of the radar during its development and provide valuable information for further improvement.

Instruments of RT-2 experiment onboard CORONAS-PHOTON and their test and evaluation IV: background simulations using GEANT-4 toolkit

Hard X-ray detectors in space are prone to background signals due to the ubiquitous cosmic rays and cosmic diffuse background radiation that continuously bombards the satellites which carry the detectors. In general, the background intensity depends on the space environment as well as the material surrounding the detectors. Understanding the behavior of the background noise in the detector is very important to extract the precise source information from the detector data. In this paper, we carry out Monte Carlo simulations using the GEANT-4 toolkit to estimate the prompt background noise measured with the detectors of the RT-2 Experiment onboard the CORONAS-PHOTON satellite.

A physical model for the origin of the diffuse cosmic infrared background and the opacity of the Universe to very high energy γ-rays

We present a physical model for origin of the cosmic diffuse infrared background (CDIRB). By utilizing the observed stellar mass function and its evolution as input to a semi-empirical model of galaxy formation, we isolate the physics driving diffuse IR emission. The model includes contributions from three primary sources of IR emission: steady-state star formation owing to isolated disk galaxies, interaction-driven bursts of star formation owing to close encounters and mergers, and obscured active galactic nuclei (AGN). We find that most of the CDIRB is produced by equal contributions from objects at z=0.5-1 and z>1, as suggested by recent observations. Of those sources, the vast majority of the emission originates in systems with low to moderate IR luminosities (L_{IR}<10^{12} $L_sun); the most luminous objects contribute significant flux only at high-redshifts (z>2). All star formation in ongoing mergers accounts for <10% of the total at all wavelengths and redshifts, while emission directly attributable to the interaction-driven burst itself accounts for <5%. We furthermore find that obscured AGN contribute <1-2% of the CDIRB at all wavelengths and redshifts, with a strong upper limit of less than 4% of the total emission. Finally, since electron-positron pair production interactions with the CDIRB represent the primary source of opacity to very high energy (VHE: E_\gamma > 1 TeV) \gamma-rays, the model provides predictions for the optical depth of the Universe to the most energetic photons. We find that these predictions agree with observations of high-energy cutoffs at TeV energies in nearby blazars, and suggest that while the Universe is extremely optically thick at >10 TeV, the next generation of VHE \gamma-ray telescopes can reasonably expect detections from out to 50-150 Mpc.

Astrophysical and cosmological implications of large volume string compactifications

We study the spectrum, couplings and cosmological and astrophysical implications of themoduli fields for the class of Calabi–Yau IIB string compactifications for which modulistabilisation leads to an exponentially large volume and an intermediate string scalems∼1011 GeV, with TeV-scale observable supersymmetry breaking. All Kähler moduli except for theoverall volume are heavier than the susy (supersymmetry) breaking scale, withm∼ln(MP/m3/2)m3/2∼(ln(MP/m3/2))2msusy∼500 TeV and, contrary to standard expectations, have matter couplings suppressedonly by the string scale rather than the Planck scale. These decay tomatter early in the history of the universe, with a reheat temperatureT∼107 GeV, and are free from the cosmological moduli problem (CMP).The heavy moduli have a branching ratio to gravitino pairs of10−30 and do not suffer from the gravitino overproduction problem. Theoverall volume modulus is a distinctive feature of these models and is anMplanck-coupledscalar of mass m∼1 MeV and subject to the CMP. A period of thermal inflation may help relax this problem. This field has alifetime τ∼1024 s and can contribute to dark matter. It may be detected through its decays toγγ ore+e−. If accessiblethe e+e− decay mode dominates, with suppressed by a factor (ln(MP/m3/2))2. We consider the potential for detection of this field through different astrophysicalsources: the Milky Way halo, the diffuse cosmic background and nearby galaxy clusters andfind that the observed gamma ray background constrains . The decays of this field may generate the 511 keV emission line from the galactic centreobserved by INTEGRAL/SPI.

TeV Blazar Gamma‐Ray Emission Produced by a Cooling Pileup Particle Energy Distribution Function

We propose a time-dependent one-zone model based on a quasi-Maxwellian distribution in order to explain the time-averaged high-energy emission of TeV blazars. The instantaneous spectra are the result of the synchrotron and synchrotron self-Compton emission of ultrarelativistic leptons. The particle energy distribution function is computed in a self-consistent way, taking into account an injection term of fresh particles, a possible pair creation term, and the radiative cooling of the particles. The source term is not a usual power law but rather a pileup distribution, which can result from the combination of stochastic heating via second-order Fermi processes and radiative cooling. To validate this approach, we have performed time-averaged fits of the well-known TeV emitter Mrk 501 during the 1997 flaring activity period, taking into account the attenuation of the high-energy component by cosmic diffuse infrared background and intrinsic absorption via the pair creation process. The model can reproduce very satisfactorily the observed spectral energy distribution. A high Lorentz factor is required to avoid strong pair production; in the case of smaller Lorentz factor, an intense flare in the GeV range is predicted because of the sudden increase of soft photon density below the Klein-Nishina threshold. The possible relevance of such a scenario is discussed.

The MEGA advanced Compton telescope project

The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to improve sensitivity at medium gamma-ray energies (0.4–50 MeV) by at least an order of magnitude over that of COMPTEL. This will be achieved with a new compact design that allows for a very wide field of view, permitting a sensitive all-sky survey and the monitoring of transient and variable sources. The key science objectives for MEGA include the investigation of cosmic high-energy particle accelerators, studies of nucleosynthesis sites using gamma-ray lines, and determination of the large-scale structure of galactic and cosmic diffuse background emission. MEGA records and images gamma-ray events by completely tracking both Compton and pair creation interactions in a tracker of double-sided silicon strip detectors and a calorimeter of CsI crystals able to resolve in three dimensions. We present initial laboratory calibration results from a small prototype MEGA telescope.

Reanalysis ofVoyagerUltraviolet Spectrometer Limits to the Extreme‐Ultraviolet and Far‐Ultraviolet Diffuse Astronomical Flux

We reexamine Voyager Ultraviolet Spectrometer (UVS) data used to establish upper limits to the 500-900 A and 900-1100 A cosmic diffuse background. The measurement of diffuse flux with the Voyager UVS data requires complex corrections for noise sources which are far larger than the astronomical signal. In the analyses carried out to date, the upper limits obtained on the diffuse background show statistical anomalies which indicate that substantial systematic errors are present. We detail these anomalies and identify specific problems with the analyses. We derive statistically robust 2 σ upper limits for continuum flux of 570 photons s-1 cm-2 sr-1 A-1 and for the 1000 A diffuse line flux of 11,790 photons s-1 cm-2 sr-1. The true limits may be substantially higher because of unknown systematic uncertainties. The new statistical limits alone are insufficient to support previous conclusions based on the Voyager data, including work on the character of interstellar dust and estimates of the diffuse extragalactic far-UV background as absorbed by intergalactic dust.

Diffuse Background Radiation

A new determination of the upper limit to the cosmic diffuse background radiation, at ~110 nm, of 300 photons s-1 cm-2 sr-1 nm-1, is placed in the context of diffuse background measurements across the entire electromagnetic spectrum, including new optical, infrared, visible, and gamma-ray background measurements. The possibility that observed excess diffuse visible radiation is due to redshifted cosmological Lyman alpha recomination radiation is explored. Also, a new standard of units for the display of spectra is advocated.

Granularity of the diffuse background observed

First results are reported from a program for measuring the field-to-field fluctuation level of the cosmic diffuse background by using differences between the two background positions of each deep exposure with the HEXTE instrument on RXTE. With 8 million live seconds accumulated to date a fluctuation level on the 15–25 keV band is observed which is consistent with extrapolations from the HEAO-1 measurements. Positive results are expected eventually at higher energies. Models of AGN origin will eventually be constrained by this program.

A Matter‐Antimatter Universe?

We ask whether the universe can be a patchwork consisting of distinct regions of matter and antimatter. We demonstrate that, after recombination, it is impossible to avoid annihilation near regional boundaries. We study the dynamics of this process to estimate two of its signatures: a contribution to the cosmic diffuse gamma-ray background and a distortion of the cosmic microwave background. The former signal exceeds observational limits unless the matter domain we inhabit is virtually the entire visible universe. On general grounds, we conclude that a matter-antimatter symmetric universe is empirically excluded.

Constraints on the Very High Energy Emissivity of the Universe from the Diffuse G[CLC]e[/CLC]V Gamma-Ray Background

Very high energy (VHE, E 100 GeV) radiation emitted at cosmological distances will pair produce on low-energy diffuse extragalactic background radiation before ever reaching us. This prevents us from directly seeing most of the VHE emission in the universe. However, a VHE γ-ray that pair produces initiates an electromagnetic pair cascade. At low energies, this secondary cascade radiation has a spectrum insensitive to the spectrum of the primary γ-radiation and, unlike the original VHE radiation, is observable. Motivated by new measurements of the extragalactic MeV-GeV diffuse γ-ray background, we discuss the constraints placed on cosmological VHE source populations by requiring that the cascade background they produce not exceed the observed levels. We use a new, accurate cascading code and pay particular attention to the dependence of the constraints on the diffuse cosmic background at infrared/optical wavelengths. Despite considerable uncertainty in this background, we find that robust constraints may still be placed on the integrated emissivity of potential VHE sources in the universe. The limits are tighter than those obtained by considering cascading on the microwave background alone and restrict significantly, for example, the parameter space available for the exotic particle physics scenarios recently proposed to explain the highest energy cosmic-ray events. If direct emission from blazar AGNs in fact accounts for most of the observed GeV background, the resulting stronger limits rule out AGN emission scenarios that produce significant power above ~300 GeV.

Diffuse Cosmic Gamma Radiation Measured byHEAO 1

The High Energy Astronomical Observatory 1 (HEAO 1) contained as one of its instruments the UCSD/MIT hard X-ray and gamma-ray instrument, a complex of actively shielded scintillation counters that operated over the nominal 10 keV to 10 MeV range. The two medium-energy detectors (MEDs) employed in this investigation covered the range 80 keV-2 MeV, had a geometrical area of 43 cm2, and were collimated to a circular field of view of 17? FWHM. During a period of operation starting in 1978 April, these detectors were operated in a manner designed to provide a precise measure of the diffuse cosmic background gamma-ray flux. Previous measurements of the spectrum in this range were not of sufficient precision to distinguish between various models explaining the hard X-ray background either as an ensemble of discrete sources or due to a global effect at cosmological distances. The detectors could be alternately blocked or unblocked with a moveable shutter of CsI, which could be operated in an active anticoincidence mode or as a passive shield. Data taken in these various modes and analyzed with respect to the varying radiation environment of the 23? inclination 400 km circular orbit permitted separation of various background effects that have plagued previous measurements of this radiation using scintillation spectrometers. Over the ~80-400 keV band, systematic errors were small and correctable, with a resulting spectrum fitted by a power law of dN/dE = (2.62 ? 0.05) ? 10-3(E/100 keV)-(2.75?0.08 photons cm-2 s-1 keV-1 sr-1. This fit, in general agreement with previous results in this energy range, joins smoothly with measurements at lower energies, and when extrapolated to higher energies, it agrees with the recently reported COMPTEL results at energies above 2 MeV obtained on the Compton Gamma Ray Observatory. The accompanying demonstration of the experimental difficulties associated with scintillation spectrometer measurements of the diffuse cosmic background provides significant clarification of the observational status of this important measurement. When account is taken of measurements with possible local background estimation uncertainties, the spectrum between ~200 keV and ~50 MeV appears more simplified than previous estimates.

Do Massive Neutrinos Ionize Intergalactic HI?

The radiative decay of massive relic 30eV neutrinos could explain several observational puzzles including the missing dark matter in the universe and the anomalous degree of ionization of interstellar matter in the Galaxy. We note that various non-standard particle physics models with extended scalar sector or minimal supersymmetry have sufficient freedom to accommodate such neutrinos. We discuss observational constraints in the immediate Solar neighborhood, in nearby regions of low interstellar absorption, in the Galactic halo, in clusters of galaxies, and in extragalactic space. Although some observations have been interpreted as ruling out this picture, we note that this is true only for models in which extreme concentrations of neutrinos occur in clusters of galaxies. An instrument is under development to measure the cosmic diffuse EUV background in the local Solar neighborhood, for flight on the Spanish Minisat satellite platform. This instrument will have the capability of providing a definitive test of the radiative neutrino decay hypothesis.

ALEXIS Observations of the Diffuse Cosmic Background in the Extreme Ultraviolet

Abstract.We present preliminary results of ALEXIS satellite observations of the extreme ultraviolet diffuse sky. ALEXIS was designed to be a half-sky monitor in three narrow wavelength bands between 13.0 and 19.0 nm. In our band centered at 172 Å we find a clear signal from the diffuse sky that is about 20 counts per second above the signal when looking at the dark earth from the same part of an orbit. This difference corresponds to an upper limit on the true cosmic diffuse background signal in this narrow band. When estimates of the geocoronal contributions, both in and out of band, are removed, our upper limit is reduced.

The average seyfert spectrum at gamma ray energies

The recent measurements of high energy spectra of radioquiet active galactic nuclei, in particular Seyfert galaxies, made with the instruments onboard Compton are discussed and compared with previous observations to obtain an average spectrum for these objects. The spectra obtained for type 1 and type 2 objects show indications in favour of the unified emission model of AGNs. The problems associated with the contribution of type 2 objects to the gamma-ray cosmic diffuse background are discussed.

ROSAT Detection of an X-ray Shadow in the 1/4-keV Diffuse Background in the Draco Nebula

The detection by the Roentgen satellite (ROSAT) x-ray telescope of a shadow in the 1/4-kiloelectron volt (C band, 0.1 to 0.284 kiloelectron volt) cosmic diffuse background is reported. The location and morphology of the local minimum in x-rays are in clear agreement with a discrete H I cloud. The shadow is very deep with a minimum level at 50 percent of the surrounding emission; therefore, a minimum of 50 percent of the observed off-cloud flux must originate on the far side of the cloud. The analysis of H I velocity components links the cloud with the Draco nebula (distance approximately 600 parsecs); it then follows that there is significant 1/4-kiloelectron volt x-ray emission at a large distance (>400 parsecs) from the galactic plane along this line of sight. The extent of the distant emission region is uncertain, and, if it indicates the existence of a hot galactic corona, it must be patchy in nature.