Recent Γ-ray Observations Research Articles

Radio haloes and relics from extended cosmic-ray ion distributions with strong diffusion in galaxy clusters

ABSTRACT A joint hadronic model is shown to quantitatively explain the observations of diffuse radio emission from galaxy clusters in the form of minihaloes, giant haloes, relics, and their hybrid, transitional stages. Cosmic-ray diffusion of order D ∼ 1031–32 cm2 s−1, inferred independently from relic energies, the spatial variability of giant-halo spectra, and the spectral evolution of relics, reproduces the observed spatio-spectral distributions, explains the recently discovered mega-haloes as enhanced peripheral magnetization, and quenches electron (re)acceleration by weak shocks or turbulence. For instance, the hard-to-soft evolution along secondary-electron diffusion explains both the soft spectra in most halo peripheries and relic downstreams, and the hard spectra in most halo centres and relic edges, where the photon index can reach α ≃ −0.5 regardless of the Mach number $\mathcal {M}$ of the coincident shock. Such spatio-spectral modelling, recent γ-ray observations, and additional accumulated evidence are thus shown to support a 2010 claim that the seamless transitions among minihaloes, giant haloes, and relics, their similar energetics, integrated spectra, and delineating discontinuities, the inconsistent $\mathcal {M}$ inferred from radio versus X-rays in leptonic models, and additional observations, all indicate that these diffuse radio phenomena are manifestations of the same cosmic-ray ion population, with no need to invoke less natural alternatives.

Fluctuation of the background sky in the Hubble Extremely Deep Field (XDF) and its origin

Abstract We performed a fluctuation analysis of the Hubble Extremely Deep Field (XDF) at four optical wavelength bands and found large fluctuations that are significantly brighter than those expected for ordinary galaxies. Good cross-correlations with flat spectra are found down to ${0{^{\prime\prime}_{.}}2}$, indicating the existence of a spatial structure even at the ${0{^{\prime\prime}_{.}}2}$ scale. The detected auto- and cross-correlations provide a lower limit of 24 nW m−2 sr−1 for the absolute sky brightness at 700–900 nm, which is consistent with previous observations. We searched for candidate objects to explain the detected large fluctuation using the catalog of the Hubble Ultra Deep Field (UDF), and found that the surface number density of faint compact objects (FCOs) rapidly increases toward the faint end. Radial profiles of FCOs are indistinguishable from the point spread function (PSF), and the effective radius based on de Vaucouleur’s law is estimated to be smaller than ${0{^{\prime\prime}_{.}}02}$. The spectral energy densities (SEDs) of FCOs follow a power law at optical wavelengths, but show greater emission and structure at $\lambda \gt 1\, \mu$m. Assuming that the FCOs are the cause of the excess brightness and fluctuations, the faint magnitude limit is 34.9 mag for the F775W band, and the surface number density reaches 2.6 × 103 arcsec−2. Recent γ-ray observations require that the redshift of FCOs must be less than 0.1, if FCOs are the origin of the excess optical and infrared background. Assuming that FCOs consist of missing baryons, the mass and luminosity of a single FCO range from 102 to 103 solar units, and the mass-to-luminosity ratio is significantly lower than 1.0 solar unit. The maximum effective radius of an FCO is 4.7 pc. These results and the good correlation between the near-infrared and X-ray background indicate that FCOs could be powered by the gravitational energy associated with black holes.

Explaining the cosmic-ray e+/( e− + e+) and [formula omitted] ratios using a steady-state injection model

We present a model of cosmic ray (CR) injection into the Galactic space based on recent γ-ray observations of supernova remnants (SNRs) and pulsar wind nebulae (PWNe) by the Fermi Large Area Telescope ( Fermi) and imaging atmospheric Cherenkov telescopes (IACTs). Steady-state injection of nuclear particles and electrons ( e −) from the Galactic ensemble of SNRs, and electrons and positrons ( e +) from the Galactic ensemble of PWNe are assumed, with their injection spectra inferred under guidance of γ-ray observations and recent development of evolution and emission models. The ensembles of SNRs and PWNe are assumed to share the same spatial distributions. Assessment of possible secondary CR contribution from dense molecular clouds interacting with SNRs is also given. CR propagation in the interstellar space is handled by GALPROP. Different underlying source distribution models and Galaxy halo sizes are employed to estimate the systematic uncertainty of the model. We show that this observation-based model reproduces the positron fraction e +/( e − + e +) and antiproton-to-proton ratio ( p ¯ / p ) reported by PAMELA and other previous missions reasonably well, without calling for any speculative sources. A discrepancy remains, however, between the total e − + e + spectrum measured by Fermi and our model below ∼20 GeV, for which the potential causes are discussed. Important quantities for Galactic CRs including their energy injection, average lifetime in the Galaxy, and mean gas density along their typical propagation path are also estimated.

The disk/jet connection in the enigmatic microquasar Cygnus X-3

AbstractSimultaneous multi-wavelength observations are crucial for understanding the physics of microquasars, especially the accretion disk/jet connection. The enigmatic microquasar Cygnus X-3 exhibits strong, relativistic jet ejection events producing radio flares up to 20 Jy. These events are preceded by a very soft X-ray state with quenched emission in the radio and hard X-ray bands. Recently, GeV flux was observed by the AGILE and Fermi γ-ray observatories during the newly-identified hypersoft state. By using an extensive database of simultaneous multi-wavelength observations gathered from Cygnus X-3 we can form a more unified picture of the nature of the source and show how the recent γ-ray observations fit into it.

Gamma-rays from neutron stars

The number of Galactic neutron stars is significant, N ∼ 108–9, but radiation from their surfaces is hard to detect. The nearest isolated neutron stars could be as close as ∼ 10 pc (the estimate sensitively depends on assumptions about initial velocities), but would probably be too old, and thus too cold, for optical detections. Thatγ-ray emission is a useful alternative window for neutron star studies was revealed through observations of pulsedγ-ray emission from the Crab and Vela pulsars (e.g. Bignami 1987). A wealth of recentγ-ray observations of neutron stars is provided by sophisticated experiments aboard the COMPTON Gamma Ray Observatory (CGRO) and several other spacecraft. We summarize the current status of the Galactic population ofγ-ray pulsars. Gamma ray bursts are also believed to be associated with neutron stars. Although data from BATSE aboard CGRO have recently challenged the standard paradigm thatγ-ray bursts (GRBs) originate on or near neutron stars in the Galactic disk, the possibility of bursts from an extended Galactic halo of high velocity neutron stars is still under consideration. Furthermore, many cosmological scenarios also invoke neutron stars as the energy source. We thus include GRBs in this review. A subset of GRBs, the soft gamma repeaters, has recently been associated with Galactic supernova remnants. Their properties and counterparts are discussed. In addition, we briefly describeγ-ray emission from slowly accreting neutron stars, and theγ-ray line afterglow resulting from production of radioactive isotopes during their birth.

High-energy neutrinos from extragalactic jets

In this paper I calculate the diffuse background of high-energy neutrinos from extragalactic jets emerging from active galactic nuclei (AGN). I assume that radio-quiet AGN have jets that are disrupted by turbulence in the vicinity of an accretion disk surrounding a central black hole and remain invisible in the radio band. In contrast, radio-loud AGN have high Mach number jets which dissipate in a strong shock far away from the AGN. It is assumed that protons are accelerated to ultra-high energies in jets. Photo-production of pions induces neutrino emission and electromagnetic cascades. The cascade spectra are in agreement with the most recent X- and γ-ray observations of AGN; in particular with the lack of γ-ray emission from radio-quiet AGN and the powerful γ-ray emission from flat-spectrum radio-loud AGN. A high energy neutrino background from radio-quiets is predicted assuming that they produce the cosmic diffuse X-ray background. However, the flux limits from the Fréjus proton decay experiment are violated by this prediction. On the other hand, recent γ-ray observations make it very plausible that the diffuse γ-ray background at least above 100 MeV is due to radio-loud AGN. A striking similarity exists between the energy fluxes of diffuse γ-rays above 100 MeV and cosmic ray protons above the ankle. This is an independent argument for proton acceleration in radio jets consistent with the explanation of the individual γ-ray spectra by hadronically induced cascades. The corresponding prediction of a neutrino flux at ultra-high energies therefore rests on a firm basis.

New Upper Limits on Intergalactic Infrared Radiation from High-Energy Astrophysics

Following our previously proposed technique, we have used the recent γ-ray observations of Mrk 421 obtained by the Whipple Observatory team and the EGRET experiment on Compton Gamma-Ray Observatory as a probe to place significant constraints on the intergalactic infrared energy density. Our 2 a upper limits are ∼10 -8 W m -2 sr -1 for the 1-5 μm range. These constraints are much better than those obtained by direct measurements. They rule out more exotic mechanisms for producing a large intergalactic IR flux and are consistent with radiation produced by stars in normal galaxies

Possible Evidence for Structured Acceleration of Cosmic Rays on a Galactic Scale from Recent γ-RAY Observations

Recent data from SAS-2 on the galactic -ray line flux as a function of longitude reveal a broad maximum in the region <= 300. These data, as unfolded here, imply that the low-energy (1-10 GeV) galactic cosmic-ray flux varies with the radial distance, , from the galactic center and is about an order of magnitude higher than the local value in a toroidal region for between 4 and 5 kpc. We further show that this enhancement can be plausibly accounted for by Fermi acceleration and compression caused by a hydrodynamic shock driven by the expanding gas in the "3-kpc" arm and invoked in some versions of galactic structure theory. Subject headings: cosmic rays - galactic structure - gamma rays