Home OALib Journal OALib PrePrints Submit Ranking News My Lib FAQ About Us Follow Us+
 Title Keywords Abstract Author All
Search Results: 1 - 10 of 100 matches for " "
 Page 1 /100 Display every page 5 10 20 Item
 CASA-MIA Collaboration Physics , 1997, DOI: 10.1103/PhysRevLett.79.1805 Abstract: Diffuse ultrahigh energy gamma-radiation can arise from a variety of astrophysical sources, including the interaction of extremely high energy cosmic rays with the 3K microwave background radiation or the collapse of topological defects created in the early Universe. We describe a sensitive search for diffuse gamma-rays at ultrahigh energies using the CASA-MIA experiment. An isotropic flux of radiation is not detected, and we place stringent upper limits on the fraction of the gamma-ray component relative to cosmic rays (less than one part in 10,000) at energies from 570 TeV to 55,000 TeV. This result represents the first comprehensive constraint on the gamma-ray flux at these energies.
 Physics , 1996, DOI: 10.1086/310883 Abstract: VHE (Very High Energy, 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 gamma-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 gamma-radiation and, unlike the original VHE radiation, IS observable. Motivated by new measurements of the extragalactic MeV-GeV diffuse gamma-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 AGN in fact accounts for most of the observed GeV background, these limits strengthen and rule out AGN emission scenarios which produce significant power above 300 GeV.
 Physics , 2013, Abstract: More than 90% of the Galactic gas-related gamma-ray emissivity above 1 GeV is attributed to the decay of neutral pions formed in collisions between cosmic rays and interstellar matter, with lepton-induced processes becoming increasingly important below 1 GeV. Given the high-quality measurements of the gamma-ray emissivity of local interstellar gas between ~50 MeV and ~4 GeV obtained with the Large Area Telescope on board the Fermi space observatory, it is timely to re-investigate this topic in detail, including the hadronic production mechanisms. The emissivity spectrum will allow the interstellar cosmic-ray spectrum to be determined reliably, providing a reference for origin and propagation studies as well as input to solar modulation models. A method for such an analysis and illustrative results are presented.
 Physics , 2012, DOI: 10.1051/0004-6361/201218882 Abstract: Long duration gamma-ray bursts (GRBs) allow us to explore the distant Universe, and are potentially the most effective tracer of the most distant objects. Our current knowledge of the properties of GRB host galaxies at redshifts >5 is very scarce. We propose to improve this situation by obtaining more observations of high-redshift hosts to better understand their properties and help enable us to use GRBs as probes of the high-redshift universe. We performed very deep photometric observations of three high-redshift GRB host galaxies, GRB 080913 at z =6.7, GRB 060927 at z =5.5 and GRB 060522 at z =5.1. In addition, we completed deep spectroscopic observations of the GRB080913 host galaxy with X-Shooter at the VLT to search for Ly-alpha emission. For the sake of the discussion, we use published results on another high-redshift GRB host, GRB 050904 at z = 6.3. The sample of GRB host galaxies studied in this paper consists of four out of the five spectroscopically confirmed GRBs at z>5. Despite our presented observations being the deepest ever reported of high-redshift GRB host galaxies, we do not detect any of the hosts, neither in photometry nor in spectroscopy in the case of GRB 080913. These observations indicate that the GRB host galaxies seem to evolve with time and to have lower SFRs at z >5 than they have at z<1. In addition, the host galaxy of GRB 080913 at z =6.7 does not show Ly-alpha emission. While the measured properties of the galaxies in our sample agree with the properties of the general galaxy population at z>5, our observations are not sufficiently sensitive to allow us to infer further conclusions on whether this specific population is representative of the general one. The characterization of high-redshift GRB host galaxies is a very challenging endeavor requiring a lot of telescope time, but is necessary to improve our understanding of the high-redshift universe.
 Physics , 2012, DOI: 10.1051/0004-6361/201219387 Abstract: This paper investigates what constraints can be placed on the fraction of Compton-thick (CT) AGN in the Universe from the modeling of the spectrum of the diffuse X-ray background (XRB). We present a model for the synthesis of the XRB that uses as input a library of AGN X-ray spectra generated by the Monte Carlo simulations described by Brightman & Nandra. This is essential to account for the Compton scattering of X-ray photons in a dense medium and the impact of that process on the spectra of obscured AGN. We identify a small number of input parameters to the XRB synthesis code which encapsulate the minimum level of uncertainty in reconstructing the XRB spectrum. These are the power-law index and high energy cutoff of the intrinsic X-ray spectra of AGN, the level of the reflection component in AGN spectra and the fraction of CT AGN in the Universe. We then map the volume of the space allowed to these parameters by current observations of the XRB spectrum in the range 3-100 keV. One of the least constrained parameters is the fraction of CT AGN. Statistically acceptable fits to the XRB spectrum at the 68% confidence level can be obtained for CT fractions in the range 5-50%. This is because of degeneracies among input parameters to the XRB synthesis code and uncertainties in the modeling of AGN spectra (e.g. reflection). The most promising route for constraining the fraction of CT AGN in the Universe is via the direct detection of those sources in high energy (>10keV) surveys. It is shown that the observed fraction of CT sources identified in the SWIFT/BAT survey, limits the intrinsic fraction of CT AGN, at least at low redshift, to 10-20% (68% confidence level). We also make predictions on the number density of CT sources that current and future X-ray missions are expected to discover. Testing those predictions will constrain the intrinsic fraction of CT AGN as a function of redshift.
 Physics , 2011, DOI: 10.1016/j.nuclphysbps.2011.04.109 Abstract: The observation of neutrinos from cosmic accelerators will be revolutionary. High energy neutrinos are closely connected to ultrahigh energy cosmic rays and their sources. Cosmic ray sources are likely to produce neutrinos and the propagation of ultrahigh cosmic rays from distant sources can generate PeV to ZeV neutrinos. We briefly review recent progress on the observations of ultrahigh energy cosmic rays and their implications for the future detections of high energy neutrinos.
 Physics , 2009, Abstract: We report the discovery of a luminous, mini radio halo of ~240 kpc dimension at the center of a distant cluster of galaxies at redshift z = 0.131. Our optical and multi-wavelength GMRT and VLA observations reveal a highly unusual structure showing a twin bubble-like diffuse radio halo surrounding a cluster of bright elliptical galaxies; very similar to the large-scale radio structure of M87, the dominant galaxy in Virgo cluster. It presents an excellent opportunity to understand the energetics and the dynamical evolution of such radio jet inflated plasma bubbles in the hot cluster atmosphere.
 Physics , 2014, DOI: 10.1103/PhysRevD.90.023007 Abstract: We study high-energy neutrino production in inner jets of radio-loud active galactic nuclei (AGN), taking into account effects of external photon fields and the blazar sequence. We show that the resulting diffuse neutrino intensity is dominated by quasar-hosted blazars, in particular, flat spectrum radio quasars, and that PeV-EeV neutrino production due to photohadronic interactions with broadline and dust radiation is unavoidable if the AGN inner jets are ultrahigh-energy cosmic-ray (UHECR) sources. Their neutrino spectrum has a cutoff feature around PeV energies since target photons are due to Ly$\alpha$ emission. Because of infrared photons provided by the dust torus, neutrino spectra above PeV energies are too hard to be consistent with the IceCube data unless the proton spectral index is steeper than 2.5, or the maximum proton energy is $\lesssim100$ PeV. Thus, the simple model has difficulty in explaining the IceCube data. For the cumulative neutrino intensity from blazars to exceed $\sim{10}^{-8}~{\rm GeV}~{\rm cm}^{-2}~{\rm s}^{-1}~{\rm sr}^{-1}$, their local cosmic-ray energy generation rate would be $\sim10-100$ times larger than the local UHECR emissivity, but is comparable to the averaged gamma-ray blazar emissivity. Interestingly, future detectors such as the Askaryan Radio Array can detect $\sim0.1-1$ EeV neutrinos even in more conservative cases, allowing us to indirectly test the hypothesis that UHECRs are produced in the inner jets. We find that the diffuse neutrino intensity from radio-loud AGN is dominated by blazars with gamma-ray luminosity of $\gtrsim10^{48}~{\rm erg}~{\rm s}^{-1}$, and the arrival directions of their $\sim1-100$ PeV neutrinos correlate with the luminous blazars detected by Fermi.
 Physics , 2015, DOI: 10.1051/0004-6361/201525677 Abstract: We compare the performance of several dust models in reproducing the dust spectral energy distribution (SED) per unit extinction in the diffuse interstellar medium (ISM). We use our results to constrain the variability of the optical properties of big grains in the diffuse ISM, as published by the Planck collaboration. We use two different techniques to compare the predictions of dust models to data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared emission spectrum to recover the dust extinction and the intensity of the interstellar radiation field (ISRF). Second, we infer the ISRF intensity from the total power emitted by dust per unit extinction, and then predict the emission spectrum. In both cases, we test the ability of the models to reproduce dust emission and extinction at the same time. We identify two issues. Not all models can reproduce the average dust emission per unit extinction: there are differences of up to a factor $\sim2$ between models, and the best accord between model and observation is obtained with the more emissive grains derived from recent laboratory data on silicates and amorphous carbons. All models fail to reproduce the variations in the emission per unit extinction if the only variable parameter is the ISRF intensity: this confirms that the optical properties of dust are indeed variable in the diffuse ISM. Diffuse ISM observations are consistent with a scenario where both ISRF intensity and dust optical properties vary. The ratio of the far-infrared opacity to the $V$ band extinction cross-section presents variations of the order of $\sim20\%$ ($40-50\%$ in extreme cases), while ISRF intensity varies by $\sim30\%$ ($\sim60\%$ in extreme cases). This must be accounted for in future modelling.
 Brian C. Lacki Physics , 2011, DOI: 10.1088/1475-7516/2012/01/054 Abstract: The only invariant speed in special relativity is c; therefore, if some neutrinos travel at even tiny speeds above c, normal special relativity is incomplete and any superluminal speed may be possible. I derive a limit on superluminal neutrino speeds v >> c at high energies by noting that such speeds would increase the size of the neutrino horizon. The increased volume of the Universe visible leads to a brighter astrophysical neutrino background. The nondetection of "guaranteed" neutrino backgrounds from star-forming galaxies and ultrahigh energy cosmic rays (UHECRs) constrains v/c at TeV--ZeV energies. I find that v/c <= 820 at 60 TeV from the nondetection of neutrinos from star-forming galaxies. The nondetection of neutrinos from UHECRs constrains v/c to be less than 2500 at 0.1 EeV in a pessimistic model and less than 4.6 at 4 EeV in an optimistic model. The UHECR neutrino background nondetection is strongly inconsistent with a naive quadratic extrapolation of the OPERA results to EeV energies. The limits apply subject to some caveats, particularly that the expected pionic neutrino backgrounds exist and that neutrinos travel faster than c when they pass the detector. They could be improved substantially as the expected neutrino backgrounds are better understood and with new experimental neutrino background limits. I also point out that extremely subluminal speeds would result in a much smaller neutrino background intensity than expected.
 Page 1 /100 Display every page 5 10 20 Item