Search results for "COSMIC"
showing 10 items of 656 documents
On the microwave background anisotropy produced by big voids in open universes
1996
The Tolman-Bondi solution of the Einstein equations is used in order to model the time evolution of the void observed in Bo\"otes. The present density contrast of the central region ($\sim -0.75$) and its radius ($\sim 30h^{-1} \ Mpc$) are fixed, while the density parameter of the Universe, the amplitude of the density contrast inside the void wall, the width of this wall and the distance from the void centre to the Local Group are appropriately varied. The microwave background anisotropy produced by Bo\"otes-like voids is estimated for a significant set of locations. All the voids are placed far from the last scattering surface. It is shown that the anisotropy generated by these voids stro…
On the universality of void density profiles
2014
The massive exploitation of cosmic voids for precision cosmology in the upcoming dark energy experiments, requires a robust understanding of their internal structure, particularly of their density profile. We show that the void density profile is insensitive to the void radius both in a catalogue of observed voids and in voids from a large cosmological simulation. However, the observed and simulated voids display remarkably different profile shapes, with the former having much steeper profiles than the latter. Sparsity can not be the main reason for this discrepancy, as we demonstrate that the profile can be recovered with reasonable accuracy even with very sparse samples of tracers. On the…
A discrimination technique for extensive air showers based on multiscale, lacunarity and neural network analysis
2011
We present a new method for the identification of extensive air showers initiated by different primaries. The method uses the multiscale concept and is based on the analysis of multifractal behaviour and lacunarity of secondary particle distributions together with a properly designed and trained artificial neural network. In the present work the method is discussed and applied to a set of fully simulated vertical showers, in the experimental framework of ARGO-YBJ, to obtain hadron to gamma primary separation. We show that the presented approach gives very good results, leading, in the 1–10 TeV energy range, to a clear improvement of the discrimination power with respect to the existing figu…
Possible detection of a radio event correlated with a γ-ray burst
1977
COSMIC bursts of electromagnetic radiation, both isolated and in connection with other impulsive astrophysical phenomena, have been sought for many years1, with only one suggestive positive result2, until, in 1973, the discovery of γ-ray bursts3 of cosmic origin prompted a new series of observations4. In 1975 we started a systematic search for electromagnetic bursts at v.h.f. and u.h.f. The system, based at Medicina (Bologna, Italy), is semi-automatic and assures highly efficient data collection. Simultaneous observations at different frequencies facilitate the evaluation of disturbances of local origin and, hopefully, their isolation. Regular observations started in July 1976, and we repor…
Ortho-H2 and the age of prestellar cores
2013
Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but they are currently difficult to distinguish. One major difference is the timescale involved in forming the prestellar cores: some theories advocate nearly free-fall speed via, e.g., rapid turbulence decay, while others can accommodate much longer periods to let the gas accumulate via, e.g., ambipolar diffusion. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based…
Rule-guided identification of cosmic-ray patterns in PLASTEX
1992
Some techniques devised in the computer science fields of pattern recognition and expert systems are being applied to the interpretation of EAS responses in the PLASTEX experiment. An attempt is made to codity in a set of rules the expertise of trained researchers who are able to recognize and classify different hit patterns even in the presence of noisy background, and in spite of imperfections in the detector response. The patterns expected to be useful include, but are not limited to, track patterns. The software described here, as a progress report, automatically finds patterns corresponding to isolated tracks, and patterns composed of tracks that connect with each other in a layer of d…
Large-scale inhomogeneities may improve the cosmic concordance of supernovae
2010
We reanalyze the supernovae data from the Union Compilation including the weak lensing effects caused by inhomogeneities. We compute the lensing probability distribution function for each background solution described by the parameters Omega_M, Omega_L and w in the presence of inhomogeneities, approximately modeled with a single-mass population of halos. We then perform a likelihood analysis in the space of FLRW-parameters and compare our results with the standard approach. We find that the inclusion of lensing can move the best-fit model significantly towards the cosmic concordance of the flat LCDM model, improving the agreement with the constraints coming from the cosmic microwave backgro…
A self-consistent approach to the reflection component in 4U 1705-44
2010
High-resolution spectroscopy has recently revealed in many neutron-star Low-Mass X-ray binaries that the shape of the broad iron line observed in the 6.4-6.97 keV range is consistently well fitted by a relativistically smeared line profile. We show here spectral fitting results using a newly developed self-consistent reflection model on XMM-Newton data of the LMXB 4U 1705-44 during a period when the source was in a bright soft state. This reflection model adopts a blackbody prescription for the shape of the impinging radiation field, that we physically associate with the boundary layer emission. © 2010 American Institute of Physics.
Light majoron cold dark matter from topological defects and the formation of boson stars
2019
We show that for a relatively light majoron ($\ll 100 $ eV) non-thermal production from topological defects is an efficient production mechanism. Taking the type I seesaw as benchmark scheme, we estimate the primordial majoron abundance and determine the required parameter choices where it can account for the observed cosmological dark matter. The latter is consistent with the scale of unification. Possible direct detection of light majorons with future experiments such as PTOLEMY and the formation of boson stars from the majoron dark matter are also discussed.
Effects of non-uniform interstellar magnetic field on synchrotron X-ray and inverse-Compton γ-ray morphology of supernova remnants
2011
Context. Observations of SuperNova Remnants (SNRs) in X-ray and γ-ray bands promise to contribute important information to our understanding of the kinematics of charged particles and magnetic fields in the vicinity of strong non-relativistic shocks and, therefore, the nature of Galactic cosmic rays. The accurate analysis of SNR images collected in different energy bands requires theoretical modeling of synchrotron and inverse Compton emission from SNRs. Aims. We develop a numerical code (remlight) to synthesize, from MHD simulations, the synchrotron radio, X-ray, and inverse Compton γ-ray emission originating in SNRs expanding in a non-uniform interstellar medium (ISM) and/or non-uniform i…