Searcharxiv⌕ Search

SEARCH · Searcharxiv

Results for “astro-ph”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8Linked to original sources
arXiv2007

Analysis of the SN1987A two-stage explosion hypothesis with account for the MSW neutrino flavour conversion

Detection of 5 events by the Liquid Scintillation Detector (LSD) on February, 23, 1987 was interpreted in the literature as the detection of neutrinos from the first stage of the two-stage supernova collapse. We pose rigid constraints on the properties of the first stage of the collapse, taking into account neutrino flavour conversion due to the MSW-effect and general properties of supernova neutrino emission. The constraints depend on the unknown neutrino mass hierarchy and mixing angle θ_{13}.

astro-ph↗
arXiv2008

Gravitational Collapse to Toroidal and Higher Genus asymptotically AdS Black Holes

We match collapsing inhomogeneous as well as spatially homogeneous but anisotropic spacetimes to vacuum static exteriors with a negative cosmological constant and planar or hyperbolic symmetry. The collapsing interiors include the inhomogeneous solutions of Szekeres and of Barnes, which in turn include the Lemaitre-Tolman and the McVittie solutions. The collapse can result in toroidal or higher genus asymptotically AdS black holes.

gr-qc↗
arXiv2011

Age-Rotation-Activity Relations for M Dwarf Stars Based on ASAS Photometric Data

Based on analysis of photometric observations of nearby M type stars obtained with ASAS, 31 periodic variables were detected. The determined periods are assumed to be related to rotation periods of the investigated stars. Among them 10 new variables with periods longer than 10 days were found, which brings the total number of slowly rotating M stars with known rotation periods to 12 objects. X-ray activity and rotation evolution of M stars follows the trends observed in G-K type stars. Rapidly rotating stars are very active and activity decreases with increasing rotation period but the period-activity relation is mass-dependent which suggests that the rotation period alone is not a proper measure of activity. The investigated stars were grouped according to their mass and the empirical turnover time was determined for each group. It increases with decreasing mass more steeply than for K type stars for which a flat dependence had been found. The resulting Rossby number-activity relation shows an exponential decrease of activity with increasing Rossby number. The analysis of space motions of 27 single stars showed that all rapidly rotating and a few slowly rotating stars belong to young disk (YD) whereas all old disk (OD) stars are slowly rotating. The median rotation period of YD stars is about 2 days and that of OD stars is equal to 47 days, i.e. nearly 25 times longer. The average X-ray flux of OD stars is about 1.7 dex lower than YD stars in a good agreement with the derived Rossby number-activity formula supplemented with rotation-age relation and in a fair agreement with recent observations but in a disagreement with the Skumanich formula supplemented with the activity-rotation relation.

astro-ph↗
arXiv2007

Gamma-ray background: a review

The gamma-ray background is still a subject under great debate. All phenomena in the universe emitting gamma-rays can contribute directly as diffuse emission or as an isotropic component from unresolved point sources. The question of the origin of the extragalactic component cannot be answered without determining the galactic emission. To discuss in detail all models resulting in gamma-ray background contributions is far beyond the scope of this paper. Therefore the focus will be on recent publications on the extragalactic high energy (>100 MeV) part of the gamma-ray background.

astro-ph↗
arXiv2007

The Minerageny of Two Groups of Zircons from Plagioclase- Amphibolite of Mayuan Group in Northern Fujian

Zircons can crystallize in a wide range of physical and chemical conditions. At the same time, they have high stability and durability. Therefore zircons can grow and survive in a variety of geological processes. In addition, the diffusivity of chemical compositions in their crystals is very low. Consequently,we can trace back the evolution history of the planetary materials containing zircon with zircon U-Th-Pb geochronology and geochemistry studies. However, this depends on our ability to decipher its genesis, namely magmatic or metamorphic origins. In this paper, magmatic and metamorphic zircons were found from plagioclase-amphibolite samples. Their geneses have been determined by zircon morphology, chemical composition zonations and geological field setting combined with their zircon U-Th-Pb ages. We have found obvious differences in micro-scale Raman spectra between these magmatic and metamorphic zircons. The magmatic zircons exhibit a high sloping background in their Raman spectra, but the metamorphic zircons exhibit a low horizontal background in their Raman spectra, which suggest that the magmatic zircons may contain a much higher concentration of fluorescent impurities than the metamorphic zircons. Moreover, reverse variation trends in Raman spectrum peak intensities from core to rim of a crystal between the magmatic and metamorphic zircons have been found. We think that this can be attributed to their reverse chemical composition zonations. These differences can be used to distinguish magmatic and metamorphic zircons.

physics.geo-ph↗
arXiv2007

A Raman spectroscopic study of zircons on micro-scale and Its significance in explaining the origin of zircons

The magmatic and metamorphic zircons were investigated with Raman spectrum microprobe analysis. We found notable differences between these two kinds of zircons exhibited by the variation trend of Raman peak intensity from core to rim of a crystal. In magmatic zircons, the intensity and the ratio H/W of Raman spectrum peaks gradually decrease from core to rim of a crystal, which is produced by an increase in metamictization degree and suggests an increase in U and Th concentrations from core to rim. In metamorphic zircons, there are two kinds of crystals according to their Raman spectra: the first group of zircons exhibits a variation trend opposite to those of magmatic zircons, tending to increase in the Raman peak intensity and H/W value from core to rim of a crystal, which is produced by a decrease in metamictization degree and indicates a decrease of U and Th concentrations from core to rim of a crystal. The second group of zircons exhibits no change in Raman peak intensity and H/W value through a crystal. The data of infrared and Raman spectra of these crystals show that they are well crystallized and have no lattice destruction induced by metamictization, and are thought to crystallize in high temperature stages of metamorphism. During these stages, the U and Th ions have been removed by metamorphic fluids from the parent rocks of these zircons. The other difference between magmatic and metamorphic zircons is the background level of their Raman spectra, which is high and sloped in magmatic zircons, but low and horizontal in metamorphic zircons. The differences between magmatic and metamorphic zircons can be used to identify the genesis of zircons and understand the origin and evolution history of their parent rocks.

physics.geo-ph↗
arXiv2010

Discovering Relic Gravitational Waves in Cosmic Microwave Background Radiation

The authority of J. A. Wheeler in many areas of gravitational physics is immense, and there is a connection with the study of relic gravitational waves as well. I begin with a brief description of Wheeler's influence on this study. One part of the paper is essentially a detailed justification of the very existence of relic gravitational waves, account of their properties related to the quantum-mechanical origin, derivation of the expected magnitude of their effects, and reasoning why they should be detectable in the relatively near future. This line of argument includes the comparison of relic gravitational waves with density perturbations of quantum-mechanical origin, and the severe criticism of methods and predictions of inflationary theory. Another part of the paper is devoted to active searches for relic gravitational waves in cosmic microwave background radiation (CMB). Here, the emphasis is on the temperature-polarization TE cross-correlation function of CMB. The expected numerical level of the correlation, its sign, statistics, and the most appropriate interval of angular scales are identified. Other correlation functions are also considered. The overall conclusion is such that the observational discovery of relic gravitational waves looks like the matter of a few coming years, rather than a few decades.

gr-qc↗
arXiv2007

Education and Outreach for the Pierre Auger Observatory

The scale and scope of the physics studied at the Auger Observatory offer significant opportunities for original outreach work. Education, outreach, and public relations of the Auger collaboration are coordinated in a task of its own whose goals are to encourage and support a wide range of efforts that link schools and the public with the Auger scientists and the science of cosmic rays, particle physics, and associated technologies. This report focuses on the impact of the collaboration in Mendoza Province, Argentina, as: the Auger Visitor Center in Malargue that has hosted over 29,000 visitors since 2001, the Auger Celebration and a collaboration-sponsored science fair held on the Observatory campus in November 2005, the opening of the James Cronin School in Malargue in November 2006, public lectures, school visits, and courses for science teachers. As the collaboration prepares the proposal for the northern Auger site foreseen to be in southeast Colorado, plans for a comprehensive outreach program are being developed in parallel, as described here.

astro-ph↗
arXiv2007

Origin and propagation of cosmic rays (some highlights)

The detection of high-energy particles, cosmic rays (CRs), deep inside the heliosphere implies that there are, at least, three distinctly different stages in the lifetime of a CR particle: acceleration, propagation in the interstellar medium (ISM), and propagation in the heliosphere. Gamma rays produced by interactions of CRs with gas, radiation, and magnetic fields can be used to study their spectra in different locations. Still, accurate direct measurements of CR species inside the heliosphere (such as their spectra and abundances) are extremely important for the understanding of their origin and propagation. In this paper, an emphasis is made on very recent advances and especially on those where GLAST and PAMELA observations can lead to further progress in our understanding of CRs.

astro-ph↗
arXiv2007

Spitzer Observations of Powerful Radio Sources

We have measured the mid-infrared radiation from an orientation-unbiased sample of powerful 3C RR galaxies and quasars using the IRS and MIPS instruments aboard the Spitzer Space Telescope. We fit the Spitzer data as well as other measurements from the literature with synchrotron and dust components. At 15 microns, quasars are typically four times brighter than radio galaxies with the same isotropic radio power. Based on our fits, half of this difference can be attributed to the presence of non-thermal emission in the quasars but not the radio galaxies. The other half is consistent with dust absorption in the radio galaxies but not the quasars.

astro-ph↗
arXiv2008

Visible and dark matter in M 31 - II. A dynamical model and dark matter density distribution

In the present paper we derive the density distribution of dark matter (DM) in a well-observed nearby disc galaxy, the Andromeda galaxy. From photometrical and chemical evolution models constructed in the first part of the study (Tamm, Tempel & Tenjes 2007 (arXiv:0707.4375), hereafter Paper I) we can calculate the mass distribution of visible components (the bulge, the disc, the stellar halo, the outer diffuse stellar halo). In the dynamical model we calculate stellar rotation velocities along the major axis and velocity dispersions along the major, minor and intermediate axes of the galaxy assuming triaxial velocity dispersion ellipsoid. Comparing the calculated values with the collected observational data, we find the amount of DM, which must be added to reach an agreement with the observed rotation and dispersion data. We conclude that within the uncertainties, the DM distributions by Moore, Burkert, Navarro, Frenk & White (NFW) and the Einasto fit with observations nearly at all distances. The NFW and Einasto density distributions give the best fit with observations. The total mass of M 31 with the NFW DM distribution is 1.19*10^12 M_sun, the ratio of the DM mass to the visible mass is 10.0. For the Einasto DM distribution, these values are 1.28*10^12 M_sun and 10.8. The ratio of the DM mass to the visible mass inside the Holmberg radius is 1.75 for the NFW and the Einasto distributions. For different cuspy DM distributions, the virial mass is in a range 6.9-7.9*10^11 M_sun and the virial radius is ~150 kpc. The DM mean densities inside 10 pc for cusped models are 33 and 16 M_sun pc^-3 for the NFW and the Einasto profiles, respectively. For the cored Burkert profile, this value is 0.06 M_sun pc^-3.

astro-ph↗
arXiv2008

Visible and dark matter in M31 - I. Properties of stellar components

We construct a structural model of the Andromeda Galaxy, simultaneously corresponding to observed photometrical and kinematical data and chemical abundances. In this paper we present the observed surface brightness, colour and metallicity distributions, and compare them to the model galaxy. In Paper II (Tempel, Tamm & Tenjes 2007) we present similar data for the kinematics, and derive the mass distribution of the galaxy. On the basis of U, B, V, R, I and L luminosity distributions, we construct the model galaxy as a superposition of four axially symmetric stellar components: a bulge, a disc, an inner halo and an extended diffuse halo. By using far-infrared imaging data of M31 and a thin dust disc assumption, we derive dust-free surface brightness and colour distributions. We find the total absorption corrected luminosity of M31 to be L_B = (3.3+/-0.7)x10^10 L_sun, corresponding to an absolute luminosity M_B = -20.8+/-0.2 mag. Of the total luminosity, 41% (0.57 mag) is obscured from us by the dust inside M31. Using chemical evolution models, we calculate mass-to-light ratios of the components, correspoding to the colour indices and metallicities. We find the total intrinsic mass-to-light ratio of the visible matter to be M/L_B=3.1-5.8 M_sun/L_sun and the total mass of visible matter M_vis =(10-19)x10^10 M_sun. The use of the model parameters for a dynamical analysis and for determining dark matter distribution is presented in Paper II.

astro-ph↗
arXiv2007

Relative abundances of cosmic ray nuclei B-C-N-O in the energy region from 10 GeV/n to 300 GeV/n. Results from ATIC-2 (the science flight of ATIC)

The ATIC balloon-borne experiment measures the energy spectra of elements from H to Fe in primary cosmic rays from about 100 GeV to 100 TeV. ATIC is comprised of a fully active bismuth germanate calorimeter, a carbon target with embedded scintillator hodoscopes, and a silicon matrix that is used as the main charge detector. The silicon matrix produces good charge resolution for protons and helium but only partial resolution for heavier nuclei. In the present paper, the charge resolution of ATIC was improved and backgrounds were reduced in the region from Be to Si by using the upper layer of the scintillator hodoscope as an additional charge detector. The flux ratios of nuclei B/C, C/O, N/O in the energy region from about 10 GeV/nucleon to 300 GeV/nucleon obtained from this high-resolution, high-quality charge spectra are presented, and compared with existing theoretical predictions.

astro-ph↗
arXiv2007

HiRes Estimates and Limits for Neutrino Fluxes at the Highest Energies

The High Resolution Fly's Eye Experiment (HiRes) measures cosmic rays (CR) at the highest energies using the air fluorescence technique. As data taking on the Dugway Proving Grounds in Western Utah is finished, the HiRes data are relevant for cosmogenic neutrinos in two different ways. We first use our best fit to the measured HiRes CR spectrum together with a model of the extragalactic CR sources to derive the expected cosmogenic neutrino and gamma ray fluxes at Earth. We then use the HiRes data directly to set competitive experimental limits on the electron and tau neutrino fluxes at the highest energies.

astro-ph↗
arXiv2007

3D Reconstruction of Extensive Air Showers from Fluorescence Data

A new method to reconstruct the 3-dimensional structure of extensive air showers, seen by fluorescence detectors, is proposed. The observation of the shower is done in 2-dimensional pixels, for consecutive time bins. Time corresponds to a third dimension. Assuming that the cosmic ray shower propagates as a plane wave front moving at the speed of light, a complex 3D volume in space can be associated to each measured charge (per pixel and time bin). The 3D description in space allows a simultaneous access to the longitudinal and lateral profiles of each shower. In the case that several eyes observe the same shower, the method gives a straight-forward combination of all the information. This method is in an early phase of development and is not used for the general reconstruction of the Auger data.

astro-ph↗
arXiv2008

On the Transfer of Adiabatic Fluctuations through a Nonsingular Cosmological Bounce

We study the transfer of cosmological perturbations through a nonsingular cosmological bounce in a special model in which the parameters of the bounce and the equation of state of matter are chosen such as to allow for an exact calculation of the evolution of the fluctuations. We find that the growing mode of the metric fluctuations in the contracting phase goes over into the growing mode in the expanding phase, a result which is different from what is obtained in analyses in which fluctuations are matched at a singular hypersurface. Consequences for Ekpyrotic cosmology are discussed in a limit when the equation of state of a fluid becomes large.

hep-th↗