共查询到20条相似文献，搜索用时 46 毫秒
1.
We analyze the arrival directions of cosmic rays with energies E 0≈(1–4)×1017 eV and zenith angles θ≤53≤ detected with the Yakutsk extensive air shower (EAS) array during 1974–2001. We show that ～10% of them form many clusters correlated with the Supergalactic plane. Enhanced particle fluxes arrive from the Supergalactic plane and the regions that are symmetrically adjacent to it at angles ±b SG ≈ 6°.5. The relatively high concentrations of clusters of galaxies and quasars that bear a relationship to the large-scale structure of the Universe are observed in these regions. 相似文献
2.
James G. Bartlett 《Astrophysics and Space Science》2004,290(1-2):105-118
After briefly recalling the physics of the thermal and kinetic Sunyaev-Zel'dovich effects, I review some recent results and discuss future directions. Present achievements include the construction of Hubble diagrams and measurements of baryon fractions with catalogs consisting of tens of clusters, leading to reasonable constraints on the Hubble constant and the matter density. Detailed millimeter spectra and corresponding peculiar velocity limits have been obtained for ～ 10 clusters. A new generation of instruments based on interferometer arrays and bolometer cameras will soon open the way to SZ surveying, and near the turn of the decade the Planck mission will provide an almost all-sky catalog of more than 10,000 clusters distributed out to redshifts beyond unity. I finish with a discussion of several aspects of surveying and of the science targeted by these projects. 相似文献
3.
A. V. Glushkov 《Astronomy Letters》2003,29(3):142-152
We present the results of a harmonic analysis of and search for clusters in the arrival directions of cosmic rays with energies E0≥5×1016 eV and zenith angles ?≤53° detected at the Yakutsk extensive air shower (EAS) array from 1974 to 2002. We show that the phase of the first harmonic periodically varies greatly and takes on nonrandom values at a confidence level ≥4σ. These phases point to the Supergalactic plane (the Local supercluster of galaxies). 相似文献
4.
Analysis of the arrival directions of extensive air showers (EASs) detected on the EAS MSU array and the prototype of the
EAS-1000 array has revealed a region of enhanced flux of cosmic rays with PeV energies toward the pulsars PSR J1840+5640 and
LAT PSR J1836+5925 at a confidence level up to 4.5σ. The first pulsar was discovered almost 30 years ago and is a well-studied old radio pulsar at a distance of 1.7 kpc from
the Solar system. The second pulsar belongs to a new class of pulsars discovered by the Fermi Gamma-Ray Observatory whose
pulsations are seen neither in the X-ray nor in the radio bands, but only in the gamma-ray energy range (gamma-ray-only pulsars).
In our opinion, the existence of a region of enhanced cosmic-ray flux in the data sets obtained on two different arrays suggests
that the pulsars can make a noticeable contribution to the flux of Galactic cosmic rays with PeV energies. 相似文献
5.
6.
Paul Sommers 《Astroparticle Physics》2001,14(4):271-286
A cosmic ray observatory with full-sky coverage can exploit standard anisotropy analysis methods that do not work if part of the celestial sphere is never seen. In particular, the distribution of arrival directions can be fully characterized by a list of spherical harmonic coefficients. The dipole vector and quadrupole tensor are of special interest, but the full set of harmonic coefficients constitutes the anisotropy fingerprint that may be needed to reveal the identity of the cosmic ray sources. The angular power spectrum is a coordinate-independent synopsis of that fingerprint. The true cosmic ray anisotropy can be measured despite non-uniformity in celestial exposure, provided the observatory is not blind to any region of the sky. This paper quantitatively examines how the accuracy of anisotropy measurement depends on the number of arrival directions in a data set. 相似文献
7.
A. V. Uryson 《Astronomy Letters》2005,31(11):755-759
We estimate the detection rate of ultra-high-energy cosmic rays on ground-based arrays by assuming that the cosmic-ray sources are active galactic nuclei. We analyze the cases of detection of clusters, several particles that arrived, within the error limits, from the same area of the sky. The adopted model is shown to explain the detection rate of clusters on the AGASA array. 相似文献
8.
Structured Pc 1 signals propagate in the ionospheric F2 region duct from their secondary sources at high latitudes to lower latitudes. Propagation directions to low latitude stations can be inferred from measurements of polarization parameters. The analysis of five events recorded at two low latitude stations (L = 1.9) are presented. Direction of arrival measurements are used to investigate the spatial and temporal structure of Pc 1 sources. Results show a close relationship between the structure of events identified in the frequency-time representation and direction of arrival measurement patterns. Multiple sources are sometimes indicated. 相似文献
9.
We discuss the capability of ‘100 GeV’ class imaging atmospheric Cherenkov telescope (IACT) arrays as future powerful instruments of ground-based gamma-ray astronomy. It is assumed that the array is gathered from individually triggered quadrangular 4-IACT ‘cells’ with a linear size of about 100 m. The multi-cell concept allows coverage of large detection areas economically, and at the same time the effective exploitation of the stereoscopic approach of determination of the shower parameters using information obtained by several IACTs simultaneously. Determination of arrival directions of γ-ray primaries on an event-by-event basis with accuracy δθ ≤ 0.1° combined with high suppression efficiency (at both the hardware and software levels) of the background hadronic showers by a factor of ≈ 103, and large, up to 1 km2 collection areas, can provide minimum detectable energy fluxes of ≥ 100 GeV γ-rays from point sources down to 10−13 erg/cm2 s which is about 3 orders of magnitude lower than the current sensitivities achieved by the satellite-borne detectors at MeV and GeV energies. High sensitivities of multi-IACT arrays would partially compensate the limited efficiency of the technique for all-sky surveys, as well as allow study of moderately extended (≤ 1°) γ-ray sources. IACT arrays with minimum detectable fluence of ≥ 100 GeV γ-rays Sγ < 10−8 erg/cm2 are well suited for effective exploration of highly sporadic nonthermal phenomena from different classes of astrophysical objects on time-scales from ≤ 1 s to several minutes. 相似文献
10.
Previously we have reported the detection of a sequence of three particle arrival events at energies above 1019 eV during one day and from a small sky region by two extensive air shower (EAS) arrays. We have shown that the probability of a chance coincidence of the three events is very low and suggested the arrival of a short-lived particle beam at the Earth. Here we refine the energies of the recorded particles and discuss a possible beam formation mechanism that explains the observational results.
相似文献11.
Due to the action of the intervening cosmic magnetic fields, ultra-high energy cosmic rays (UHECRs) can be deflected in such a way as to create clustered energy-ordered filamentary structures in the arrival direction of these particles, the so-called multiplets. In this work we propose a new method based on the spherical wavelet transform to identify multiplets in sky maps containing arrival directions of UHECRs. The method is illustrated in simulations with a multiplet embedded in isotropic backgrounds with different numbers of events. The efficiency of the algorithm is assessed through the calculation of Type I and II errors. 相似文献
12.
P. Stewart 《Astrophysics and Space Science》1992,191(2):175-184
The behaviour of stellar orbits is examined under the influence of a fixed triaxial potential and a tidal force. Changes in the kinetic energies in the principal directions are computed as a function of tidal interaction times; the important resonances are identified. Resonant interactions are discussed in relation to clusters of galaxies. 相似文献
13.
The measurement of large scale anisotropies in cosmic ray arrival directions is generally performed through harmonic analyses of the right ascension distribution as a function of energy. These measurements are challenging due to the small expected anisotropies and meanwhile the relatively large modulations of observed counting rates due to experimental effects. In this paper, we present a procedure based on the shuffling technique to carry out these measurements, applicable to any cosmic ray detector without any additional corrections for the observed counting rates. 相似文献
14.
In the present paper various observational data are used to compare seven most common models proposed to describe the behaviour
of the interstellar extinction in different parts of the sky. The data for field stars, open and globular clusters are used
to select the most adequate model at different distances and for different directions in the Galaxy. The results of this study
allow to select the best models for particular distances and directions. They also make it possible to propose a synthesized
model describing the interstellar extinction.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
R. Campana E. Bernieri E. Massaro F. Tinebra G. Tosti 《Astrophysics and Space Science》2013,347(1):169-182
The minimal spanning tree (MST) algorithm is a graph-theoretical cluster-finding method. We previously applied it to γ-ray bidimensional images, showing that it is quite sensitive in finding faint sources. Possible sources are associated with the regions where the photon arrival directions clusterize. MST selects clusters starting from a particular “tree” connecting all the point of the image and performing a cut based on the angular distance between photons, with a number of events higher than a given threshold. In this paper, we show how a further filtering, based on some parameters linked to the cluster properties, can be applied to reduce spurious detections. We find that the most efficient parameter for this secondary selection is the magnitude M of a cluster, defined as the product of its number of events by its clustering degree. We test the sensitivity of the method by means of simulated and real Fermi-Large Area Telescope (LAT) fields. Our results show that $\sqrt{M}$ is strongly correlated with other statistical significance parameters, derived from a wavelet based algorithm and maximum likelihood (ML) analysis, and that it can be used as a good estimator of statistical significance of MST detections. We apply the method to a 2-year LAT image at energies higher than 3 GeV, and we show the presence of new clusters, likely associated with BL Lac objects. 相似文献
16.
Riccardo Campana Enrico Massaro Dario Gasparrini † Sara Cutini † Andrea Tramacere 《Monthly notices of the Royal Astronomical Society》2008,383(3):1166-1174
We developed a source detection algorithm based on the Minimal Spanning Tree (MST), that is a graph-theoretical method useful for finding clusters in a given set of points. This algorithm is applied to γ-ray bi-dimensional images where the points correspond to the arrival direction of photons, and the possible sources are associated with the regions where they clusterize. Some filters to select these clusters and to reduce the spurious detections are introduced. An empirical study of the statistical properties of MST on random fields is carried out in order to derive some criteria to estimate the best filter values. We also introduce two parameters useful to verify the goodness of candidate sources. To show how the MST algorithm works in practice, we present an application to an EGRET observation of the Virgo field, at high Galactic latitude and with a low and rather uniform background, in which several sources are detected. 相似文献
17.
The detection of high‐energy astro‐physical neutrinos of extraterrestrial origin by the IceCube neutrino observatory in Antarctica has opened a unique window to the cosmos that may help to probe both the distant Universe and our cosmic backyard. The arrival directions of these high‐energy events have been interpreted as uniformly distributed on the celestial sphere. Here, we revisit the topic of the putative isotropic angular distribution of these events applying Monte Carlo techniques to investigate a possible anisotropy. A modest evidence for anisotropy is found. An excess of events appears projected towards a section of the Local Void, where the density of galaxies with radial velocities below 3000 km s–1 is rather low, suggesting that this particular group of somewhat clustered sources are located either very close to the Milky Way or perhaps beyond 40 Mpc. The results of further analyses of the subsample of southern hemisphere events favour an origin at cosmological distances with the arrival directions of the events organized in a fractal‐like structure. Although a small fraction of closer sources is possible, remote hierarchical structures appear to be the main source of these very energetic neutrinos. Some of the events may have their origin at the IBEX ribbon. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
18.
The spatial distribution of magnetic CP stars is studied using the catalog of CP stars compiled by the Special Astrophysical Observatory of the Russian Academy of Sciences. It is shown that the overwhelming majority of these objects are at distances closer than 500 pc from the sun. The axes of rotation and the magnetic axes of the field CP stars are distributed randomly, while a preferential orientation of the magnetic axes in certain directions is seen for stars that are members of open clusters. The closeness of the magnetic models for CP stars that are members of clusters may be evidence that there is a general factor which influences the origin of magnetic fields in stars. The magnetic field of the galaxy may be such a factor. 相似文献
19.
E. Gaburov A. Gualandris S. Portegies Zwart 《Monthly notices of the Royal Astronomical Society》2008,384(1):376-385
We study the circumstances under which first collisions occur in young and dense star clusters. The initial conditions for our direct N -body simulations are chosen such that the clusters experience core collapse within a few million years, before the most massive stars have left the main sequence. It turns out that the first collision is typically driven by the most massive stars in the cluster. Upon arrival in the cluster core, by dynamical friction, massive stars tend to form binaries. The enhanced cross-section of the binary compared to a single star causes other stars to engage the binary. A collision between one of the binary components and the incoming third star is then mediated by the encounters between the binary and other cluster members. Due to the geometry of the binary–single star engagement the relative velocity at the moment of impact is substantially different than in a two-body encounter. This may have profound consequences for the further evolution of the collision product. 相似文献
20.
S. K. Gupta P. V. Ramana Murthy S. C. Tonwar P. R. Vishwanath 《Astrophysics and Space Science》1985,115(1):163-183
Detection of cosmic sources of very high energy gamma rays based on the atmospheric Cerenkov technique is discussed. Very high energy gamma-rays initiate, on entering the terrestrial atmosphere, electron-photon cascade showers with in turn produce Cerenkov photons in the air. Parabolic reflectors are used to focus these photons onto fast photomultipliers. Two methods of deployment of parabolic reflectors are in vogue: one in which all the reflectors are located close to each other in a compact array and the other in which the reflectors are spread out farther apart forming a distributed array. In the latter mode, the arrival direction of individual showers can be determined accurately by using the measured relative arrival times between different detectors. Detailed studies with the distributed array helped us to understand the various parameters in the two designs and evaluate their relative merits in reaching the ultimate goals of lowering the energy threshold and improving the signal to background ratio for the detection of gamma-ray sources. It is found that the relative superiority among the two types of arrays is a function of the exponent assumed for the differential power law energy spectrum for the gamma ray source. It is also seen that with the type of reflectors commonly used in atmospheric Cerenkov work, lower energy thresholds can be achieved with use of larger aperture. 相似文献