Distribution Of Arrival Directions Research Articles

Searches for clustering in the time integrated skymap of the ANTARES neutrino telescope

This paper reports a search for spatial clustering of the arrival directions of high energy muon neutrinos detected by the ANTARES neutrino telescope. An improved two-point correlation method is used to study the autocorrelation of 3058 neutrino candidate events as well as cross-correlations with other classes of astrophysical objects: sources of high energy gamma rays, massive black holes and nearby galaxies. No significant deviations from the isotropic distribution of arrival directions expected from atmospheric backgrounds are observed.

PROBING OORT CLOUD AND LOCAL INTERSTELLAR MEDIUM PROPERTIES VIA DUST PRODUCED IN COMETARY COLLISIONS

The Oort Cloud remains one of the most poorly explored regions of the Solar System. We propose that its properties can be constrained by studying a population of dust grains produced in collisions of comets in the outer Solar System. We explore the dynamics of micron-size grains outside the heliosphere (beyond ~250 AU), which are affected predominantly by the magnetic field of the interstellar medium (ISM) flow past the Sun. We derive analytic models for the production and motion of small particles as a function of their birth location in the Cloud and calculate the particle flux and velocity distribution in the inner Solar System. These models are verified by direct numerical simulations. We show that grains originating in the Oort Cloud have a unique distribution of arrival directions, which should easily distinguish them from both interplanetary and interstellar dust populations. We also demonstrate that the distribution of particle arrival velocities is uniquely determined the mass distribution and dust production rate in the Cloud. Cometary collisions within the Cloud produce a flux of micron-size grains in the inner Solar System of up to several per square meter per year. The next-generation dust detectors may be sensitive enough to detect and constrain this dust population, which will illuminate us about the Oort Cloud's properties. We also show that the recently-detected mysterious population of large (micron-size) unbound particles, which seems to arrive with the ISM flow is unlikely to be generated by collisions of comets in the Oort Cloud.

Extensive Air Showers’ Arrival Direction Distribution by TBS Array

Arrival zenith angle distribution for the Extensive Air Showers (EAS) with a wide range of number of charged particles is studied using the experimental data obtained using the EAS 4-detector array TBS in Tbilisi. The station is a part of the GELATICA net in Georgia (GEorgian Large-area Angle and Time Coincidence Array). This experiment is devoted to the study of possible correlations in the arrival times and directions of separate EAS events over large distances and to the Primary Cosmic Ray energy spectrum investigation at very high energies. It is shown that the distribution function with the exponential dependence of showers’ flux on absorbing atmospheric depth provides a good approximation for the angular distribution despite the existing azimuth anisotropy of array. The dependence of the EAS absorption path estimation on the angular measure cutoff boundary is studied.

CORRELATIONS OF THE ARRIVAL DIRECTIONS OF ULTRA-HIGH ENERGY COSMIC RAYS WITH EXTRAGALACTIC OBJECTS AS OBSERVED BY THE TELESCOPE ARRAY EXPERIMENT

We search for correlations between positions of extragalactic objects and arrival directions of Ultra-High Energy Cosmic Rays (UHECRs) with primary energy $E \ge 40$ EeV as observed by the surface detector array of the Telescope Array (TA) experiment during the first 40 months of operation. We examined several public astronomical object catalogs, including the Veron-Cetty and Veron catalog of active galactic nuclei. We counted the number of TA events correlated with objects in each catalog as a function of three parameters: the maximum angular separation between a TA event and an object, the minimum energy of the events, and the maximum redshift of the objects. We determine combinations of these parameters which maximize the correlations, and calculate the chance probabilities of having the same levels of correlations from an isotropic distribution of UHECR arrival directions. No statistically significant correlations are found when penalties for scanning over the above parameters and for searching in several catalogs are taken into account.

Cosmic Rays of Extreme Energies

The origin of the highest energy cosmic rays is a challenging mystery defying decades of observations. The sources of these extremely energetic particles should be among the most powerful extragalactic sources. Extremely energetic cosmic rays (EECRs) reach interaction energies orders of magnitude beyond the LHC probing the frontiers of particle physics. Possible explanations for their origin have narrowed down with the confirmation of a GZK-like spectral feature. Hints of anisotropies in the distribution of arrival directions raise hopes for observing source images directly, however, composition measurements reported by Auger suggest a surprising interpretation. A clear resolution of this mystery calls for much larger statistics at extremely high energies beyond the reach of current observatories. A future space program can achieve several orders of magnitude in additional exposure to EECRs. The first step in such a program is the Extreme Universe Space Observatory at the Japanese Experiment Module (JEM-EUSO). JEM-EUSO is a large field of view ultraviolet telescope to be deployed at the International Space Station. JEM-EUSO can increase the annual exposure to EECRs by an order of magnitude.

Medium scale anisotropy in the TeV cosmic ray flux observed by ARGO-YBJ

Measuring the anisotropy of the arrival direction distribution of cosmic rays provides important information on the propagation mechanisms and the identification of their sources. In fact, the flux of cosmic rays is thought to be dependent on the arrival direction only due to the presence of nearby cosmic ray sources or particular magnetic-field structures. Recently, the observation of unexpected excesses at TeV energy down to an angular scale as narrow as ∼10° raised the possibility that the problem of the origin of Galactic cosmic rays may be addressed by studying the anisotropy. The ARGO-YBJ experiment is a full-coverage extensive air showers array, sensitive to cosmic rays with the energy threshold of a few hundred GeV. Searching for small-size deviations from the isotropy, the ARGO-YBJ Collaboration explored the declination region δ∼−20°–80°, making use of about 3.7×1011 events collected from November 2007 to May 2012. In this paper, the detection of different significant (up to 13 standard deviations) medium-scale anisotropy regions in the arrival directions of cosmic rays is reported. The observation was performed with unprecedented detail. The relative excess intensity with respect to the isotropic flux extends up to 10−3. The maximum excess occurs for proton energies of 10–20 TeV, suggesting the presence of unknown features of the magnetic fields the charged cosmic rays propagate through, or some contribution of nearby sources never considered so far. The observation of new weaker few-degree excesses throughout the sky region 195°≤R.A.≤290° is reported for the first time.1 MoreReceived 29 April 2013DOI:https://doi.org/10.1103/PhysRevD.88.082001© 2013 American Physical Society

HERMES: Simulating the propagation of ultra-high energy cosmic rays

The study of ultra-high energy cosmic rays (UHECR) at Earth cannot prescind from the study of their propagation in the Universe. In this paper, we present HERMES, the ad hoc Monte Carlo code we have developed for the realistic simulation of UHECR propagation. We discuss the modeling adopted to simulate the cosmology, the magnetic fields, the interactions with relic photons and the production of secondary particles. In order to show the potential applications of HERMES for astroparticle studies, we provide an estimation of the surviving probability of UHE protons, the GZK horizons of nuclei and the all-particle spectrum observed at Earth in different astrophysical scenarios. Finally, we show the expected arrival direction distribution of UHECR produced from nearby candidate sources. A stable version of HERMES will be released in the next future for public use together with libraries of already propagated nuclei to allow the community to perform mass composition and energy spectrum analysis with our simulator.

Strangelets and the TeV–PeV cosmic-ray anisotropies

Several experiments (e.g., Milagro and IceCube) have reported the presence in the sky of regions with significant excess in the arrival direction distributions of Galactic cosmic rays in the TeV-to-PeV energy range. Here we study the possibility that these hotspots are a manifestation of the peculiar nature of these cosmic rays, and of the presence of molecular clouds near the sources. We propose that stable quark matter lumps or so-called strangelets can be emitted in the course of the transition of a neutron star to a more compact astrophysical object. A fraction of these massive particles would lose their charge by spallation or electron capture in molecular clouds located in the immediate neighborhood of their source, and propagate rectilinearly without decaying further, hence inducing anisotropies of the order of the cloud size. With reasonable astrophysical assumptions regarding the neutron star transition rate, strangelet injection and neutralization rates, we can reproduce successfully the observed hotspot characteristics and their distribution in the sky.

On the influence of galactic magnetic fields on the arrival direction distribution of ultra-high-energy cosmic rays

We study the influence of galactic magnetic field (GMF) on both the correlation between ultra-high-energy cosmic rays (UHECRs) and active galactic nuclei (AGNs) and the clustering of UHECRs in the direction of Centaurus A (Cen A), both of which are observed in the arrival direction distribution of 69 UHECRs with energy E ≥ 55 EeV detected by the Pierre Auger Observatory. If UHECRs are dominated by protons, the correlation and the clustering are not affected significantly. However, if UHECRs are dominated by iron nuclei, the influence of the GMF becomes significant, and the correlation and the clustering may be fake due to the GMF while the arrival direction distribution outside of the GMF is compatible with the isotropy.

REVISIT OF CORRELATION ANALYSIS BETWEEN ACTIVE GALACTIC NUCLEI AND ULTRA-HIGH ENERGY COSMIC RAYS

We update the previous analysis of correlation between ultra-high energy cosmic rays (UHECR) and active galactic nuclei (AGN), using 69 UHECR events with energy E ≥ 55 EeV released in 2010 by Pierre Auger observatory and 862 AGN within the distance d ≤ 100 Mpc listed in the 13th edition of Véron-Cetty and Véron AGN catalog. To make the test hypothesis definite, we use the simple AGN source model in which UHECR are originated both from AGN, with the fraction fA, and from the isotropic background. We treat all AGN as equal sources of UHECR, and introduce the smearing angle θs to incorporate the effects of intervening magnetic fields. We compare the arrival direction distributions observed by Pierre Auger Observatory (PAO) and expected from the model by the correlational angular distance distribution (CADD) method. CADD method rules out the AGN dominance model with a small smearing angle (fA ≳ 0.7 and θs ≲ 6°). Concerning the isotropy, CADD shows that the distribution of PAO data is marginally consistent with isotropy. The best fit model lies around the AGN fraction fA = 0.4 and the moderate smearing angle θs = 10°. For the fiducial value fA = 0.7, the best probability of CADD was obtained at a rather large smearing angle θs = 46°. Our results imply that for the whole AGN to be viable sources of UHECR, either an appreciable amount of additional isotropic background or the large smearing effect is required. Thus, we try to bin the distance range of AGN to narrow down the UHECR sources and found that the AGN residing in the distance range 60–80 Mpc have good correlation with the updated PAO data. It is an indication that further study on the subclass of AGN as the UHECR source may be quite interesting.

Anisotropy studies in the cosmic ray proton flux with the PAMELA experiment

Using data taken by the Pamela experiment during 5 years of operation we studied the anisotropy in the arrival direction distribution of cosmic ray protons with rigidity above 40 GV. In this survey we used two different and independent techniques to investigate the large and medium anisotropy patterns in the proton spectrum. With both methods the observed distribution of arrival directions is consistent with the isotropic expectation and no significant evidence of strong anisotropies has been observed.

TIME-AVERAGE-BASED METHODS FOR MULTI-ANGULAR SCALE ANALYSIS OF COSMIC-RAY DATA

Over the past decade, a number of experiments dealt with the problem of measuring the arrival direction distribution of cosmic rays, looking for information on the propagation mechanisms and the identification of their sources. Any deviation from the isotropy may be regarded to as a signature of unforeseen or unknown phenomena, mostly if well localized in the sky and occurring at low rigidity. It induced experimenters to search for excesses down to angular scales as narrow as 10°, disclosing the issue of properly filtering contributions from wider structures. A solution commonly envisaged was based on time-average methods to determine the reference value of cosmic-ray flux. Such techniques are nearly insensitive to signals wider than the time window in use, thus allowing us to focus the analysis on medium- and small-scale signals. Nonetheless, the signal often cannot be excluded in the calculation of the reference value, which induces systematic errors. The use of time-average methods recently revealed important discoveries about the medium-scale cosmic-ray anisotropy, present both in the northern and southern hemispheres. It is known that the excess (or deficit) is observed as less intense than in reality and that fake deficit zones are rendered around true excesses because of the absolute lack of knowledge a priori of which signal is true and which is not. This work is an attempt to critically review the use of time-average-based methods for observing extended features in the cosmic-ray arrival distribution pattern.

OBSERVATION OF COSMIC-RAY ANISOTROPY WITH THE ICETOP AIR SHOWER ARRAY

We report on the observation of anisotropy in the arrival direction distribution of cosmic rays at PeV energies. The analysis is based on data taken between 2009 and 2012 with the IceTop air shower array at the South Pole. IceTop, an integral part of the IceCube detector, is sensitive to cosmic rays between 100 TeV and 1 EeV. With the current size of the IceTop data set, searches for anisotropy at the 10^-3 level can, for the first time, be extended to PeV energies. We divide the data set into two parts with median energies of 400 TeV and 2 PeV, respectively. In the low energy band, we observe a strong deficit with an angular size of about 30 degrees and an amplitude of (-1.58 +/- 0.46 (stat) +/- 0.52 (sys)) x 10^(-3) at a location consistent with previous observations of cosmic rays with the IceCube neutrino detector. The study of the high energy band shows that the anisotropy persists to PeV energies and increases in amplitude to (-3.11 +/- 0.38 (stat) +/- 0.96 (sys)) x 10^(-3).

Revisit to centaurus a as a point source of ultra-high-energy cosmic rays

The void regions found in the arrival direction distribution of ultra-high-energy cosmic rays (UHECRs) observed by the Pierre Auger Observatory (PAO) may be an important clue in UHECR source hunting. We claim that there is a circular void band structure around the Centaurus A (Cen A) direction. We incorporated this observed void structure in the background contribution into the Cen A dominance model to improve the fitting of the arrival direction distribution. The best fit is obtained for the Cen A contribution fraction fC = 0.14 (The observed fraction at PAO is fC,PAO = 0.27, which means that 18 out of 69 observed UHECR are contributed by Cen A.) and the smearing angle θs = 12° with a maximum likelihood of Lmax= 0.68. The estimate for the intergalactic magnetic fields in the vicinity of the Cen A direction based on the UHECRs contributed by Cen A is B ≈ 24Z−1(lc/Mpc)−1/2 nG.

Multi-scale TeV cosmic-ray anisotropy observed with the ARGO-YBJ experiment

The distribution of the cosmic-ray arrival direction provides essential information about the source distribution, the propagation medium and the diffusive process that primaries undergo. To the current knowledge of the local interstellar medium, cosmic rays in the rigidity range 1012 – 1013 V are re-accelerated within few hundreds of astronomic units, what makes the TeV anisotropy an important probe to investigate the outer space until that horizon. We report the observation of anisotropy in the TeV cosmic-ray arrival direction measured with the ARGO-YBJ experiment, at angular scales in the range 10° – 180° and with intensity 10−-4 – 10−-3. Findings of other experiments are confirmed and new details on the cosmic-ray distribution in the declination range −10° – 70° are given.

Search for cosmic ray electron-positron anisotropies with the Pamela data

Using data taken by the Pamela experiment during 5 years of operation we studied the anisotropy in the arrival direction of cosmic ray electrons and positrons with energy above 40 GeV. We report on a study of anisotropy in the e± flux at different angular scales extending from 30° up to 90°, furthermore a directional analysis has been performed around the Sun direction. The observed distribution of arrival directions is consistent with the isotropic expectation at any angular scale used in this study and no significant evidence of strong anisotropies has been observed, also the analysis around the Sun direction did not show any significant excess.

CENTAURUS A AS A POINT SOURCE OF ULTRAHIGH ENERGY COSMIC RAYS

We probe the possibility that Centaurus A (Cen A) is a point source of ultra-high energy cosmic rays (UHECR) observed by PAO, through the statistical analysis of the arrival direction distribution. For this purpose, we set up the Cen A dominance model for the UHECR sources, in which Cen A contributes the fraction $f_{\rm C}$ of the whole UHECR with energy above $5.5\times10^{19}\,{\rm eV}$ and the isotropic background contributes the remaining $1-f_{\rm C}$ fraction. The effect of the intergalactic magnetic fields on the bending of the trajectory of Cen A originated UHECR is parameterized by the gaussian smearing angle $\theta_s$. Using the correlational angular distance distribution (CADD), we identify the excess of UHECR in the Cen A direction and fit the CADD of the observed PAO data by varying two parameters $f_{\rm C}$ and $\theta_s$ of the Cen A dominance model. The best-fit parameter values are $f_{\rm C}\approx0.1$ (The corresponding Cen A fraction observed at PAO is $f_{\rm C,PAO}\approx0.15$, that is, about 10 out of 69 UHECR.) and $\theta_s=5^\circ$ with the maximum probability $P_{\rm max}=0.29$. Considering the uncertainty concerning the assumption of isotropic background in the Cen A dominance model, we extend the viable parameter ranges to the $2\sigma$ band, $0.09\lesssim f_{\rm C,PAO}\lesssim 0.25$ and $0^\circ\lesssim \theta_s\lesssim 20^\circ$. This result supports the existence of a point source extended by the intergalactic magnetic fields in the direction of Cen A. If Cen A is actually the source responsible for the observed excess of UHECR, the average deflection angle of the excess UHECR implies the order of $10\,{\rm nG}$ intergalactic magnetic field in the vicinity of Cen A.

Measurement of Cosmic Ray Spectrum and Anisotropy with ARGO-YBJ

The combined measurement of the cosmic ray (CR) energy spectrum and anisotropy in their arrival direction distribution needs the knowledge of the elemental composition of the radiation to discriminate between different origin and propagation models. Important information on the CR mass composition can be obtained studying the EAS muon content through the measurement of the CR rate at different zenith angles. In this paper we report on the observation of the anisotropy of galactic CRs at different angular scales with the ARGO-YBJ experiment. We report also on the study of the primary CR rate for different zenith angles. The light component (p+He) has been selected and its energy spectrum measured in the energy range (5 - 200) TeV for quasi-vertical events. With this analysis for the first time a ground-based measurement of the CR spectrum overlaps data obtained with direct methods for more than one energy decade, thus providing a solid anchorage to the CR spectrum measurements carried out by EAS arrays in the knee region. Finally, a preliminary study of the non-attenuated shower component at a zenith angle $\theta >$ 70$^{\circ}$ (through the observation of the so-called horizantal air showers) is presented.

Future plans for the Telescope Array experiment

The Telescope Array (TA) experiment is world's first and only air shower detector to be directly calibrated by an on-site accelerator beam. For wider and deeper understanding of cosmic rays via high precision measurements, we have several future plans for TA experiment. The first extension plan is an on-going project, called as TA low energy extension (TALE), to extend sensitive energy range to 10 16.5 eV in order to study second knee, predicted galactic-extragalactic transition of dominant sources and air shower phenomena comparing with LHC measurements. The second proposition is exchanges of FDs and SDs between TA and Pierre Auger Observatory, toward understanding systematic uncertainties of these experiments and to solve discrepancies in energy scales and Xmax. The third plan is a huge air shower array, the world observatory, consisting of a huge number of SDs and/or FDs for world's largest exposure and finest accuracy to open a new window on astronomy with ultra high energy particles. surface detectors (SDs) arrayed with a spacing of 1.2km between each SD in an area of approximately 680km 2 , and air fluorescence detectors (FDs) in three stations located around SDs facing inward and looking over array. The full operation of detectors have been started in March 2008. In TA experiment we have most valuable calibration facility which is a electron linear accelerator, called Electron Light Source (ELS) (2). ELS installed 100m away from south-east FD station (called BRM station) shoots calibrated electron beams for FDs. The typical energy per electron and number of electrons are 40MeV and 10 9 , respectively. With this equipment we will achieve an end to end calibration for FDs. After four years of very stable operations, we have successfully released our results in this conference, although some of these are still preliminary. The results are briefly summarized as follows: The energy spectrum has a clear ankle at 10 18.7 eV and a sharp cutoff above 10 19.7 eV. The flux is consistent with HiRes result. The shape of spectrum curve, it means spectral indexes and break points on curve, are fitted to spectrum curves by HiRes and that by Auger, However, to fit our curve with that by Auger we need to shift primary energies about 20%. Our preliminary results of mass composition analyses, which are averaged Xmax and their distributions, show that protons dominate primary composition of observed showers. This preliminary conclusion contradicts Auger's results (3), which show a transition of dominant composition from light to heavy, derived from averaged Xmax values and their distributions. From arrival direction distribution analyses, we do not observe

CONSTRAINTS ON THE ORIGIN OF COSMIC RAYS ABOVE 10 18 eV FROM LARGE-SCALE ANISOTROPY SEARCHES IN DATA OF THE PIERRE AUGER OBSERVATORY

A thorough search for large scale anisotropies in the distribution of arrival directions of cosmic rays detected above $10^{18}$ eV at the Pierre Auger Observatory is reported. For the first time, these large scale anisotropy searches are performed as a function of both the right ascension and the declination and expressed in terms of dipole and quadrupole moments. Within the systematic uncertainties, no significant deviation from isotropy is revealed. Upper limits on dipole and quadrupole amplitudes are derived under the hypothesis that any cosmic ray anisotropy is dominated by such moments in this energy range. These upper limits provide constraints on the production of cosmic rays above $10^{18}$ eV, since they allow us to challenge an origin from stationary galactic sources densely distributed in the galactic disk and emitting predominantly light particles in all directions.