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Observation of Anisotropy in the Arrival Direction Distribution of TeV Cosmic Rays with HAWC  [PDF]
S. Y. BenZvi,D. W. Fiorino,S. Westerhoff,for the HAWC Collaboration
Physics , 2015,
Abstract: The High-Altitude Water Cherenkov (HAWC) Observatory, located 4100 m above sea level near Sierra Negra (19$^\circ$ N) in Mexico, is sensitive to gamma rays and cosmic rays at TeV energies. The arrival direction distribution of cosmic rays at these energies shows significant anisotropy on several angular scales, with a relative intensity ranging between 10$^{-3}$ and 10$^{-4}$. We present the results of a study of cosmic-ray anisotropy based on more than 86 billion cosmic-ray air showers recorded with HAWC since June 2013. The HAWC cosmic-ray sky map, which has a median energy of 2 TeV, exhibits several regions of significantly enhanced cosmic-ray flux. We present the energy dependence of the anisotropy and the cosmic-ray spectrum in the regions of significant excess.
Observation of Anisotropy in the Arrival Direction Distribution of Cosmic Rays above TeV Energies with Icecube  [PDF]
S. Toscano,for the IceCube Collaboration
Physics , 2011, DOI: 10.1016/j.nima.2012.01.040
Abstract: Between May 2009 and May 2010, the IceCube neutrino detector recorded 32 billion of atmospheric muons generated in air showers produced by cosmic rays in the TeV energy range. With such high statistics sample it is possible to observe, for the first time in the southern hemisphere, an energy dependence in the Galactic cosmic ray anisotropy up to a few hundred TeV. This study shows that the same large-scale anisotropy observed at median energies around 20 TeV is not present at 400 TeV; the anisotropy observed at 400 TeV shows substantial differences with respect to that at lower energy. In addition to the large-scale features observed at 20 TeV in the form of strong dipole and quadrupole moments, the data include several localized regions of excess and deficit on scales between 10{\degree} to 30{\degree}. The features observed at both large and small scale are statistically significant, but their origin is currently unknown.
A New Analysis Method for Reconstructing the Arrival Direction of TeV Gamma-rays Using a Single Imaging Atmospheric Cherenkov Telescope  [PDF]
R. W. Lessard,J. H. Buckley,V. Connaughton,S. Le Bohec
Physics , 2000, DOI: 10.1016/S0927-6505(00)00133-X
Abstract: We present a method of atmospheric Cherenkov imaging which reconstructs the unique arrival direction of TeV gamma rays using a single telescope. The method is derived empirically and utilizes several features of gamma-ray induced air showers which determine, to a precision of 0.12 degrees, the arrival direction of photons, on an event-by-event basis. Data from the Whipple Observatory's 10 m gamma-ray telescope is utilized to test selection methods based on source location. The results compare these selection methods with traditional techniques and three different camera fields of view. The method will be discussed in the context of a search for a gamma-ray signal from a point source located anywhere within the field of view and from regions of extended emission.
Observation of the anisotropy in arrival direction of Cosmic Rays with IceCube  [PDF]
S. Toscano for the IceCube Collaboration
Physics , 2010,
Abstract: The IceCube Neutrino Observatory is a kilometer-scale detector currently under construction at the South Pole. In its final configuration the detector will comprise 5160 Digital Optical Modules (DOMs) deployed on 86 strings between 1.5-2.5 km deep within the ice. While still incomplete, the detector has already recorded tens of billions of cosmic ray muons with a median energy of 20 TeV. This large sample has been used to study the arrival direction distribution of the cosmic rays. We report the observation of an anisotropy in the cosmic rays arrival direction at two different angular scales. The observed large scale anisotropy seems to be a continuation of similar structures observed in the Northern Sky by several experiments. IceCube observes also significant features on the angular scale of $20^{\circ} - 30^{\circ}$ that might be part of the larger scale structure.
The HAWC Gamma-Ray Observatory: Observations of Cosmic Rays  [PDF]
HAWC Collaboration,A. U. Abeysekara,R. Alfaro,C. Alvarez,J. D. álvarez,R. Arceo,J. C. Arteaga-Velázquez,H. A. Ayala Solares,A. S. Barber,B. M. Baughman,N. Bautista-Elivar,E. Belmont,S. Y. BenZvi,D. Berley,M. Bonilla Rosales,J. Braun,R. A. Caballero-Lopez,K. S. Caballero-Mora,A. Carrami?ana,M. Castillo,U. Cotti,J. Cotzomi,E. de la Fuente,C. De León,T. DeYoung,R. Diaz Hernandez,J. C. Díaz-Vélez,B. L. Dingus,M. A. DuVernois,R. W. Ellsworth,A. Fernandez,D. W. Fiorino,N. Fraija,A. Galindo,F. Garfias,L. X. González,M. M. González,J. A. Goodman,V. Grabski,M. Gussert,Z. Hampel-Arias,C. M. Hui,P. Hüntemeyer,A. Imran,A. Iriarte,P. Karn,D. Kieda,G. J. Kunde,A. Lara,R. J. Lauer,W. H. Lee,D. Lennarz,H. León Vargas,E. C. Linares,J. T. Linnemann,M. Longo,R. Luna-GarcIa,A. Marinelli,H. Martinez,O. Martinez,J. Martínez-Castro,J. A. J. Matthews,P. Miranda-Romagnoli,E. Moreno,M. Mostafá,J. Nava,L. Nellen,M. Newbold,R. Noriega-Papaqui,T. Oceguera-Becerra,B. Patricelli,R. Pelayo,E. G. Pérez-Pérez,J. Pretz,C. Rivière,D. Rosa-González,H. Salazar,F. Salesa,F. E. Sanchez,A. Sandoval,E. Santos,M. Schneider,S. Silich,G. Sinnis,A. J. Smith,K. Sparks,R. W. Springer,I. Taboada,P. A. Toale,K. Tollefson,I. Torres,T. N. Ukwatta,L. Villase?or,T. Weisgarber,S. Westerhoff,I. G. Wisher,J. Wood,G. B. Yodh,P. W. Younk,D. Zaborov,A. Zepeda,H. Zhou
Physics , 2013,
Abstract: We describe measurements of GeV and TeV cosmic rays with the High-Altitude Water Cherenkov Gamma-Ray Observatory, or HAWC. The measurements include the observation of the shadow of the moon; the observation of small-scale and large-scale angular clustering of the TeV cosmic rays; the prospects for measurement of transient solar events with HAWC; and the observation of Forbush decreases with the HAWC engineering array and HAWC-30.
Surveying the TeV sky with HAWC  [PDF]
Robert J. Lauer,for the HAWC collaboration
Physics , 2015,
Abstract: The High altitude Water Cherenkov (HAWC) Observatory has been completed and began full operation in early 2015. Located at an elevation of 4,100 m near the Sierra Negra volcano in the state of Puebla, Mexico, HAWC consists of 300 water tanks instrumented with 4 PMTs each. The array is optimized for detecting air showers produced by gamma rays with energies between 100 GeV and 100 TeV and can also be used to measure charged cosmic rays. A wide instantaneous field of view of ~2 steradians and a duty cycle >95% allow HAWC to survey two-thirds of the sky every day. These unique capabilities make it possible to monitor variable gamma-ray fluxes and search for gamma-ray bursts and other transient events, providing new insights into particle acceleration in galactic and extra-galactic sources. In this contribution, we will present first results from more than one year of observations with a partial array configuration. We will discuss how HAWC can map the gamma-ray sky as well as probe other physics including cosmic ray anisotropies and the search for signatures of dark matter annihilation.
Towards a Measurement of the $e^+e^-$ Flux above 1 TeV with HAWC  [PDF]
Segev BenZvi,Daniel Fiorino,Zigfried Hampel-Arias,Mehr Un Nisa,for the HAWC Collaboration
Physics , 2015,
Abstract: The High-Altitude Water Cherenkov (HAWC) Observatory records the air showers produced by cosmic rays and gamma rays at a rate of about 20 kHz. While the events observed by HAWC are 99.9% hadronic cosmic rays, this background can be strongly suppressed using topological cuts that preferentially select electromagnetic air showers. Using this capability of HAWC, we can create a sample of air showers dominated by gamma rays and cosmic electrons and positrons. HAWC is one of the few operating observatories capable of measuring showers produced by electron and positron primaries above 1 TeV, and can record these showers from two-thirds of the sky each day. We describe the sensitivity of HAWC to leptonic cosmic rays, and discuss prospects for the measurement of the combined $e^+e^-$ flux and possible approaches for positron and electron charge separation with the HAWC detector.
A minimal width of the arrival direction distribution of ultra-high energy cosmic rays detected with the Yakutsk array  [PDF]
A. A. Ivanov
Physics , 2015, DOI: 10.1088/0004-637X/804/2/122
Abstract: This paper presents the results of searches for anisotropy in arrival directions of ultra-high energy cosmic rays detected with the Yakutsk Array during the 1974--2008 observational period together with available data from other giant extensive air shower arrays working at present. A method of analysis based on a comparison of the minimal width of distributions in equatorial coordinates is applied. As a result, a hypothesis of isotropy in arrival directions is rejected at the $99.5\%$ significance level. The observed decrease in the minimal width of distribution can be explained by the presence of cosmic ray sources in energy intervals and sky regions according to the recent indications inferred from data of the Yakutsk Array and Telescope Array experiments.
Anisotropy of TeV Cosmic Rays and the Outer Heliospheric Boundaries  [PDF]
P. Desiati,A. Lazarian
Physics , 2011, DOI: 10.1088/0004-637X/762/1/44
Abstract: Cosmic rays in the energy range from about 10's GeV to several 100's TeV are observed on Earth with an energy-dependent anisotropy of order 0.01-0.1%, and a consistent topology that appears to significantly change at higher energy. The nearest and most recent galactic cosmic ray sources might stochastically dominate the observation and possibly explain a change in orientation of the anisotropy as a function of energy. However, the diffusion approximation is not able to explain its non-dipolar structure and, in particular, the significant contribution of small angular scale features. Particle propagation within the mean free path in the local interstellar medium might have a major role in determining the properties of galactic cosmic rays, such as their arrival distribution. In particular, scattering on perturbations induced in the local interstellar magnetic field by the heliosphere wake, may cause a re-distribution of anisotropic cosmic rays below about 100 TeV toward the direction of the elongated heliotail and of the local interstellar magnetic field in the outer heliosphere. Such scattering processes are considered responsible of the observed TeV cosmic ray global anisotropy and its fine angular structure.
Full-Sky Analysis of Cosmic-Ray Anisotropy with IceCube and HAWC  [PDF]
The HAWC Collaboration,The IceCube Collaboration
Physics , 2015,
Abstract: During the past two decades, experiments in both the Northern and Southern hemispheres have observed a small but measurable energy-dependent sidereal anisotropy in the arrival direction distribution of galactic cosmic rays. The relative amplitude of the anisotropy is $10^{-4} - 10^{-3}$. However, each of these individual measurements is restricted by limited sky coverage, and so the pseudo-power spectrum of the anisotropy obtained from any one measurement displays a systematic correlation between different multipole modes $C_\ell$. To address this issue, we present the preliminary status of a joint analysis of the anisotropy on all angular scales using cosmic-ray data from the IceCube Neutrino Observatory located at the South Pole ($90^\circ$ S) and the High-Altitude Water Cherenkov (HAWC) Observatory located at Sierra Negra, Mexico ($19^\circ$ N). We describe the methods used to combine the IceCube and HAWC data, address the individual detector systematics and study the region of overlapping field of view between the two observatories.
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