The unexpectedly high flux of cosmic-ray positrons detected at Earth may originate from nearby astrophysical sources, dark matter, or unknown processes of cosmic-ray secondary production. We report ...the detection, using the High-Altitude Water Cherenkov Observatory (HAWC), of extended tera–electron volt gamma-ray emission coincident with the locations of two nearby middle-aged pulsars (Geminga and PSR B0656+14). The HAWC observations demonstrate that these pulsars are indeed local sources of accelerated leptons, but the measured tera–electron volt emission profile constrains the diffusion of particles away from these sources to be much slower than previously assumed. We demonstrate that the leptons emitted by these objects are therefore unlikely to be the origin of the excess positrons, which may have a more exotic origin.
To search for ultra-high-energy photons in primary cosmic rays, air shower observables are needed that allow a good separation between primary photons and primary hadrons. We present a new ...observable, Fγ, which can be extracted from ground-array data in hybrid events, where simultaneous measurements of the longitudinal and the lateral shower profile are performed. The observable is based on a template fit to the lateral distribution measured by the ground array with the template taking into account the complementary information from the measurement of the longitudinal profile, i.e. the primary energy and the geometry of the shower. Fγ shows a very good photon-hadron separation, which is even superior to the separation given by the well-known Xmax observable (the atmospheric depth of the shower maximum). At energies around 1 EeV (10 EeV), Fγ provides a background rejection better than 97.8 % (99.9 %) at a signal efficiency of 50 %. Advantages of the observable Fγ are its technical stability with respect to irregularities in the ground array (i.e. missing or temporarily non-operating stations) and that it can be applied over the full energy range accessible to the air shower detector, down to its threshold energy. Furthermore, Fγ complements nicely to Xmax such that both observables can well be combined to achieve an even better discrimination power, exploiting the rich information available in hybrid events.
To search for ultra-high-energy photons in primary cosmic rays, air shower observables are needed that allow a good separation between primary photons and primary hadrons. In this paper, we present a ...new observable, Fγ, which can be extracted from ground-array data in hybrid events, where simultaneous measurements of the longitudinal and the lateral shower profile are performed. The observable is based on a template fit to the lateral distribution measured by the ground array with the template taking into account the complementary information from the measurement of the longitudinal profile, i.e. the primary energy and the geometry of the shower. Fγ shows a very good photon-hadron separation, which is even superior to the separation given by the well-known X max observable (the atmospheric depth of the shower maximum). At energies around 1 EeV (10 EeV), Fγ provides a background rejection better than 97.8 % (99.9 %) at a signal efficiency of 50 %. Advantages of the observable Fγ are its technical stability with respect to irregularities in the ground array (i.e. missing or temporarily non-operating stations) and that it can be applied over the full energy range accessible to the air shower detector, down to its threshold energy. Finally and furthermore, Fγ complements nicely to X max such that both observables can well be combined to achieve an even better discrimination power, exploiting the rich information available in hybrid events.
ABSTRACT A survey of the inner Galaxy region of Galactic longitude and latitude is performed using one-third of the High Altitude Water Cherenkov Observatory, operated during its construction phase. ...To address the ambiguities arising from unresolved sources in the data, we use a maximum likelihood technique to identify point source candidates. Ten sources and candidate sources are identified in this analysis. Eight of these are associated with known TeV sources but not all have differential fluxes that are compatible with previous measurements. Three sources are detected with significances >5 after accounting for statistical trials, and are associated with known TeV sources.
The sources of ultrahigh energy cosmic rays (CRs) are not yet known. However, the discovery of anisotropic CRs above 57 X 1018 eV by the Pierre Auger Observatory suggests that a direct source ...detection may soon be possible. The near-future prospects for such a measurement are heavily dependent on the flux of the brightest source. In this work, we show that the flux of the brightest source above 57 X 1018 eV is expected to comprise 10% or more of the total flux if two general conditions are true. The conditions are: (1) the source objects are associated with galaxies other than the Milky Way and its closest neighbors, and (2) the CR particles are protons or heavy nuclei such as iron and the Greisen-Zatsepin-Kuz'min effect is occurring. The Pierre Auger Observatory collects approximately 23 events above 57 X 1018 eV per year. Therefore, it is plausible that, over the course of several years, tens of CRs from a single source will be detected.
The sources of ultrahigh energy cosmic rays (CRs) are not yet known. However, the discovery of anisotropic CRs above 57 x 10{sup 18} eV by the Pierre Auger Observatory suggests that a direct source ...detection may soon be possible. The near-future prospects for such a measurement are heavily dependent on the flux of the brightest source. In this work, we show that the flux of the brightest source above 57 x 10{sup 18} eV is expected to comprise 10% or more of the total flux if two general conditions are true. The conditions are: (1) the source objects are associated with galaxies other than the Milky Way and its closest neighbors, and (2) the CR particles are protons or heavy nuclei such as iron and the Greisen-Zatsepin-Kuz'min effect is occurring. The Pierre Auger Observatory collects approximately 23 events above 57 x 10{sup 18} eV per year. Therefore, it is plausible that, over the course of several years, tens of CRs from a single source will be detected.
The High-Altitude Water Cherenkov (HAWC) Observatory is sensitive to gamma rays and charged cosmic rays at TeV energies. The detector is still under construction, but data acquisition with the ...partially deployed detector started in 2013. An analysis of the cosmic-ray arrival direction distribution based on 4.9 x 10 super(10) events recorded between 2013 June and 2014 February shows anisotropy at the 10 super(-4) level on angular scales of about 10degrees. The HAWC cosmic-ray sky map exhibits three regions of significantly enhanced cosmic-ray flux; two of these regions were first reported by the Milagro experiment. A third region coincides with an excess recently reported by the ARGO-YBJ experiment. An angular power spectrum analysis of the sky shows that all terms up to l = 15 contribute significantly to the excesses.
We introduce a method to constrain the characteristic angular size of the brightest cosmic-ray sources observed above 57 × 1018 eV. By angular size of a source, we mean the effective angular extent ...over which cosmic-rays from that source arrive at earth. The method is based on the small-scale (<10°) self-clustering of cosmic-ray arrival directions. The method is applicable to sparse data sets in which strong localizations of CR* directions are not yet observed. We show that useful constraints on the source size can be made in the near future and that these constraints are not strongly dependent on the assumed spatial distribution and luminosity function of the cosmic-ray sources. We suggest that an indication of the source size is quite telling. For example, an indication of the source size can be used to infer limits on the particle charge and intervening magnetic fields (not independently), both of which are not well constrained so far. This is possible because the source size is similar in scale to the magnetic deflection.
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