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Ultrahigh-energy cosmic ray composition from the distribution of arrival directions

Rita C. dos Anjos, Jorge F. Soriano, Luis A. Anchordoqui, Thomas C. Paul, Diego F. Torres, John F. Krizmanic, Timothy A. D. Paglione, Roberto J. Moncada, Frederic Sarazin, Lawrence Wiencke, and Angela V. Olinto
Phys. Rev. D 98, 123018 – Published 26 December 2018
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  • INTRODUCTION
  • EXPERIMENTAL DATA
  • NUCLEUS PHOTODISINTEGRATION
  • DEFLECTIONS ON MAGNETIC FIELDS
  • TEST STATISTICS
  • POEMMA SENSITIVITY
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
    • References

    Abstract

    The sources of ultrahigh-energy cosmic rays (UHECRs) have been difficult to catch. It was recently pointed out that while sources of UHECR protons exhibit anisotropy patterns that become denser and compressed with rising energy, nucleus-emitting sources give rise to a cepa stratis (onionlike) structure with layers that become more distant from the source position with rising energy. The peculiar shape of the hot spots from nucleus accelerators is steered by the competition between energy loss during propagation and deflection on the Galactic magnetic field (GMF). Here, we run a full-blown simulation study to accurately characterize the deflections of UHECR nuclei in the GMF. We show that while the cepa stratis structure provides a global description of anisotropy patterns produced by UHECR nuclei en route to Earth, the hot spots are elongated depending on their location in the sky due to the regular structure of the GMF. We demonstrate that with a high-statistics sample at the high-energy end of the spectrum, like the one to be collected by NASA’s Probe Of Extreme Multi-Messenger Astrophysics mission, the energy dependence of the hot spot contours could become a useful observable to identify the nuclear composition of UHECRs. This new method to determine the nature of the particle species is complementary to those using observables of extensive air showers and therefore is unaffected by the large systematic uncertainties of hadronic interaction models.

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    • Received 22 October 2018

    DOI:https://doi.org/10.1103/PhysRevD.98.123018

    © 2018 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Cosmic rays & astroparticles
    Gravitation, Cosmology & Astrophysics

    Authors & Affiliations

    Rita C. dos Anjos1,2, Jorge F. Soriano1,3, Luis A. Anchordoqui1,3,4, Thomas C. Paul1, Diego F. Torres5,6,7, John F. Krizmanic8, Timothy A. D. Paglione3,4,9, Roberto J. Moncada4,10, Frederic Sarazin11, Lawrence Wiencke11, and Angela V. Olinto12,13

    • 1Department of Physics & Astronomy, Lehman College, City University of New York, New York 10468, USA
    • 2Departamento de Engenharias e Exatas, Universidade Federal do Paraná, 85950-000 Palotina, Brazil
    • 3Department of Physics, Graduate Center, City University of New York, New York 10016, USA
    • 4Department of Astrophysics, American Museum of Natural History, New York 10024, USA
    • 5Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Magrans s/n, 08193 Barcelona, Spain
    • 6Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain
    • 7Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
    • 8NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    • 9Department of Earth and Physical Sciences, York College, City University of New York, New York 11451, USA
    • 10Department of Physics, University of Miami, Coral Gables, Florida 33146, USA
    • 11Department of Physics, Colorado School of Mines, Golden, Colorado 80401, USA
    • 12Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637, USA
    • 13Enrico Fermi Institute and Kavli Institute for Cosmological Physics, University of Chicago, Chicago, Illinois 60637, USA

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    Issue

    Vol. 98, Iss. 12 — 15 December 2018

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    Images

    • Figure 1
      Figure 1

      Circles representing the composition-layered structure of hot spots at different energies, for proton sources (top) and nuclei sources (bottom). The radii of the circles respect the proportions of the angular sizes given by (1), for protons (black), helium (magenta), nitrogen (yellow), silicon (green) and iron (red), and for 40 (left), 70 (center) and 100 EeV (right).

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    • Figure 2
      Figure 2

      Sky map in Galactic coordinates of the Li-Ma significances of overdensities in 20° radius windows for 840 events recorded by Auger with E>EAuger and 130 events recorded by TA with E>ETA [35]. The color scale indicates the significance in units of standard deviations; negative values follow the convention of indicating the (positive) significance of deficits.

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    • Figure 3
      Figure 3

      Comparison of UHECR event locations with starburst (left) and radiogalaxies (right) in Galactic coordinates. The green points indicate the arrival directions of 231 events with E>52  EeV and zenith angle θ<80° detected by the Pierre Auger Observatory from January 1, 2004 up to March 31, 2014 [44]. The blue points indicate the arrival directions of 72 events with E>57  EeV and θ<55° recorded from May 11, 2008 to May 4, 2013 with TA [22]. The stars indicate the location of nearby starburst (left) and radiogalaxies (right). The shaded regions delimit angular windows around the sources of angular radius of 15°.

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    • Figure 4
      Figure 4

      Photodisintegration MFP on the CMB and EBL. The horizontal dashed lines indicate the distance to nearby starbursts and radiogalaxies.

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    • Figure 5
      Figure 5

      Sky maps in Mollweide projection of the distribution of arrival directions for selected SBGs (top) and radiogalaxies (bottom). The sky maps are in Galactic coordinates.

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    • Figure 6
      Figure 6

      Sky maps in Mollweide projection of the distribution of arrival directions for selected starbursts shown in Fig. 4 here indicated with yellow star. In all the cases we adopted a hard injection spectrum ∝E−1.5, setting a threshold for the minimum energy of Emin/EeV=40, 70, 100 from top to bottom. We have also imposed the cuts given in Table 2. The sky maps are in Galactic coordinates.

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    • Figure 7
      Figure 7

      Sky maps in Mollweide projection of the distribution of arrival directions for selected radiogalaxies also shown in Fig. 4 here indicated with a yellow star. In all the cases we adopted a hard injection spectrum ∝E−1.5, setting a threshold for the minimum energy of Emin/EeV=40, 70, 100 from top to bottom. We have also imposed the cuts given in Table 2. The sky maps are in Galactic coordinates.

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    • Figure 8
      Figure 8

      Probability distribution function histograms for the test statistic empirical distribution for the null hypothesis (proton), helium and iron, with N=20.

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    • Figure 9
      Figure 9

      CDF for the test statistic distribution for null hypotheses for various N. The dashed lines indicate the choice α=0.05 and the corresponding critical values of the test statistic.

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    • Figure 10
      Figure 10

      Power of the statistical test for different alternative hypotheses, i.e., different nuclei and number of events per hot spot. The horizontal axis on the top indicates the projected timescale for POEMMA.

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    • Figure 11
      Figure 11

      Power of the statistical test for a nitrogen alternative hypothesis as a function of the angular radius of the window around the source. The number of events in the sky is fixed to have an expected number of 10 events in a 13° radius window. The curve shows a fit to the data with an error function.

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    • Figure 12
      Figure 12

      Power of the statistical test for a nitrogen alternative hypothesis for a hard (γ=1.5) and a soft (γ=5) spectrum, as a function of the number of events in the hot spot. All the other parameters are unaltered with respect to those in Fig. 10.

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    • Figure 13
      Figure 13

      Ratio of the expected exposure of POEMMA after five years of operation and the exposure collected by the surface array of the Pierre Auger Observatory as reported at the 35th International Cosmic Ray Conference [24].

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