Using the “least-deflected” subsamples of ultra-high energy cosmic rays to constrain source population(s)

Abstract

Context. The source population(s) of ultra-high energy cosmic rays (UHECRs) with energies of ≥57 × 1018 eV (57 EeV) are still unknown and continue to be a subject of debate. The published arrival directions and energies of 303 UHECRs from the Pierre Auger Observatory and Telescope Array enable correlations with astronomical sources. However, deflections of UHECRs in the Galactic Magnetic Field (GMF) can be considerable, especially for heavy UHECR compositions. Aims. We aim to constrain the source(s) of UHECRs by focusing on the subset of UHECRs that experience minimal deflections by the Galactic magnetic field (GMF). Methods. We used Monte Carlo simulations of UHECR trajectories in a total of eight posited GMF models (and six UHECR compositions from H to Fe) to identify two subsamples of about 40 UHECRs. The H compositions of these subsamples suffer deflections of less than a few degrees in one (or most) of the GMFs. Both the total UHECR sample and the “least-deflected” (vs. the remaining “more-deflected”) subsamples of UHECRs were cross-correlated with catalogs of astronomical sources (at D ≲ 100 Mpc) and neutrino arrival directions. Results. For H compositions, the full sample of UHECRs is most closely correlated with Cen A, nearby (< 75 Mpc) radiogalaxies, and optically selected active galactic nuclei (AGNs); the least-deflected sub sample shows a most consistent correlation with nearby (D ≲ 25 Mpc) galaxies, although some GMF models show a preference for the IceCube muon-neutrino arrival detections. For oxygen compositions, the out-of-galaxy arrival directions of least-deflected UHECRs fall on the extragalactic plane (and thus close to nearby galaxies) in many GMFs. For Fe compositions, the out-of-galaxy UHECR arrival directions typically originate in a region ∼30 deg offset from Cen A, except in the case of the JF12 model; this offset can be decreased by re-scaling (by ∼50%) the magnitudes of the dipole and disk fields of the relevant GMFs.

Constraining astrophysical sources of intermediate-mass Ultra-high energy cosmic rays (UHECRs)

“…We are continuing the work of [1] (e.g., Fig. 1), who found an association between the (GMF deflection-corrected) arrival directions of the 'least-deflected' UHECRs with nearby galaxies when considering an oxygen composition.…”

“…Specifically, the angular deflections range from near zero to a few tens of degrees for H, increasing to hundreds of degrees for Fe. This is comparable to, or significantly larger than the error of the UHECR arrival direction determination (few degrees; e.g., [17], [18], [31], [1]).…”

“…As in [1], for calculation of the correlations of the original sources and the arrival directions, four classical models from [17] were used (AS-S, AS-A, BS-S, BS-A), three models proposed for [30] (AS-S Ring, AS-S Arm, and BS-S), and a model proposed for [31] (hereafter JF12).…”

“…The code used is a program to calculate UHECR trajectories in GMFs, started by Nagar & Matulich in 2007, and improved by [1]. We use Monte-Carlo simulations to backtrack UHECR trajectories in 8 different GMF models.…”

“…To correlate GMF-corrected arrival directions with candidate astronomical sources we use several relevant astronomical catalogs and the required distance cutoff of each mass composition. The catalogs are described in [1] and consist of The 105-Month Swift-BAT [27], Van Velzen radio galaxy [25], the Zaw optical AGN [34], the updated nearby galaxy [26], and a nearby supermassive black hole (SMBH) [35] catalog. Apart from the positions in these catalogs we also use fluxes and luminosities of the individual sources in our correlations.…”