Feasibility of ultra-high-energy cosmic ray backtracking through sparse local measurements of the Galactic magnetic field

Abstract

Planned and ongoing campaigns for the acquisition of high-quality local measurements of the Galactic magnetic field (GMF) at interstellar cloud locations have generated intense interest in the use of such measurements to accurately backtrack Ultra High-Energy Cosmic Rays (UHECR) through the Milky Way, a crucial aspect of charged-particle astronomy. However, the inherent sparsity of these measurements raises concerns regarding the feasibility of this approach. We assessed the achievable accuracy of UHECR backtracking using mock sparse local GMF data derived from the Jansson & Farrar 2012 (JF12) GMF model and mock UHECR events. We created mock UHECR datasets that trace back within a 3 degree angular range from the galaxy M82 (a hypothesized UHECR source), and we investigated the impact on such backtracking attempts of varying GMF measurement sparsity and of varying GMF strength, which we emulated by rescaling the strength of the ordered components of the JF12 model. We found that: (a) for an average GMF strength of $1μG$, satisfactory backtracking results for magnetic rigidities of $10^{20}$ eV can be obtained even with very sparse measurements ($ \sim 1600$ pc); (b) when the average GMF strength is significantly increased ($\sim$ factor of 10) the accuracy of backtracking breaks down at measurement spacings of 400 pc. These findings emphasize on one hand that sparsity is not an automatic deal-breaker for the utility of local GMF measurements in UHECR backtracking. On the other hand, we also confirm that important challenges remain on the path from sparse local GMF measurements to precise charge-particle astronomy, especially in directions of high-strength ordered magnetic fields. This underscores the importance of using all available complementary magnetic field measurements and sophisticated reconstruction techniques to enable accurate backtracking of UHECR.