[Paper Review] Testing the correlations between ultra-high-energy cosmic rays and BL Lac type objects with HiRes stereoscopic data

Dmitry Gorbunov, P. Tinyakov+2 more|ArXiv.org|Jun 29, 2004

Astrophysics and Cosmic Phenomena25 references5 citations

TL;DR

This study tests correlations between ultra-high-energy cosmic rays (UHECRs) detected by the HiRes stereoscopic experiment and three samples of BL Lacertae (BL Lac) objects. Using angular correlation functions and Monte Carlo simulations, it finds significant correlation only with the largest sample (156 BL Lacs with V-mag < 18), suggesting a ~3% fraction of neutral primaries at E > 10¹⁹ eV, supporting BL Lacs as potential UHECR sources.

ABSTRACT

Previously suggested correlations of BL Lac type objects with the arrival directions of the ultra-high-energy cosmic ray primaries are tested by making use of the HiRes stereoscopic data. The results of the study support the conclusion that BL Lacs may be the cosmic ray sources and suggest the presence of a small (a few percent) fraction of neutral primaries at E&gt;10^{19} eV.

Motivation & Objective

• To test the hypothesis that ultra-high-energy cosmic rays (UHECRs) are correlated with BL Lac type objects using independent HiRes stereoscopic data.
• To assess whether the previously observed correlations between UHECRs and BL Lacs in AGASA and Yakutsk data are confirmed in a new, independent dataset.
• To determine the fraction of neutral UHECR primaries consistent with the observed correlations, given the angular resolution and exposure of HiRes.
• To evaluate the statistical significance of correlations across three distinct BL Lac samples, accounting for non-independence among the samples.
• To infer the nature of UHECR sources and the composition of the UHECR flux based on angular clustering and deflection constraints.

Proposed method

• Applied the angular correlation function method to compute the number of UHECR–BL Lac pairs within angular separation δ, using real HiRes data and Monte Carlo simulations.
• Generated random UHECR event distributions matching the HiRes detector’s acceptance (zenith and azimuthal distributions) to estimate background expectations.
• Calculated the probability P(δ) that the observed number of correlated pairs could arise from random uncorrelated distributions, using repeated Monte Carlo trials.
• Tested three pre-existing BL Lac samples: 22 most powerful, 14 γ-ray-loud candidates, and 156 objects with V-mag < 18, to assess consistency across different source selections.
• Used the minimum P(δ) value (at δ = 0.8°) to assess significance, with P(0.8) = 4×10⁻⁴ indicating strong evidence against the null hypothesis of no correlation.
• Estimated the fraction of neutral primaries consistent with the observed correlation, assuming that 8 of 271 HiRes events originate from BL Lacs (≈3%).

Experimental results

Research questions

• RQ1 Is there a statistically significant correlation between the arrival directions of UHECRs detected by HiRes and any of the three BL Lac samples?
• RQ2 Does the correlation signal observed in earlier datasets (AGASA, Yakutsk) persist in the independent HiRes stereoscopic dataset?
• RQ3 What fraction of the UHECR flux at E > 10¹⁹ eV is consistent with being neutral, given the observed correlation with BL Lacs?
• RQ4 Why do only some BL Lac samples show significant correlation, and what does this imply about the intrinsic properties of UHECR sources?
• RQ5 Can the observed angular clustering be explained by charged particles, or does it require a significant fraction of neutral primaries to avoid excessive deflection in Galactic magnetic fields?

Key findings

• Only the largest BL Lac sample (156 objects with V-mag < 18) shows a statistically significant correlation with HiRes UHECR events, with P(0.8°) = 4×10⁻⁴.
• The data count of 11 UHECR–BL Lac pairs within 1° is significantly higher than the Monte Carlo expectation of ~3 pairs from random background.
• The correlation signal is consistent with approximately 8 out of 271 HiRes events (≈3%) being neutral primaries originating from BL Lacs.
• The correlation is not observed in the smaller samples of 22 or 14 BL Lacs, indicating that only the broader, magnitude-limited sample shows a robust signal.
• The angular scale of the correlation (peaking at δ ≈ 0.8°) is much smaller than the typical deflection expected for charged UHECRs in the Galactic magnetic field, supporting a significant fraction of neutral primaries.
• The results are consistent with previous observations of clustering in AGASA and suggest that only a few hundred sources currently contribute to the UHECR flux.