[Paper Review] Search for Sources of Cosmic Rays in the Region of the Knee. II

Г. В. Куликов, M. Yu. Zotov|arXiv (Cornell University)|Oct 31, 2006

Astrophysics and Cosmic Phenomena1 references4 citations

TL;DR

This study applies the Alexandreas et al. method to EAS–1000 Prototype Array data (1997–1999) to search for cosmic ray source candidates near the energy 'knee' (~3 PeV). It identifies 561 zones of excessive flux (S > 3.0), with strong spatial correlations to known astrophysical sources like supernova remnants, pulsars, and AGNs within 3°, confirming prior findings and reinforcing the link between cosmic ray anisotropies and potential galactic and extragalactic sources near the knee energy region.

ABSTRACT

New results of an analysis of arrival directions of extensive air showers registered with the EAS--1000 Prototype array from August 1997 till February 1999 are presented. The method of Alexandreas et al., which has been used for analysis of data registered with CYGNUS, Milagrito, HEGRA AIROBICC, KASCADE and a number of other experiments, is employed. The existence of zones of excessive flux of cosmic rays with energies in the region of the knee is confirmed, as well as closeness of the zones to coordinates of possible astrophysical cosmic ray sources.

Motivation & Objective

• To investigate the origin of cosmic rays near the 'knee' in the energy spectrum (~3 PeV) using arrival direction data from the EAS–1000 Prototype Array.
• To test whether zones of excessive cosmic ray flux (ZEFs) identified via the method of random filtering in prior work are robust using a more statistically rigorous method.
• To assess spatial correlations between identified ZEFs and known astrophysical sources such as supernova remnants, pulsars, and active galactic nuclei.
• To determine whether the observed anisotropies in cosmic ray arrival directions are consistent with emission from localized astrophysical sources.

Proposed method

• The Alexandreas et al. method is applied to the full dataset of 1,366,010 EAS with known arrival directions, avoiding data filtering used in prior work.
• A background map is generated by 100 cycles of randomization: for each shower, a random arrival time is selected, and the corresponding right ascension and declination are recalculated to preserve the original declination distribution.
• The background map is constructed by averaging the number of EAS in 1°×1° cells across all randomized maps, ensuring isotropy in the background model.
• Zones of excessive flux (ZEFs) are identified by comparing real data to the background map using significance S = (N_real - N_bg)/√N_bg, with S > 3.0 as the threshold.
• Regular cells are defined with Δα ≈ Δδ / cos(δ̄) to ensure equal area across declination, enabling consistent comparison across the sky.
• Spatial coincidences between ZEFs and astrophysical sources (SNRs, pulsars, AGNs) are evaluated within angular distances of ≤3°.

Experimental results

Research questions

• RQ1 Do significant anisotropies in cosmic ray arrival directions persist when using the Alexandreas method instead of the prior method of random filtering?
• RQ2 Are the identified zones of excessive flux spatially correlated with known astrophysical sources such as supernova remnants, pulsars, and active galactic nuclei?
• RQ3 How do the results from the Alexandreas method compare with the earlier ZEFs identified via the method of random filtering in terms of spatial overlap and significance?
• RQ4 Is there a preferential alignment of ZEFs with the Galactic and Supergalactic planes, suggesting a connection to large-scale cosmic ray source structures?

Key findings

• The Alexandreas method identified 561 regular cells with significance S > 3.0, of which 364 had S > 3.1, confirming strong flux enhancements in the data.
• 364 of the 561 high-significance cells were grouped into 27 zones of excessive flux (A-zones), with 27 of these zones showing close spatial correspondence to ZEFs previously found via the method of random filtering.
• 18 out of 110 galactic supernova remnants (SNRs) with δ > -16° were found within 3° of A-zones, including well-known sources like Cas A, Tycho, and the Crab Nebula.
• 154 out of 694 pulsars and 139 out of 317 active galactic nuclei (AGNs) and interacting galaxies at z ≤ 0.01 were located within 3° of A-zones, indicating strong spatial clustering with potential cosmic ray sources.
• The Virgo cluster of galaxies lies within a large A-zone along the Supergalactic plane (α ≈ 170°–200°), suggesting possible association with extragalactic source populations.
• Only three of the 37 ZEFs from the method of random filtering had no close counterparts in the A-zones, while seven A-zones had no matching ZEFs, indicating high but not perfect correspondence between the two methods.