Correlation of the highest-energy cosmic rays with the positions of nearby active galactic nuclei

TLDR: In this paper, the Pierre Auger Observatory data was used to confirm the anisotropy of the arrival direction of the highest-energy cosmic rays with the highest energy, which are correlated with the positions of relatively nearby active galactic nuclei (AGN) at a confidence level of more than 99%.

About: This article is published in Astroparticle Physics.The article was published on 2008-04-01 and is currently open access. It has received 415 citations till now. The article focuses on the topics: Pierre Auger Observatory & Ultra-high-energy cosmic ray.

Figures

Fig. 7. Comparison of the average log-likelihood per event in different energy intervals (calculated using the smoothed distribution of AGN) between real data and simulated samples (bottom lines isotropic distribution, top lines AGN distribution from V-C catalog).

Fig. 4. Distribution of angular separations to the closest AGN within 71 Mpc. The 6 events with jbj 6 12 have been shaded in grey. The average expectation for an isotropic flux is shown as the dashed line histogram. 0 20

Fig. 6. Number of pairs as a function of maximum separation angle a for the subset of 21 events with E > 57 EeV and jbj > 12 (points) and average expectation for AGN in the V-C catalogue with z 6 0:017 and the same cut in galactic latitude. The error bars on the AGN expectations represent the 90% confidence limit dispersion.

Fig. 8. Distribution of the deflections for protons in the BSS-S model of the galactic magnetic field. Left panel: 1000 directions drawn from an isotropic flux in proportion to the exposure of the Observatory, for E ¼ 60 EeV. Right panel: deflections of the 27 arrival directions of the observed events with E > 57 EeV.

Fig. 1. Likelihood ratio R as a function of the number of events observed in the prescribed test. The null hypothesis (isotropy) was rejected at the 99% likelihood threshold with 10 events. Shaded regions indicate expectations from isotropy at the 68% and 95% confidence limit.

Table 2 Number of AGN with z 6 0:017 in each of the exposure bands indicated in Fig. 2

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Correlation of the highest-energy cosmic rays with the positions of nearby active galactic nuclei" ?

Data collected by the Pierre Auger Observatory provide evidence for anisotropy in the arrival directions of the cosmic rays with the highest-energies, which are correlated with the positions of relatively nearby active galactic nuclei ( AGN ) [ Pierre Auger Collaboration, Science 318 ( 2007 ) 938 ]. The authors discuss the prospect of unequivocal identification of individual sources of the highest-energy cosmic rays within a few years of continued operation of the Pierre Auger Observatory. The angular scale of the correlation observed is a few degrees, which suggests a predominantly light composition unless the magnetic fields are very weak outside the thin disk of their galaxy. 

Q2. What is the effect of the interaction between protons and cosmic microwave background photons?

Protons with higher energies interact with cosmic microwave background photons to produce pions [3,4], which leads to a significant attenuation of their flux from more distant sources. 

Q3. How many arrival directions are expected to correlate with AGN positions?

Nineteen out of 21 arrival directions correlate with AGN positions while 5.0 are expected to do so by chance if the flux were isotropic. 

Q4. How much uncertainty in S is resulting from the adjustment of the shower size?

The uncertainty in S resulting from the adjustment of the shower size, the conversion to a reference angle, the fluctuations from shower-to-shower and the calibration curve amounts to about 18%. 

Q5. What is the horizon scale of the cosmic rays injected at the sources?

Regarding the possibility that the cosmic rays injected at the sources are heavy nuclei, attenuated mainly by photodisintegration processes, one may note that nuclei of the iron group have horizons only slightly smaller than the proton horizons, but intermediate mass nuclei (A ’ 20–40) have significantly smaller horizons (e.g. the horizon for a threshold energy of 60 EeV is about 60 Mpc for 28Si nuclei [43]). 

Q6. Why is the horizon smaller for decreasing nuclearmass?

The smaller horizon for decreasing nuclearmass is due to the corresponding decrease in the threshold required to excite the giant-dipole resonance for photodisintegration. 

Q7. What is the probability of a cosmic ray coming from a distant source?

Their scan in energies is motivated by the assumption that the highest-energy cosmic rays are those that are least deflected by intervening magnetic fields, and that they have a smaller probability to arrive from very distant sources due to the GZK effect [3,4]. 

Q8. What is the alternative method for detecting the evolution of the correlation signal?

An alternative standard technique in sequential analysis could also have been used to monitor the evolution of the correlation signal: the sequential likelihood ratio test [30,31]. 

Q9. How many events were expected to correlate with the AGN?

With this set of parameters there are 22 events among the 27 with E > 57 EeV that correlate with at least one of the 310 selected AGN, while only 7.4 were expected, on average, to do so by chance if the flux was isotropic (p ¼ 0:28). 

Q10. What is the reason for the steepening of the cosmic ray spectrum at the highest energy?

This provides evidence that the observed steepening of the cosmic ray spectrum at the highest-energies is due to the ‘‘GZK effect”, and not to acceleration limits at the sources. 

Q11. Why are deviations from the estimates above expected?

Deviations of the horizon scale from the estimates above are expected, in particular due to local departures of the sources from uniformity in spatial distribution, intrinsic luminosity, and spectral features. 

Q12. Why was the largest deviation from isotropic expectations in the data set?

The largest departure from isotropic expectations (minimum value of the probability P) in the complete data set was found to be due to correlation with AGN at a distance smaller than 71 Mpc and for cosmic rays with energies above 57 EeV. 

Q13. What is the way to determine if AGN are the sources of cosmic rays?

A significant increase in ultra-high energy cosmic ray statistics combined with searches for counterparts in a multi-wavelength and multi-messenger campaign should improve their ability to distinguish if AGN are the sources of cosmic rays or tracers of the sources. 

Q14. What is the probability that the correlation arose from an isotropic flux?

The chance probability that the observed correlation arose from an isotropic flux is much larger than P min, as alreadydiscussed in Section 2.3, because a scan was performed over a large parameter space to find the minimum of P.To account for the effects of the scan the authors built simulated sets each with equal number of arrival directions (81 in their case) drawn from an isotropic flux in proportion to the relative exposure of the Observatory, and counted the fraction of simulated sets which had, anywhere in the parameter space and under the same scan, equal or smaller values of P min than the minimum found in the data [18]. 

Q15. How many events were required to confirm the results at a statistically significant level?

Since the authors could not predict how many events would be required to confirm the results at a statistically significant level from the exploratory scan, the authors adopted a running prescription (with a pre-defined stopping rule) for conducting a sequential analysis with individual tests to be applied after the detection of each subsequent event passing their selection criteria.