📝 Original Info
- Title: Do Ultrahigh Energy Cosmic Rays Come from Active Galactic Nuclei and Fermi $gamma$-ray Sources?
- ArXiv ID: 1004.1877
- Date: 2014-11-20
- Authors: Researchers from original ArXiv paper
📝 Abstract
We study possible correlations between ultrahigh energy cosmic rays (UHECRs), observed by Auger, AGASA and Yakutsk, and nearby active galactic nuclei (AGNs) and $Fermi$ sources. We consider the deflection effects by a Galactic magnetic field (GMF) model constrained by the most updated measurements. We found that the average deflection angles of UHECRs by the Galactic magnetic fields are less than $4^\circ$. A correlation between the Auger cosmic-ray events and nearby AGNs with a significance level of $\sim 4\sigma$ was found for the Auger UHECR data sets with or without deflection correction. No correlation was found between the AGASA/Yakutsk events with nearby AGNs. Marginal correlations between the Auger events and the $Fermi$ sources, and between AGASA events and $Fermi$ AGNs were found when the deflections calculated by the GMF model were considered. However, no correlation was found between the Yakutsk data and $Fermi$ sources. Some $Fermi$ sources are close to the arrival directions of UHECR events detected by Auger, AGASA, and Yakutsk, most of which are probably chance coincidence rather than objects producing UHECRs in the nearby Universe. Four $Fermi$ sources, NGC 4945, ESO 323-G77, NGC 6951, and Cen A, within 100~Mpc have UHECR events within $3.1^{\circ}$ from their positions, which could potentially be cosmic ray accelerators. However, the association can only be confirmed if more UHECRs are preferably detected in these directions.
💡 Deep Analysis
Deep Dive into Do Ultrahigh Energy Cosmic Rays Come from Active Galactic Nuclei and Fermi $gamma$-ray Sources?.
We study possible correlations between ultrahigh energy cosmic rays (UHECRs), observed by Auger, AGASA and Yakutsk, and nearby active galactic nuclei (AGNs) and $Fermi$ sources. We consider the deflection effects by a Galactic magnetic field (GMF) model constrained by the most updated measurements. We found that the average deflection angles of UHECRs by the Galactic magnetic fields are less than $4^\circ$. A correlation between the Auger cosmic-ray events and nearby AGNs with a significance level of $\sim 4\sigma$ was found for the Auger UHECR data sets with or without deflection correction. No correlation was found between the AGASA/Yakutsk events with nearby AGNs. Marginal correlations between the Auger events and the $Fermi$ sources, and between AGASA events and $Fermi$ AGNs were found when the deflections calculated by the GMF model were considered. However, no correlation was found between the Yakutsk data and $Fermi$ sources. Some $Fermi$ sources are close to the arrival directi
📄 Full Content
The spectrum, origin, and composition of ultrahigh energy cosmic rays (UHECRs) with energies 10 19 eV (=10 EeV) are a long standing mystery in high-energy astrophysics (Hillas 1984). Greisen (1966) and Zatsepin & Kuz'min (1966) showed a theoretical distant limit for the cosmic rays with energies of order 10 20 eV traveling through the microwave background radiation field, which is called the GZK effect. Because of the GZK effect, particles with energies above 10 EeV are able to reach our Earth only from nearby sources within about 100 Mpc. Another barrier in the investigation of the UHECR origin is the deflections of UHE-CRs by the magnetic fields. Due to the poor knowledge of the extragalactic and intergalactic magnetic fields, the deflections of UHECRs have not yet understood. Dolag et al. (2004Dolag et al. ( , 2005) ) suggested that the deflections by extragalactic magnetic fields are generally less than 1 • , while Ryu et al. (2010) and Sigl et al. (2003) claimed that could be larger than 10 • . The Galactic magnetic fields (GMFs) are relatively better known (e.g. Han et al. 2006;Sun et al. 2008) and are widely discussed in the studies of UHECR origin (e.g. Stanev 1997;Tinyakov & Tkachev 2002;Prouza & Šmída 2003;Nagar & Matulich 2009). Kachelrieß et al. (2007) concluded that the deflections of UHECRs by the GMFs cannot be neglected even for the protons of E = 10 20 eV, since the deflection angles are comparable with the angular resolution of current experiments. Nagar & Matulich (2009) tried seven GMF models to study the correlations between UHE-CRs and source population(s). However, no halo component was included in the four GMF models they used and another three GMF models adopted from Sun et al. (2008) 1 National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China; hjl@nao.cas.cn 2 Graduate University of the Chinese Academy of Sciences, Beijing 100049, China have a strong halo component about 7 µG. Observational constraints on the Galactic magnetic field strength (Han & Qiao 1994;Han et al. 1999;Morris & Serabyn 1996) and the configuration of disk magnetic fields (Han et al. 2006;Han 2009) should be carefully considered in the GMF model.
Since the discovery of UHECRs (Linsley 1963), many equipments have been used to search for these events, including Fly’s Eye (Bird et al. 1994), Yakutsk Extensive Air Showers Array (Ivanov et al. 2003a;Pravdin et al. 2005), Akeno Giant Air Shower Array (AGASA; Hayashida et al. 2000;Takeda et al. 2003), High Resolution Fly’s Eye cosmic-ray detector (HiRes; Abbasi et al. 2004Abbasi et al. , 2008) ) and Pierre Auger Observatory (PAO; Abraham et al. 2004;The Pierre Auger Collaboration 2007, 2008). The existence of the GZK cutoff has been observed by the HiRes and Auger (Abbasi et al. 2008;The Pierre Auger Collaboration 2008).
Some objects have been suggested to be possible sources of UHECRs, e.g., pulsars (Blasi et al. 2000), active galactic nuclei (AGNs) and subclasses of AGNs (Protheroe & Szabo 1992;Farrar & Biermann 1998;Tinyakov & Tkachev 2001a,b;Virmani et al. 2002;Gorbunov et al. 2002Gorbunov et al. , 2004;;Abbasi et al. 2006;Farrar et al. 2009), radio lobes of FR II galaxies (Rachen & Biermann 1993;Hardcastle et al. 2009), and γ-ray bursts (Waxman 1995;Milgrom & Usov 1995). However, the real sources of UHECRs are not known yet. AGNs are favored as the most probable sources for accelerating particles to the extreme energies (Hillas 1984) for a long time.
Recently, The Pierre Auger Collaboration (2007Collaboration ( , 2008) studied the correlation between the arrival directions of UHECRs and the positions of nearby AGNs in the Véron- Cetty & Véron (2006) AGN catalog (hereafter VCV catalog). They concluded that the arrival directions of cosmic rays with energies above ∼ 60 EeV are anisotropic and UHECRs have a good correlation with the positions of nearby AGNs (z < 0.018). The intriguing result attracted much attention. Ivanov (2008) found the correlation between Yakutsk UHECRs and the nearby VCV AGNs ( 100 Mpc). George et al. (2008) investigated the correlation between the Swift Burst Alert Telescope AGN catalog with the Auger UHECR events, and found a correlation at a significance level of 98% when the AGNs were weighted by their hard X-ray flux and the Auger experiment exposure. However, some associated AGNs of Auger events may not have enough energy to accelerate particles to ultrahigh energies (Moskalenko et al. 2009). The High Resolution Flys Eye Collaboration searched for possible correlation between the HiRes UHECRs and AGNs located in the northern hemisphere; however, no significant correlation was found.
The F ermi high energy γ-ray sources are also possible UHECR sources. The recently released F ermi Large Area Telescope First Source Catalog (1FGL) contains 1451 γ-ray point sources (Abdo et al. 2010) with nearly uniform sky coverage (Atwood et al. 2009). Mirabal & Oya (2010) first investigat
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