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Extensive air showers with the small muon content in the region of ultrahigh energies
Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 245–248 www.elsevierphysics.com
Extensive air showers with the small muon content in the region of ultrahigh energies A.A. Mikhailov, N.N. Efremov, E.S. Nikiforovaa
∗
a
Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy, 31 Lenin Ave., 677980 Yakutsk, Russia Ultrahigh-energy extensive air showers registered at the Yakutsk EAS array have been considered. The showers without the muon component are detected. Almost one half of them forms the clusters, the arrival directions of which correlate with the nearest pulsars. The classification of showers by muon composition has been carried out and the existence of four classes of showers has been shown. The problem of mass composition and origin of ultrahigh-energy cosmic rays are discussed.
1. INTRODUCTION We have analyzed the muon content in extensive air showers (EASs) detected at the Yakutsk array with energies E > 5 × 1018 eV, zenith angles smaller than 60◦ , and axes lying within the array perimeter. The accuracy of determining the arrival angle and energy of a shower is equal to 5◦ -7◦ and 30%, respectively. 2. RESULTS AND DISCUSSION The detection time of EASs observed at the Yakutsk array was separated into 6-h time intervals. The time intervals when the muon detectors did not operate were excluded from the analysis. First, we selected showers without muon component, i.e., showers for which the readings of the muon detectors were absent (equal to zero) within the limits of the detection threshold, which was equal to 1 GeV. For a zero reading of muon detectors, the probability that this muon detector does not trigger for the expected particle num ber N is estimated as P = (P1i + P2i ), where P1i is the probability that no muon reaches the ith detector and P2i is the probability that one particle reaches the detector but it does not trigger. At P > 10−3 , this shower was excluded from consideration. ∗ grants Russian FBR 04-02-16287, Ministry of Education RF 01-30
0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.11.016
Figure 1a shows the observed particle density in a shower according to the data of scintillation and muon detectors as a function of the distance, r, from the shower axis (closed circles and open circles show the electron-photon and muon components, respectively). This shower was detected on December 8, 1994 with an energy E ∼ 2 × 1019 eV and zenith angle θ ∼ 18.1◦. The muon density (muons per meter squared) at all five muon points is equal to zero (open circles). The probability that no particle reaches the muon detector with a total area of 100 m2 is P ∼ 10−20 . The expected densities of the electron-photon component of usual showers are shown by the solid line according to experimental data [1]. Each detection of a shower without the muon component was carefully checked. A shower without muons was selected if, for 30 min before and after it, the muon detectors that gave zero reading triggered and detected particles from another shower (see in detail [2], [3]). We found 19 showers without muon component and 6 showers for which the muon density at distances larger than 100 m was more than 3σ lower than the average expected value. For example, the shower detected on April 24, 1991 (see Fig.1b), has a muon density of 0.22 muons per meter squared at a distance of 851 m from the shower axis, which is 4.4σ lower than the value of 1.4 muons per meter squared expected from
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Figure 1. Four classes of showers: a) showers without muons, b) showers depleted in muons, c) showers with usual muon content, d) showers with high muon content. The closed circles show the electron-photon component and open circles show the muon component. Solid and dashed lines are the expected lateral distribution functions of the electron-photon component and muon component of showers, respectively.
a usual shower. The expected densities of the muon component are shown by the dashed lines. Both fractions of EASs without muons and EASs with low muon content (depleted in muons) were ∼ 1%. Figure 1c shows the data for showers with the usual muon content, for which the particle densities are within 3σ around the expected average value. Such showers constitute a majority of ∼ 97%. Fig. 1d shows the data for the shower with maximum observed energy E = 1.2 × 1020 eV that was detected at the Yakutsk array on May 7, 1989, and contained only muons [4]. The fraction of showers where the muon density was 3σ higher than the expected average value (showers with high muon content) was ∼ 1%. Show-
Figure 2. Distribution of showers without muons (closed circles) and depleted in muon (open circles) in the equatorial coordinates: declination d and right ascension RA. Large circles denote clusters 1-5.
ers with high muon content are observed only for the highest energies. In the showers with energy E > 4 × 1019 eV it is not found any event without muons. Earlier we found out the correlation for directions of arrival of showers with the usual muon content with energy E ∼ 1019 eV with a site of pulsars which are located along force lines of the Local Arm of the Galaxy [5]. In this case, the directions of arrival of showers without muons correlate with pulsars irrespectively of their arrangement concerning force lines of the magnetic field of the Local Arm. From this we assume that showers without muons are formed from neutral particles. Detection of the correlation of directions of arrival of usual showers and showers without muons with a site of pulsars indicate that sources of ultrahigh-energy cosmic rays are pulsars. It is shown [6] that the muon content of showers reflects the mass composition of particles which have formed EAS. Probably, showers with the usual muon content are formed by the usual charged particles, and showers without muons neutral particles, showers with the high muon contents - heavy nuclei. The last statement confirms our earlier conclusion obtained by an other method [7].
A.A. Mikhailov et al. / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 245–248
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Table 1 The arrival date, energy and coordinates of showers without the muon component which composing clusters and the pulsars correlated with them. The distances to the pulsars and their lifetime are given. N Date E, EeV RA, deg. δ, deg. Pulsar, PSR D, kpc lg T, year 1 27.01.1992 13.3 148.3 70.0 0809+74 0.3 8.1 1 18.03.1994 47.4 92.5 74.0 1 08.12.1994 21.3 123.1 76.5 1 10.05.2001 7.1 136.2 70.0 2 25.04.1996 8.6 81.7 60.2 0450+55 0.7 6.3 2 26.03.1998 8.3 85.4 57.6 3 17.11.2000 5.7 322.4 51.4 2217+47 2.4 6.5 3 13.01.2002 7.7 326.7 44.3 4 13.03.1992 8.7 334.7 69.8 2241+69 2.3 6.7 4 30.05.1996 5.0 322.9 65.8
Fig. 2 shows the distribution of 19 showers with E > 5 × 1018 eV without muons (closed circles) and 6 showers depleted in muons (open circles) in the equatorial coordinates (δ is declination and RA is right ascension). The distribution of these showers over the celestial sphere is isotropic. One cluster with four showers and four doublets are observed (see also the Table). The last doublet no. 5 consists of showers without muons and depleted in muons. This doublet is not included in the list of the doublets consisting of showers without muons (Table). The arrival directions of all four showers of the first cluster (see Table and Fig.1) are within an angular range of 9◦ around the PSR 0809+74 pulsar [8], which is located at a distance of 0.3 kpc from the Earth. The probability that arrival directions of 4 showers from 19 are within < 9◦ from pulsar PSR 0809+74 is equal to P ∼ 5 × 10−3 . The method of determination of probability is given in detail in [9]. The arrival directions of doublets nos. 2-4 are within an angular range of 9◦ around the PSR 0450+55, 2221+47, and 2241+65 pulsars, respectively, which are located at a distance less than 2.4 kps from the Earth (see the Table). As seen from the Table, the pulsars are located at an angular distance less than ∼ 1.5σ from the arrival directions of the showers of the clusters (σ is the accuracy to determine the shower arrival angle).
3. CONCLUSION Ultrahigh-energy showers can be conventionally classified among 4 classes in terms of the muon content: 1) showers without muons(Fig.1a) ∼ 1%, 2) showers depleted in muons (Fig.1b) ∼ 1%, 3) showers with usual muon content (Fig.1c) ∼ 97%, 4) showers with high muon content (Fig.1d) < 1%. Showers with high muon content are observed only for the highest energies. This fact can be important for determining the composition and origin of cosmic rays with extremely high energies. The arrival directions of the clusters without muons correlate with nearest pulsars. Pulsars are most likely sources of cosmic rays. REFERENCES 1. V.P. Egorova, A.V. Glushkov, A.A. Ivanov et al. Nuclear Physics B (Proc. Suppl.), 3 (2004) 136. 2. A.A. Mikhailov, E.S. Nikiforova. JETP Lett., 72 (2000) 229. 3. A.A. Mikhailov, N.N. Efremov, E.S. Nikiforova. JETP Lett., 83 (2006) 265. 4. N.N. Efimov, T.A. Egorov, A.V. Glushkov et al., in Proc. of ICRR Intern. Symp. on Astrophysical Aspects of the Most Energetic Cosmic Rays, Kofu (1990) 20.
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A.A. Mikhailov et al. / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 245–248
5. A.A. Mikhailov, Izv. AN, s. fiz., 63 (1999) 556. 6. G.B. Khristiansen, G.V. Kulikov, Yu.A. Fomin, Space radiation of ultrahigh energy, Moscow. Atomizdat, (1975) 258. 7. A.A. Mikhailov. JETP Lett., 72 (2000) 233. 8. J.N. Taylor, R.N. Manchester and A.G. Lyne, Astrophys. J. Suppl., 88 (1993) 529 9. N.N. Efimov, A.A. Mikhailov. Astropart. Physics, 2 (1994) 329.
Extensive air showers with the small muon content in the region of ultrahigh energies A.A. Mikhailov, N.N. Efremov, E.S. Nikiforovaa
∗
a
Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy, 31 Lenin Ave., 677980 Yakutsk, Russia Ultrahigh-energy extensive air showers registered at the Yakutsk EAS array have been considered. The showers without the muon component are detected. Almost one half of them forms the clusters, the arrival directions of which correlate with the nearest pulsars. The classification of showers by muon composition has been carried out and the existence of four classes of showers has been shown. The problem of mass composition and origin of ultrahigh-energy cosmic rays are discussed.
1. INTRODUCTION We have analyzed the muon content in extensive air showers (EASs) detected at the Yakutsk array with energies E > 5 × 1018 eV, zenith angles smaller than 60◦ , and axes lying within the array perimeter. The accuracy of determining the arrival angle and energy of a shower is equal to 5◦ -7◦ and 30%, respectively. 2. RESULTS AND DISCUSSION The detection time of EASs observed at the Yakutsk array was separated into 6-h time intervals. The time intervals when the muon detectors did not operate were excluded from the analysis. First, we selected showers without muon component, i.e., showers for which the readings of the muon detectors were absent (equal to zero) within the limits of the detection threshold, which was equal to 1 GeV. For a zero reading of muon detectors, the probability that this muon detector does not trigger for the expected particle num ber N is estimated as P = (P1i + P2i ), where P1i is the probability that no muon reaches the ith detector and P2i is the probability that one particle reaches the detector but it does not trigger. At P > 10−3 , this shower was excluded from consideration. ∗ grants Russian FBR 04-02-16287, Ministry of Education RF 01-30
0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.11.016
Figure 1a shows the observed particle density in a shower according to the data of scintillation and muon detectors as a function of the distance, r, from the shower axis (closed circles and open circles show the electron-photon and muon components, respectively). This shower was detected on December 8, 1994 with an energy E ∼ 2 × 1019 eV and zenith angle θ ∼ 18.1◦. The muon density (muons per meter squared) at all five muon points is equal to zero (open circles). The probability that no particle reaches the muon detector with a total area of 100 m2 is P ∼ 10−20 . The expected densities of the electron-photon component of usual showers are shown by the solid line according to experimental data [1]. Each detection of a shower without the muon component was carefully checked. A shower without muons was selected if, for 30 min before and after it, the muon detectors that gave zero reading triggered and detected particles from another shower (see in detail [2], [3]). We found 19 showers without muon component and 6 showers for which the muon density at distances larger than 100 m was more than 3σ lower than the average expected value. For example, the shower detected on April 24, 1991 (see Fig.1b), has a muon density of 0.22 muons per meter squared at a distance of 851 m from the shower axis, which is 4.4σ lower than the value of 1.4 muons per meter squared expected from
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Figure 1. Four classes of showers: a) showers without muons, b) showers depleted in muons, c) showers with usual muon content, d) showers with high muon content. The closed circles show the electron-photon component and open circles show the muon component. Solid and dashed lines are the expected lateral distribution functions of the electron-photon component and muon component of showers, respectively.
a usual shower. The expected densities of the muon component are shown by the dashed lines. Both fractions of EASs without muons and EASs with low muon content (depleted in muons) were ∼ 1%. Figure 1c shows the data for showers with the usual muon content, for which the particle densities are within 3σ around the expected average value. Such showers constitute a majority of ∼ 97%. Fig. 1d shows the data for the shower with maximum observed energy E = 1.2 × 1020 eV that was detected at the Yakutsk array on May 7, 1989, and contained only muons [4]. The fraction of showers where the muon density was 3σ higher than the expected average value (showers with high muon content) was ∼ 1%. Show-
Figure 2. Distribution of showers without muons (closed circles) and depleted in muon (open circles) in the equatorial coordinates: declination d and right ascension RA. Large circles denote clusters 1-5.
ers with high muon content are observed only for the highest energies. In the showers with energy E > 4 × 1019 eV it is not found any event without muons. Earlier we found out the correlation for directions of arrival of showers with the usual muon content with energy E ∼ 1019 eV with a site of pulsars which are located along force lines of the Local Arm of the Galaxy [5]. In this case, the directions of arrival of showers without muons correlate with pulsars irrespectively of their arrangement concerning force lines of the magnetic field of the Local Arm. From this we assume that showers without muons are formed from neutral particles. Detection of the correlation of directions of arrival of usual showers and showers without muons with a site of pulsars indicate that sources of ultrahigh-energy cosmic rays are pulsars. It is shown [6] that the muon content of showers reflects the mass composition of particles which have formed EAS. Probably, showers with the usual muon content are formed by the usual charged particles, and showers without muons neutral particles, showers with the high muon contents - heavy nuclei. The last statement confirms our earlier conclusion obtained by an other method [7].
A.A. Mikhailov et al. / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 245–248
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Table 1 The arrival date, energy and coordinates of showers without the muon component which composing clusters and the pulsars correlated with them. The distances to the pulsars and their lifetime are given. N Date E, EeV RA, deg. δ, deg. Pulsar, PSR D, kpc lg T, year 1 27.01.1992 13.3 148.3 70.0 0809+74 0.3 8.1 1 18.03.1994 47.4 92.5 74.0 1 08.12.1994 21.3 123.1 76.5 1 10.05.2001 7.1 136.2 70.0 2 25.04.1996 8.6 81.7 60.2 0450+55 0.7 6.3 2 26.03.1998 8.3 85.4 57.6 3 17.11.2000 5.7 322.4 51.4 2217+47 2.4 6.5 3 13.01.2002 7.7 326.7 44.3 4 13.03.1992 8.7 334.7 69.8 2241+69 2.3 6.7 4 30.05.1996 5.0 322.9 65.8
Fig. 2 shows the distribution of 19 showers with E > 5 × 1018 eV without muons (closed circles) and 6 showers depleted in muons (open circles) in the equatorial coordinates (δ is declination and RA is right ascension). The distribution of these showers over the celestial sphere is isotropic. One cluster with four showers and four doublets are observed (see also the Table). The last doublet no. 5 consists of showers without muons and depleted in muons. This doublet is not included in the list of the doublets consisting of showers without muons (Table). The arrival directions of all four showers of the first cluster (see Table and Fig.1) are within an angular range of 9◦ around the PSR 0809+74 pulsar [8], which is located at a distance of 0.3 kpc from the Earth. The probability that arrival directions of 4 showers from 19 are within < 9◦ from pulsar PSR 0809+74 is equal to P ∼ 5 × 10−3 . The method of determination of probability is given in detail in [9]. The arrival directions of doublets nos. 2-4 are within an angular range of 9◦ around the PSR 0450+55, 2221+47, and 2241+65 pulsars, respectively, which are located at a distance less than 2.4 kps from the Earth (see the Table). As seen from the Table, the pulsars are located at an angular distance less than ∼ 1.5σ from the arrival directions of the showers of the clusters (σ is the accuracy to determine the shower arrival angle).
3. CONCLUSION Ultrahigh-energy showers can be conventionally classified among 4 classes in terms of the muon content: 1) showers without muons(Fig.1a) ∼ 1%, 2) showers depleted in muons (Fig.1b) ∼ 1%, 3) showers with usual muon content (Fig.1c) ∼ 97%, 4) showers with high muon content (Fig.1d) < 1%. Showers with high muon content are observed only for the highest energies. This fact can be important for determining the composition and origin of cosmic rays with extremely high energies. The arrival directions of the clusters without muons correlate with nearest pulsars. Pulsars are most likely sources of cosmic rays. REFERENCES 1. V.P. Egorova, A.V. Glushkov, A.A. Ivanov et al. Nuclear Physics B (Proc. Suppl.), 3 (2004) 136. 2. A.A. Mikhailov, E.S. Nikiforova. JETP Lett., 72 (2000) 229. 3. A.A. Mikhailov, N.N. Efremov, E.S. Nikiforova. JETP Lett., 83 (2006) 265. 4. N.N. Efimov, T.A. Egorov, A.V. Glushkov et al., in Proc. of ICRR Intern. Symp. on Astrophysical Aspects of the Most Energetic Cosmic Rays, Kofu (1990) 20.
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A.A. Mikhailov et al. / Nuclear Physics B (Proc. Suppl.) 175–176 (2008) 245–248
5. A.A. Mikhailov, Izv. AN, s. fiz., 63 (1999) 556. 6. G.B. Khristiansen, G.V. Kulikov, Yu.A. Fomin, Space radiation of ultrahigh energy, Moscow. Atomizdat, (1975) 258. 7. A.A. Mikhailov. JETP Lett., 72 (2000) 233. 8. J.N. Taylor, R.N. Manchester and A.G. Lyne, Astrophys. J. Suppl., 88 (1993) 529 9. N.N. Efimov, A.A. Mikhailov. Astropart. Physics, 2 (1994) 329.