Regions of an excessive flux of cosmic rays according to data of the FIAN and MSU arrays
E.N. Gudkova, M.Yu. Zotov, N.N. Kalmykov, G.V. Kulikov, N.M. Nesterova, V.P. Pavlyuchenko
RRegions of an excessive flux of cosmic raysaccording to data of the FIAN and MSU arrays
E.N. Gudkova , M.Yu. Zotov , N.N. Kalmykov , G.V. Kulikov ,N.M. Nesterova , V.P. Pavlyuchenko
1. P.N. Lebedev Physical Institute of the Russian Academy of Sciences2. D.V. Skobeltsyn Institute of Nuclear Physics,M.V. Lomonosov Moscow State University(To be published in the Proceedings of the 33rd Russian Cosmic Ray Conference,Dubna, 11–15 August, 2014)
Abstract
Results of a blind search for localised regions of an excessive flux of cosmic rays in the energyrange from 50 TeV to 20 PeV with the data of the FIAN KLARA-Chronotron experiment,the EAS MSU array and the Prototype of the EAS-1000 array are presented. A number ofregions with a significant excess of the registered flux over an expected isotropic backgroundare found. Some of the regions are present in at least two of the data sets considered.
Results of the Super-Kamiokande, Tibet-AS γ , Milagro, IceCube and some other experiments re-vealed localised regions of excess of primary cosmic rays (PCRs) in the TeV-PeV energy rangeand thus attracted considerable attention because the large-scale distribution of arrival directionsof such PCRs is almost isotropic, see the recent review [1]. In the present work, we continue theseries of anisotropy studies of PCRs at medium angular scales basing on the data of the KLARA-Chronotron experiment carried out at the Tian Shan station of the Lebedev Physical Institute ofthe Russian Academy of Sciences (FIAN) (43 . ◦ N, 76 . ◦ E, P = 690 g cm ) and two experimentscarried out at Lomonosov Moscow State University (MSU), one with the EAS MSU array, anotherone with the prototype of the EAS-1000 array (PRO-1000 in what follows) (55 . ◦ N, 37 . ◦ E) [2–9]. To the contrary to the previous works, the data of all three experiments are analysed with aunified method, namely the shuffling technique, which has demonstrated its effectiveness in similaranisotropy studies. This allows us to compare the results basing on the common ground.
The KLARA-Chronotron experiment was carried out by FIAN together with the Central Insti-tute for Physics Research (KFKI) of the Hungarian Academy of Sciences at a separate installationlocated at the Tian Shan scientific station [3]. The experiment was specifically designed for con-tinuous studies of anisotropy of PCRs and worked simultaneously but independently of the mainFIAN EAS array. Twenty three million events with energies from 50 TeV to 0.5 PeV with zenithangles in the 20 ◦ –60 ◦ range registered in 1978–1982 were selected for the present analysis. Halfa million events registered with the EAS MSU array in 1984–1990 [10] and 1.3 million events ofPRO-1000 (1997–1999) [11] were selected of the MSU data sets. 95% of these events correspond toPCRs in the energy range approximately 0.2–20 PeV in the first case, and from 50 TeV to 5 PeVin the latter one. Thus, the energy range of events registered with the FIAN array is fully covered1 a r X i v : . [ a s t r o - ph . H E ] J a n y the PRO-1000 data. The MSU events selected for the analysis have zenith angles < ◦ . Theaccuracy of arrival directions in all three experiments is estimated to be of the order of 3 ◦ .The analysis of the data was performed over the whole fields of view of the experiments, namelyfor declination δ > − ◦ for the FIAN experiment and δ > ◦ for those of MSU. The fields ofview were covered with a grid with 0 . ◦ × . ◦ cells and scanned with circular regions of differentradii from 2 ◦ to 8 ◦ . For the further analysis, we selected regions in which the expected number ofevents exceeded 10000, 400 and 200 for the FIAN, PRO-1000 and EAS MSU data sets respectively.The (pre-trial) statistical significance S of deviation of the real number of events registered in aparticular region from the expected background was calculated with the formula by Li and Ma,traditionally used in anisotropy studies [12].Regions found in the FIAN and MSU data sets such that the registered number of events insidethem exceeds the expected background by S > δ ≈ ◦ –28 ◦ . The maximum deviation from theexpected background flux is observed in a circle of radius 4 . ◦ centred at (87 . ◦ , 18 . ◦ ). 78061events are registered inside the circle with the expected 77061.2 events, which gives the pre-trialsignificance S = 3 .
6. REF 2 intersects with an extended region in the PRO-1000 data set ( δ ≈ ◦ –38 ◦ ) and a small region in the EAS MSU data. This part of the celestial sphere is interesting due tonumerous potential sources of PCRs of TeV-PeV energies, among them the Crab Nebula (SN1054),the supernova remnants IC443, PKS 0607+17, S147 and a number of energetic pulsars includingGeminga [13,14], see Fig. 2.Region 4 is located near the Supergalactic plane. In this case, the maximum deviation fromthe isotropic flux almost reaches S = 4. The EAS MSU and PRO-1000 data sets also demonstratesimilar extended REFs, for which the excess of the registered flux over the expected backgroundexceeds 3.8 standard deviations. Possible sources of PCRs with TeV–PeV energies are not knownin this part of the celestial sphere except for the close pulsar B0809+74 (0.43 kpc from the Solarsystem), which is capable to accelerate protons up to ∼ . ◦ centred at (301 . ◦ , 45 . ◦ ), also draws certain interest. 26970 events are registered within this circlewith the expected number equal to 26327.2, which gives S = 3 .
9. The region adjoins a REF in theEAS MSU data set, which has a similar size. Both regions are located in the direction to the Cygnussuperbubble, which contains multiple supernova remnants, energetic pulsars and OB-associations,see Fig.2. Some of these objects are located at distances < ∼ . ∩ -like curve shows the Supergalacticplane. There are a considerable number of localised regions with angular sizes up to several dozen degreeswith an excessive flux of PCRs in the energy range from 50 TeV to 20 PeV in the data sets obtainedby the KLARA-Chronotron experiment (FIAN) and with the EAS MSU and PRO-1000 arrays, andsome of the regions found in different data sets overlap or intersect. As a whole, anisotropy ofPCRs found with the FIAN data is closer to that for PRO-1000, which is possibly due to the factthat both experiments covered close energy intervals.A direct comparison of the presented results with the results on anisotropy of PCRs at similarangular scales obtained by other experiments is not straightforward since the most pronounced The probability of overlapping regions to appear by chance equals a product of the respective probabilities forthe data sets since all three sets were obtained independently. γ for energies 50 TeV and300 TeV [20].It is difficult to give an unambiguous answer to the question about the origin of the REFs butthe regions that have close counterparts in two or all three data sets are undoubtedly interesting,especially in case they are located in parts of the celestial sphere with possible sources of PCRs of theconsidered energies. Still, an existence of close supernova remnants or pulsars in this case does notmean they are the reason of the REFs. In a ∼ µ G magnetic field, a 1 PeV proton has a gyroradiusof just 1 pc, which is much less than the distance to any of the possible sources. Neutrons of theseenergies must also be excluded from consideration because of the too short lifetime in free state.The fraction of air showers initiated by gamma-quanta in this energy range is likely to be negligiblysmall [21, 22] and cannot lead to the appearance of the discovered REFs. All this makes explainingthe existence of local inhomogeneities of the flux of TeV–PeV PCRs a rather complicated task. Atthe moment, the most popular models are those based on the influence of various configurations ofGalactic magnetic fields, see [1] for a review. There is no doubt that further studies of anisotropyof PCRs in the TeV–PeV energy range are of considerable interest both for the cosmic ray physicsand for understanding the structure of the Galactic magnetic field.The authors thank members of the staff of KFKI, MSU and the Tian Shan station of FIAN whotook part in constructing the arrays and performing the experiments. The work was done with afinancial support by the grant of the Government of Russian Federation (contract 14.25.31.0010)and grants of the Russian Foundation for Basic Research 14-02-00372, 13-02-12175-ofi-m and 13-02-00214.