Important informations on the origin and the propagation mechanisms of cosmic rays may be provided by the measurement of the anisotropies of their arrival direction. In this paper the observation of anisotropy regions at different angular scales is reported. In particular, the observation of a possible anisotropy on scales between $\sim10^{\circ}$ and $\sim30^{\circ}$ may be a key-detection for speculations on the presence of unknown features of the magnetic fields the charged cosmic rays propagate through, as well as to potential contributions of nearby sources to the total flux of cosmic rays. Evidence of new weaker few-degree excesses throughout the sky region $195^{\circ}\leq$ R.A. $\leq 315^{\circ}$ is also reported.
As the most part of cosmic rays (CRs) are charged nuclei, their arrival direction is deflected and made isotropic by the action of galactic magnetic field (GMF) that they propagate through before reaching the Earth atmosphere. In such a field, the gyroradius of CRs is given by r a.u. = 100 R TV , where r a.u. is in astronomic units and R TV is in TeraVolt. It must be taken as a reference value, because the GMF is the superposition of regular field lines and chaotic contributions, the strength of both them being still under debate. Data available today gives for the local total intensity the value B = 2 ÷ 4 µG.
Actually, deviations from the isotropy are expected to occurr as a consequence of the particular realization of the random distribution of cosmic ray sources and magnetic field lines in the galaxy [1]. In this framework, the amplitude of the anisotropy is proportional to the rigidity, that is why it is expected to be seen at high energies (above few hundreds TeV). However, different experiments [2][3][4][5][6][7] observed an energydependent “large scale” anisotropy in the sidereal time frame, well below that threshold. The amplitude is about 10 -4 -10 -3 , suggesting the existence of two distint broad regions, one showing an excess of CRs (called “tail-in”), distributed around 40 • to 90 • in Right Ascension (R.A.). The other a deficit (the “loss cone”), distributed around 150 • to 240 • in R.A.. The origin of these anisotropies is still unknown. Some authors claim that the observations may be due to a combined effect of the regular and turbolent GMF [8], or to local uni-and bi-dimensional inflows [9]. Other studies suggest that it can be explained within the diffusion approximation taking into account the role of the few most nearby and recent sources [1,10].
Easy to understand, more beamed the anisotropies and lower their energy, more difficult to fit the standard model of CRs and GMF to experimental results. That is why the evidence of the existence of a medium angular scale anisotropy contained in the tail-in region by the Tibet ASγ [11] and Milagro [12] collabo-rations in rcent years was rather surprising. Similar small scale anisotropies has been recently claimed to be observed by the Icecube experiment in the Southern hemisphere [7]. So far, no theory of CRs in the Galaxy exists which is able to explain few degrees anisotropies in the rigidity region 1-10 TV leaving the standard model of CRs and that of the local GMF unchanged at the same time.
From the experimental viewpoint, observing anisotropy effects at the level of 10 -4 with an air shower array is a difficult job, because of the intrinsic difficulties that this kind of apparatus has to cope with in estimating the exposure.
Finally, the observation of a possible small angular scale anisotropy region contained inside a larger one rely on the capability for suppressing the anisotropic structures at larger scales without, at the same time, introducing effects of the analysis on smaller scales.
In this paper the observation of CR anisotropy at different angular scales with ARGO-YBJ is reported as a function of the primary energy.
The ARGO-YBJ experiment, located at the Yang-BaJing Cosmic Ray Laboratory (Tibet, P.R. China, 4300 m a.s.l., 606 g/cm 2 ), is an air shower array able to detect the cosmic radiation at an energy threshold of a few hundred GeV. The full detector is in stable data taking since November 2007 with a duty cycle greater than 85%. The trigger rate at the threshold is 3.6 kHz. The detector characteristics and performance are described in [13].
In order to study the anisotropy at different angular scales the isotropic background of CRs has been estimated with two methods: the equi-zenith angle method [14] and the direct integration method [15].
The equi-zenith angle method, used to study the large scale anisotropy, is able to eliminate various spurious effects caused by instrumental and environmental variations, such as changes in pressure and temperature that are hard to control and tend to introduce systematic errors in the measurement. The method uses data coming from all the angular scales, so that potential small structures are not separated from the underlying large scale modulation.
The direct integration method, based on timeaverage, rely on the assumption that the local distribution of the incoming CRs is slowly varying and the time-averaged signal may be used as a good estimation of the background content. Time-averaging methods act effectively as a high-pass filter, not allowing to inspect features larger than the time over which the background is computed (i.e., 15 • /hour×∆t in R.A.). The time interval used to compute the average spans ∆t= 3 hours and makes us confident the results are reliable for structures up to ≈35 • wide.
The observation of the CR large scale anisotropy by ARGO-YBJ is shown in the figure 1 To quantify the scale of the anisotropy we studied the 1-D R.A. projections integrating the
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