Anisotropy in the cosmic radiation at TeV energy
aa r X i v : . [ a s t r o - ph . H E ] F e b Frascati Physics Series Vol. XLVI (2007), pp. 000-000
HADRON07: XII Int. Conf. on Hadron Spectroscopy – Frascati, October 8-13, 2007Parallel Session
ANISOTROPY IN THE COSMIC RADIATION AT TEVENERGY
Roberto Iuppa
University of Tor Vergata and INFN, sez.ne Roma Tor Vergatavia della Ricerca Scientifica 1, Rome - 00133, Italy
Abstract
In recent years very important results were obtained from cosmic ray experi-ments about the arrival direction distribution of primaries in the TeV energyrange. As most of these particles are charged nuclei, they are deflected by themagnetic field they pass through before reaching the Earth surface, the effectof the Lorentz force being inversely proportional to the particle energy. As faras the local interstellar medium is known, the gyroradius of a 10 TeV protonis expected to be only 100 a.u., small enough to make the arrival directiondistribution isotropic. Since 1930s a ”large scale” (90-120) anisotropy is knownto exist, generally interpreted as the combined effect of sources far away andmagnetic fields nearby. Nonetheless, in the last decade experiments like Tibet-ASg, Milagro, ARGO-YBJ and IceCube discovered structures as wide as 10-30all over the sky at 10 TeV energy, what is unexplainable within the standardmodel of cosmic rays. In this paper a review of the most recent experimentalresults about cosmic ray anisotropy is given, together with the status of thert of theoretical efforts aimed at interpreting them within the current cosmicray paradigma.
As CRs are mostly charged nuclei, their paths are deflected and highly isotropizedby the action of galactic magnetic field (GMF) they propagate through beforereaching the Earth atmosphere. The GMF is the superposition of regular fieldlines and chaotic contributions and the local total intensity is supposed tobe B = 2 ÷ µ G 1). In such a field, the gyro-radius of CRs is given by r a.u. ≈ TV , where r a.u. is in astronomic units and R TV is the rigidity inTeraVolt. Clearly, there is very little chance of observing a point-like signalfrom any radiation source below 10 eV, as they are known to be at leastseveral hundreds parsecs away.If it is true that magnetic fields are the most important “isotropizing”factor when they randomly vary on short distances, it is clear as much thatsome particular features of the magnetic field at the boundary of the solarsystem or farther might focus CRs along certain lines and the observed arrivaldirection distribution turns out to be consequently an-isotropic.Different experiments observed an energy-dependent “large scale” anisotropywith amplitude spanning 10 − to 10 − , from tens GeV to hundreds TeV, sug-gesting the existence of two distinct broad regions, an excess named “tail-in” (distributed around 40 ◦ to 90 ◦ in Right Ascension (R.A.) and a deficit named “loss cone” (distributed around 150 ◦ to 240 ◦ in R.A.).Moreover, in the last decade smaller excesses ( ∼ ◦ wide) were found toexist in the CR arrival direction distribution.The origin of the galactic CR anisotropy is still unknown, but the study ofits evolution over the energy spectrum has an important valence to understandthe propagation mechanisms and the structure of the magnetic fields throughwhich CRs have traveled. In 2006 the Tibet AS γ experiment, located at Yangbajing (4300 m a.s.l.),published the first 2D high-precision measurement of the CR anisotropy in theorthern hemisphere in the energy range from few to several hundred TeV2). In the figure 1 the CR intensity map observed by Tibet AS γ is shown(panel (a)), together with some theoretical model. The Tibet AS γ collaborationcarried out the first measurement of the energy and declination dependencesof the R.A. profiles in the multi-TeV region with a single EAS array, revealingfiner details of the known anisotropy. They found that the first harmonicamplitude is remarkably energy-independent in the range 4 - 53 TeV and allthe components of the anisotropy fade out for CR energy higher than a fewhundred TeV, showing a co-rotation of galactic CRs with the local Galacticmagnetic environment.The Milagro collaboration published in 2009 a 2D display of the siderealanisotropy projections in R.A. at a primary CR energy of about 6 TeV 3).They observed a steady increase in the magnitude of the signal over seven years,in disagreement with the Tibet AS γ results 4). It is worth noting that theenergy at which the Tibet AS γ and Milagro results were obtained ( ∼
10 TeV)is too high for Sun effects play an important role.In 2007, modeling the large scale anisotropy of 5 TeV CR, the Tibet-AS γ collaboration ran into a “skewed” feature over-imposed to the broad structureof the so-called tail-in region 5). They modeled it with a couple of intensityexcesses in the hydrogen deflection plane 6 , ◦ -30 ◦ wide. Aresidual excess remained in coincidence with the helio-tail. See the figure 1 (d)and its caption for more details.Afterwards the Milagro collaboration claimed the discovery of two local-ized regions of excess 10 TeV CRs on angular scales of 10 ◦ with greater than12 σ significance 8). The figure 2 reports the pre-trial significance map ofthe observation. Regions “A” and “B”, as they were named, are positionallyconsistent with the “skewed feature” observed by Tibet-AS γ .The strongest and most localized of them (with an angular size of about10 ◦ ) coincides with the direction of the helio-tail. The fractional excess of re-gion A is ∼ × − , while for region B it is ∼ × − . The deep deficitsbordering the excesses are due to a bias in the reference flux calculation. Thiseffect slightly underestimates the significance of the detection. The Milagrocollaboration excluded the hypothesis of gamma-ray induced excesses. In ad-dition, they showed the excess over the large scale feature without any datahandling (see the figure 2 of 8)).igure 1: Anisotropy maps of galactic CRs observed and reproduced at themodal energy of 7 TeV by the Tibet-AS γ experiment 4). (a): the observedCR intensity; (b): the best-fit large scale component; (c): the significancemap of the residual anisotropy after subtracting the large scale component;(d): the best-fit medium scale component; (e): the best-fit large+mediumscale components; (f): the significance map of the residual anisotropy aftersubtracting the large and the medium scale component. The solid black curvesrepresent the galactic plane. The dashed black curves represent the HydrogenDeflection Plane. The helio-tail direction is indicated by the black filled circle.The open cross and the inverted star with the attached characters “F” and“H” represent possible orientations of the local interstellar magnetic field. Theopen triangle with “B” indicates the orientation of the best-fit bi-directionalcosmic-ray flow obtained in the reference 4).Figure 2: Significance map for the Milagro data set without any cuts to removethe hadronic CR background. A 10 ◦ bin was used to smooth the data, and thecolor scale gives the statistical significance. The solid line marks the Galacticplane, and every 10 ◦ in Galactic latitude are shown by the dashed lines. Theblack dot marks the direction of the helio-tail, which is the direction oppositethe motion of the solar system with respect to the local interstellar matter.he excesses in both regions are harder than the spectrum of the isotropicpart of CRs.Easy to understand, more beamed the anisotropies and lower their energy,more difficult to fit the standard model of CRs and galactic magnetic field toexperimental results. In addition, the observation of a possible small angularscale anisotropy region contained inside a larger one rely on the capability forsuppressing the smooth global CR anisotropy at larger scales without, at thesame time, introducing effects of the analysis on smaller scales.Nonetheless, this observation has been confirmed by the ARGO-YBJ ex-periment 9 ,
10) at median energy of the isotropic CR proton flux of aboutE p ≈ ≈ Some authors suggested that the large scale anisotropy can be explained withinthe diffusion approximation taking into account the role of the few most nearbyand recent sources 11 , ∼ Current experimental results show that the main features of the anisotropy areuniform in the energy range (10 - 10 eV). Structures are there in everyregion of the harmonic domain down to angular scales as narrow as 10 ◦ . Sofar, no theory of CRs in the Galaxy exists which is able to explain both largecale and few degrees anisotropies leaving the standard model of CRs and thatof the local galactic magnetic field unchanged at the same time. The author wishes to thank Prof. R. Santonico and Dr. G. Di Sciascio for theirsupport in data analysis and critical review of theoretical models.
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