Krone-Martins, A G O

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29th International Cosmic Ray Conference Pune (2005) 7, 4750The observational signatures of ultra-high energy cosmic rays in theGalactic CenterAlberto G. O. Krone-Martins and Gustavo Medina-TancoInstituto de Astronomia, Geof·sica e Ciencias Atmosf·ericas, Universidade de Sao Paulo, BrasilPresenter: Gustavo Medina-Tanco (gustavo@astro.iag.usp.br), bra-medinatanco-G-abs4-he14-oralThe central region of our Galaxy is believed to contain a supermassive black hole coincident with the radiosource Sgr A. This region has been observed by several gamma-ray observatories as a source of gammas atenergies above 200 GeV, probably extending into the TeV region. In this work we study the propagation ofprotons inside a model of the magnetic field present in the central 400 parsecs of our Galaxy. The particles areinjected in the field with two different configurations, one assuming that their source is located within a sphereof 15 parsecs in the Galactic Center, and other injecting the particles at a spherical boundary surrounding themagnetized bulge. For both cases we study the propagation of 1 EeV protons inside 200 pc from the GalacticCenter, their trapping time and estimate the neutron and gamma fluxes resultant from their interaction with thelocal interstellar medium.1. IntroductionWithin the central hundreds parsecs of our Galaxy a great number of phenomena are taking place due to amyriad of different astrophysical objects. The taxonomy of those objects reveals the presence of a centralsupermassive blackhole surrounded by stars in formation process, hot X-ray halos, supernova remnants and agreat molecular zone of about 200 parsecs in radius lying down in the Galactic Plane [1]. Although the regionis obscured at optical and UV wavelenghts, the study of the Galactic Center indicates the presence of gammaray emmiters (TeV and GeV energy ranges), X-Ray, radio, infrared [1] and, probably, cosmic ray sources [2]as well.Inside this region the field has been observed or inferred by several different techniques, such as Zeeman effectin maser and radio, infrared polarization and radio synchrotron emission.The Galactic Magnetic Field (GMF) structure and strength plays a central role in the study of charged particlescrossing or trapped in our Galaxy. The regular component of this field inside the Galactic disk has a strengthof the order of few   G [3] and may be the result of a large scale S0 dynamo. Furthermore, there are evidencesof antisymmetric rotation measurements at high Galactic latitudes which are compatible with a toroidal field inthe Galactic halo. This toroidal component possibly extends into the inner Galaxy, inside the central molecularzone [4].On the other hand, the observation of non-thermal radio filaments in the Galactic center region [1], have longbeen thought as indicatives of a larger scale poloidal component rooted in a central dipole [5, 6, 8].The combination of the toroidal and dipole components in the Galactic center region strongly suggests that anA0 dynamo is operating in the halo and bulge of our Galaxy.According to some observations, the strength of the inner Galaxy fields can reach up to 3 mG [7] at scales oftens of parsecs, perhaps remaining at the level of several tens of   G up to several hundred parsecs from theGalactic center.Therefore, the extension of the regular magnetic structure observed (or inferred ) at large scales into the centralfew hundred parsec of our Galaxy is potentially interesting for cosmic ray propagation studies since the gyro-radius of particles with energies as high as  eV would be much smaller than the characteristic scale of the48 A.G.O. Krone-Martins and G. Medina-Tancoregion (see, Fig. 1 ), leading to severe bending or even partial confinement and the corresponding build up ofa local ultra-high energy cosmic ray excess density.In this work the propagation of charged particles inside the central 400 parsecs of our Galaxy and its corre-sponding observational manifestations are analyzed.Figure 1. The Larmor radius of proton and Fe nuclei as a function of energy at 	 and  pc from the GalacticCenter in the vicinity of the Galactic Plane.2. Propagation at EeV: estimates and simulationsIf the intensity of the magnetic field in the central region of the Galaxy is considerably higher than in the restof the disk, as it seems to be the case, particles coming mainly from the extragalactic cosmic ray flux may bepartially confined for a relatively long time. Since the interstellar medium (ISM) in this region is also muchdenser than in the rest of the Galaxy, cosmic ray particles might have the chance to interact with the ISM andproduce neutral particles, neutrons, neutrinos and gammas, that could be detected at Earth.As an example, let us consider particles n the energy range  –  eV and a confinement region of ﬁﬀﬂﬃpc. If the diffusion coefficient can be approximated by  ﬀﬂ!#"%$&#' the diffusion time scale across the regionwould be (*)+ﬀ,.-/&# , while the light travel time is 0"12ﬀ,3&#$ , and one can estimate a density enhancementfactor inside this region with respect to the original cosmic ray density as 4ﬁﬀ5()6&#0/"17ﬀ$89&: which, forour example, would amount to 4;ﬀ<>= .The ISM density at the Galactic center, on scales of tens of parsecs is difficult to estimate, but a conservativevalue is ?A@B>CDﬀEﬃ- cm F=. The column density along the size of the region,  , is GHﬀJI%KML#?N@B>COPﬀQSR TMUFV- gm/cm - , and the mass traversed by a typical cosmic ray at 1 EeV would be WXﬀY4%GZﬀQ>Tgm/cm - .Since the proton-proton interaction length, GL[L\ﬀQ gm/cm - , is comparable, almost every cosmic ray protonwould have at least one interaction with the ISM during confinement.Since the cosmic ray flux at 1 EeV is of the order of ] = _^a`bc#dfeﬀ ﬃ-6gh i m FV- sec FN sr FN eV - , onecan estimate the number of particles with energies between 1 and 2 EeV entering a confinement region of size5ﬀjk pc as lm`n]jojbp#qr(s]toj>pe*ﬀjﬃ-ru sec F% . Each one of these particles will experience, onaverage,  interaction with the ISM.Therefore, since the multiplicity for neutron production in p-p interaction is ﬀv/&Q, the number of neutronsper year reaching Auger from this ”point-like” source would be wfxyrzZﬀJ`/&Qe{|lﬂX`}(\~3%8#&Qkefﬀ'kThe observational signatures of ultra-high energy cosmic rays in the Galactic Center 49neutrons/yr. In the same area of the sky, the background arriving to the detector would be *%Ł6  ﬃ\rn*kp::#A yr % .These numbers were estimated for the particular case of   ﬃ pc but, for 7Hﬃ pc they can be scaled as:mŁr <\k8¡ ¢[£ neutron/yr and  %Łr p¤ﬃS  particles/yr. These fluxesare plotted in figure 2 as a function of the size of the region, 3¥§¦ .It can be seen that, if the size of the high magnetic field intensity region is some tens of pc, the Galacticcenter should be detectable above the background after few years of integration by an experiment like Auger.Inverting the argument, the detection of such a source, should allow to infer the characteristic fields in theinnermost regions of our Galaxy.Figure 2. Left vertical axis: secondary neutron ux arising from a connement region of size ¨ª©« , ¬­®¯ , as a functionof ¨§©S« in the 1 to 2 EeV range expected at a °±±± km ² array like Auger (units are particles per yr). Also shown forcomparison is the all particle ux expected in the same region of the sky, ¬´³µkµ®¯ . Right vertical axis: angular size subtendedby the neutron source as a function of its size.In order to make a more reliable assessment of this and other effects, we explicitly tracked protons of differentenergies injected in the central region of our Galaxy at two different positions. In the first simulation group,the protons were injected at random positions over the surface of a 15 parsecs radius sphere centered in theGalactic Center. Those particles were generated with random directions. This first picture represents the central15 parsecs of our Galaxy as the source of those protons. As the magnetic field in this region is probably verytangled, the momentum of the protons that succeed in escaping from that region points completely randomdirections.The second scenario considers the extragalactic flux of protons impinging on an artificial spherical boundarysurrounding the inner magnetized region of the Galactic bulge.The model assumed for the magnetic field at the center of the Galaxy takes into account the observed valuesfor this field and the regular structure inferred from polarimetric and radio studies. We estimate the total fieldas a composition of a large scale dynamo, a dipolar field acting in the innermost region and a magnetic fieldcreated by a flat 2D ring of electric current located in the galactic plane.50 A.G.O. Krone-Martins and G. Medina-TancoInitial results confirm the confinement effect at   EeV, see fig. 3, and the neutron flux estimated above.Further details will also be presented about neutrino and gammas fluxes.Figure 3. Tracks of some few protons injected with parallel momenta at three different energies, at the border of the centralbulge region. The particle orbits show the possibility of some degree of connement at scales of tens of parsec at energiesas high as ¶ ±ﬃ·¹¸rº ²» eV. This does not preclude connement at higher energies in a smaller central region depending ondetails of the magnetic eld structure there.asdgh3. AcknowledgmentsThis work was partially supported by the Brazilian Agencies CNPq, CAPES and FAPESP.References[1] Morris M., E. Serabyn, Annu. Rev. of Astron. and Astrophys. 34, 645 (1996)[2] Hayashida N. et al., Astropart.Phys. 10, 303 (1999)[3] Beck R., et al., Annu. Rev. of Astron. and Astrophys. 34, 155 (1996)[4] Novak G., et al., Astrophys. J, 583, L83 (2003)[5] Yusef-Zadeh F., Morris M. (1987) Astrophys. J. 320, 545[6] Sofue Y., Reich W., Inoue M., Seiradakis J.H. (1987) Publ. Astron. Soc. Japan 39, 95.[7] Chuss D. T., et al., Astron. Nachr. 324, No S1, 173 (2003)[8] Sofue Y., C. Lang, The Central Parsecs of the Galaxy, ASP Conference Series, Vol. 186 (1999)

The observational signatures of ultra-high energy cosmic rays in the Galactic Center

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