O'Neill, A C

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29th International Cosmic Ray Conference Pune (2005) 7, 373376Limit on the flux of ultra-high energy photons from the HiRes StereoDataA.C. O’Neill and S. Westerhoff for the HiRes CollaborationPhysics Dept, Columbia University, New York, NY, USAPresenter: A.C. O’Neill (oneill@phys.columbia.edu), usa-oneill-A-abs1-he14-oralTop-down models for the origin of extragalactic cosmic rays predict a substantial photon fraction in the cosmicray flux above 10 eV. The HiRes air fluorescence detector is capable of discriminating between photon andhadronic events and can place a limit on the photon fraction.When modeling the passage of primary photons towards the earth, we have to take into account thatphotons above 10 eV are capable of pair production in the geomagnetic field, which causes an electromag-netic shower to occur before atmospheric interactions begin. We have created a Monte Carlo simulation ofthis preshower physics and use it to calculate the photon fraction in the HiRes stereo data above 10 eV. Wecompare our limit to predictions of various top-down models.1. IntroductionGamma Rays above 10 eV have sufficient energy to interact with a geomagnetic photon and pair pro-duce before entering the atmosphere. The resulting pair can emit several hundred secondary photons viaBremsstrahlung before atmospheric effects become important. This geomagnetic preshower has the effect ofreducing the depth of shower maximum (X  ). This change is particularly important when considered inconjunction with the Landau-Pomeranchuk-Migdal[1] effect which reduces the Bremsstrahlung cross sectionconsiderably at this energy.The change in shower development due to geomagnetic preshowering is important for air fluorescence detectorssince X 	 is used in determining the composition of the primary particle. By reducing the atmospheric depthof the shower maximum the effect also increases the sensitivity of air fluorescence detectors to high-energygamma rays. These factors must both be considered in calculating a limit on the gamma ray flux.We have created a Monte Carlo simulation of gamma rays in the geomagnetic field and applied it to measuringthe fraction of photons present in the HiRes stereo data above 40 EeV. This is similar in concept to simulationscreated by Homala et al[2] and Vankov et al[3].2. Pair Production and Bremsstrahlung in the Geomagnetic FieldThe process of pair production at high energy (or in strong magnetic fields) is well understood. The mean freepath for a photon of energyin a transverse magnetic field  is given by [4] ﬁﬀﬃﬂ ﬁ!"$#&%('*),+.-0/132/4!65(1)where!87:9ﬁﬀﬃﬂ (2)and ﬁﬀ;ﬂ<>='=))?%A@. At low gamma energy, the electron and positron receive approximately equal energy,but at large values of ! the energy distribution of the pair becomes highly assymetric.374 A.C. O’Neill and S. WesterhoffThe magnetic Bremsstrahlung process for a relativistic electron is also well described in the literature[5]: )B6CEDF G,HCJILK CED;K MONPCJI Q(3)where the Bremsstrahlung spectrum R 2 JS 9S  5 is given byR2 S 9ST5VUXWXY2[Z59\29^]_5/S(4)UXW , the classical Bremsstrahlung radiation per unit distance isUXWa` #Eb/F//9cF7:/9edfS(5)and the function Y 2[Z 5 isY2[Z5 gih j` #2)lk\Z54mon2pZ5kZ4\/-rqs2pZ5)lk\Z t' (6)The auxillary Bremsstrahlung functionn2pZ5 is given byn2pZ5Z_uwvx-rys2J5Q (7)and the parameter\is\j9$7z9ﬁﬀ;ﬂ ' (8)We use the International Geomagnetic Reference Field to get   .Incoming photon primaries are simulated by a Monte Carlo code implementing the above results. The electro-magnetic preshower is then used as an input to the CORSIKA air shower simulation, and the results processedusing the HiRes detector simulator.3. Effect of Preshower on Shower DevelopmentBecause of the directionality of the geomagnetic field, the preshower effect is highly dependent on arrival di-rection. We have identified the directions of maximum and minimum preshower probability, which correspondto the maximum and minimum transverse magnetic field. At the HiRes detector in Utah the maximum is at 10degrees East of North, 30 degrees above the horizontal and the minimum is found at 10 degrees West of South,60 degrees above the horizontal.Figure 1 shows the elongation rate ( {[| }e.~ as a function of energy) for preshowered gammas from these twodirections, along with that of protons and iron.[6]It can be seen that the preshower effect does not cause {p| ~ for photons to be close to {[| .~ for hadrons.The reduced depth does however improve the acceptance, particularly for less inclined showers.The large spread in the |  distribution for gamma showers from the North between ),%  and )?%  eVis due to some showers which have pair produced and preshowered and some which did not cause a pairproduction event before entering the atmosphere.Limit on  -ux from HiRes ... 375log(Energy) (eV)19 19.2 19.4 19.6 19.8 20 20.2)2> (g/cmmax<X700800900100011001200130014001500 Iron (QGSJet)Protons (QGSJet)Gammas (North)Gammas (South)Figure 1. Elongation Rate for Preshowered Gamma Rays at the HiRes Location4. Calculating the Fraction of Gamma Rays in the HiRes DataRather than using the simulated elongation rate to determine composition, we have applied a likelihoodmethod[7] to determine a limit on the number of photons in the HiRes data above the GZK limit. This involvescomparing the simulated gamma | distribution with the observed distribution, and has the advantage ofreducing dependence on factors such as the spectrum of simulated events or hadronic model used in simulation.The simulated gamma ray showers have been compared to the 50 events above 40 EeV in the HiRes stereodata.5. ConclusionsOne can place a useful upper limit on the flux of ultra high energy photons using the HiRes stereo data whichwill have interesting implications to many top-down models of super-GZK cosmic rays. The likelihood methodallows us to calculate a limit on the number of photons in the HiRes data.Further results will be presented at the conference.376 A.C. O’Neill and S. Westerhoff6. AcknowledgementsThis work is supported by US NSF grants PHY-9321949, PHY-9322298, PHY-9904048, PHY-9974537, PHY-0098826, PHY-0140688, PHY-0245428, PHY-0305516, PHY-0307098, and by the DOE grant FG03-92ER40732. We gratefully acknowledge the contributions from the technical staffs of our home institutions.The cooperation of Colonels E. Fischer and G. Harter, the US Army, and the Dugway Proving Ground staff isgreatly appreciated.References[1] K. Kim, C. Song, and P. Sokolsky, Prepared for 26th International Cosmic Ray Conference (ICRC 99),Salt Lake City, Utah, 17-25 Aug 1999.[2] P. Homola et al., (2003), astro-ph/0311442.[3] H. P. Vankov, N. Inoue, and K. Shinozaki, Phys. Rev. D67, 043002 (2003), astro-ph/0211051.[4] A. Sokolov and I. Ternov, Radiation From Relativistic Electrons, AIP translation series, 2nd rev. ed. ed.(Springer Verlag, 1986).[5] T. Erber, Rev. Mod. Phys. 38, 626 (1966).[6] The High Resolution Fly’s Eye, R. U. Abbasi et al., Astrophys. J. 622, 910 (2005), astro-ph/0407622.[7] The High Resolution Fly’s Eye (HiRes), B. Connolly et al., Prepared for 29th International Cosmic RayConference (ICRC 2005), Pune, India 3-10 Aug 2005.

Limit on the flux of ultra-high energy photons from the HiRes Stereo Data

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