Electromagnetic showers in a strong magnetic field

We present the results concerning the main shower characteristics in a strong magnetic field obtained through shower simulation. The processes of magnetic bremsstrahlung and pair production were taken into account for values of the parameter $\chi \gg 1$. We compare our simulation results with a recently developed cascade theory in a strong magnetic field.Comment: 11 pages, 9 eps figures, LaTex2e, Iopart.cls, Iopart12.clo, Iopams.st

The nature of the highest energy cosmic rays

Ultra high energy gamma rays produce electron--positron pairs in interactions on the geomagnetic field. The pair electrons suffer magnetic bremsstrahlung and the energy of the primary gamma ray is shared by a bunch of lower energy secondaries. These processes reflect the structure of the geomagnetic field and cause experimentally observable effects. The study of these effects with future giant air shower arrays can identify the nature of the highest energy cosmic rays as either gamma-rays or nuclei.Comment: 15 pages of RevTeX plus 6 postscript figures, tarred, gzipped and uuencoded. Subm. to Physical Review

Characteristics of geomagnetic cascading of ultra-high energy photons at the southern and northern sites of the Pierre Auger Observatory

Cosmic-ray photons above 10^19 eV can convert in the geomagnetic field and initiate a preshower, i.e. a particle cascade before entering the atmosphere. We compare the preshower characteristics at the southern and northern sites of the Pierre Auger Observatory. In addition to a shift of the preshower patterns on the sky due to the different pointing of the local magnetic field vectors, the fact that the northern Auger site is closer to the geomagnetic pole results in a different energy dependence of the preshower effect: photon conversion can start at smaller energies, but large conversion probabilitites (>90%) are reached for the whole sky at higher energies compared to the southern Auger site. We show how the complementary preshower features at the two sites can be used to search for ultra-high energy photons among cosmic rays. In particular, the different preshower characteristics at the northern Auger site may provide an elegant and unambiguous confirmation if a photon signal is detected at the southern site.Comment: 25 pages, 14 figures, minor changes, conclusions unchanged, Appendix A replaced, accepted by Astroparticle Physic

Simulation of Ultra-High Energy Photon Propagation in the Geomagnetic Field

The identification of primary photons or specifying stringent limits on the photon flux is of major importance for understanding the origin of ultra-high energy (UHE) cosmic rays. We present a new Monte Carlo program allowing detailed studies of conversion and cascading of UHE photons in the geomagnetic field. The program named PRESHOWER can be used both as an independent tool or together with a shower simulation code. With the stand-alone version of the code it is possible to investigate various properties of the particle cascade induced by UHE photons interacting in the Earth's magnetic field before entering the Earth's atmosphere. Combining this program with an extensive air shower simulation code such as CORSIKA offers the possibility of investigating signatures of photon-initiated showers. In particular, features can be studied that help to discern such showers from the ones induced by hadrons. As an illustration, calculations for the conditions of the southern part of the Pierre Auger Observatory are presented.Comment: 41 pages, 9 figures, added references in introduction, corrected energy in row 1 of Table 3, extended caption of Table

Ultra-High Energy Gamma Rays in Geomagnetic Field and Atmosphere

The nature and origin of ultra-high energy (UHE: reffering to > 10^19 eV) cosmic rays are great mysteries in modern astrophysics. The current theories for their explanation include the so-called "top-down" decay scenarios whose main signature is a large ratio of UHE gamma rays to protons. Important step in determining the primary composition at ultra-high energies is the study of air shower development. UHE gamma ray induced showers are affected by the Landau-Pomeranchuk-Migdal (LPM) effect and the geomagnetic cascading process. In this work extensive simulations have been carried out to study the characteristics of air showers from UHE gamma rays. At energies above several times 10^19 eV the shower is affected by geomagnetic cascading rather than by the LPM effect. The properties of the longitudinal development such as average depth of the shower maximum or its fluctuations depend strongly on both primary energy and incident direction. This feature may provide a possible evidence of the UHE gamma ray presence by fluorescence detectors.Comment: 27 pages, 12 figures, submitted to Phys.Rev.

Lorentz invariance violation in top-down scenarios of ultrahigh energy cosmic ray creation

The violation of Lorentz invariance (LI) has been invoked in a number of ways to explain issues dealing with ultrahigh energy cosmic ray (UHECR) production and propagation. These treatments, however, have mostly been limited to examples in the proton-neutron system and photon-electron system. In this paper we show how a broader violation of Lorentz invariance would allow for a series of previously forbidden decays to occur, and how that could lead to UHECR primaries being heavy baryonic states or Higgs bosons.Comment: Replaced with heavily revised (see new Abstract) version accepted by Phys. Rev. D. 6 page

Discovery of Very High Energy γ\gamma-Rays from Markarian~180 Triggered by an Optical Outburst

The high-frequency-peaked BL Lacertae object Markarian~180 (Mrk~180) was observed to have an optical outburst in 2006 March, triggering a Target of Opportunity observation with the MAGIC telescope. The source was observed for 12.4 hr and very high energy $\gamma$-ray emission was detected with a significance of 5.5 $\sigma$. An integral flux above 200 GeV of $(2.3\pm0.7)\times10^{-11} {cm}^{-2} {s}^{-1}$ was measured, corresponding to 11% of the Crab Nebula flux. A rather soft spectrum with a photon index of $-3.3\pm0.7$ has been determined. No significant flux variation was found.Comment: Accepted by ApJ Letters, minor revision

First bounds on the very high energy gamma-ray emission from Arp 220

Using the Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC), we have observed the nearest ultra-luminous infrared galaxy Arp 220 for about 15 hours. No significant signal was detected within the dedicated amount of observation time. The first upper limits to the very high energy $\gamma$-ray flux of Arp 220 are herein reported and compared with theoretical expectations.Comment: Accepted for publication in Ap

Systematic search for VHE gamma-ray emission from X-ray bright high-frequency BL Lac objects

All but three (M87, BL Lac and 3C 279) extragalactic sources detected so far at very high energy (VHE) gamma-rays belong to the class of high-frequency peaked BL Lac (HBL) objects. This suggested to us a systematic scan of candidate sources with the MAGIC telescope, based on the compilation of X-ray blazars by Donato et al. (2001). The observations took place from December 2004 to March 2006 and cover sources on the northern sky visible under small zenith distances zd < 30 degrees at culmination. The sensitivity of the search was planned for detecting X-ray bright F(1 keV) > 2 uJy) sources emitting at least the same energy flux at 200 GeV as at 1 keV. In order to avoid strong gamma-ray attenuation close to the energy threshold, the redshift of the sources was constrained to values z<0.3. Of the fourteen sources observed, 1ES 1218+304 and 1ES 2344+514 have been detected in addition to the known bright TeV blazars Mrk 421 and Mrk 501. A marginal excess of 3.5 sigma from the position of 1ES 1011+496 was observed and has been confirmed as a source of VHE gamma-rays by a second MAGIC observation triggered by a high optical state (Albert et al. 2007). For the remaining sources, we present here the 99% confidence level upper limits on the integral flux above ~200 GeV. We characterize the sample of HBLs (including all HBLs detected at VHE so far) by looking for correlations between their multi-frequency spectral indices determined from simultaneous optical, archival X-ray, and radio luminosities, finding that the VHE emitting HBLs do not seem to constitute a unique subclass. The absorption corrected gamma-ray luminosities at 200 GeV of the HBLs are generally not higher than their X-ray luminosities at 1 keV.Comment: 15 pages, 7 figures, 5 tables, submitted to ApJ (revised version

Unfolding of differential energy spectra in the MAGIC experiment

The paper describes the different methods, used in the MAGIC experiment, to unfold experimental energy distributions of cosmic ray particles (gamma-rays). Questions and problems related to the unfolding are discussed. Various procedures are proposed which can help to make the unfolding robust and reliable. The different methods and procedures are implemented in the MAGIC software and are used in most of the analyses.Comment: Submitted to NIM