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Very high energy particle astronomy
Expected performance |
| Very high energy particle astronomy |
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Besides the traditional signals of astronomy, TeVγ rays, VHEν , and VHE protons (or EHECR) can be astronomical messengers from the Universe. The recent observation of TeVγ and EHECRs has strongly suggested the existence of extragalactic VHE proton asccelerators.
In addition to TeVγ s and EHECRs, the accelerated proton interacts, producing the osberved VHEγ s and unobserved VHEν s through neutral and charged π decays, and subsequently escapes the ambient materials or photons. There are many theoretical predictions of the GUT era in the Big Bang Universe and the decay of massive relic particles in the dark halo of the Galaxy as the VHE particle radiators. The comprehensive observation of high energy particles should be a key to identify the profound physics of the sources. Ashra can take an image of air-shower (AS) through two kinds of yielded lights, Cerenkov and fluorescence, with the 1 arcmin resolution in the entirely all sky coverage of field of view. These advanced features of Ashra can provide us the systematic exploration into extragalactic VHE particle radiators in the Universe. The entirely all sky survey allows us possibility of the independent discoveries of unknown TeVγ sources or transient phenomena like VHE explosion or flare. Also the all sky survey for EHECRs with Ashra provides us a chance of making a complete map of proton acceleration sites. This makes a precision test of the EHECR clusters suggested by AGASA with much higher statistics. It is an extremely exciting research to check positional and luminous correlation between VHEν clusters and origins of TeVγ s or EHECRs to probe the physics processes at the VHE particle radiators. For instance, observing a VHEν cluster associating with neither VHEγ nor EHECR sources strikingly means discovery of unknown VHEν object surrounded target photons or materials which make the source obscured for photons as well as nucleons. Super-massive dark halo models can be tested through the anisotoropy measurements for VHE particles. The observation of VHEν or EHEγ which can survive distant propagation suggests new physics which comprise such questions as Lorents invariance, grand unification theory of particle physics, quantum gravity theory and a universe containing new large extra dimensions. Upward and horizontal τ ASs emerging from the Earth crust or mountain chains offer the most powerful signal of VHEν s at PeV and higher energy. They also provide us a chance of super-long base line "ντ apperance" experiment for ν oscillation. For VHEν s, the target mass can reach fairly more than 100km3-water-equivalent, taking into account geometrical trigger and reconstruction efficiencies, which is at least two order of magnitude larger than the traditional and constructing water Cerenkov ν telescope. Using Earth clust or mountain as target mass allows us huge detection acceptance for VHEν events. To discriminate the VHEν AS signals from contamination of abandant cosmic ray proton ASs, we really need to measure the AS tracks emerging from the Earth crust or mountain with their directions as precisely and harmetically as possible. |
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Expected perfomance
TeVγ We estimated trigger efficiencies for TeVγ AS events by the detector simulation of Ashra. We limited the region of 16x16 macrocells(6.4ox6.4o) which should sufficiently include one event of AS Cerenkov image and assume the trigger gate time to be 20~ns, the observation height to be 1600m above sea level, and the night sky background as described above. The trigger frequency from night sky background in the limited region is required to be less than 10Hz considering realistic readout speed enhancing the threshold energy for TeVγ s to be as lower as possible. We realistically took into account the design parameters in the optical system of Ashra, the gains and decay times of phosphor screens in the photoelectric image pipeline system. As a result, for γ s with the energy of 1TeV, the trigger efficiency is evaluated to be about 50% (Fig.1). The Cerenkov imaging method makes the image of AS lateral distribution which has a shape of wide ellipse on the imaging device and dominantly limits the event-by-event determination error of the arrival direction to be 0.1o level. The precise image resolution itself cannot help it so much. The all-sky survey of Ashra has an excellent advantage that we don't need to share valuable observation time with interesting objects, which improves pointing accuracy as increasing observation time. For air Cerenkov observation, the all-sky survey has epoch-making meaning from two terms of the opportunity of independent discovery and the continuous observation. 
FIg.1 Trigger efficiency EHECR The most significant advantage of air-fluorescence method is to continuously measure the longitudinal development of AS event by event. In this case, the high resolution of Ashra makes a great sense because the inclination angle of the AS-detector plane directly depends on the image resolution of the fluorescence detector. We evaluated the accuracy of the arrival derection determination using air-fluorescence stereo reconstruction for EHECR AS eventsas shown in Fig.2. The results show that the arrival direction can be determined with the accuracy better than 1~arcmin above 1018.5eV and striking accuracy of 0.3 arcmin for the EHECR events with 1020eV. 
Fig.2 Arrival direction resolution with the stereo reconstruction for EHECRs. For the detection sensitivity for EHECR events with Ashra, any cut on the AS track length is not necessary because of sufficiently fine pixel resolution. The more distant events with the shorter track in the field of view dominates as increasing the energy of EHECR AS events. The finer trigger pixel field of view of 0.4o and the track recognition with the 1-arcmin resolution make significant advantage on the detection sensitivity for EHECR. Using only three stations, the detection sensitivity for EHECRs with Ashra-2 can compete with the next gigantic AS array, Auger and the arrival direction accuracy with Ashra is better by two order of magnitude than that with Auger. VHE ν Several traditional water Cerenkov detector projects has been planned orstarted for the first detection of VHEν . The AS sampling array Auger also proposed to detect using the signature of deeply penetrating holizontal air shower. Fig.3 shows comparison of sensitivities among projects aimed for the VHEν observation. Also in Fig.3 shown is two kinds of theoretical upper bounds assuming transparent and obscured hadron accelerator sites. 
Fig.3 VHEν flux predictions and expected sensitivities. 90%CL upper limit (straight line) assuming E-2 flux. Flux (curved line) from which one event per year is expected in one decade of energy. Two theoretical upper bounds (W & B Limit, Obscured) assuming extreme optical thickness cases for neutron. Ashra detects ASs emerging from the Earth clust or mountain, deeply penetrating atmosphere with air-fluorescence technique. Although the observing energy range of Ashra is complementary to water Cerenkov detectors, there is no intrinsic atmospheric neutrino background and high resolution image of AS ensure the high purityand high confidence level of VHEν identification as well as the great sensitivity. |
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