The exciting results from H.E.S.S. point to a new population of gamma-ray
sources at energies E > 10 TeV, paving the way for future studies and new
discoveries in the multi-TeV energy range. Connected with these energies is the
search for sources of PeV cosmic-rays (CRs) and the study of multi-TeV
gamma-ray production in a growing number of astrophysical environments. TenTen
is a proposed stereoscopic array (with a suggested site in Australia) of
modest-sized (10 to 30m^2) Cherenkov imaging telescopes with a wide field of
view (8 to 10deg diameter) optimised for the E~10 to 100 TeV range. TenTen will
achieve an effective area of ~10 km^2 at energies above 10 TeV. We outline here
the motivation for TenTen and summarise key performance parameters.Comment: 4 pages, 2 figures, proceedings of the 30th ICRC, Merida, Mexico,
  200

Clay, R.

Dawson, B.

Denman, J.

Protheroe, R.

Rowell, G.

Smith, A. G. K.

Stamatescu, V.

Thornton, G.

Wild, N.

English

arXiv

Submitted to Cornell University’s online archive www.arXiv.org in 2007 by Gavin Rowell.The exciting results from H.E.S.S. point to a new population of gamma-ray sources at energies E > 10 TeV, paving the way for future studies and new discoveries in the multi-TeV energy range. Connected with these energies is the search for sources of PeV cosmic-rays (CRs) and the study of multi-TeV gamma-ray production in a growing number of astrophysical environments. TenTen is a proposed stereoscopic array (with a suggested site in Australia) of modest-sized (10 to 30m^2) Cherenkov imaging telescopes with a wide field of view (8 to 10deg diameter) optimised for the E~10 to 100 TeV range. TenTen will achieve an effective area of ~10 km^2 at energies above 10 TeV. We outline here the motivation for TenTen and summarise key performance parameters

Smith, A.

Adelaide Research &amp; Scholarship

arXiv:0710.2027v1  [astro-ph]  10 Oct 200730TH INTERNATIONAL COSMIC RAY CONFERENCETenTen: A New Array of Multi-TeV Imaging Cherenkov TelescopesG. ROWELL, V. STAMATESCU, R. CLAY, B. DAWSON, J. DENMAN, R. PROTHEROE, A. SMITH,G. THORNTON, N. WILDSchool of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australiagrowell@physics.adelaide.edu.auAbstract: The exciting results from H.E.S.S. point to a new population of γ-ray sources at energiesE>10 TeV, paving the way for future studies and new discoveries in the multi-TeV energy range. Con-nected with these energies is the search for sources of PeV cosmic-rays (CRs) and the study of multi-TeVγ-ray production in a growing number of astrophysical environments. TenTen is a proposed stereoscopicarray (with a suggested site in Australia) of modest-sized (10 to 30m2) Cherenkov imaging telescopeswith a wide field of view (8◦ to 10◦ diameter) optimised for the E∼10 to 100 TeV range. TenTen willachieve an effective area of∼10 km2 at energies above 10 TeV. We outline here the motivation for TenTenand summarise key performance parameters.Motivation for Multi-TeV StudiesGround-based γ-ray astronomy operating in the∼0.1 to ∼10 TeV range has become a mainstreamastronomical discipline due to the exciting resultsfrom H.E.S.S. [29] in the Southern Hemisphereover recent years. In the Northern Hemisphere theMAGIC [35] and MILAGRO [37] telescopes arenow producing results and similar high impact cansoon be expected from VERITAS [36]. The TeVsource catalogue extends to over 30 individualsources, and we are now able to perform detailedstudies of extreme environments capable of CRparticle acceleration. Several key points can beextracted from the results of H.E.S.S. and otherswhich motivate development of a new dedicatedinstrument for studies at multi-TeV (E >few TeV)energies (see also [16, 28]):Increasing variety of TeV Sources: The numberof environments established as sources of gammaradiation (to energies exceeding ∼ 10 TeV inmany cases) is growing and include, in the caseof Galactic sources — shell-type supernova rem-nants (SNRs), pulsar-wind-nebulae (PWN), com-pact binaries and/or X-ray binaries, young stellarsystems/clusters and molecular clouds acting astargets for CRs in their vicinity. There are alsoseveral Galactic sources (all extended) that to-datehave no known counterpart at lower energies, andso remain unidentified. Extragalactic sources com-prise active galaxies with jets aligned along ourline of sight, the so-called Blazars, and at least onemisaligned Blazar.Hard Photon Spectra: The majority of newGalactic sources generally exhibit hard power lawphoton spectra dN/dE ∼ E−Γ where Γ < 2.5without indication of cutoffs, suggesting that theiremission extends beyond 10 TeV.Extended Sources & Large Field of View: Themajority of Galactic sources are extended (up toseveral degrees in scale) in morphology, provid-ing insight into γ-ray production and transport pro-cesses, especially when coupled with multiwave-length results. Effective studies of these sourcestherefore require large instantaneous fields of view,not only to encompass sources of interest but toalso allow adequate selection of regions for CRbackground estimation. The 5◦ FoV cameras inH.E.S.S. for example have been designed withthese aspects in mind. This large FoV has permit-ted highly successful surveys of the inner SouthernGalactic Plane within just a few years [20, 22], andalso the establishment of degree-scale morphologyin several strong sources.Limited Multi-TeV Sensitivity of Present In-TENTEN: A NEW MULTI-TEV ARRAYstruments: Current intruments operate with a∼0.1 TeV threshold energy and effective collectionarea (E > 10 TeV) of less than 1 km2. The major-ity of source studies are therefore made in the 0.1 to∼10 TeV band. The fluxes of these new sources arein the few to ∼15% Crab flux range, reflecting theintrumental sensitivities. With the limited observa-tional opportunities due to the ever growing sourcecatalogues, the accumulation of high statistics inthe multi-TeV band is difficult. Fig. 1 illustratesthis point where a new weak TeV source is revealedto the north of HESS J1825−137 after only deep(> 50 hr) observations. Further, detailed studiesof this weak source, motivated perhaps by its rel-atively rare coincidence with a MeV/GeV EGRETsource, would not be practical with H.E.S.S. Suchstudies would require >100 hr of observationsand face stiff competition from other source pro-grammes.E > 10 TeV Sources Already Exist: For thoseGalactic sources (two shell-type SNRs and sev-eral PWN [21, 23, 25, 24, 26]) with strong fluxes(> 15% Crab) and/or deep observation times (≥50 hr), photon spectra have been established to en-ergies∼50 TeV or greater, demonstrating that par-ticle acceleration to energies exceeding 100 TeVis occurring in these types of objects. MILAGROhas also recently revealed degree-scale emission(with total flux exceeding 1 Crab) at energies above10 TeV in the Cygnus and other regions of theNorthern Galactic Plane [6, 7], highlighting the po-tential of future all-sky-monitors in the multi-TeVrange. These points provide clear observational ev-idence that rich astrophysics potential awaits in theE > 10 TeV range.In addition there are strong theoretical grounds forpushing deep into the multi-TeV domain:Particle Acceleration to the knee and beyond:The desire to understand particle acceleration tothe CR knee (E ∼ 1 PeV) energy and beyondremains a key motivation for multi-TeV studies.While it is generally accepted that CRs can beaccelerated in shell-type SNRs [41] to energiesEmax ∼few×1014 eV [33] (via the diffusive shockacceleration process), there is considerable uncer-tainty as to how particles can reach the knee en-ergy and beyond (eg. [30, 32]) in so-called Pe-vatrons. Several ideas have been put forward, forexample: strong amplification of pre-shock mag-Figure 1: H.E.S.S. image of the PWNHESS J1825−137 [24] resulting from 52 hrobservations. A considerably weaker source isrevealed to the north of HESS J1825−137, andis possibly a counterpart to the EGRET source3EG J1826−1302. This weaker source is unlikelyto be studied further in depth by H.E.S.S. Thisimage nicely illustrates a variety of Galactic sourcetypes in the same FoV — a strong extended PWN;weak extended source with MeV/GeV counterpartand a pointlike compact binary (LS 5039).netic fields [3]; local Gamma-Ray-Bursts (GRBs)[5, 4]; and superbubbles which combine the ef-fects of many SNRs and maybe Wolf-Rayet/OBstellar winds [9, 1, 13, 8]. Extragalactic sourceswith large-scale kpc shocks such as galaxy clusters(eg. [19]) and AGN jets and giant lobes (eg. [2])could also be sources. Only observations around100 TeV and greater can begin to solve the mys-tery of PeV CR acceleration.E > 10 TeV — Easier Separation of Hadronic& Electronic Components: A major complica-tion in interpreting present results in the 0.1 to∼10 TeV range concerns the separation of γ-ray components from accelerated hadrons (fromsecondary pi◦-decay) and those from acceleratedelectrons (most commonly from inverse-Comptonscattering). Multiwavelength information, in par-ticular at radio and X-ray energies, can provideconstraints on these components but often onerequires model-dependent assumptions to decidethe nature of the parent particles. At energiesE >10 TeV the electronic component can be sup-30TH INTERNATIONAL COSMIC RAY CONFERENCEpressed due to strong radiative synchrotron en-ergy losses suffered by electrons in magnetisedpost-shock environments, such as that in shell-type SNRs. In addition, the Klein-Nishina effecton the inverse-Compton cross-section can signif-icantly reduce the efficiency of this process. Ex-cept in those cases where a strong source of elec-trons exists, such as in PWN, interpretation ofE > 10 TeV spectra may therefore be much moreconfidently interpreted as arising from acceleratedhadrons.Probing Local Intergalactic/Interstellar PhotonFields: E > 10 TeV photons can indirectlyprobe ambient soft photon fields. In the ∼10 to∼100 TeV energy range, absorption on the cosmicinfra-red background (CIB) in the 10 to 100 µmrange dominates with mean free paths extendingbeyond 1 Mpc. Constraints on the (nearby) inter-galactic CIB via γ-ray spectral studies of nearbyextragalactic sources, such as M 87 (an establishedTeV source) can yield important information con-cerning star and galaxy formation in our local in-tergalactic neighbourhood [27]. Constraints on theinterstellar CIB may also be possible via E >10 TeV spectral studies of Galactic source popu-lations [12].TenTen: Initial Simulation Study &PerformanceGiven that source fluxes rapidly decrease with en-ergy (typically via a power law), any dedicatedinstrument operating in the multi-TeV energy do-main would need to achieve a very large effectivecollection area Aeff ∼10 km2. While there areseveral promising ways to achieve 10 km2 usingground-based techniques, earlier simulations [14]have shown that a proven, technically straightfor-ward, and yet highly sensitive method would em-ploy stereoscopy in an array of 30 to 50 modest-sized imaging atmospheric Cherenkov telescopes(IACTs) in a cell-based approach. Each telescopewould have mirror area 10 to 30 m2, field ofview (FoV) 5◦ to 10◦, and inter-telescope spac-ing within a single cell ≥200 metres (in contrastto ∼100 m employed by arrays such as H.E.S.S.).The large FoV, limited practically by optical aber-rations, allows events to trigger out to core dis-tances ≥200 m, thereby increasing the effectivecollection area of a cell. We propose here such anarray, known as TenTen, which stands for 10 km2above 10 TeV. Similar and other ideas for 100 TeVstudies have also been suggested [39, 40, 34].Our initial simulation study [17] examined the per-formance of a single cell of 5 telescopes, eachwith mirror area 23.8 m2 (84x60 cm spherical mir-ror facets) and 1024 pixel camera spanning 8.2◦diameter (with pixel diameter 0.25◦). The lay-out of the cell has the outer four telescopes ar-ranged in a square of side length L with a sin-gle telescope at the centre (similar to the HEGRAIACT-System layout[15]). Gamma-ray and pro-ton extensive air shower simulations (30◦ zenith— with CORSIKA v6.204 [11] and SIBYLL [10])coupled with telescope responses (based on [31])were used to investigate basic performance param-eters of the cell. An observation altitude 200 ma.s.l. was chosen since we are investigating sitesin Australia. For E > 10 TeV, low-altitude sitesmay be beneficial compared to mid/high altitudesdue to the larger distances between telescopes andshower maxima. Details concerning this study in-cluding ongoing work are summarised in our com-panion paper/poster [18]. Briefly, we found thatfor a cell with side length L = 300 m, an on-axis γ-ray effective collection area Aeff exceed-ing 1 km2 for E > 30 TeV can be achieved ina single cell, and that somewhat similar cosmic-ray background rejection power and arc-minute an-gular resolution is achieved in the E > 10 TeVrange as H.E.S.S. and the HEGRA IACT-Systemachieve(d) in their respective energy ranges. Anenergy threshold in the 1 to few TeV range is alsoindicated. This encouraging result suggests thatexpanding the array to (for example) ∼10 suffi-ciently spaced cells (so that there are no commonevents between cells) could yield collection areas∼10 km2, exceeding that of H.E.S.S. by factorsapproaching 50 at 100 TeV. The approximate fluxsensitivity (based on the improvement in collec-tion area over H.E.S.S.) and energy coverage ofTenTen is depicted in Fig. 2. Flux sensitivitiesfor large extended sources such as the 2◦ diame-ter shell type SNR RX J0852.0−4622 would be∼ 10−12 erg cm−2 s−1 at 10 TeV. For point-likesources, the accessible fluxes would be a factor of10 to 20 lower again (less than ∼ 10−13 erg cm−2s−1 above 10 TeV). Importantly, a 1 to a few TeVTENTEN: A NEW MULTI-TEV ARRAY6 7 8 9 10 11 12 13 14101010101010                                                                INTEGRALEnergy Flux (erg/cm  s)2Energy (eV)10 10 10 10 10 10 10 1010−10−11−12−13−8−9GLASTCOMPTELEGRETWhippleHEGRA"100 GeV Arrays"TenTen (approx)ICCELESTEHEGRAcompared to the Crab NebulaDetector sensitivities5@5SMEGAFigure 2: Approximate pointsource energy flux sensitivity forTenTen (assuming 10 cells of5 telescopes discussed in text)in comparison with other instru-ments in the γ-ray regime andthe flux from the Crab Nebula.An observation time of 50 hr andsignal significance of 5σ is de-manded. The TenTen sensitivity isestimated from its collection areaimprovement over H.E.S.S. (simi-lar to “100 GeV arrays”) from ourinitial simulation study.energy threshold would also allow detailed studiesof existing γ-ray sources that are at the thresholdof detection for H.E.S.S. Despite our simulationsbeing limited to E ≤ 100 TeV, we also expecthigh collection area above this energy. Note thatin the 200 TeV to 100 PeV range, γ-ray absorptionon the cosmic microwave background (CMB) be-comes important, with mean free path ≤100 kpc[38], limiting the focus of γ-ray astronomy in thisenergy range to Galactic sources.ConclusionsWe have outlined the motivation for a new ar-ray of IACTs achieving 10 km2 at E > 10 TeVand described some important performance param-eters. This array, known as TenTen, could also beconsidered complementary to future MeV to TeVγ-ray instruments such as GLAST, HESS-II andMAGIC-II. Studies are currently underway to fur-ther optimise individual telescopes (optics, elec-tronics, camera design), overall layout parameters,and site potential in Australia.References[1] Bykov A.M. and Toptygin I.N. Astron. Lett., 27:625, 2001.[2] Hillas A.M. Ann. Rev. Astron. Astrophys., 22:425, 1984.[3] Bell A.R. and Lucek S.G. MNRAS, 321:433, 2001.[4] Dermer C.D. and Holmes J.M. ApJ, 628:L21, 2005.[5] Waxman E. Phys. Rev. Lett., 75:386, 1995.[6] Abdo A.A. et al. ApJ, 658:L33, 2007.[7] Adbo A.A. et al. ApJ submitted, arXiv:astro-ph/0705.0707, 2007.[8] Binns B. et al. ApJ, 634:351, 2006.[9] Drury L.O’C. et al. Space Sci. Rev., 99:29, 2001.[10] Fletcher R.S. et al. Phys. Rev. D, 50:5710, 1994.[11] Heck D. et al. Report FZKA 6019 (1998)Forschingzentrum Karlsruhe, http://www-ik.fzk.de/corsika/physics description/corsika phys.html, 1998.[12] Moskalenko I.V. et al. ApJ, 640:L155, 2005.[13] Parizot E. et al. A&A, 424:747, 2004.[14] Plyasheshnikov A.V. et al. J Phys. G: Nucl. Part. 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Ten Ten: A New Array of Multi-TeV Imaging Cherenkov Telescopes

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TenTen: A New Array of Multi-TeV Imaging Cherenkov Telescopes

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