CRIS 2010 100 years of Cosmic Ray Physics: from Pioneering Experiments to Physics in Space 1 Higher Statistics UHECR observatories: a new era for a challenging astronomy Etienne Parizot APC University Paris Diderot (Paris 7) Input and reflections from many colleagues and friends in the field (notably Guillaume Decerprit and Denis Allard at APC)
Critical time 2 Auger has accumulated ~20 000 km 2 sr yr (since Jan. 2004) Statistics will double (on the same sky) in ~3-4 years, triple in 7-8 years Auger has delivered very important results, and more key results are to come at lower energies, but possibly not dramatically new results at UHE Crisis? The results delivered do not allow us to conclude much about the main questions concerning UHECRs (and CRs in general) Is there a way to do better? Is it worth it?
3 Part I: The Auger era
Auger results - I 4 Spectrum
Auger results - I 5 Spectrum there is a significant flux suppression! but what do we learn from this spectrum? essentially nothing :-( it was expected, from very basic arguments, therefore it is very generic, therefore it is compatible with almost any model and how do we know if this is the GZK cutoff? we don t! horizon effect? E max effect? (anisotropy could tell but they don t really yet)
Fitting the spectrum 6 E 3!(E) (ev 2 m -2 s -1 sr -1 ) 10 25 10 24 10 23 pure proton sources Auger data 18 18,5 19 19,5 20 20,5 log 10 E (ev)
Fitting the spectrum 7 E 3!(E) (ev 2 m -2 s -1 sr -1 ) 10 25 10 24 10 23 pure Fe sources Auger data 18 18,5 19 19,5 20 20,5 log 10 E (ev)
Fitting the spectrum 8 10 25 CNO only (at sources) E max = Z # 10 20.3 ev " = 2.0 pure CNO sources E 3!(E) (ev 2 m -2 s -1 sr1) 10 24 10 23 H He total H CNO 18,4 18,8 19,2 19,6 20 20,4 log 10 E ev
Fitting the spectrum No clear information about source composition 9 Kotera, Allard & Olinto (2010)
Fitting the spectrum No clear information about source spectrum 10 Kotera, Allard & Olinto (2010) (pure proton) 2
Fitting the spectrum No clear information about sources ( Just a horizon effect!) 11 Even more statistics won t help! (cosmic variance!)
Auger results - II 12 Anisotropy
Auger results - II 13 Anisotropy what does it tell us?
Auger results - II 14 Anisotropy w/o period I with period I
Auger results - II 15 Anisotropy only one conclusion: it is not conclusive!
Auger results - II 16 Anisotropy Cen-A excess hot spot? source?
Auger results - II 17 Anisotropy
Auger results - III 18 Composition transition towards a heavier composition or (strong!) modification of hadronic physics
Does this all make any sense? 19 Yes, definitely! Nature is not contradictory Nature cannot be contradictory! Are we facing conflicting results? No, why? Transition to heavier nuclei is rather natural (NB: a change in the underlying hadronic physics would also not be too unexpected ) Anisotropy with large deflections is very much possible (and indeed the observed anisotropy is not strong, esp. at small angular scales!) Are we facing a crisis? Yes, definitely! Auger is working remarkably well, yet essentially nothing has been learned about UHECR sources More and more models are proposed!
A UHECR Crisis? 20 wēijī danger UHECRs are Fe, deflections are too large, statistics won t help, we will never pinpoint sources and identify counterparts we d better stop this impossible quest now opportunity face the problem, understand its origin, think deeper or wider, and find a solution Change era!
21 Part II: The post-auger era (Beware, however, the Auger era is not finished ) (statistics will grow, and important results are still expected)
22 Motivations and expectations NB: here, I won t talk about actual experiments and projects (see e.g. next talk) Auger North JEM-EUSO radio detection of UHE showers? any other idea? Is the UHECR science so interesting after all, and is it worth the effort? What can be done, and what would we learn?
Back to basics: central questions 23 Puzzling existence of UHECRs what are the sources? (NB: could be presently unknown sources!) how are the particles accelerated? what constraints do they put on the physics of the sources (and perhaps physics in general)? How are UHECRs related to general CR science, high-energy astrophysics and astroparticle physics? not an isolated science! don t forget the bigger picture Are these questions worth the effort? do we stop here or do we go forward?
A new era for UHECRs 24 Old era: study of the overall spectrum all particles from all sources everywhere in the universe (important stage, but reaches its limits) New era: study of individual sources is it possible? is it worth it? We must check deflections and source density Yes! key informations about the sources! effectively part of the bigger picture
Deflections and source density 25 Anisotropy studies with low statistics enhance the power of autocorrelation function studies, to constrain two important parameters: the typical angular deflection of UHECRs (from their sources to the Earth) and the number density of sources in the local universe compare the data not only to isotropic expectations, but to generic model expectations! Check for Decerprit, Busca & EP (2010) (very soon ;-) ) Basic idea: Few sources, small deflections many clusters of UHECRs Many sources, large deflections few clusters of UHECRs
Overview of the method 26 choose a given source density and a given size for the typical magnetic deflections of UHECRs Build a particular realization of the source configuration with that density (Monte-Carlo from a uniform source distribution, or from a distribution similar to that of nearby galaxies, etc.) Build a random data set (with a given number of events) from that particular source distribution (implementing propagation effects) Compute the angular auto-correlation function of that data set, and determine its level of clustering (by comparison with isotropic expectations) Repeat that for many realizations of sources with the chosen source density and deflection angle Repeat that all for a whole range of source densities and deflections Compare the level of clustering of your actual (Auger, HiRes, TA ) data set with that of all types of models, i.e. all types of sources densities and deflection angles Do a statistical analysis and reject/accept regions of the parameter space (source density/deflection plane)
Example 27 Compatibility with isotropy (50 events above 60 EeV, from an a priori isotropic source distribution)
Example 28 Increasing statistics 100 events
Example 29 Increasing statistics 300 events
Example 30 Increasing statistics 600 events
Power of the method 31 Constraints on a given non isotropic model (Investigation of a data set built from a realization of sources with density 10-5 Mpc -3 and 5 deflections) (with 50 events)
Power of the method 32 Constraints on a given non isotropic model (Investigation of a data set built from a realization of sources with density 10-5 Mpc -3 and 5 deflections) (with 250 events)
a priori isotropic source distribution 33 Compatibility with isotropy (50 events above 60 EeV, from an a priori isotropic source distribution)
More realistic a priori distribution 34 Compatibility with isotropy (50 events above 60 EeV, from a source distribution similar to galaxies)
Preliminary conclusion 35 Assuming an a priori uniform source distribution deflections are large, or source density very large Assuming a galaxy-like source distribution deflections are large anyway! (not very realistic!) (quite plausible) NB: expected if heavy nuclei! (or protons with large B fields ) no contradiction between Fe and anisotropy (on the contrary!) Is that the end of UHECR science? NO! individual sources can still be observed!
36 The GZK benediction Reduces the typical travel distance of UHECRs from theit sources to the Earth Lower deflections! Reduces the number of sources Larger angular separation! (+ high energy!) Restricts observation to local universe and eliminates the isotropic background Study of local sources individually Price to pay: very low flux Exposure target: ~10 6 km sr yr
37 The GZK benediction The GZK cut-off is our friend: let s use it! Look into the GZK regime Fight cosmic variance by focusing on cosmic variance! Go from general to particular Observe individual sources
Looking for the brightest sources 38 Claim: at the highest energies, the sky must be dominated by the contribution of only a few sources With higher statistics, we simply cannot miss them Even with large deflections (suggested by current data), the hottest spots will be identified! Proof: Simulations! So what? What do we do with one or few hot spots?
Looking for the brightest sources 39 Method Assume a given density of sources Build a particular realization of the source configuration with that density Build a random data set (with a very large number of events to avoid large Poissonian fluctuations) from that particular source distribution (implementing propagation effects) Determine the fraction of events that come from what turned to be the brightest source for that source configuration Do the same for the second brightest source, third brightest source, etc. Repeat all this for another source configuration with the same density, to explore cosmic variance Repeat all the above for a different source density (and luminosity distribution, to explore this important parameter) NB: Most (very!) conservative case: all sources have the same intensity
Single source contribution Brightest sources above 60 EeV 40 Source 1 Source 2 Source 3
Single source contribution Brightest source above 60 EeV 41 Source 1 Source 2 Source 3
Single source contribution Brightest source above 70 EeV 42 Source 1 Source 2 Source 3
Single source contribution Brightest source above 80 EeV 43 Source 1 Source 2 Source 3
Single source contribution Brightest source above 90 EeV 44 Source 1 Source 2 Source 3
Single source contribution Brightest source above 100 EeV 45 Source 1 Source 2 Source 3
Single source contribution Synthetic view 46 fraction of total flux 0,16 0,14 0,12 0,1 0,08 0,06 0,04 0,02 Single source contribution (10-4 sources/mpc 3 ) source 1 source 2 source 3 0 50 60 70 80 90 100 110 E (EeV) th
Single source contribution Synthetic view 47 fraction of total flux 0,4 0,35 0,3 0,25 0,2 0,15 0,1 0,05 Single source contribution (10-5 sources/mpc 3 ) source 1 source 2 source 3 0 50 60 70 80 90 100 110 E (EeV) th
Single source contribution Synthetic view 48 0,6 0,5 Single source contribution (10-6 sources/mpc 3 ) source 1 fraction of total flux 0,4 0,3 0,2 source 2 0,1 source 3 0 50 60 70 80 90 100 110 E (EeV) th
Single source contribution Synthetic view 49 0,6 0,5 Single source contribution (hottest source) 10-6 Mpc -3 fraction of total flux 0,4 0,3 0,2 10-5 Mpc -3 0,1 10-4 Mpc -3 0 50 60 70 80 90 100 110 E (EeV) th
Single source contribution Synthetic view 50 0,6 0,5 Single source contribution (hottest source) 10-6 Mpc -3 fraction of total flux 0,4 0,3 0,2 NB: dispersion in intrinsic luminosity is inevitable significantly larger effective source density larger contribution of hottest sources! 10-5 Mpc -3 0,1 10-4 Mpc -3 0 50 60 70 80 90 100 110 E (EeV) th
Sources? What are they worth? 51 Finding sources is definitely worth the effort! even without identification with known sources Remember the UHECR sources might be new sources! Remember the UHECR sources might be transient! (e.g. GRBs ) even without pinpointing the sources within arc seconds or even degrees! Key parameters: source density extremely important! individual source power! maximum energy? magnetic deflections efficient constraints on UHECR source phenomenology
UHECR? What are they worth? 52 Important part of a bigger picture involving high-energy physics, high-energy astrophysics, astroparticle physics Particle acceleration in the universe is still poorly understood! The key ingredient of high-energy astrophysics! UHECRs are the most challenging can be particularly useful! Remember that CR sources are unknown at all energies! UHECRs remain a key element in the solving of this 100 years old puzzle! smallest deflections small number of sources easier to identify! (GZK is our friend ) most challenging particles strongest constraints! the same sources could be responsible for all CRs!!! holistic model realistic for a source spectrum in E -2.3 and a mixed UHECR source composition!
UHECR? What are they worth? 53 It might be that low-e CRs and usual sources won t help much Low-E CRs are not challenging: many sources can do it. seeing more TeV sources, or reaching a detailed modelling of SNRs, may not help: we already know that they accelerate particles, and still don t know the origin of CRs even if we finally prove that relativistic hadrons are in these sources, how will we know if they are the cosmic rays? (NB: known sources cut around 1 PeV or below these are not the CR sources!) solving the problem may really require studying UHECRs (as well as the GCR/EGCR transition ) we are on the edge of observing individual sources over the sky For all reasonable source densities and UHECR deflections, sources are within reach of detectors with statistics ~100 events at ~80 EeV For all reasonable source densities and UHECR deflections, sources are within reach of detectors with statistics ~100 events at ~80 EeV
Let s change era! 54 Observe individual sources not so much to be done with the overall spectrum UHECR science is at a critical stage Auger has spoken, clear and loud, but has not given the answer Larger statistics are required (NB: we know how much ) This will require new techniques/ideas (fluorescence) JEM-EUSO ready to try the space road with known and proven technique R&D towards new possibilities? (Radio detection of showers? ) Leave the paradoxical stage where new data actually lead to more and more possible models (triggering new ideas)! Reach the stage where new data start killing more and more models Crisis = opportunity!
55 Thank you very much