Annu. Rev. Astron. Astrophys201049 1056-8700/97/0610-00 The Astrophysics of Ultrahigh Energy Cosmic Rays 1 1 0 2 Kumiko Kotera and Angela V. Olinto n Department of Astronomy and Astrophysics, a J Kavli Institute for Cosmological Physics, 2 The University of Chicago, Chicago, IL 60637, USA 2 ] E H Key Words . h cosmicaccelerators,magneticfields,particleastrophysics,neutrinos,gamma-rays,compact p objects, active galaxies, cosmic background radiation, gamma-ray bursts - o r Abstract t s a Theoriginofthehighestenergycosmicraysisstillunknown. Thediscoveryoftheirsources [ willrevealtheworkingsofthemostenergeticastrophysicalacceleratorsintheuniverse. Cur- 1 rent observations show a spectrum consistent with an origin in extragalactic astrophysical v sources. Candidatesourcesrangefromthebirthofcompactobjectstoexplosionsrelatedto 6 5 gamma-rayburstsortoeventsinactivegalaxies. Wediscussthemaineffectsofpropagation 2 from cosmologically distant sources including interactions with cosmic background radia- 4 tionandmagneticfields. Weexaminepossibleaccelerationmechanismsleadingtoasurvey . 1 ofcandidatesourcesandtheirsignatures. Newquestionsarisefromanobservedhintofsky 0 1 anisotropies and an unexpected evolution of composition indicators. Future observations 1 may reach the necessary sensitivity to achieve charged particle astronomy and to observe : v ultrahigh energy photons and neutrinos, which will further illuminate the workings of the i universe at these extreme energies. In addition to fostering a new understanding of high- X energy astrophysical phenomena, the study of ultrahigh energy cosmic rays can constrain r a the structure of the Galactic and extragalactic magnetic fields as well as probe particle interactionsatenergiesordersofmagnitudehigherthanachievedinterrestrialaccelerators. 1 Introduction Theobservationthatcosmicrayscanexceed1020eVposessomeinterestingandchallenging questions: Where do they come from? How can they be accelerated to such high energies? Whatkindofparticlesarethey? Whatisthespatialdistributionoftheirsources? Whatdo they tell us about these extreme cosmic accelerators? How strong are the magnetic fields thattheytraverseontheirwaytoEarth? Howdotheyinteractwiththecosmicbackground 1 Figure 1: AllparticlecosmicrayfluxmultipliedbyE2 observedbyATIC(Ahnetal.2008),Proton (Grigorovetal.1971),RUNJOB(Apanasenkoetal.2001),TibetAS-γ (Chen2008),KASCADE (Kampertetal.2004),KASCADE-Grande(Apeletal.2009),HiRes-I(Abbasietal.2009), HiRes-II(Abbasietal.2008b),andAuger(Abrahametal.2010b). LHCenergyreachofp−p collisions(intheframeofaproton)isindicatedforcomparison. radiation? What secondary particles are produced from these interactions? What can we learn about particle interactions at these otherwise inaccessible energies? Here we review recent progress towards answering these questions. The dominant component of cosmic rays observed on Earth originate in the Galaxy. As showninFigure1,thestudyofthisstrikingnon-thermalspectrumrequiresalargenumber of instruments to cover over 8 orders of magnitude in energy and 24 in flux. Galactic cosmicraysarelikelytooriginateinsupernovaremnants(see,e.g.,Hillas2006,forarecent update on the origin of Galactic cosmic rays). A transition from Galactic to extragalactic cosmic rays should occur somewhere between 1 PeV (≡1015 eV) and 1 EeV (≡1018 eV). Progress on determining this transition relies both on the study of the highest energies reached in Galactic accelerators as well as the search for extragalactic accelerators that produce ultrahigh energy cosmic rays (UHECRs). We begin with a brief summary of recent observations (Section 2), which reveal a spec- 2 Kotera & Olinto Figure 2: SpectrumofUHECRsmultipliedbyE3 observedbyHiResI(Abbasietal.2009)andAuger (Abrahametal.2010b). Overlaidaresimulatedspectraobtainedfordifferentmodelsofthe Galactictoextragalactictransitionanddifferentinjectedchemicalcompositionsandspectral indices,s,describedinSection2.1and4. trumwhoseshapesupportsthelong-heldnotionthatsourcesofUHECRsareextragalactic. As shown in Figure 2, the crucial spectral feature recently established at the highest ener- giesisasteepdeclineinfluxaboveabout30EeV.Thisfeatureisreminiscentoftheeffectof interactionsbetweenextragalacticcosmicraysandthecosmicbackgroundradiation,named the Greisen-Zatsepin-Kuzmin (GZK) cutoff (Greisen 1966; Zatsepin & Kuzmin 1966). An- other important feature shown in Figure 2 is the hardening of the spectrum at a few EeV, called the ankle, which may be caused by the transition from Galactic to extragalactic cosmic rays or by propagation losses if UHECRs are mostly protons. As discussed in Section 2, recent reports of a trend toward a heavier composition from a few EeV up to 40 EeV together with hints of anisotropies in the sky distribution above 60 EeV raise new and unexpected puzzles. An anisotropic sky distribution is expected for trans-GZK energies (i.e., energies above 60 EeV), if UHECRs are mainly protons, due to a combination of the GZK effect (which limits trans-GZK observed sources to lie within a few 100 Mpc), the anisotropic distribution of source bearing galaxies on 100 Mpc scales, Astrophysics of UHECRs 3 andthelowmagneticdeflectionoflighttrans-GZKnucleibytheGalacticandextragalactic magnetic fields. Therefore, the report of correlations between UHECRs above 55 EeV and thedistributionofnearbyactivegalaxies(Abrahametal.2007)canbesimplyinterpretedas protons from nearby sources within the so-called GZK sphere. However, composition indi- catorsfromshowerdevelopmentobservationsargueforatransitiontoaheaviercomponent from a few EeV up to 40 EeV (Abraham et al. 2010a). Heavy nuclei dominated injection models are quite rare in the astrophysical literature of candidate sources (see Section 6) and if iron is the main component at the highest energies, Galactic magnetic fields should wash out most anisotropic patterns around 60 EeV. Another possible interpretation of the observedshowerdevelopmentpropertiesisachangeinhadronicinteractionsabove100TeV centerofmass(TeV≡1012 eV),anorderofmagnitudehigherenergythanwillbereached by the Large Hadron Collider (LHC) at CERN. A new puzzle is born: an injection at the source dominated by heavy nuclei is astrophysically unexpected, while significant changes in hadronic interactions represent novel particle physics. To help discriminate between possible interpretations of recent results, we review in Section 3 the well-known physics of the propagation of ultrahigh energy cosmic rays: their interaction with the cosmic background radiation and the effect of cosmic magnetic fields. The effect of propagation on the observed spectrum, sky distribution, and composition depends on the source redshift evolution, the injected spectrum and composition, and the evolutionofcosmicbackgroundsandmagneticfields. Thespectrumiscut-offduetophoto- pion production of protons and photo-dissociation of nuclei off cosmic backgrounds. The composition simplifies to either proton or iron (or a mixture of the two) at trans-GZK energies. Anisotropies in the sky distribution of sources are blurred by magnetic fields for heavier primaries while protons keep most of the original anisotropies at trans-GZK energies. DifferentscenariosforthetransitionbetweencosmicrayscreatedintheGalaxyandthose from extragalactic sources are discussed in Section 4. Specific acceleration mechanisms envisioned for reaching these extremely high energies are the topic of Section 5, including shockacceleration,unipolarinductors,andotherproposals. InSection6,wesurveyknown astrophysicalsitesthatarereasonablecandidatesforUHECRsources,fromcompactobjects such as, neutron stars (or magnetars), to gamma-ray bursts and active galaxies. Possible signatures of different candidate sources are discussed in light of future observations of UHECRs and other messengers of the extreme universe. Withasignificantincreaseintheintegratedexposuretocosmicraysabove60EeV,next generation observatories may reach the sensitivity necessary to achieve charged particle astronomy and to observe ultrahigh energy photons and neutrinos, which will further illu- minatetheworkingsoftheuniverseatthemostextremeenergies. Weendwiththeongoing and future search plans for the cosmic sources of ultrahigh energy particles. Duetothelimitedspace,wereferreadersinterestedindetailsofobservationaltechniques to Letessier-Selvon & Stanev (2011), Beatty & Westerhoff (2009), Bluemer et al. (2009), and Nagano & Watson (2000). Previous reviews on the astrophysics of UHECRs can be found in Cronin (2005), Olinto (2000), Berezinsky et al. (1990b), and Hillas (2006, 1984), while Bhattacharjee & Sigl (2000) also include a survey of UHECRs from cosmological 4 Kotera & Olinto Figure 3: Evolutionoftheexposuresofpast,current,andplannedUHECRobservatoriesovertime: Fly’s Eye(Baltrusaitisetal.1985),AGASA(Chibaetal.1992),HiRes(Boyeretal.2002),Pierre AugerObservatory(Abrahametal.2004),TA(Nonakaetal.2009;Tokunoetal.2009). ProjectedexposuresforAugerNorth(Blu¨meretal.2010)ifconstructionstartin2016and JEM-EUSO(Takahashietal.2009)iflaunchedin2017including20%dutycycle. relics. Stanev(2010)publishedarecentmonographonUHECRswhileGaisser(1991)covers cosmicraysoflowerenergies. Recentreviewscoverthecloselyrelatedhighenergyneutrinos (Anchordoqui& Montaruli 2010)and highenergy gamma-rays (Hinton &Hofmann 2009). 2 Cosmic Ray Observations at Ultrahigh Energies After many decades of efforts to discover the origin of cosmic rays, current observatories are now reaching the necessary exposure to begin unveiling this longstanding mystery (see Figure 3 for a the history of exposures for the largest observatories). The first detection of UHECRs dates back to Linsley (1963), but it was only during the 1990s that an interna- tional effort began to address these questions with the necessary large-scale observatories. The largest detectors operating during the 1990s were the Akeno Giant Air Shower Ar- ray (AGASA), a 100 km2 ground array of scintillators in Japan (Chiba et al. 1992), and the High Resolution Fly’s Eye (HiRes) a pair of fluorescence telescopes that operated in Utah until 2006 (Boyer et al. 2002). During their lifetimes, AGASA reached an exposure Astrophysics of UHECRs 5 of 1.6×103 km2 sr yr (or 1,600 L1) while HiRes reached twice that. To date, the highest energyrecordedeventwasa320EeVfluorescencedetection(Birdetal.1994)bythepioneer fluorescence experiment Fly’s Eye (Baltrusaitis et al. 1985). Completed in 2008, the Pierre Auger Observatory is the largest observatory at present (Abrahametal.2004). ConstructedintheprovinceofMendoza,Argentina,byacollabora- tionof18countries,itconsistsofa3,000km2arrayofwaterCherenkovstationswith1.5km spacinginatriangulargridoverlookedbyfourfluorescencetelescopes. Thecombinationof thetwotechniquesintoahybridobservatorymaximizestheprecisioninthereconstruction of air showers, allowing for large statistics with good control of systematics. The largest observatory in the northern hemisphere, the Telescope Array (TA), is also hybrid (Nonaka et al. 2009; Tokuno et al. 2009). Situated in Utah, it covers 762 km2 with scintillators spaced every 1.2 km overlooked by three fluorescence telescopes. 2.1 Spectrum Theobservedcosmicrayspectrum(Figure1)canbedescribedbyabrokenpowerlaw,E−s, with spectral index s=2.7 below the knee at ∼ 1 PeV (=1015 eV) and s(cid:39)3 between the knee and the ankle around 3 EeV (Apel et al. 2009). Above the ankle, s (cid:39) 2.6 followed by the recently established flux suppression above about 30 EeV. With exposures around 103L,themeasuredspectraatenergieswheretheGZKeffectwasanticipatedhadconflicting results: AGASA reported no flux supression at trans-GZK energies (Takeda et al. 1998), while early results from HiRes were consistent with the GZK prediction (Abbasi et al. 2004). By2006,HiResaccumulatedenoughstatisticsforthefirstsignificantobservationof the GZK suppression (Abbasi et al. 2008b), as displayed in Figure 2. This was confirmed by the Auger Observatory (Abraham et al. 2008b) with a recent update starting at 1 EeV (Abrahametal.2010b)andbasedon1.3×104Lexposure(showninFigure2). Thedisplayed error bars are statistical errors while the reported systematic error on the absolute energy scaleisabout22%. Thissystematicerrorallowsforoverallenergyshiftsthatmakethetwo observations consistent within the estimated errors. The highest energy event reported by Auger thus far is of 142 EeV (Abreu et al. 2010). Figure 2 also shows the observed spectrum fit by different models of UHECR sources (taken from Kotera et al. 2010b and references therein). In the mixed composition and iron dominated models (Allard et al. 2007), the ankle indicates a transition from Galactic to extragalactic cosmic rays (see Section 4), the source evolution is similar to the star formation rate (SFR), and the injection spectra are relatively hard (s ∼ 2−2.1). In the proton dominated models in the figure, the ankle is due to pair production propagation losses (Berezinsky & Grigorieva 1988), named “dip models” (Berezinsky et al. 2006), and the injection spectra are softer for a wide range of evolution models. Models with proton primariescanalsofitthespectrumwithharderinjectionwithatransitionfromGalacticto extragalactic at the ankle (Wibig & Wolfendale 2004). TheconfirmedpresenceofaspectralfeaturesimilartothepredictedGZKcutoff,settles the question of whether acceleration in extragalactic sources can explain the high-energy 1InhonorofUHECRpioneerJohnLinsley,weusetheexposureunitL=1km2 sryr. 6 Kotera & Olinto Figure 4: Fractionofcosmicraysthatsurvivespropagationoveradistance>D,forprotonsabove40,60, and100EeVandforHe,CNO,andFeabove60EeV.Blacksolidlineshowswhere50%ofagiven speciescanoriginateforagivenatomicmassandenergy. Attrans-GZKenergies(E(cid:38)60EeV), onlyprotonsandironsurvivethepropagationoverD(cid:38)50Mpc. AdaptedfromAllardetal. (2007). spectrum,endingtheneedforexoticalternativesdesignedtoavoidtheGZKfeature. How- ever,thepossibilitythattheobservedsofteningofthespectrumismainlyduetothemaxi- mumenergyofaccelerationatthesource,E ,isnotaseasilydismissed. Aconfirmation max that the observed softening is the GZK feature, awaits supporting evidence from the spec- tral shape, anisotropies, and composition at trans-GZK energies and the observation of produced secondaries such as neutrinos and photons. 2.2 Anisotropies in the Sky Distribution The landmark measurement of a flux suppression at the highest energies encourages the searchforsourcesinthenearbyextragalacticuniverseusingthearrivaldirectionsoftrans- GZK cosmic rays. Above GZK energies, observable sources must lie within about 100 Mpc, the so-called GZK horizon or GZK sphere (Harari et al. 2006; Allard et al. 2007 Astrophysics of UHECRs 7 Figure 5: ArrivaldirectionsofcosmicrayswithenergyE≥55EeVdetectedbyAuger(blackdots)inan Aitoff-HammerprojectionoftheskyinGalacticcoordinatesrestrictedto|b|>10◦ (Abreuetal. 2010). Shadedareasrepresentasmootheddensitymap(5◦ smoothingangle)ofthe2MRS galaxieswithin200MpcovertheAugerObservatoryfieldofview. and references therein). This effect is shown in Figure 4 where the fraction of cosmic rays that arrive on Earth from a given distance is plotted for different energy protons (> 40, 60, and 100 EeV) and for different nuclei (He, CNO, and Fe) arriving with energies above 60 EeV. At these trans-GZK energies, light composite nuclei are promptly dissociated by cosmic background photons (see Section 3), while protons and iron nuclei may reach us from sources at distances up to about 100 Mpc. Since matter is known to be distributed inhomogeneouslywithinthisdistancescale,thecosmicrayarrivaldirectionsshouldexhibit ananisotropicdistributionabovetheGZKenergythreshold,providedinterveningmagnetic fieldsarenottoostrong. Atthehighestenergies,theisotropicdiffusefluxfromsourcesfar beyond this GZK horizon should be strongly suppressed. Attempts to detect anisotropies at ultrahigh energies date back to the mid 1990s when hints of correlations with the local large scale structure and with distant BL Lacs objects were claimed and debated (see Section 6.2). With the increase in the number of observed ultrahigh energy events, these early claims have not been substantiated while different correlations have been recently reported. The most recent discussion of anisotropies in the sky distribution of ultrahigh energy events began with the report that the arrival directions of the 27 cosmic rays observed by Auger with energies above 57 EeV exhibited a statistically significant correlation with the anisotropically distributed galaxies in the 12th VCV (V´eron-Cetty & V´eron 2006) catalog of active galactic nuclei (AGN) (Abraham et al. 2007, 2008a). The correlation was most significantforAGNwithredshiftsz<0.018(distances<75Mpc)andwithin3.1◦ separa- tion angles. An independent dataset confirmed the anisotropy at a confidence level of over 99% (Abraham et al. 2007, 2008a). The prescription established by the Auger collabora- tion tested the departure from isotropy given the VCV AGN coverage of the sky, not the 8 Kotera & Olinto hypothesis that the VCV AGN were the actual UHECR sources. In particular, a lack of events from the Virgo region showed that assuming the VCV AGN to be the sources gives a bad match to the observed event distribution (Gorbunov et al. 2008). No corresponding correlation was observed in the northern hemisphere by HiRes (Abbasi et al. 2008c) where the distribution of their 13 trans-GZK events is consistent with isotropy. Figure 5 shows a map of the 69 events used in the recently published Auger update whichincludesanother42trans-GZKevents(Abreuetal.2010). Withthenewevents,the correlation with the VCV catalog is not as strong for the same parameters as the original period (20 events correlate out of the original 27 while only 12 correlate out of the new 42). Thedataaftertheprescriptionperiodshowsadeparturefromisotropyatthe3σlevel. With the currently estimated correlation fraction of 38%, a 5σ significance will require at least another four years of Auger observations (Abreu et al. 2010). The VCV catalog is not a catalog produced by an instrument or survey strategy, but an extensive compilation of known AGN in the literature. A better set of catalogs which giveamorehomogeneousandstatisticallycompletesurveyofthenearbyuniverseoverthe large field of view of Auger has become recently available. In particular, the Swift-BAT catalog of AGN (Tueller et al. 2010) and the 2MASS Redshift Survey (2MRS) catalog of galaxies (Huchra et al. 2005) are two catalogs where correlations may become more meaningful (George et al. 2008; Abreu et al. 2010). Figure 5 shows the example of Auger data superimposed on a density map generated with the 22,000 galaxies within 200 Mpc of the 2MRS catalog (Abreu et al. 2010) with Galactic latitude |b| > 10◦. The Auger trans-GZKeventstendtoalignbetterwiththedistributionofgalaxiesin2MRS(andwith Swift-BATAGN)thanwiththeisotropicscenario,howeverasignificanceoftheanisotropy or a source class identification is hard to access with the current limited statistics (Abreu et al. 2010). Finally,theanisotropyreportedbythetestwiththeVCVcatalogmayindicatetheeffect of the large scale structure in the distribution of source harboring galaxies or it may be due to a nearby source. An interesting possibility is the cluster of Auger events around the direction of Centaurus A, the closest AGN (at ∼ 3.8 Mpc). The clustering around the Cen A region may also be due to the Centaurus cluster (as shown by the dark red region of Figure 5) which is much further away but on the general direction of Cen A (Kashti & Waxman 2008; Kotera & Lemoine 2008b). Only much higher statistics will tell if Cen A is the first UHECR source to be identified. 2.3 Composition ThethirdkeymeasurementthatcanhelpresolvethemysterybehindtheoriginofUHECRs istheircompositionasafunctionofenergyobservedonEarth. Compositionmeasurements can be made directly up to energies of ∼ 100 TeV with space-based experiments (see, e.g., Ahnetal.2010). Forhigherenergies,compositionisderivedfromtheobserveddevelopment and particle content of the extensive airshower created by the primary cosmic ray when it interacts with the atmosphere. Presently,thebestindicatorofthecompositionoftheprimaryparticleisthedepthinthe Astrophysics of UHECRs 9 Figure 6: (cid:104)Xmax(cid:105)andRMS(Xmax)asafunctionofprimaryenergy,asmeasuredbyAugerfluorescence detectors(Abrahametal.2010a). MCsimulationsfromdifferenthadronicinteractionmodelsare displayedforprimaryprotons(blue)andprimaryironnuclei(red). atmosphereoftheshowermaximum,X ,givening/cm2. Theaverageshowermaximum, max (cid:104)X (cid:105), scales approximately as ln(E/A), where E is the energy and A is the atomic max mass of the primary cosmic ray which generated the shower (see, e.g., Letessier-Selvon & Stanev 2011 and references therein). On average the shower maximum for protons occurs deeper in the atmosphere than that for the same energy iron nucleus, (cid:104)Xp (cid:105) > (cid:10)XFe (cid:11). max max In addition, proton showers fluctuate more about (cid:104)X (cid:105) providing another measure of max composition,forexample,therootmeansquarefluctuationsabout(cid:104)X (cid:105). Anotheruseful max measureofcompositionistheparticlecontentoftheshowersuchasthenumberofmuons: proton showers have fewer muons than showers caused by heavier nuclei with the same energy. In practice, observed shower maxima and particle numbers are compared with Monte Carlo airshower simulations which involve an extrapolation to higher energies of hadronic interactions known at energies of laboratory accelerators ((cid:46) TeV). Observationsofshowerpropertiesfromthekneetojustbelowtheankleindicateageneral trend from light primaries dominating at the knee to heavier primaries dominating up to ∼0.1EeV(see,e.g.,Bluemeretal.2009). Theseobservationsfollowexpectationsthatthe knee is created by a rigidity2 dependent end of Galactic cosmic rays which may be due to 2Rigidityisdefinedasparticlemomentumdevidedbycharge,R≡p/Z∝E/Z 10 Kotera & Olinto