The highest-energy cosmic raysappear to come from nearbyactive galactic nucleiThough it's more reossuring thon surprising, this new result from theenormous Auger ground orroy suggests the discovery potential ofcosmic-roy astronomy.

The Pierre Auger Observatory hasreported its first major result. Designedto study Liltrahigh-t nergy cosmic rays,the 3000-km- facility studs the highplains abutting the Andes in western Ar-gentina with 1600 water-Cherenkov de-tectors watched over by four fluores-cence telescopes (see figures 1 and 2).

Construction began in 1999 under theleadership of James Cronin (Universityof Chicago) and Alan Watson (Univer-sity of Leeds, UK), and the Auger col-laboration has been acaimulating datasince 2004. Now, with 1400 of the 1600ground-array detectors in full operation,the collaboration has puhlished evi-dence that the highest-energy cosmicrays appear to be coming from relativelynearby active galactic nuclei (AGNs)JCronin calls it "the beginning of cosmic-ray astronomy."

Cosmic rays with energies above10' eV are very attractive to astron-omers. Like photons and neutrinos,they point back to their sources. Unlikephotons and neutrinos, cosmic rays arecharged particles—either protons orheavier nuclei. So their trajectories arebent and ordinarily scrambled by thepatchy magnetic fields that pervade ourgalaxy and intergalactic space. But the

radius of curvature in a magnetic fieldis proportional to a particle's momen-tum. AlO' -lO^ -eV proton is thought tobe a good pointer because it would bedeflected by only a few degrees in thecourse of a 500-miIlion-light-year (MIy)journey through the presumed nano-gauss fields of intergatactic space.

Questions and answersCosmic rays with energies above6 X 10'"* eV pose two intriguing ques-tions: What mechanism could possiblyaccelerate individual nuclei to suchenormous energies —far in excess ofanything remotely conceivable with amanmade accelerator? And further-more, how could those particles, onceaccelerated, maintain such energieswhile traversing large intergalactic dis-tances? The two questions are related,and the new Auger result provides theclearest answer yet.

The fact that the arrival directions ofthose ultrahigh-energy cosmic raysshow no preference for the Milky Wayhas long made it clear that they don'toriginate within our own galaxy. But inthe absence of exotic new physics,there's an upper limit on how far theycould have come. Above a threshold en-

ergy of 6 X 10'" eV-the so-called GZKcutoff, named after its predictors, Ken-neth Greisen, Georgii Zatsepin, andVadim Kuzmin —a proton coursingthrough the ubiquitous cosmic mi-crowave background loses energy sto-chastically by creating pions in coUi-.sions with the low-energy photons ofthe CMB.

Not literally a cutoff, the GZK effectis a rather abrupt suppression at thepion-production threshold, confirmedlast year by the High Resolution Fly'sEye (HiRes) fluorescence-telescope col-laboration (see PHYSICS TODAY, May2007, page 17). TTie suppression reflectsthe fact that a prottm cannot remainabove the threshold energy for morethan about 500 Mly, Cosmologicallyspeaking, that's still the local neighbor-hood. For higher arrival energies andheavier nuclei, the effective horizon iseven more constricting.

Cosmic rays with energies belowlO' eV are thought to originate from su-pernovae and their remnant shockwaves within our galaxy. But to accel-erate a proton to 1O'''-1O'" eV wouldseem to require much larger astrophys-ical systems—large enough to confinethe proton magnetically while it's being

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Figure 1. Covering 3000 km' of arid,high plains near the Argentinean townof Malargue 1000 km west of BuenosAires, the Pierre Auger Observatory hastwo ways of recording the extensive airshowers initiated by high-energy cosmicrays: A ground array of 1600 water-Cherenkov detectors (the dots spaced1.5 km apart) records the arrival ofcharged shower particles. And, on cleardark nights, four fluorescence telescopescan image the atmospheric fluorescenceexcited by a shower. Each telescope'sfield of view is suggested on the map bya fan of 20-km-long green lines. Theblue area is the fxirt of the ground arraythat was already in operation this pastSeptember. (Adapted from ref. 1.)

16 January 2008 Physics Today © 2008 American Institute o( Physics. S-0031-9228-0801-320-3

subjected to repeated shocks.Active galactic nuclei are the prime

suspects, even though it's not known indetail how an AGN would accomplishthe task. Most galaxies harbor a super-massive central hlack hole. But an AGNdiffers from the relatively sedate nucleiof galaxies like our own in that its blackhole is feeding so ravenously on sur-rounding material that fhe nucleus isloud af radio and x-ray frequencies andtypically sports an enormous pair of jetsof relafivistic material.

How would one demonstrate thafcosmic rays arriving at energies abovethe GZK threshold do indeed come fromAGNs within a few hundred Mly of us?And what if that were not the case? Spec-ulative alternatives to AGNs includeyoung starburst galaxies undergoingprodigious star formation. And eventhough there's already clear evidence ofthe GZK suppression,- it's possible thatexotic new particle physics is circum-venting the suppression to some extent.For example, massive primordial parh-cles impervious to the CMB might be de-caying on our doorstep to produce ultra-high-energy protons.

Recording showersUnfortunately for observers, the flux ofultrahigh-energy cosmic rays is frus-tratingly meager. At the fop of the at-mosphere, one can expect about onecosmic-ray particle with energy abovefhe GZK cutoff per square kilometerper century. That's why the Auger ob-servatory has to cover so much ground.Although the ground array has alreadyrecorded almost a million cosmic-rayevents above its effective threshold en-ergy of about 10"* eV, the new paperconcentrates on the few dozen of high-est energy—most of them above theGZK cutoff.' That harvest exceeds theworld total of such ulf rah igh-energycosmic rays previously reported byfluorescence telescopes and smallerground arrays.

When a high-energy proton or nu-cleus (called the cosmic-ray primary)encounters the atmosphere, it initiatesan extensive air shower which, bythe time it reaches fhe ground, con-sists mostly of relativistic electrons,positrons, and muons. Inside each ofAuger's ground-array detectors, photo-multiplier tubes record the pulse ofCherenkov light generated by the pas-sage of the shower particles through the10 tons of water. The spacing betweenadjacent detectors is 1.5 km, and fheshower generated by a 6 x lO' -eV pri-mary is wide enough to encompass adozen of them. The microsecond differ-ences between the arrival times of the

Figure 2. One of the Auger ground array's 1600 water-Cherenkov detectors,with the Andes looming on the western horizon. Each defector holds 10 tonsof water monitored by phototubes for Cherenkov light pulses. One can seethe detector's battery pack (left), its north-facing solar panel, and its electron-ics dome topped by a communications mast.

shower at each of those detectors yieldthe incident direction of the primary towithin a degree or so.

Determining the primary's incidentenergy is less straightforward. Themore energetic the primary, fhe greateris the density of shower particles hittingthe ground at a given distance from theshower axis. But translating showerdensity directly into energy has to relyon uncertain computer models of howulfrahigh-energy nuclear collisionsevolve into extensive air showers.

Instead, the Auger team used hybridevents—the small fraction of cosmic-ray showers recorded by one of thefluorescence telescopes as well as by theground array—to calibrate the ground-array density data. The telescopes,which image the fluorescent emissionfrom excited nitrogen molecules in theair shower, function only on clear,moonless nights. But when they do seea cosmic-ray shower, they provide arather straightforward measure of itstotal energy that requires no question-able computer simulahon.

Seeking sourcesTo test the hypothesis that cosmic raysabove the GZK cutoff come from localAGNs, the Auger collaboration availeditself of a catalog that gives the locationsof some 700 AGNs closer than 300 Mly.To avoid fhe lure of statistical fluctua-tions that mimic true correlations, theteam chose to analyze the accumulatingcosmic-ray data in two sequentialbatches. The first, exploratory batch,comprising all events recorded through

May 2006, was used to establish cutsand tolerances for testing the hypothe-sis on the subsequent confirmatorybatch.

Trying different cuts on the ex-ploratory batch, the Auger team foundthe strongest AGN correlation signalwhen the minimum cosmic-ray energywas set at 5.6 X 10' eV and source iden-tification required that the incident di-rection be within 3.1° of an AGN closerto us than 240 Mly. Of the 15 events inthe exploratory batch that survived theenergy cut, 12 also survived the AGNsource cuts; by pure chance one wouldhave expected only 3. Those cuts werethen adopted for testing the confirma-tory batch (see figure 3).

Of the 13 events above 5.6 x 10" eVin the confirmatory batch, 8 fell within3.1° of a sufficiently local AGN. If theunderlying flux were isotropic, theprobability of seeing such good appar-ent correlation with the AGNs just bychance would be about one in 500. "Soat least we've succeeded in excludingisotropy for the highest-energy cosmicrays with a confidence level of betterthan 99%," says Auger project managerPaul Mantsch of Fermilab.

The correlation between the cosmic-ray events and local AGNs gets evenbetter when one takes account of un-avoidable biases in the AGN catalog.Figure 3 shows an obvious dearth ofcataloged local AGNs in the vicinity ofour own galactic plane. That's an obser-vational bias easily explained by thegalaxy's obscuring dust. The dustwould make it harder to identify AGN

18 January 2008 Physics Today www.physicstoday.org

' Figure 3. Arrival directions of the 27 cosmic rays with energies above5.7 X 1 0 " eV recorded by the Auqer ground array are indicated by red dote,each with a radius of 3.1 ' on this sky map in celestial coordinates. Because theobservatory is in Argentina, almost oil of them are below the equator. Star-shaped points mark 472 active galactic nuclei known (from their redshifts) to becloser than 240 million light-years. Yellow stars mark the three closest AGNs,from which one might ultimately expect the most events. (Centaurus A Is only8 Miv away.) The blue dashed curve indicates the supergalactic plane, alongwhich local AGNs and other nearby galaxies cluster. The green dashed curve isthe plane of our own galaxy, through which it's hard to see AGNs. The whiteregion of northern sky is inaccessible to Auger. (Adapted from ref. 1.)

sources for cosmic rays arriving fromthat direction. "Indeed, if we ignore theevents closest to the galactic plane,"says Mantsch, "our full sample ends upwith 19 events out of 21 identified withlocal AGNs."

What ahout the admixture of heavynuclei in cosmic-ray tlux? The data ex-hihit strong correlation for any choiceof energy cut near the GZK pion-production threshold calculated ex-plicitly for protons. Another indicationthat the observed correlation is due pri-marily to protons is its manifestation atseparation angles as small as 3°. Because

bending in a magnetic field increaseswith charge, an incoming heavy nucleuspoints much less reliably to its source.

"We can't yet claim to have proventhat AGNs within the GZK horizonare the actual sources," says Augerspokesman Watson. Figure 3 shows aclustering of cosmic-ray events near thesupergalactic plane, a sheetlike struc-ture that encompasses our local super-cluster of galaxies as well as severalneighhoring superclusters, But that'swhere ail sorts of other galaxies minglewith our local AGNs. So it might he thatthe sources are starburst galaxies or

other astrophysical systems in the samecrowded environs.

With its full complement of ground-array detectors soon to he in operation,the Auger collaboration hopes to ad-dress these and other questions withahout 30 new events ahove the GZK en-ergy per year. An interesting issue, forexample, is whether these cosmic rayscome only from some recognizable suh-class of AGNs. Ultimately ohservershope that, with enough statistics, energyspectra of cosmic rays from individualsources will elucidate the mechanismsby which protons and heavy nuclei areaccelerated to uttrahigh energies.

In the light of Auger's puhlished re-sult, the HiRes team has reexamined itsultrahigh-energy cosmic-ray data accu-mulated over a decade before the facil-ity's shutdown last year. "With 13events ahove the GZK cutoff," says theteam's Gordon Thomson (Rutgers Uni-versity), "we've found no significantcorrelation between their arrival direc-tions and local AGNs."

The HiRes team and a Japanesegroup have joined forces to operate anew cosmic-ray observatory in Utahthat combines fluorescence telescopeswith a 1000-km- ground array. Galledthe Telescope Array Experiment, thenew facility hegan taking data in No-vember. The Auger collaboration is alsoturning its attention to the northern sky.It's seeking funds to start building asecond 3000-km- observatory at a sitenear the town of Lamar in southeasternColorado.

Bertram Schwarzschild

References1. Pierre Auger collaboration. Science 318,

938 (2007).2. R. U. Abbasi et al., http://arxiv.org/

abs/astro-ph/0703099.

Quantum spin Hall effect shov\ s up in a quantumvsrell insulator, just as predictedThe effect, vrhich occurs without a magnetic field, is a ne^ and topologically distinct electronic state

Five years ago, two groups of theo-rists made a bold proposal: If you applya voltage to the ends of a semiconduct-ing strip, spin-up electrons will accu-mulate along one edge, while spin-down electrons will accumulate alongthe other.

Spin segregation had already beenpredicted to occur when electrons scatteroff impurities, an effect known as the ex-trinsic spin Hail effect. What made thenew proposal intriguing is the mecha-nism: Under the right conditions, a ma-terial's intrinsic band structure, not its ex-

trinsic impurities, would sort the spins.Whether this intrinsic spin Hall ef-

fect could be observed was controver-sial. Even in that paragon ofpurity, epitaxiaily grown gallium ar-senide, scattering seemed likely tosmother the effect (see PHYSICS TODAY,February 2005, page 17).

Despite the initial controversy—andperhaps because of it—the two papersset off an explosion of interest in band-based spin flow. Together, the papershave already garnered nearly 2000 cita-tions. Shuichi Murakami and Naoto

Nagaosa of the University of Tokyo andShou-Cheng Zhang of Stanford Univer-sity wrote one of the papers.' The otherwas by Jairo Sinova of Texas A&M Uni-versity, Allan MacDonald of the Uni-versity of Texas at Austin, and their col-laborators.-

Among the ideas the papersspawned was the possibility that the in-trinsic spin Hal! effect, like the classicalHall effect, has a quantum cousin. Thequantum Hal! effect shows up when astrong magnetic field is applied to afrigid semiconducting strip. Electrons

www.physicstoday.org January 2008 Physics Today 19