Centaurus A at Ultra-High Energies

0.21 MB·

by  R. W. Clay

Centaurus A at Ultra-High Energies
Roger W. ClayA,C, Benjamin J. WhelanA, and Philip G. EdwardsB
ASchool of Chemistry and Physics, Universityof Adelaide, Adelaide SA 5005
BCSIRO ATNF,Narrabri Observatory,Locked Bag 194, Narrabri NSW 2390
Cemail: [email protected]
Abstract: We review the importance of Centaurus A in high energy astrophysics as a
nearbyobjectwithmanyofthepropertiesexpectedofamajorsourceofveryhighenergy
0 cosmicrays andgamma-rays. Weexamineobservational techniquesandthe results sofar
1 obtained in the energy range from 200GeV to above 100EeV and attempt to fit those
0
data with expectations of Centaurus A as an astrophysical source from VHE to UHE
2
energies.

n
Keywords: acceleration of particles — galaxies: active — gamma rays: observations — cosmic rays
a
J

6 1 Introduction
which may contain regions such as its extended radio
] lobes, or supermassive central black hole, with physi-
E The field of very high energy astrophysics deals with cal properties which enable cosmic ray acceleration to
H processes associated with the acceleration and inter- exceed energy limitations which apply in galaxies like
actions of particles at energies above those accessible the Milky Way. For this reason, overalmost 40 years,
.
h with spacecraft observatories, characteristically above CenAhasbeenthetargetofobservationalsearchesfor
p afew100GeV,uptothehighestparticleenergiesfound evidence that it is a significant VHE or UHE source.
- in nature, above 100EeV. The massive particles at WebrieflyreviewthetechniquesusedtostudyCenA,
o
these energies are known as cosmic rays and at the reviewthereasonswhyCenAisanattractiveobserva-
r
t top of the energy range are referred to as ultra high tional target, and examine the observational progress
s
energy(UHE).Theobservedenergeticphotons,which which has been made.
a
[ are seen at lower energies, are known as veryhigh en-
ergy (VHE) gamma-rays.
1 The all sky cosmic ray spectrum exhibits a very 2 Particle Acceleration
v
steep dependence of flux against energy. It extends
3 Processes and Sites
over 30 orders of magnitude of flux and ten orders of
1
magnitudeinenergytoabove100EeVwithratherlit-
8
tledeviationfromafeaturelesspowerlawrelationship. Thereisageneralexpectationthatcosmicrayparticles
0
There is a steepening at PeV energies, known as the primarily receive energy through diffusive shock ac-
.
1 “knee” (Hillas 1984). At energies in the EeV range, celeration (Berezinskii et al. 1990; Protheroe & Clay
0 there is then a flattening known as the “ankle”. The 2004). This is a process whereby charged particles
0 knee is thought to represent either an energy limit to diffusively cross an astrophysical shock front multiple
1 the acceleration ability of most galactic sources or a times, receiving a boost in energy with each crossing.
v: limittotheabilityofourMilkyWaygalaxytosecurely This concept has been shown to havethehappychar-
i contain and build upan internal cosmic ray flux. The acteristic that a power law cosmic ray energy spec-
X ankle is thought to represent a change from predomi- trum results. Such acceleration processes were first
r nantlygalacticsourcedcosmicraystoanextragalactic proposed by Fermi who noted that head on collisions
a flux(Gaisser & Stanev 2006). Itisthusmost reason- with moving magnetic clouds resulted in a transfer of
able to look at energies above a few EeV for direct energy to already energetic particles, and that head
observational evidence of Centaurus A (Cen A) as a on collisions were statistically preferred over others
cosmic ray source. (Berezinskii et al.1990). Fermi’soriginalprocessproved
Theoriginsofcosmicraysarenotsecurelyknown. to beveryslow and therealisation that multiple(sta-
It is thought that supernovae or supernova remnants tistical) crossings of a shock front provided a much
(SNR) are the most probable origins of cosmic rays faster (“first order”) process appeared to provide a
which originate in the Milky Way and that such par- practical acceleration model. In such a picture, there
ticles are energised through diffusive shock accelera- isaclearrequirementthat(asubsetof) theaccelerat-
tion (Protheroe & Clay 2004). There appear to be ingparticlesarerepeatedlyscatteredacrosstheshock.
severe limitations to energies accessible through this Anenergyupperlimitoftheprocessresultsfromprop-
process and thehighest energy cosmic rays are postu- erties ofthesourceregion which finallyfail toprovide
lated to originate in some different environment out- sufficient scattering at the highest energies.
side our galaxy (Hillas 1984). Cen A, our closest ac- An alternative non-stochastic scenario is that ac-
tive galaxy, is a relatively local extragalactic object celeration is associated with the voltage drop created
1
2 Publications of theAstronomical Society of Australia
byarapidlyspinningsupermassiveblackholethreaded a Cen A source will not provide a direct undeflected
bymagneticfieldsinducedbycurrentsflowinginasur- beam, although protons could interact in some inter-
rounding disk or torus (Levinson 2000). In this case, mediate matter, resulting in a “halo” around the di-
the maximum achievable energy is apparently in the rection of a source.
UHE region although more detailed modelling will be Cosmicrayinteractionscanadditionallyproducea
requiredtoclearlydeterminelimitsimposedbyenergy fluxofhighenergyneutrinos. Thesecould bethrough
loss mechanisms such as curvatureradiation. interactionswiththeCMBorwithparticlesandfields
CenAcontainsasupermassiveblackholeandalso close to the source. In the latter case, LUNASKA
exhibits evidence of substantial shocks with evidence (James et al. 2009) or northern UHE neutrino detec-
for particle acceleration associated with their related tors such as ANTARES (Brown et al. 2009) might
jets (Hardcastle et al. 2007). We shall see below that search for signals from thedirection of Cen A.
it has regions which are capable of scattering parti-
cles magnetically as required. Whether those neces-
4 Interactions with Photon
sary conditions are sufficient for the acceleration of
particles to ultra high energies is the question to be Fields
answered observationally.
Our Universe is known to be pervaded by the cos-
3 Particle Propagation and At- mic microwave background (CMB) having a photon
number density a thousand times that of characteris-
tenuation
tic plasma densities. Despite the low energies of mi-
crowave photons, VHE photons and UHE nuclei see
At very high energies, astrophysical particles are ca- them as significant targets over modest astrophysical
pable of having inelastic and elastic collisions with distances. Photons from Cen A are expected to be
particles and fields in the source, and between the severely attenuated (Protheroe 1986a,b) over a range
source and our observatories within the Milky Way ofenergies,withaspectralcut-offbeginningintheen-
galaxy. These interactions are important to under- ergyrange130to200TeVdependingonthestrengthof
standing the astrophysics of the particles which are the intergalactic magnetic field (Clay et al. 1994). In
observed. Althoughlimited in size, sourceregions can this absorption feature, attenuation lengths of a few
contain strong magnetic fields, intense photon fields kpc are expected for its deepest point at a little over
over a great energy range, and a high plasma den- 1PeV.Theabsorptionfeatureprogressivelyweakensat
sity. In intergalactic space, our knowledge of fields higher energies and, for sources within our galaxy, at
andenergydensitiesislimitedbutweexpect,atleast, muchhigherenergiestheabsorptiondipmaybepassed
thattherewillbesomemagneticfields,starlight,infra- and photon attenuation may be reduced (Protheroe
red radiation, and the cosmic microwave background 1986a). However, for photons from the more distant
(Driveret al. 2008). Closer to home, messenger par- CenAwithamuchgreaterpathlength,theabsorption
ticles will transit whatever fields are contained in our will be strong up to 10EeV (Protheroe 1986b). The
localgroupofgalaxies,ourgalactichaloandtheplane Pierre Auger Observatory (see Section 11) is capable
oftheMilky Way. Inthelattercontext,it isexpected of selecting photons from its overall detected flux at
thattheknownmagneticfieldsofourgalaxy (withan such energies and a search for photons from Cen A
underlyingregularfieldatlevelsofafewµGbutupto above 10EeV would seem worthwhile.
10µG if a random component is included (Sun et al. Cosmic raynucleiwill alsointeract with theCMB
2008)) will have deflected all charged cosmic rays by but this is not important until much higher energies
significant amounts (Stanev 1997). Astrophysical an- than for photons, and the attenuation length is much
gular uncertainties then exceed instrumental ones for greater. This attenuation phenomenon is convention-
charged cosmic rays. ally named the GZK effect after the people (Greisen,
It is possible for accelerated protons to interact ZatsepinandKuzmin)whoproposeditin1966forcos-
(most likely in a source region) and convert to neu- micrayprotonsinteractingontheCMB(Berezinskii et al.
trons. This,forinstance,isanargumentforapossible 1990). The interaction has a proton energy thresh-
cosmicrayexcessinthedirectionofourgalacticcentre old of about 60EeV, a factor of 100,000 times greater
since the neutrons will not suffer deflection in galac- than energies associated with thephoton attenuation.
ticplanemagneticfieldswhichwouldcertainlyscatter There are also interaction processes for other nuclei
protons out of a directional beam (Clay 2000). Iso- on the CMB which become important at about this
lated neutrons decay within a few minutes when at energy. However,whilstthecharacteristicattenuation
rest but cosmic ray neutrons with a high relativistic lengthduetotheGZKeffectisoftheorderof100Mpc,
gamma factor will survive long distances in the lab- the interaction mean free path is of the order of the
oratory frame. However, at the distance of Cen A, distance to Cen A. This is due to the modest energy
neutrons with energies below 400EeV (just above the lossperinteraction. ThusbothVHEgamma-raysand
highest energy cosmic ray recorded from any direc- UHEcosmicrayssourced from CenAareexpectedto
tion (Bird et al. 1995)) would decay before reaching show evidence of interactions with theCMB.
us. We note that such decay is statistical and some It appears that attenuation compatible with the
neutrons might be observable even at lower energies GZK effect is evident in the all sky cosmic ray spec-
buttheresultingfluxwould begreatly attenuatedbe- trumofthePierreAugerObservatory(Abraham et al.
low 100EeV. It seems that neutrons generated within 2008a),althoughevidenceforapropagationcut-offre-
www.publish.csiro.au/journals/pasa 3
quiresanassumptionthatthecosmicraysourcespec- the daughter electrons and positrons. The mean free
trum does not contain a similar feature. The current paths for those processes are related and are between
datasetattheseenergiesissmallanditisquestionable 30and40gcm−2(comparedtothethicknessofthever-
if any presently observed events from the direction of tical atmosphere of about 1000gcm−2). This rather
Cen A could show a statistically convincing evidence simple cascade develops “exponentially” in particle
fortheexistenceof,oralackof,GZKattenuation(al- number until ionisation energy losses begin to inhibit
though see Section 12). furtherdevelopment. Thecascadethusreachesamax-
imum particle number (often stated as a number of
5 Magnetic Fields “electrons”,Ne). Suchcascadesarestatisticalinchar-
acter but the short interaction mean free paths com-
paredtothetotalatmosphericdepthresultinarather
Charged cosmic ray particles will have their propaga- smooth developmentprofile.
tion directions changed in their passage through as-
Primary nucleialso initiate cascades buttheseare
trophysical magnetic fields. That deflection will de-
more complex and irregularly structured. They begin
pend on the particle rigidity, the ratio of momentum
with a strong interaction which produces pions. The
and charge. At the energies of interest here, this is
neutralpionsdecaytoapairofgamma-rayswhichthen
effectively the ratio of the energy and the charge. A
cascade as we have just seen. However, the charged
convenient rule of thumb is that the radius of curva-
pions are likely to decay (they may interact again if
ture of a 1PeV proton trajectory perpendicular to a
the atmospheric conditions are conducive) to muons.
uniform 1 microgauss magnetic field is 1pc. Charac-
Those muons will most likely continue to traverse the
teristicgalactic fieldsareat theselevelsorjust above,
atmosphere without further major interactions, just
but their structure and, particularly, their extension
suffering a continuous energy loss from their ionisa-
out of the galactic plane are poorly known. Also,
tionandexcitationofatmosphericgases. Thiscascade
their strength within our local group of galaxies and
now has three components. They are: the remnants
the remaining intergalactic space between ourselves
of the original particle which only loses a fraction of
and Cen A is largely unconstrained by observation
itsinitialenergyateachinteraction(thenuclearcore),
(Beck 2008). There is evidence that richer groups
the muons, and the electromagnetic cascades. A key
may bepervadedbymultipleµGlevelmagnetic fields
point is that the nuclear core continues to inject en-
(Clarke et al. 2001; Feretti& Johnston-Hollitt 2004)
ergythroughfurtherinteractions,resultingintheiniti-
but this may not apply in our local region. This lack
ation of superimposed electromagnetic cascades. The
of knowledge is a major problem since, even at pro-
overall “shower” particle number is then a superposi-
ton energies of 100EeV, we are still dealing with a
tion ofelectron numbersinsuccessiveelectromagnetic
radius of curvature of only 100kpc in a characteristic
cascades, building and decaying, plus the integrated
magnetic field. As a result, we are unable to specify
numbers of muons. This picture is further compli-
whether charged cosmic ray propagation over a dis-
cated by the fact that the cosmic ray beam (at least
tance of 3.8Mpc from Cen A is diffusive (with an un-
at the lower energies) is a mixture of nuclear compo-
certain magnetic turbulence scale size) or whether we
nents (Gaisser & Stanev 2006). These various nuclei
can assume roughly linear propagation. A clear scat-
havetheirowninteraction mean freepathsforinitiat-
tered cosmic ray signal from Cen A would give us in- ing cascades (longest for protons — 80gcm−2 — and
valuable information regarding our local extragalactic
shorter for moremassive nuclei) and,though difficult,
magneticfields(thoughofcoursethisrequiresaknowl-
and probably not possible on an eventby event basis,
edge of theintrinsic source size).
thisoffersameansforstudyingthebeamcomposition,
or its change with energy.
6 The Air Shower Technique All cascades contain charged particles which scat-
terthroughinteractingwithatmosphericgas. Asare-
Ouratmosphereisopaquetoprimaryradiation at en- sult,theelectromagneticcascadesspreadlaterallywith
ergies with which veryhigh energy astrophysicsdeals. a characteristic distance of below 100m for the nu-
Also, the flux of astronomical particles becomes suffi- mericallydominantelectromagneticcomponent. How-
cientlylowatthoseenergiesandabovesuchthatdirect ever,someelectromagneticparticlescanscattertovery
satellite observation ceases to be effective for reason- large “core distances” and ground-based detecting ar-
ablespacecraft detectorcollecting areas. Theeffective rayssuchasthePierreAugerObservatoryrecordpar-
wayofworkingathigherenergiesisthroughtheuseof ticlesatkilometresfromalateralextensionoftheorig-
ouratmosphereasatargetandobservingthecascades inal cosmic ray trajectory. The muons scatter rather
of particles, known as “air showers” or “extensive air little but retain their direction from their initiating
showers (EAS)”, which are produced as incident par- interaction which results in a characteristic spread of
ticles deposit their energy, first as conversion to sec- hundredsofmetresatsealevel. Again,averyfewcan
ondaryparticlemassandkineticenergyandtheninto be found kilometres from the core.
atmospheric gas excitation and ionisation. Agood in- Air shower cascades are studied by sampling a se-
troductiontothephysicalprocessesinairshowerscan lection of their components. This is efficient in terms
be found in Allan (1971). of enabling the detection of rare events (at the high-
Primary gamma-rays initiate cascades which, to a est energies the flux may be measured in terms of
first approximation, develop through successive pro- km−2century−1). This sampling can beaccomplished
cesses of pair production and then bremsstrahlung of with a sparse array of ground-based charged particle
4 Publications of theAstronomical Society of Australia
detectors which sample the cascade at a single devel- strained, cosmic ray images, together, in some cases,
opmentlevel. Therequirementthatparticlesreachthe witharequirementthatpointsourcesunderstudyare
ground limits this technique to energies above about at known positions in theimage.
100TeV at sea level or to detector arrays located at PeV gamma-ray studies have more commonly ex-
very high altitudes (Amenomori et al. 2000). Alter- plicitly used a muon veto in which cascades with sig-
natively, that observational energy threshold can be nificant muon numbers have been rejected. This ap-
reduced by detecting the bright beam of forward di- proachhashadmixedsuccess. Atevenhigherenergies
rected Cˇerenkov light produced in theatmosphere us- intheEeVrange,gamma-rayinitiatedshowersareex-
ing large optical photon collecting telescopes which pected to reach maximum development deeper in the
“image” those photons onto a photomultiplier “cam- atmosphere than cosmic ray showers and work is on-
era”. TheVHEgamma-raytelescopessuchasH.E.S.S. going to select potential gamma-ray cascades on this
(Aharonian et al. 2005) fall into this category. At the basis. Presently, thePierre Auger Observatory claims
highest energies, where a low level of light emission upper limits to the UHE photon fraction using this
per shower particle is not a limiting factor, isotropic method (Abraham et al. 2009).
nitrogen fluorescence light, produced by the cascade
excitingatmosphericgas,canbesuccessfullyrecorded.
Thisenablesalargecollectionareatobeachievedwith 8 Searches at TeV Energies
large mirrors viewing the cascade from the side. The
Pierre Auger Observatory employs this technique to-
The first searches for VHE gamma-ray emission from
gether with a large array of ground-based large area
Cen A were made with the Narrabri Stellar Inten-
particle detectors.
sity Interferometer. The IntensityInterferometer con-
sisted of two 6.5m diameter segmented optical reflec-
torsmountedona188mdiametercirculartrack. The
7 Differentiating between
interferometertookadvantageoftheBose-Einsteinsta-
Gamma-Rays and Cosmic tisticalnatureoflight,withopticalphotonsfromstars
tendingtoarriveinclumps. Thecorrelationsininten-
Rays
sity between the two reflectors enabled the diameters
of bright stars to be inferred. The light pool from
Inrecentyears,thestudyofveryhighenergygamma- Cˇerenkovradiation producedbyVHEcosmicrayshas
rays has become an important component of astro- a similar extent to the track diameter, and so calcu-
physics. This has become possible partly through im- lations and tests were performed to confirm that the
provementsininstrumentalsensitivityandangularres- Cˇerenkovlight signalwas notcontaminatingthemea-
olution but, also, through significant improvements surements of steller diameters (HanburyBrown et al.
inthesoftware discrimination between gamma-ray in- 1969). It was recognised, however, that the Inten-
ducedextensiveairshowersandthenumericallydomi- sity Interferometer could also be put into service as
nantcosmic rayshowers. Thegamma-ray showers are a Cˇerenkov detector. Grindlay et al. (1975b) used a
predominantly electromagnetic with interaction pro- 120m separation between reflectors and operated the
cesses(pairproductionandbremsstrahlung)whichoc- two as a stereo detector. They also employed a novel
cur at rather small intervals in the atmosphere. The backgroundrejectionscheme,usingoff-axisphotomul-
resulting cascades are rather simple and smooth. On tipliers to detect Cˇerenkov light from the penetrat-
theotherhand,cosmic raysinitiate andfeed cascades ing muoncomponent of cosmic-ray initiated cascades,
through a nuclear process with a much longer mean whichallowed∼30%ofrecordedeventstoberejected.
free path and their interactions producemuons in ad- Between 1972 and 1974, a sample of 11 sources, in-
dition to electromagnetic particles. The cascade de- cluding pulsars, X-ray binaries, the Galactic Centre,
velopment is then rather irregular and also has an ir- and AGN, were observed, with source selection based
regular geometrical spread due to the muons, which onX-rayandSAS-2(30MeVto100MeV)gamma-ray
can travel, with rather little scattering, at significant results. Atime-averaged4.5σ excesswasdetectedina
angles away from the central cascade core. Differen- totalobservingtimeof51hronCenA(Grindlay et al.
tiation between gamma-ray and cosmic ray initiated 1975a),correspondingtoaintegralfluxabove300GeV
cascadesconventionallydependsonvetoingcosmicray of (4.4±1)×10−11cm−2s−1. As the outer radio lobes
cascades. Thisisachievedeitherbydetectinganirreg- were well outside the beam, these were excluded as
ular shower development (or irregular Cˇerenkov im- the source. Possible variability of the gamma-ray sig-
age), a development which peaks at an atmospheric nal (Grindlay et al. 1975a), coupled with theoretical
depth characteristic of nuclei for a given total energy modelling, also excluded the inner radio lobes as the
(or particle content), or a muon content greater than gamma-raysource,withaproposedmodelofthegamma-
expected for a gamma-ray cascade. rayfluxarisingfrominverseComptonscatteringinthe
VHEgamma-rayastronomyusingtheatmospheric nucleus of Cen A being favoured (Grindlay 1975).
Cˇerenkov technique has proved to be very efficient in SubsequentVHEobservationsoverthenext35years
producingimageswhichhavegood gamma-raytocos- yielded negative results. The Durham group, based
mic ray discrimination. This is due to careful Monte at Narrabri, observed between March 1987 and April
Carlo modelling of thecascade and imaging processes 1988 with their MkIII telescope for a total of 44 hrof
todevelopsuitableimagecuts(Aharonian et al.2005). good on-source data. The 3σ flux upper limit above
Thesearebroadlybasedonvetoingthelarger,lesscon- 300GeVwas7.8×10−11cm−2s−1 (Carramin˜ana et al.
www.publish.csiro.au/journals/pasa 5
1990). InMarch1997,6.75hoursofobservationswere icance was 2.7σ, a value not unexpected by chance
madewiththeDurhamMk6telescope,whichprovided giventhenumberofbinsexamined,butwhichencour-
betterdiscriminationagainstthecosmicraybackground, aged further investigation. There was some evidence
with a 3σ fluxupperlimit above 300GeV of in the binned data for excess event numbers toward
5.2×10−11cm−2s−1 (Chadwick et al. 1999). both outer radio lobes (Clay et al. 1984). Some sup-
The JANZOS group observed Cen A from New porting evidence was also noted in two other South-
Zealandfor56.9hrbetweenApril1988andJune1989, ernHemisphereexperiments(Farley and Storey 1954;
reporting a 95% confidence level upper limit on the Kamata et al.1968),thoughwithdifferingenergythresh-
flux above 1TeV of 2.2×10−11cm−2s−1 (Allen et al. olds, angular resolutions and years of operation. It
1993a). was also acknowledged that, at PeV energies, the sig-
Interest in VHE emission from Cen A was rekin- nalcouldnotbeduetogamma-raysfromCenAasthe
dledbytheEGRETdetectionofCenAinthe30MeV path length for interactions of PeV gamma-rays with
to 30GeV range (Steinleet al. 1998; Sreekumar et al. CMB photons is only 10kpc(Clay et al. 1984).
1999). The CANGAROO 3.8m telescope was used in TheJANZOSexperimentcombinedTeVtelescopes
March and April1999 to record a total of 45hrof on- withaPeVscintillatorarray,andasearchforgamma-
and off-source data. The resulting 3σ flux upper lim- rays above 100TeV was conducted with data taken
itsabove1.5TeVwere5.5×10−12cm−2s−1 forapoint between October 1987 and January 1992. No signifi-
sourceatthecoreofthegalaxy,and1.3×10−11cm−2s−1 cant excess was found over this complete time range,
for an extended region of radius 14′ centred on the but an excess was observed over 48 days from April
core (Rowell et al. 1999). In March and April 2004, to June 1990, with the excess concentrated in events
further observations were made with three 10m tele- withenegiesbelow200TeV,consistentwiththeeffects
scopesoftheCANGAROO-IIIarray. From10.6hours of the expected absorption at higher energies. Monte
of on-source data, upper limits were set for the sev- Carlo simulations were used to derive a probability of
eral regions of interest: for the core of Cen A the 2σ 2% for the observed 3.8σ excess to arise by chance
upper limit above 424GeV was 4.9×10−12cm−2s−1 (Allen et al. 1993b). A contour map of the signifi-
(Kabukiet al. 2007). cance showed a peak that coincided, within the an-
TheH.E.S.S. groupachieved adetectionof Cen A gular resolution of the array, with the core of Cen A.
with over 120hr of observation between April 2004 Buckland Park data taken between March 1988 and
and July 2008. Their measured integral flux above February1989wasexaminedandnosignificantexcess
250GeV was (1.56±0.67)×10−12cm−2s−1. The de- found from Cen A (Bird & Clay 1990) — this being
tection is concentrated on the central galaxy region, consistent with the JANZOSresult for thisperiod.
not the lobes. This excess “only matches the position A larger Buckland Park data-set, from 1984 to
of the core, the pc/kpc inner jets and the inner radio 1989,wassplit,apriori,intothreeeventsizebins,and
lobes” (Aharonian et al. 2009). aexcessfoundinthelowest sizebin,correspondingto
TheH.E.S.S.fluxisafactorofalmost30belowthe energies below 150TeV (Clay et al. 1994) (hence be-
original report of Grindlay et al. (1975a). However, low the CMB absorption feature). A confidence level
the Intensity Interferometer observations were made of 99.4% was claimed for the excess. A Kolmogorov-
during an extended period of enhanced X-ray emis- Smirnov test indicated there was no significant evi-
sion in the early 1970s. Although X-ray monitoring dence for enhanced periods of emission over this five-
wasinfrequentduringthe1980s,itappearsCenAhas year period. A contour map of the excess showed a
beeninarelativelyquiescentstateformostofthelast peak suggestive of a point source compatible with the
30 years(Bond et al. 1996; Turneret al. 1997; Steinle core of Cen A.
2006). A low X-ray state would plausibly result in a PeV detections and upper limits are also plotted
low flux of inverse Compton scattered VHE gamma- in Figure 1.
rays.
TeVdetectionsandupperlimitsareplottedinFig-
ure 1. 10 SUGAR
The Sydney University Giant Air Shower Recorder
9 Searches at PeV Energies
(SUGAR)(Winn et al.1986),apartfromhavingacre-
ativeacronym, wasnotable andimportant in pioneer-
Searches at PeV energies are made using air shower ingthestartofaneweraofcosmicraystudy. Likethe
arrays of particle detectors. The Buckland Park air huge Pierre Auger Observatory (below), with an en-
showerarraywasusedbetween1978and1981tostudy closedareaof70km2SUGAR’sdesignrecognisedthat
theanisotropyofcosmicraysaboveanenergyof1PeV. thefluxofcosmicraysatthehighestenergiesissolow
The directional accuracy of the array was 3◦×sec(θ), that its ground detectors required large separations
whereθ isthezenithangle. Thestudyconfirmedthat and could not realistically be connected by cable to
the overall cosmic ray flux shows no strong sidereal theirdirectneighbours. Theyrequiredameasureoflo-
isotropy at these energies, and the work was then ex- calautonomy. InthecaseofSUGAR,thiswasthrough
tendedtosearchformorelocalizedexcesses. Theleast therealisationofalocalcoincidencebetweentwomuon
isotropicdeclinationbandwasthatbetween−40◦ and detectors at each station and the tape recording of
−45◦, and the largest excess in bins of 1hr in right their data plus time stamping for later global array
ascension coincided with Cen A. The overall signif- analysis. The detector sites were also autonomous in
6 Publications of theAstronomical Society of Australia
D 79
E b=30°
C F
A H G IJ 12664
B
I
b=0°
K
l=-40° l=-80°
Figure 2: SUGAR events with energies
above 60EeV in the vicinity of Centaurus A
(SUGAR Catalogue 1986). The events are
Figure 1: Reported detections (solid circles) and
labelled by their energy in EeV. The red star
upper limits (arrows) to the flux from Cen A at
indicates the position of Cen A.
TeV and PeV energies. (The labels are as follows:
A, H.E.S.S., Aharonian et al. 2005; B, H.E.S.S.,
Aharonian et al. 2009; C, Narrabri, Grindlay et
al. 1975a; D, Durham, Carraminana et al. 1990
E, Durham, Chadwick et al. 1999; F, JANZOS, of Malargu¨e, Argentina, with planning of a North-
ern Site in Colorado, USA, underway (Harton et al.
Allen et al. 1993a;G, CANGAROO, Rowellet al.
2009). The southern site covers an area of approxi-
1999; H, CANGAROO-III, Kabuki et al. 2007; I,
mately 3000km2 (Suomija¨rvi et al. 2009). The PAO
JANZOS, Allen et al. 1993b; J, Buckland Park,
employs two cosmic ray detection methods through
Clay et al. 1994; K, Buckland Park, Clay et al.
the Surface Detector (SD) and the Fluorescence De-
1984.) The Chadwick et al. 1999 point (E) has
tector(FD)(Abraham et al. 2004;Bellido et al.2005;
been moved horizontally from the actual 300GeV Allekotte et al. 2008).
energy threshold for clarity.
The SD consists of more than 1600 autonomous
particle detector stations which employ thewater-
Cˇerenkov detection technique. The particle detectors
operatebyrecordingCˇerenkovlightemittedwhenrela-
terms of power, generating their own power thermo- tivisticchargedparticlesinanairshowerpassthrough
electrically. Previous arrays had detected real time 1.2mdeeppurifiedwaterenclosedinlarge-area(10m2)
coincidencesbetweenspaceddetectorstoinitiatedata tanks. These stations are arranged on a triangular
acquisitionfollowingthearrivalofasuitablyenergetic grid, with 1.5km spacing. This spacing results in the
shower. SUGARoperated for long enough todetect a SD being fully efficient for detecting showers with a
significant numberof showers with energies above the primaryenergyofabove3×1018EeVatzenithangles
GZK cut-off energy and, until the commissioning of of 60◦ or less (Suomija¨rvi et al. 2009). Statistical en-
the Pierre Auger Observatory, was the only array at ergyuncertaintiesfromtheSDareapproximately17%,
the highest energies with Cen A in its field of view. with an additional systematic uncertainty of 7% at
SUGAR was shown to have a problem with photo- 1019eV(increasingto15%at1020eV)arisingfromcal-
multiplier afterpulsing which could makesome energy ibrationwithFDenergies(seebelow)(Di Guilio et al.
assignmentsuncertain,althoughitsdirectiondetermi- 2009). Directionaluncertaintiesareapproximately1.5◦
nations would not have been affected by that. Data at energies around 3EeV, reducing to less than 1◦ for
from this array was used for studying a possible as- energies above approximately 10EeV (Bonifazi et al.
sociation of the highest energy cosmic rays with the 2009). It has a duty cycle of slightly less than 100%
direction of our galactic centre (Bellido et al. 2001). givingacurrentintegratedexposureofmorethan12,000km2sryr,
No excess from the direction of Cen A was evident in increasing by approximately 350km2sryr per month
that analysis. Figure 2shows theSUGARhighest en- (Schu¨ssler et al. 2009).
ergy events (SUGARCatalogue 1986) in the vicinity
TheFDmakesuseof theair fluorescencemethod.
of Cen A(for comparison with Figure 3for thePierre
Twenty-four telescopes are separated into 4 groups of
Auger Observatory).
6 telescopes (each group being termed a FD ‘site’),
whichoverlook differentsectionsoftheSD.Eachtele-
11 The Pierre Auger Obser- scope views 30◦ in azimuth, giving each site a 180◦
◦
azimuthalfieldofview,and28.6 inelevation. Ineach
vatory
telescopeacameraconsistingofanarrayofphotomul-
tiplier tubes, viewing separate regions of sky, collects
The Pierre Auger Observatory (PAO) is the largest thelightemittedbynitrogenmoleculesexcitedbythe
cosmic ray detector ever built. It currently consists EAS. Using pulse timing information from triggered
of a Southern Hemisphere site located near the town pixels, the axis along which the shower front propa-
www.publish.csiro.au/journals/pasa 7
12 Observational Results at the
gatescanbereconstructed. Theenergyoftheshower,
apartfromasmallamountof‘invisibleenergy’carried Highest Energies
by neutrinos and muons which are not visible to the
FD, is proportional to the integrated light flux along
ThePierreAugerObservatoryistheonlysystempresently
the shower’s path. This allows an effectively calori-
recording data at EeV energies from the direction of
metric measurement of particle energy (Bellido et al.
Centaurus A. It presented its first skymap in 2007
2005). Thestatisticalenergyuncertaintiesareapprox-
(Abraham et al. 2007) displaying the 27 highest en-
imately 9%, with a systematic uncertainty of approx-
ergy events at that time. This map appears to show
imately 22% arising from factors such as limitations
eventclusteringinthegeneraldirectionofCenA.Data
of knowledge of the instantaneous atmospheric pro-
in that skymapwhich are in thevicinityof Cen A are
file and aerosol content (Di Guilio et al. 2009). The
shown in Figure 3 (Abraham et al. 2008b) which il-
requirement of clear, moonless nights for the opera-
lustrates apparent clustering around the direction of
tionoftheFDmeansthatitsdutycycleisabout13%
Cen A.
(Schu¨ssler et al. 2009).
Thecolocation of theFD and SD allows eventsto
be observed by both detectors. The events for which
148
this occurs are termed ‘hybrids’. For these events,
80 b=30°
the timing information from a triggered SD station
is added to that from theFD trace to reconstruct the 84
arrival direction (Bellido et al. 2005). This allows a
70
greater accuracy in determining the shower axis than 63 6958
is possible with either method alone, and the average
hybrid directional uncertainty is 0.6◦ (Bonifazi et al. 64 6679 b=0°
2009). An additional advantageof thehybridmethod
isthatitallowstheSDenergyscaletobedetermined.
By a comparison of independent SD and FD recon- l=-40° l=-80°
structionsofthesameevents,SDenergiescanbecali-
bratedagainst theessentially calorimetric valuesfrom Figure 3: Auger events above 57EeV in the vicin-
theFD (DiGuilio et al. 2009). ity of Centaurus A (Abraham et al. 2008b). The
events are labelled by their energy in EeV.
Composition studiesareperformed primarily with
theFDthroughmeasurementsofthepositionofshower
maximum,Xmax. Thisvalueindicatestheslantdepth
intheatmosphere,ingcm−2,atwhichthefluxoffluo- TheAugerpaperinterpretsthedatasetasawhole
rescentlightfromtheEASreachesitsmaximum. From as being statistically associated with the directions of
shower to shower the value of Xmax fluctuates due local AGNs. This was on the basis of an a priori
to the statistical nature of shower initiation and de- search “prescription”. The Pierre Auger Collabora-
velopment. On average, however, nuclei are expected tionhasnotyetdevelopedadiscoveryprescriptionfor
to have smaller Xmax values than protons at a given Cen A and, as a result, noa priori statistical analysis
energy, and fluctuations in Xmax are expected to be is presently possible. However, one can comment on
smaller. Consequently, FD measurements are used to theproperties of the dataset.
study the behaviour, as a function of energy, of both Hillas(2009)hasindependentlyexaminedthisAuger
hXmaxi (the energy dependence of which is termed data set and confirms the conclusion that the highest
the ‘elongation rate’) and the RMS of Xmax to look energyeventsareassociatedwithrathertypicalSeyfert
forpossiblechangesinthecomposition oftheprimary galaxies in clusters at distances of typically ∼50Mpc.
particles(Bellido et al.2009). Itshouldbenoted,how- The clustering of events near Cen A is confirmed and
ever,thattheinterpretationoftheseresultsisnotclear anorigininCenA,oralternatively,NGC5090consid-
duetouncertaintiesinhadronicinteractionphysicsat ered. ThecloseproximityofCenA,however,ledHillas
such high energies (Bellido et al. 2009; D’Urso et al. toconcludethatCenAisprobablyaninactive1020eV
2009). accelerator as more distant galaxies play a larger role
thanmighthavebeenexpectedonthebasisofasimple
ThelargedutycycleoftheSDmakestheprospect inverse squarelaw fluxdependence.
ofutilizing ittodetermineCR composition highlyde- Morerecently(Hagueet al.2009),thePierreAuger
sirable. This is a somewhat more difficult task than Collaborationhasshowncontinuingevidenceforacon-
with the FD, however, as a direct measurement of centrationofthehighestenergyeventsinthedirection
Xmax is not possible with the SD. Methods such as of Cen A. Those data show an excess of events above
studying the risetime of the particle signal in the SD 55EeV within 18◦ of Cen A which is well above the
stations,theshowerfrontradiusofcurvature,theratio 68%confidenceintervalforasamplefromanisotropic
of the muonic to electromagnetic contributions to the distribution. Inthatrange,12eventsarefoundwhere
signal and the azimuthal asymmetry in station signal 2.7 are expectedon thebasis of an isotropic flux. Ap-
aroundtheshoweraxisarecurrentlybeinginvestigated proximately 30% of the Auger events show some evi-
fortheirsuitabilityincompositiondeterminationwith dence of being members of such clustering out to 30◦
theSD (Wahlberg et al. 2009). from Cen A,approximately thesame fraction (10/27)
8 Publications of theAstronomical Society of Australia
as found in the original Auger skymap.
Onthebasis of thisevidence,onemight speculate 1
that Cen A is the source of a substantial fraction of
E
theextragalacticcosmicraysatenergiesabovethean- ≤ s 0.8
kle of the cosmic ray energy spectrum. In that case, nt
e
onenotes that theangular size of thecosmic ray “im- Ev 0.6
age”isappreciablygreaterthantheknownphysicaldi- of
n
mensions of theastronomical source on the sky. That o 0.4
cannot be explained by instrumental errors since, as acti
noted above, the known angular resolution of the ob- Fr 0.2
◦
servatory at theseenergies is below 1 (Bonifazi et al.
2009) and one naturally invokes magnetic scattering 0
60 80 100 120 140 160
in intergalactic space or within our galactic region. If
Energy (EeV)
thescatteringoccurs in intergalactic space, onemight
plausibly assume a10kpcturbulencecell size, leading
Figure 4: Energy distribution for events in-
toatotalscatteringdeflectionoftheorderof20times
◦
the scattering in an individual cell (after the passage side and outside a 25 circle centred on Cen A
of approximately 400 individual cells). To fit the ob- (Abraham et al.2008b). Thedottedredlineisfor
◦
served excess around Cen A, this requires a turbulent the events more than 25 from Cen A.
intergalactic field of strength 0.1µG.
At these energies, any deflections from the direc-
tionofCenAinknownregulargalacticplanemagnetic
fields are likely to be modest (a few degrees (Stanev the propagation is dominated by regular intergalac-
1997))buttheextentandstrengthofmagneticfieldsin tic fields and that the particles are deflected as in a
any galactic haloaround theMilky Way areunknown magnetic spectrometer, with the true source being in
(Sun et al. 2008) and could plausibly besubstantialif the direction from which an “infinite” energy particle
the dimensions of the halo are large. The Auger ex- would have been seen. Since the excess has its high-
cessappearstolimitthepossibilityofsucheffectsfrom est energy particles furthest north from the galactic
regularfieldssincethereseemstobenogreatasymme- plane, one would invoke a magnetic field parallel to
tryperpendiculartothegalactic planealthoughthere that plane (perpendicular to the supergalactic plane)
may be an oval axis in that direction for the excess with a “true” source region some distance further to
just discussed. the north such that theangular deflection is inversely
proportional to the particle rigidity. In this scenario,
InascenarioinwhichtheobjectCenAistheorigin
it could bethat thedeflectingmagnetic field is ahalo
of the Pierre Auger “Centaurus A” excess, one must
field of the Milky Way. This would require a prod-
explain the apparent similarity between the distribu-
uct of magnetic field strength for the regular compo-
tionofenergieswithintheexcesswhencomparedwith
nent of the field and its spatial extent of the order of
thetotalityofAugerevents. Usingthepublishedevent
50µGkpc. This may not be incompatible with mod-
data (Abraham et al. 2008b), there is no significant
elsofmagneticfieldsingroupsofgalaxiesorextended
difference between the two spectra (Figure 4) on the
galactic halos.
basis of a Kolmogorov Smirnov test (at > 99% level),
admittedly with a very limited event list. If that con-
tinues to be the case, one might have to abandon the 13 Centaurus A as a Cosmic
GZK cut-off as the source of a deficit of events above
60EeV and argue for a source acceleration limitation. Ray Source
An examination of event data presented by the
Pierre Auger Collaboration with their 2007 sky map As we saw, it is usual to think of cosmic rays being
of the highest energy events shows a possible system- accelerated to their observed energy in a rather slow
atic variation in particle energy across the Cen A ex- statistical process. There are alternative possibilities,
cess,withthehighestenergyeventsbeingatthehigh- such as a single acceleration through a very large po-
est (positive) Galactic latitudes. Such an effect could tential step or a “top down” model in which an ultra-
be due to chance, but it could also count as evidence energeticparticleistheresultofthedecayofanexotic
against the excess being associated with Cen A since highly massive initial particle. If the process is some-
there is not now a symmetry in the event energies thinglikediffusiveshockacceleration,theacceleration
aboutthecentralregion. Anexplanationcouldbethat site mustbesuchthat itsscattering fieldsare capable
the cosmic ray source for the highest energy events ofreturningacceleratingparticlesmanytimesacrossa
is in the northern lobe, or there could be contamina- shock front. This would appear to require local mag-
tionfromanothersourceinthesupergalacticplane,or netic fields with products of strengths and physical
simply that the combination of intergalactic scatter- dimensions such that a particle radius of gyration at
ing and the passage through structured regular fields thehighestenergiescanbecontainedwithinthephys-
combine to producethe effect. ical boundaries of the field. This is often expressed
Theenergyvariationwithdirectioncouldalsosug- through one of the“Hillas diagrams” (Hillas 1984).
gest that there is another possible approach to un- When it comes to considering Cen A as a source,
derstanding the “Centaurus A” excess. This is that possible extremes of the spectrum of sites would be
www.publish.csiro.au/journals/pasa 9
withinthemodeststrengthmagneticfieldsenclosedin below a level proportional to its radio emission since,
one or other of its giant radio lobes, or (at the other including all such objects, Cen A dominates the total
extreme) within very strong fields close to the cen- sum of the radio fluxes typically by at least an order
tral engine. Somewhere within a jet, or the southern of magnitude (Nagar & Matulich 2008). This is due
shock,could also becandidatesitesspecifictoCen A. toitsproximitytous,anditcouldbethatsomeofthe
Theradiusofgyrationofacosmicrayproton(inkpc) other objects are more effective at accelerating par-
is numerically close to its energy (in units of EeV) ticles to the highest energies. We noted that Cen A
dividedbythemagneticfield strength(inunitsofmi- may be a variable source at high energies, which may
crogauss). Containment within an acceleration region support this idea (Hillas 2009).
will requirethat the region is significantly larger than Recently,Rieger & Aharonian (2009)haveconsid-
that radius of gyration. Since it is the particle rigid- ered Cen A as a VHE gamma-ray and UHE cosmic-
ity which is relevant,this requirementwould be eased raysource,andconcludethatadvectiondominatedac-
in proportion to the nuclear charge for heavier nuclei. cretion disk models can account for the production of
This would mean that the acceleration of protons in the TeV emission close to the core via inverse Comp-
200kpclobesofCenA(thewholeofalobe)wouldre- ton scattering of sub-mm disk photons by accelerated
quiremagneticfieldsfillingalobeatmicrogausslevels electrons. As it is unlikely that protons could be ac-
inordertoaccelerate particlestothemeasuredAuger celerated to EeV energies in this region, they propose
limit of about 200EeV. shearacceleration alongthekpc-scalejetastheorigin
Under such conditions ((Protheroe & Clay 2004) for theseparticles.
equation41),atimeoftheorderof100millionyearsis
requiredfortheaccelerationprocess. Thiswouldseem
to be approximately the limit of possible acceleration 14 Conclusions
both under a 108year estimate of AGN lifetimes and
an estimate of microgauss strength fieldsin thelobes. Centaurus A has been a popular potential source of
Thecosmicrayenergyspectrumextendsover30orders cosmic rays for close to half a century. A number of
of magnitude in flux and very few accelerated cosmic cosmic ray and VHEgamma-ray searches for excesses
rays are required to reach the highest energies - they from that region have been made. An early search by
are statistical anomalies. The source magnetic field Grindlay et al. (1975a,b) with VHE gamma-rays was
must be strong enough and large enough in scale for encouraging, showing evidence for a positive observa-
the highest energy particles to be capable of one last tion at a time of a large X-ray flare, and, very re-
diffusivescattering across the shock front. cently,H.E.S.S.hasalsoprovidedevidencethatCenA
An alternative extreme of the spectrum of possi- is a VHE gamma-ray source. The Buckland Park air
bleCen Aacceleration sites mightbewithinthemost shower array found a signal with appropriate spectral
central volume of the AGN, close to the black hole. characteristics which included evidence for CMB ab-
ThemajorityofTeVandPeVdetectionsareconsistent sorptionatsub-PeVenergies. Also,recently,thePierre
with an excess concentrated on the central galaxy re- Auger Observatory has shown evidence of a cluster-
gion,andnottheouterlobes. Acentralregionwithdi- ingofUHEcosmic rayeventsaroundCen Aalthough
mensions of, say,10pcwould requirea shock contain- without evidence for a different spectrum to that of
ment field strength approaching 1Gauss. This would other directions. Taken with observations which have
besubstantialbutnotunreasonable. Adifficultywith produced upper limits and recognising that none of
such aregion would betoaccelerate particles toUHE theseobservations hadwell defineda priori statistical
energies over a substantial period of time within a analysis procedures, one cannot say with confidence
dense photon field containing photons with energies that Centaurus A is a major source at UHE energies.
substantiallyabovethoseoftheCMB.Thisisbecause However, it is the nearest example to us of one of the
cosmicrayenergylossinteractionswouldbesignificant few classes of objects which have been identified as
from at least EeV energies. This attenuation at ener- havingastructurepossiblycapableofacceleratingpar-
giesbelowtheankleofthecosmicrayspectrummakes ticlestothehighestobservedenergies. Accelerationof
itdifficulttoseehowtheAugerCenAspectrumcould cosmic ray particles to the highest measured energies
bear similarity with the conventional spectrum from in Cen A would be at the limit of parameters associ-
other directions. atedwith theacceleration process. Areductioninthe
Cen A is a radio galaxy with highly extended jets cosmic ray flux (from the general direction of Cen A)
and lobes. As we noted, it could be that such lobes aboveabout60EeVfromthepowerlawatlowerener-
play a key part in accelerating particles to the high- giescouldbeasourceeffectratherthanaGZKcut-off,
est observed energies. Nagar & Matulich (2008) have which would (and may well) otherwise apply to more
discussed the possible role of objects with that mor- distant sources.
phologyassourcesoftheAugerhighestenergyevents. For the future, we clearly require more data. Fur-
They note that there is a number of such sources (5) therwork at VHE(H.E.S.S.) energies tobetterdefine
in the general vicinity of Cen A out of a total of 10 thesourceregion and any possible extendedstructure
inthe“field ofview”ofthePAO.Theyalso notethat would begreat progress. An extension of thegamma-
such objects seem to be statistically closely related to ray spectrum upwards towards the CMB absorption
thedirectionsof thehighest energy PAOevents. This feature using theCˇerenkov technique,with better an-
propositionseemstobearguablebut,ifthisisthecase, gular uncertainty than Buckland Park, such as the
the contribution of Cen A to the total flux must be TenTen concept (Rowell et al. 2008), would be a ma-
10 Publications of theAstronomical Society of Australia
jor asset. Also, solid confirmation and understanding Clarke, T. E., Kronberg, P. P., & Bohringer, H. 2001,
of the Pierre Auger “Centaurus A excess” is urgently ApJ, 547, L111
needed.
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