Table Of Content1407: New Results of the HEGRA Air Shower
Detector Complex
AxelLindner ([email protected]), for the HEGRA Collaboration1
University of Hamburg, Germany
100m
8
9
9 Abstract. Recenthighlight resultsob- 4
50 2
1 tainedwiththeairshowerdetectorcom-
plex HEGRA at the Canary island La
n
a Palma are presented. Observations and 0 3
J searchesforγsourcesaboveanenergyof
1
500GeV are covered, where special em-
5
phasisisputontherecentflaringactiv-
1 -50 6 5
ities of the Blazar Mkn 501. A second
1 main research topic of the experiment
v is the measurement of the energy spec-
-100
1 trum and the elemental composition of
1 the charged cosmic rays. Data covering -100 -50 0 50 100m
3 the energy region of the ”knee” around 243 Scintillation Counters (1 m 2 ) 6 Cherenkov Telescopes
1 3PeV are presented. 77 AIROBICC Detectors ( / 0.4 m) 17 Geiger Towers (18 m 2 )
0 Fig.1. The layout of the HEGRA ex-
8
periment at LaPalma
9 1 The Setup
/
h The HEGRA (High Energy Gamma twoparts:imagingairCherenkovtele-
p scopes (IACTs) observe selected tar-
RayAstronomy)[1]experimentislo-
p- catedattheastronomicalobservatory gets with an energy threshold of 500
e of the Canary island LaPalma at a GeV, while three arrays of detectors
h monitor the whole sky above the in-
height of 2200m. It is operated by
:
v a collaboration of the German Max- stallationintheenergyrangebeyond
i Planck-Institutesfu¨rKernphysik(Hei- 15TeV.
X
r delberg) and fu¨r Physik (Mu¨nchen), 1.1 The HEGRA IACTs
a the univers. of Hamburg, Kiel, Wup-
Since the end of 1996 six IACTs are
pertal(Germany)andMadrid(Spain)
operational.Fouridenticaltelescopes
andtheYerevanPhysicsInstitute(Ar-
andanupgradedprototypecanoper-
menia). The experimentalsetup cov-
ers a square area of 40000m2 (Fig- ateasasystem.Thisuniqueinstalla-
tionobservesthe airCherenkovlight
ure1).
imagesofEASinastereomode.The
TheHEGRAexperimentmeasuresex-
segmented mirror area of each sys-
tensive air showers (EAS) initiated
tem telescope is 8.5m2 large. Each
byhighenergeticcosmicphotonsand
camera consist of 271 pixels (photo-
nuclei.Itcanroughlybedividedinto
multipliers each viewing 0.25◦ of the
1 supp.by theBMBF, DFG, CICYT sky) covering a field of view of 4.3◦.
2 A.Lindner
Forphotonsabovetheenergythresh- tillator triggered events, which has
old of 500GeV the direction of the been lowered to 5TeV after a DAQ
primary photon is deduced from the upgrade in spring 1997. Air showers
orientationoftheairCherenkovlight above11TeVtriggerAIROBICC.The
imageinthecamera,wherethestereo angularresolutionforphotoninduced
◦
observationsallowforanangularres- showersatthresholdamountsto0.3
olution of better than 0.1◦. Photon for AIROBICC triggered events and
inducedEASareseparatedfromshow- 0.9◦ for scintillator data (above 20
erstriggeredbyprimarynucleibythe TeV).
analysisoftheimageshapes.Theex-
2 TeV Photon Sources
cellent angularresolutionand the ef-
ficient photon separation lead to a Themostexcitingresultin1997were
nearlybackgroundfreeobservationof the measurements with the imaging
strongTeVpointsourcesliketheCrab air Cherenkov telescopes (IACT) of
nebula (the galactic standard candle the emission of TeV photons with a
of TeV astrophysics). The energy of very high and variable flux from the
primary photons can be determined extragalactic source Mkn501 [3, 4].
witharesolutionofbetterthan20%, This object in a distance of 500 mil-
thus detailed energy spectra can be lion lightyears belongs to the Blazar
measured [2]. The sensitivity of the class of active galactic nuclei (AGN)
telescope system can be summarized and is thoughtto containprobablya
8
such,thatwithin 100hobservationa blackholeof10 solarmasses.Blazars
detection on the 5σ level is possible areAGNswhichemitrelativisticjets
for a point source with a flux of only pointed towards the observer. Under
3% of the Crab. certain conditions they can also be
detected in the TeV energy regime.
1.2 The HEGRA Arrays TeVγ’sfromMkn501werefirstmea-
The HEGRA arrayssample the EAS sured in 1995 [5] with a flux corre-
showerfront. 243 scintillator huts on sponding to 8% of the galactic Crab
a grid with 15m spacing and a more nebula(thestrongestknownTeVemit-
dense part around the center regis- ter before). The activity rose to 0.3
ter charged particles. The so called Crab in 1996 [6]. In 1997 the mean
AIROBICCarrayof77stations(con- flux lies around 2.0 Crab, but flares
sisting of 20cm diameter open pho- with intensities ofup to 10 times the
tomultipliers attached to a Winston Crab flux have been registered ap-
cone) provides a non imaging mea- prox. once a month (Fig.2). The en-
surement of the air Cherenkov light. ergy spectrum in comparison to the
AGeigertowerarrayof17stationsis spectrumoftheCrabnebulaisshown
located in the central part of the ar- in Figure3. The Mkn 501 spectrum
ray.They allow for the measurement followsa powerlawup to10TeV. At
of the electromagnetic energy of an higher energies the study of system-
air shower at detector level and the aticdetectoreffectsisnotfinishedyet
identification of muons. The energy so that the data points are not in-
thresholdforverticalphotoninduced cluded in the plot.
showers was around 20 TeV for scin- Due to the absorptionofhighenergy
1
Measuring (cid:13)-Ray Energy Spectra with the HEGRA IACT System
W. Hofmann
HEGRA Collaboration
Max-Planck-Institut fu(cid:127)r Kernphysik, Heidelberg
Abstract
Thestereoscopicreconstructionofairshowersviewedbymultipleimagingatmospheric
Cherenkovtelescopes(IACTs)allowsamoreprecisereconstructionofshowerenergiesand
hence an improved determination of energy spectra. Reconstruction techniques and in
particular new systematic checks are discussed on the basis of the large sample of(cid:13)-rays
1407: New Results of HEGRA 3
fromMkn 501 detected with the HEGRA IACT system.
]z250 an object very similar to Mkn501 at
mH200 HEGRA nearly the same distance, but not as
1. Introduction [Rate 150 spectacular as Mkn501 in 1997, and
100 Crab theCrabnebula.NoevidenceforTeV
50 photonemissioncouldbefoundfrom
OneofthepremisesofstereoscopicimagingofairshowersusingmultipleatmosphericCherenkov
0 other AGNs. Also no TeV photons
telescopesisanimprovedreconstru5c2t5iono5f50show57e5rene6r00giesa62n5dhenceofsourcespectra. Stereo-
Date [Mjd-50000] couldbedetectedfromthesupernova
scopic imaging allows a rather precise reconstruction of the core location; using the known
Fig.2.TheHEGRAIACT system trig- remnants (SNR) IC443 and G78+2.
relation between light yield,core distance, and shower energy, the energy is estimatedby suit-
ger rate of photons from Mkn501 in a The upper limits provide significant
ablyaveragingthemeasuremepnatrstboyft1h9e97in.dAivpirdilu1aslttceolerrsecsopponesd.sTtoheHcoEnGstRraAinstysstoenmmoofdimelsagwinhgich try to
atmospheric Cherenkov telesc5o3p9eso(nDtahuemhoertizoanl.t,a1l9a9x7is),iJsuolyp1esrtattoionaelxspinlacienltahtee 1a9cc9e6l,eraantdion(cid:12)rostf the bulk
results on energy spectra have63b0e.en shown at various occasions; foroefxacmhaprglee,dFcigo.sm1ischroawysstinhethe shells
current version of the Mkn 501 spectrum (Aharonian et al., 1997). PofurSpNoRses.of the present pa-
]]s s 10 HEGRA SourcesearcheswiththeHEGRAar-
22 rays (with a higher energy threshold
mm
V cV c 1 Mkn 501 and less flux sensitivity compared to
ee the IACTs) have not been success-
TT -1
11/ / 10 ful up to now. Only small excesses
11
--00 -2 have been found from the Crab neb-
[[1110 ula,HerculesX-1andasampleofnear
x x
FluFlu10-3 (Cx r0a.b1) Blazars, but all need further confir-
mation.
-4
10 3 Charged Cosmic Rays
1 10 The energyspectrum ofchargedcos-
EE [[TTeeVV]] micrays(CR)followsapowerlawup
toroughly3PeV,wheretheCRspec-
Fig.3. The energy spectra of Mkn501 trumbecomessteeperandfollowsan-
(averaged over all measurements in
other power law. The understanding
1997) and of the Crab nebula. Data be-
Fig. 1: Typical energy spectra measured using the HEGRA telesocfotpheisssyostceamll.edE“rkrnoeres”afreeature may
yond 10TeV are not shown.
statistical only. provide a clue to understand the ori-
gin of CR. HEGRA developed new
per isto providebackground ipnhfoortmonastivoinaoinntetrhaecttieocnhnwiiqthueisnfursaerdedtormeceotnhsotdrsuctot smpeeacsturare,athnedetnoergyspec-
photons resulting in pair production trum and the coarse composition in
1
Talk presented at the Workshop \Towards a Major Atmospheric Cherenkov Detector V", Kruger Park,
these data will provide an important the energy region around the knee.
SouthAfrica,1997
tool to measure indirectly the inter- One method is describedin morede-
galacticinfraredbackgroundlightin- tail:theairCherenkovlightmeasure-
tensity, whichis tightly connected to ments of AIROBICC allow to recon-
thehistoryofstarformation[7].Mul- struct the distance between the de-
tiwavelengthobservationsofMkn501 tector and the air shower maximum.
[8] indicate that the measurements Together with the registerednumber
maybeexplainedbysynchrotronemis- of charged particles at detector level
sionofe± andinverseComptonscat- orthetotalamountofCherenkovlight
tering. Other sources observed with the energy contained in the electro-
theCherenkovtelescopesareMkn421, magnetic shower component can be
4 A.Lindner
determined. This is combined with intergalacticabsorptionofthesepho-
theenergypernucleonoftheprimary tons.
nucleus estimated from the depth of AsmallevidenceforTeVphotonsre-
theshowermaximumtocalculatethe lated to the direction of the highest
primaryenergyandthenucleonnum- energetic (320EeV) CR event regis-
ber. While the primary energy can tered so far [9] was detected, which
be determined for cosmic ray nuclei may hint at an intergalactic cascade
with an accuracy of 30%, the deter- initiated by extremely energetic par-
mination of the nucleon number is ticles.CurrentlyIACTmeasurements
possible only in a coarsemanner due in this region are under way.
to natural fluctuations in the shower
5 Summary
developments. Figure4 shows the re-
The last years have established air
constructed (preliminary) fraction of
showermeasurementsasanewbranch
light nuclei (H and He) in the CR.
ofhighenergyastrophysics.BothTeV
The results below 1PeV are in good
γ observations of extragalactic and
galacticsourcesaswellasanalysesof
charged cosmic rays have been per-
formed successfully and will enhance
ourunderstandingofthe nonthermal
universe even more in the near fu-
ture.
References
Fig.4. The fraction of light nuclei [1] A.Lindner et al.(HEGRA),
in cosmic rays. “JACEE” marks re- Proc.25thICRC5, 113 (Durban
sultsobtainedbydirect balloonmeasure- 1997) and references therein
ments. [2] W.Hofmann,
astro-ph/9710297 (1997)
agreement to data obtained by bal- [3] F.Aharonian et al.(HEGRA),
loonandsatellitemeasurements.The astro-ph/9706019 (1997),
decrease near the knee is in agree- acc.byAst.&Astrophys.Lett.
ment with models describing the CR [4] R.J.Protheroe et al.,
acceleration in supernova remnants, astro-ph/9710118 (1997)
[5] J.Quinn et al.(WHIPPLE),
but also a constant composition is
ApJL.456, L83 (1996)
not excluded.
[6] S.M.Bradbury et al.(HEGRA),
Ast.&Astrophys.320, L5 (97)
4 Other HEGRA Results
[7] D.MacMinn, J.R.Primack,
SpaceSci.Rev75, 413 (1996)
No Gamma Ray Burst (GRB) coun-
F.W.Stecker, O.C.DeJager,
terpartshavebeenfoundforenergies
astro-ph/9710145 (1997)
above 5TeV, neither coincident with [8] E.Pian et al.(BEPPO-SAX),
the GRBs or for long time measure- astro-ph/9710331 (1997)
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extragalactic origin of GRBs, where Astrophys.J. 4411, 144 (1995)
no TeV emission is expected due to
