H.E.S.S. observations of gamma-ray bursts in 2003-2007

0.23 MB·

by  F. Aharonian

Astronomy&Astrophysicsmanuscriptno.1072 c ESO2009
(cid:13)
January21,2009
H.E.S.S. observations of γ-ray bursts in 2003–2007
F.Aharonian1,13,A.G.Akhperjanian2,U.BarresdeAlmeida8 ⋆,A.R.Bazer-Bachi3,B.Behera14,W.Benbow1,
K.Bernlo¨hr1,5,C.Boisson6,A.Bochow1,V.Borrel3,I.Braun1,E.Brion7,J.Brucker16,P.Brun7,R.Bu¨hler1,
T.Bulik24,I.Bu¨sching9,T.Boutelier17,S.Carrigan1,P.M.Chadwick8,A.Charbonnier19,R.C.G.Chaves1,
9 A.Cheesebrough8,L.-M.Chounet10,A.C.Clapson1,G.Coignet11,M.Dalton5,B.Degrange10,C.Deil1,
0 H.J.Dickinson8,A.Djannati-Ata¨ı12,W.Domainko1,L.O’C.Drury13,F.Dubois11,G.Dubus17,J.Dyks24,M.Dyrda28,
0 K.Egberts1,D.Emmanoulopoulos14,P.Espigat12,C.Farnier15,F.Feinstein15,A.Fiasson15,A.Fo¨rster1,G.Fontaine10,
2
M.Fu¨ßling5,S.Gabici13,Y.A.Gallant15,L.Ge´rard12,B.Giebels10,J.F.Glicenstein7,B.Glu¨ck16,P.Goret7,

n C.Hadjichristidis8,D.Hauser14,M.Hauser14,S.Heinz16,G.Heinzelmann4,G.Henri17,G.Hermann1,J.A.Hinton25,
a
J A.Hoffmann18,W.Hofmann1,M.Holleran9,S.Hoppe1,D.Horns4,A.Jacholkowska19,O.C.deJager9,I.Jung16,
5 K.Katarzyn´ski27,S.Kaufmann14,E.Kendziorra18,M.Kerschhaggl5,D.Khangulyan1,B.Khe´lifi10,D.Keogh8,
1 Nu.Komin7,K.Kosack1,G.Lamanna11,J.-P.Lenain6,T.Lohse5,V.Marandon12,J.M.Martin6,
O.Martineau-Huynh19,A.Marcowith15,D.Maurin19,T.J.L.McComb8,M.C.Medina6,R.Moderski24,E.Moulin7,
]
E M.Naumann-Godo10,M.deNaurois19,D.Nedbal20,D.Nekrassov1,J.Niemiec28,S.J.Nolan8,S.Ohm1,J-F.Olive3,
H E.deOn˜aWilhelmi12,29,K.J.Orford8,J.L.Osborne8,M.Ostrowski23,M.Panter1,G.Pedaletti14,G.Pelletier17,
P.-O.Petrucci17,S.Pita12,G.Pu¨hlhofer14,M.Punch12,A.Quirrenbach14,B.C.Raubenheimer9,M.Raue1,29,
.
h S.M.Rayner8,M.Renaud1,F.Rieger1,29,J.Ripken4,L.Rob20,S.Rosier-Lees11,G.Rowell26,B.Rudak24,
p
C.B.Rulten8,J.Ruppel21,V.Sahakian2,A.Santangelo18,R.Schlickeiser21,F.M.Scho¨ck16,R.Schro¨der21,
-
o U.Schwanke5,S.Schwarzburg18,S.Schwemmer14,A.Shalchi21,J.L.Skilton25,H.Sol6,D.Spangler8,Ł.Stawarz23,
tr R.Steenkamp22,C.Stegmann16,G.Superina10,P.H.Tam14,J.-P.Tavernet19,R.Terrier12,O.Tibolla14,C.vanEldik1,
s G.Vasileiadis15,C.Venter9,L.Venter6,J.P.Vialle11,P.Vincent19,M.Vivier7,H.J.Vo¨lk1,F.Volpe10,29,S.J.Wagner14,
a
[ M.Ward8,A.A.Zdziarski24,andA.Zech6

1 (Affiliationscanbefoundafterthereferences)
v
7 Preprintonlineversion:January21,2009
8
1
2 ABSTRACT
.
1 Aims.Very-high-energy(VHE;>100GeV)γ-raysareexpectedfromγ-raybursts(GRBs)insomescenarios.Exploringthisphotonenergyregime
0 isnecessaryforunderstandingth∼eenergeticsandpropertiesofGRBs.
9
Methods.GRBs have been one of the prime targets for the H.E.S.S. experiment, which makes use of four Imaging Atmospheric Cherenkov
0
Telescopes(IACTs)todetectVHEγ-rays.Dedicatedobservationsof32GRBpositionsweremadeintheyears2003–2007andasearchforVHE
:
v γ-raycounterpartsoftheseGRBswasmade.Dependingonthevisibilityandobservingconditions,theobservationsmostlystartminutestohours
i aftertheburstandtypicallylasttwohours.
X Results.Resultsfromobservations of22GRBpositionsarepresentedandevidenceofaVHEsignalwasfoundneitherinobservationsofany
r individualGRBs,norfromstackingdatafromsubsetsofGRBswithhigherexpectedVHEfluxaccordingtoamodel-independentrankingscheme.
a UpperlimitsfortheVHEγ-rayfluxfromtheGRBpositionswerederived.ForthoseGRBswithmeasuredredshifts,differentialupperlimitsat
theenergythresholdaftercorrectingforabsorptionduetoextra-galacticbackgroundlightarealsopresented.
Keywords.gammarays:bursts–gammarays:observations
1. Introduction ofGRBs,andprovidevaluableinformationabouttheirphysicalprop-
erties.TheseMWLafterglowobservations aregenerallyexplainedby
Gamma-ray bursts (GRBs) are the most energetic events in the γ-ray synchrotronemissionfromshockedelectronsintherelativisticfireball
regime.Depending ontheirduration (e.g.T90),GRBsarecategorized model(Piran,1999;Zhang&Me´sza´ros,2004).Aplateauphaseisre-
into long GRBs (T90 > 2 s) and short GRBs (T90 < 2 s). First de- vealed in many of the Swift/XRT light curves, the origin of which is
tectedinlate1960s(Klebesadeletal.,1973),GRBsremainedmysteri- still not clear (Zhangetal., 2006). Observations of GRBs at energies
ousforthreedecades.BreakthroughsinunderstandingGRBscameonly >10GeV may testsome of the ideasthat havebeen suggested toex-
after the discovery of longer-wavelength afterglows with the launch plaintheX-rayobservations(Fanetal.,2008).
of BeppoSAX in 1997 (vanParadijsetal., 2000). Multi-wavelength
Intheframeworkoftherelativisticfireballmodel,photonswithen-
(MWL) observations have proved to be crucial in our understanding
ergiesupto 10TeVorhigherareexpectedfromtheGRBafterglow
∼
phase(Zhang&Me´sza´ros,2004;Fan&Piran,2008).Possibleleptonic
Send offprint requests to: P.H. Tam, e-mail: radiationmechanismsincludeforward-shockedelectronsup-scattering
[email protected] self-emittedsynchrotronphotons(SSCprocesses;Dermeretal.,2000;
⋆ supportedbyCAPESFoundation,MinistryofEducationofBrazil Zhang&Me´sza´ros, 2001; Fanetal., 2008) or photons from other
2 F.Aharonianetal.:H.E.S.S.observationsofγ-rayburstsin2003–2007
shocked regions(Wangetal.,2001). Physicalparameters, such asthe collectionareaincreasesfrom 103m2at100GeVtomorethan105m2
∼
ambientdensityofthesurroundingmaterial(n),magneticfieldequipar- at1TeVforobservations atazenithangle(Z.A.)of 20 .Thesystem
◦
titionfraction(ǫ ),andbulkLorentzfactor(Γ )oftheoutflow,may hasapointsourcesensitivityabove100GeVof 1.4 10 11ergcm 2s 1
B bulk ∼ × − − −
be constrained by observations at these energies (Wangetal., 2001; (3.5%ofthefluxfromtheCrabnebula)fora5σdetectionina2hob-
Pe’er&Waxman,2005). servation.EachH.E.S.S.cameraconsistsof960photomultipliertubes
Apossibleadditional contributiontoVHEemissionrelatestothe (PMTs),whichintotalprovideafieldofview(FoV)of 5◦.Thisrela-
∼
X-rayflarephenomenon. X-rayflaresarefoundinmorethan 50%of tivelylargeFoVallowsforthesimultaneousdeterminationoftheback-
the Swift GRBs during the afterglow phase (Chincarinietal., 2007). groundeventsfromoff-sourcepositions,sothatnodedicatedoffrunis
The energy fluence of some of them (e.g. GRB 050502B) is com- needed (Aharonianetal., 2006c). Theslew rateof the array is 100◦
∼
parable to that of the prompt emission. Most of them are clustered perminute,enablingittopointtoanyskypositionwithin 2minutes.
∼
at 102–103s after the GRB (see Figure 2 in Chincarinietal., 2007), The H.E.S.S. array is currently the only IACT array in the Southern
wh∼ilelateX-rayflares(>104s) arealsoobserved; when these happen HemisphereusedforanactiveGRBobservingprogramme2.
they can cause an increase in the X-ray flux of an order of magni- ThetriggersystemoftheH.E.S.S.arrayisdescribedinFunketal.
tudeormoreoverthepower-lawtemporaldecay(Curranetal.,2008). (2004).Thestereoscopictechniqueisused,i.e.acoincidenceofatleast
The cause of X-ray flaresis still a subject of debate, but correspond- twotelescopes triggering withinawindow of (normally) 80 nanosec-
ing VHE γ-ray flares from inverse-Compton (IC) processes are pre- ondsisrequired.Thislargelyrejectsbackgroundeventscausedbylocal
dicted (Wangetal., 2006; Galli&Piro, 2007; Fanetal., 2008). The muonsthattriggeronlyasingletelescope.
accompanying external-Compton flare may be weak if the flare orig- The observations reported here were obtained over the period
inated behind the external shock, e.g. from prolonged central engine March2003toOctober2007. TheobservationsoftwoGRBsin2003
activity (Fanetal., 2008). However, in the external shock model, the weremadeusingtwotelescopeswhilethesystemwasunderconstruc-
expected SSC flare is very strong at GeV energies and can be read- tion.Before2003July,eachofthetwotelescopestookdataseparately.
ilydetectedusingaVHEinstrumentwithanenergythresholdof 100 Stereo analysis was then performed on the data, which requires co-
GeV(Galli&Piro,2008),suchastheH.E.S.S.array,foratypical∼GRB incidence of events to be determined offline using GPS time stamps.
atz 1.Therefore,VHEγ-raydatatakenduringanX-rayflaremayhelp After the installation of the central trigger system in 2003 July, the
for∼distinguishingtheinternal/externalshockoriginoftheX-rayflares, stereo multiplicity requirement was determined on-line. All observa-
andmaybeusedasadiagnostictoolforthelatecentralengineactivity. tionssince2004havemadeuseofthecompletedfour-telescopearray
Waxman&Bahcall (2000) and Muraseetal. (2008) suggest that andthestereotechnique(Aharonianetal.,2006c).
GRBsmaybesourcesofultra-high-energycosmicrays(UHECRs).In Mostofthedataweretakenin28minuterunsusingwobblemode,
thiscase,π-decaysfromproton-γinteractionmaygenerateVHEemis- i.e.theGRBpositionisplacedatanoffset,θoffset,of±0◦.5or0◦.7(inR.A.
sion.TheVHEγ-rayemissionproducedfromsuchahadroniccompo- andDecl.)relativetothecentreofthecameraFoVduringobservations.
nentisgenerallyexpectedtodecaymoreslowlythantheleptonicsub- Onboard GRB triggers distributed by the Swift satellite, as well
MeV radiation(Bo¨ttcher&Dermer, 1998).Dermer (2007)suggests a astriggersfromINTEGRALandHETE-II confirmedbyground-based
combined leptonic/hadronic scenario to explain the rapidly-decaying analysis, are followed by H.E.S.S. observations. Upon the reception
phase and plateau phase seen in many of the Swift/XRT light curves. of a GCN3 notice from one of these satellites (with appropriate indi-
ThismodelcanbetestedwithVHEobservationstakenminutestohours cations4 that the source is a genuine GRB), the burst position is ob-
aftertheburst. servedifZ.A.<45◦ (toensureareasonablylow-energythreshold)dur-
MostsearchesforVHEγ-raysfromGRBshaveobtainednegative ing H.E.S.S.d∼ark time5. An automated program isrunning on siteto
results (Connaughtonetal., 1997; Atkinsetal., 2005). There may be keep the shift crew alerted of any new detected GRBs in real time.
indicationsofexcessphotoneventsfromsomeobservations,butthese Dependingontheobservationalconstraintsandthemeasuredredshifts
resultsarenotconclusive(Amenomorietal.,1996;Padillaetal.,1998; oftheGRBsreportedthroughGCNcirculars6,observationsoftheburst
Atkinsetal., 2000; Poirieretal., 2003). Currently, the most sensitive positionsarestartedupto 24hoursafterthebursttime,typicallywith
∼
detectorsintheVHEγ-rayregimeareIACTs.Horanetal.(2007)pre- an exposure time of 120 minutes in wobble mode. The remarkably
≈
sentedupperlimitsfrom7GRBsobservedwiththeWhippleTelescope nearby, bright GRB 030329 was an exceptional case. It was not ob-
during the pre-Swift era. Upper limitsfor 9 GRBs with redshifts that served until 11.5 days after the burst because of poor weather, which
were either unknown or >3.5 were also reported by the MAGIC col- prohibitedobservationanyearlier.
laboration (Albertetal., 2007). In general, these limitsdo not violate
a power-law extrapolation of the keV spectra obtained with satellite-
basedinstruments.However,mostGRBsarenowbelievedtooriginate 3. TheGRBobservations
at cosmological distances, thereforeabsorption of VHEγ-raysbythe
Thirty-twoGRBswereobservedwithH.E.S.S.duringtheperiodfrom
EBL(Nikishov,1962)mustbeconsideredwheninterpretingtheselim-
March2003toOctober2007.Afterapplyingasetofdata-qualitycri-
its.
teriathatrejectsobservationrunswithnon-optimalweatherconditions
Inthispaper, observations of 22γ-rayburstsmade withH.E.S.S.
and hardware status, 22 GRB observations were selected for analysis
during the years 2003–2007 are reported. They represent the largest
andaredescribedinthissection.
sample of GRB afterglow observations made by an IACT array and
result in the most stringent upper limits obtained in the VHE band.
ThepromptphaseofGRB060602Bwasobservedserendipitouslywith 3.1.PropertiesoftheGRBs
H.E.S.S.Theresultsofobservationsbefore,during,andafterthisburst
arepresentedinAharonianetal.(2009). Foreachburst,theobservationalpropertiesasobtainedfromthetrigger-
ingsatelliteareshowninTable1.Theseincludetriggernumber,energy
band, fluenceinthatenergyband, andthedurationof theburst(T ).
90
2. TheH.E.S.S.experimentandGRBobservation Whenever there were follow-up observations in the X-ray, optical, or
strategy
2 http://www.lsw.uni-heidelberg.de/projects/hess/HESS/grbs.phtml
The H.E.S.S. array1 is a system of four 13m-diameter IACTs lo- 3 The Gamma ray bursts Coordinates Network,
catedat1800mabovesealevelintheKhomasHighlandofNamibia http://gcn.gsfc.nasa.gov/
(23 1618 S,16 3000 E).Eachofthefourtelescopesislocatedata 4 which include, e.g., burst position incompatible with known
◦ ′ ′′ ◦ ′ ′′
cornerofasquarewithasidelengthof120m.Thisconfigurationwas sources,andahighsignal-to-noiseratiooftheburst
optimizedformaximumsensitivityto 100GeVphotons.Theeffective 5 H.E.S.S.observationsaretakenindarknessandwhenthemoonis
∼
belowthehorizon.ThefractionofH.E.S.S.darktimeisabout0.2
1 http://www.mpi-hd.mpg.de/hfm/HESS/HESS.html 6 http://gcn.gsfc.nasa.gov/gcn3 archive.html
F.Aharonianetal.:H.E.S.S.observationsofγ-rayburstsin2003–2007 3
Table1.PropertiesofGRBsobservedwithH.E.S.S.fromMarch2003toOctober2007.
GRB Satellite Trigger R.A.a Decl.a Errora Energyband Fluenceb Tb XcOcRc z Rankd
90
number ( ) (keV) (10 8ergcm 2) (s)
′′ − −
071003 Swift 292934 20h07m24s.25 +10 5648.8 5.7 15–150 830 150 √ √ √ 1.604e 5
◦ ′ ′′
070808 Swift 287260 00h27m03s.36 +01 1034.8 1.9 15–150 120 ∼ 32 √ √ . 9
◦ ′ ′′
070724A Swift 285948 01h51m13s.96 -18 3540.1 2.2 15–150 3 ∼0.4 √ 0··.4·57f 21
◦ ′ ′′
070721B Swift 285654 02h12m32s.95 -02 1140.6 0.9 15–150 360 ∼340 √ √× × 3.626g 10
◦ ′ ′′
070721A Swift 285653 00h12m39s.24 -28 2200.6 2.3 15–150 7.1 3∼.868 √ √ ×. 20
◦ ′ ′′
070621 Swift 282808 21h35m10s.14 -24 4903.1 2 15–150 430 33 √ . ··· 1
◦ ′ ′′
070612B Swift 282073 17h26m54s.4 -08 4508.7 4.7 15–150 168 13.5 √ × . ··· 15
◦ ′ ′′
070429A Swift 277571 19h50m48s.8 -32 2417.9 2.4 15–150 91 163.3 √ √× . ··· 3
◦ ′ ′′
070419B Swift 276212 21h02m49s.57 -31 1549.7 3.5 15–150 736 236.4 √ √ . ··· 7
◦ ′ ′′
070209 Swift 259803 03h04m50s -47 2230 168 15–150 2.2 0.09 . 0··.3·14?h 22
◦ ′ ′′
061110A Swift 238108 22h25m09s.9 -02 1530.7 3.7 15–150 106 40.7 √× √× . 0.758i 11
◦ ′ ′′
060526 Swift 211957 15h31m18s.4 +00 1711.0 6.8 15–150 126 298.2 √ √ . 3.21j 8
◦ ′ ′′
060505 Swift 208654 22h07m04s.50 -27 4957.8 4.7 15–150 94.4 4 √ √ . 0.0889k 18
◦ ′ ′′
060403 Swift 203755 18h49m21s.80 +08 1945.3 5.5 15–150 135 30∼.1 √ . 16
◦ ′ ′′
050801 Swift 148522 13h36m35s -21 5541 1 15–150 31 19.4 √ √× 1··.5·6l 2
◦ ′ ′′
050726 Swift 147788 13h20m12s.30 -32 0350.8 6 15–150 194 49.9 √ √ ×. 13
◦ ′ ′′
050509C HETE-II H3751 12h52m53s.94 -44 5004.1 1 2–30 60 25 √ √ √ ··· 19
◦ ′ ′′
050209 HETE-II U11568 08h26m [phone] 46 . . ··· 14
◦ ′
041211Bm HETE-II H3622 06h43m12s [phone] 1000 >100 . × . ··· 4
◦ ′ ′′
041006 HETE-II H3570 00h54m50s.23 +01 1404.9 0.1 30–400 713 20 √ √× √ 0··.7·16n 6
◦ ′ ′′
030821 HETE-II H2814 21h42m -44 52 o 30–400 280 ∼23 . . . 17
◦
030329 HETE-II H2652 10h44m49s.96 +21 3117.44 10 3 30–400 10760 33 √ √ √ 0··.1·687p 12
◦ ′ ′′ −
a R.A.,Decl.,andthepositionalerrors(90%containment)weretakenfromGCNReports(http://gcn.gsfc.nasa.gov/report archive.html)for
GRB061110A–GRB071003andGCNCircularsotherwise.
b Fluence and T data for GRB 050726 – GRB 070612B were taken from Sakamotoetal. (2008) except that T of GRB 060505 was
90 90
takenfromPalmeretal.(2006).FluenceandT dataofGRB030329andGRB030821weretakenfromSakamotoetal.(2005),andthoseof
90
GRB041006fromShirasakietal.(2008).OtherdataweretakenfromGCNCircularsandHETEpages(http://space.mit.edu/HETE/Bursts).
c X:X-ray,O:optical,R:radio;“√”indicatesthedetectionofacounterpart,“ ”anulldetection,and“.”thatnomeasurementwasreported
inthecorrespondingenergyrange,fromhttp://grad40.as.utexas.edu/grblog.php ×
d TherelativeexpectedVHEfluxforeachGRBisrankedaccordingtotheempiricalschemedescribedinSect.3.3
e Perleyetal.(2008)
f Cucchiaraetal.(2007)
g Malesanietal.(2007)
h RedshiftofacandidatehostgalaxyBerger&Fox(2007).
i Fynboetal. (2007)
j Berger&Gladders(2006)
k Ofeketal.(2006)
l RedshiftaccordingtodePasqualeetal.(2007),basedonafterglowmodelling
m AlthoughthisburstwasreferredtoasGRB041211invariousGCNCirculars,thepropernameGRB041211B(e.g.,inPe´langeonetal.,
2006)shouldbeusedtodistinguishitfromanotherburst,GRB041211A(=H3621)whichoccurredearlieronthesameday(Pe´langeon,A.,
privatecommunication).
n Soderbergetal.(2006)
o Thepositionerrorofthisburstislarge,seeFig.3
p Staneketal.(2003)
radiobands,whetheradetectionhasoccurred(denotedbyatick√)or 3.2.H.E.S.S.observations
not(denotedbyacross )isalsoshown.Ifnoobservationatagiven
wavelengthwasreported×,adot(.)isshown.Thereportedredshifts(z) Foreachburst,thestarttime,Tstart,oftheH.E.S.S.observationsafterthe
of10GRBsarealsopresented,ofwhich6arelowerthanone.Twoob- burstisshowninTable2.Sinceanobservingstrategytostartobserving
servedbursts,GRB070209andGRB070724A,areshortGRBswhile the burst position up to 24 hours after the burst timeis applied, the
∼
the rest are long GRBs. The population of short GRBshas a redshift meanTstartisoftheorderof10hours.The(good-quality)exposuretime
distribution(Bergeretal.,2007)significantlylessthanthatofthelong oftheobservationsusingNteltelescopesforeachburstisincluded.The
GRBs(Jakobssonetal.,2006).Therefore,onaveragetheyarelikelyto meanZ.A.oftheobservationsisalsopresented.
sufferfromalowerlevelofEBLabsorption.
3.3.Therankingscheme
X-rayflaresweredetectedfromthreeoftheGRBsintheH.E.S.S. Asmentionedintheintroduction,thereisnolackofmodelspredicting
sample. They occurred at 273s after the burst for GRB 050726, 284s VHEemissionfromGRBs.However,theevolutionofthepossibleVHE
for GRB 050801, and 2.6 105s for GRB 070429A (Curranetal., γ-ray emission with time is model-dependent. To give an empirical,
×
2008).Unfortunately,theflaresoccurredoutsidethetimewindowsof model-independentestimateoftherelativeexpectedVHEfluxofeach
theH.E.S.S.observations. GRB(whichalsodependsonT ),itisassumedthat:(1)therelative
start
4 F.Aharonianetal.:H.E.S.S.observationsofγ-rayburstsin2003–2007
VHEsignalscalesastheenergyreleasedinthepromptemission,taken than the corresponding error circles. Therefore, point-source analyses
asatypicalenergymeasureofaGRB.HenceF F where were performed for all GRBs except GRB 030821, the error box of
F isthefluenceintheSwift/BATband.FVHoErb∝urs1ts5−n1o50tkterViggered whichismuchbiggerthantheH.E.S.S.PSF(seeSect.5.4foritstreat-
15 150keV
byB−AT,themeasuredfluenceisextrapolatedintothisenergyband;(2) ment).
thepossibleVHEsignal fadesastimegoeson, asobserved inlonger
wavelength (e.g. X-ray) data. In particular, the VHE flux follows the
averagedecayoftheX-rayfluxandthereforeF F t 1.3 4.2.Energythreshold
where t denotes the time after the burst and 1.V3HEis∝the1a5−v1e5r0akgeVe×X-−ray
Theenergythreshold, E ,isconventionallydefinedasthepeakinthe
afterglowlate-timepower-lawdecayindex(Nouseketal.,2006).Since th
differential γ-ray rate versus energy curve of a fictitious source with
in most cases the exposure time of the observations is much shorter
photon index Γ (Konopelkoetal., 1999). This curve is a convolution
thanT (thestarttimeofthecorrespondingH.E.S.S.observationsaf-
start of theeffective area withthe expected energy spectrum of thesource
terthetrigger),theexpectedfluxatT canbeusedasameasureof
start asseenonEarth.Suchenergythresholds,obtainedbythestandard-cut
thestrengthoftheVHEsignal,andthereforeoftherelativepossibility
analysisandthesoft-cutanalysisforeachGRBobservation,areshown
ofdetectingaVHEsignalfromthatGRB.BysettingttoT ,wehave
start inTable2,assumingΓ=2.6.TheenergythresholddependsontheZ.A.
FVHE ∝F15−150keV×Ts−ta1r.t3 (1) oisftthheeeonbesregryvatthiorensshaonldd.tMheoarneaolvyesri,ssuosfet-dc.uTthaenalalyrgseisrtghiveeZs.Aa.l,otwheerhvigahlueer
The rank of each GRB according to equation 1 is shown in the last
of E thanthatofstandard-cutanalysis.Notethatγ-rayphotonswith
columninTable1.Notethatredshiftinformation(availableforonlya th
energiesbelowE canbedetectedbythetelescopes.
few GRBs), and thus the corresponding EBL absorption, isnot taken th
intoaccountintherankingscheme.
4.3.Opticalefficiencyoftheinstrument
4. DataAnalysis
Thedatapresentedwerealsocorrectedforthelong-termchangesinthe
opticalefficiencyoftheinstrument.Theopticalefficiencyhasdecreased
Calibration of data, event reconstruction and rejection of the cosmic-
overaperiodofafewyears.Thishaschangedtheeffectiveareaanden-
raybackground (i.e.γ-rayevent selectioncriteria)wereperformedas
ergythresholdoftheinstrument.Specifically,theenergythresholdhas
described in Aharonianetal. (2006c), which employs the techniques
increasedwithtime.UsingimagesoflocalmuonsintheFoV,thiseffect
describedbyHillas(1996).
inthecalculationoffluxupperlimitsiscorrected(c.f.Aharonianetal.,
Gamma-like events were then taken from a circular region (on-
2006c).
source)ofradiusθ centredattheburstpositiongiveninTable1.The
cut
backgroundwasestimatedusingthereflected-regionbackgroundmodel
asdescribedinBergeetal.(2007),inwhichthenumberofbackground
5. Results
eventsintheon-sourceregion(N )isestimatedfromn off-source
off region
regions located at the same θ as the on-source region during the
offset Noevidence of asignificantexcess ofVHEγ-rayeventsfromanyof
sameobservation.Thenumberofγ-likeeventsisgivenbyN αN
on− off the GRB positions given inTable 1 during the period covered by the
whereN isthetotalnumberofeventsdetectedintheon-sourceregion
on H.E.S.S.observationswasfound. Thenumber ofon-source (N )and
andα=1/nregionthenormalizationfactor. off-source events (N ), normalization factor (α), excess, andonstatis-
IndependentanalysesofvariousGRBsusingdifferentmethodsand tical significance9 ofoffthe excess in standard deviations (σ) are given
backgroundestimates(Bergeetal.,2007)yieldedconsistentresults.
for eachof the21 GRBsinTable2. TheresultsforGRB 030821 are
given inSect. 5.4. Figure1 shows the distribution of thesignificance
obtained fromthe soft-cut analysis of the observations of each of the
4.1.Analysistechnique
21GRBs.AGaussiandistributionwithmeanzeroandstandarddevi-
TwosetsofanalysiscutswereappliedtosearchforaVHEγ-raysignal ationone, whichisexpectedinthecaseof nodetection, isshownfor
fromobservationaldatatakenwiththreeorfourtelescopes.Theseare comparison.Thedistributionofthestatisticalsignificanceisconsistent
‘standard’cuts(Aharonianetal.,2006c)and‘soft’cuts7(thelatterhave with this Gaussian distribution. Thus no significant signal was found
lowerenergy thresholds, asdescribed inAharonianetal.,2006a).For from any of the individual GRBs. A search for serendipitous source
standard (soft) cuts, θ = 0.11 (θ = 0.14 ). While standard cuts discoveriesintheH.E.S.S.FoVduringobservationsoftheGRBsalso
cut ◦ cut ◦
areoptimizedforasourcewithapower-lawspectrumofphotonindex resulted in no significant detection. The 99.9% confidence level (c.l.)
Γ=2.6,softcutsareoptimizedforasourcewithasteepspectrum(Γ= fluxupperlimits(aboveEth)havebeencalculatedusingthemethodof
5.0),andhavebettersensitivityatlowerenergies.SinceEBLabsorption Feldman&Cousins(1998)forbothstandardcuts(assumingΓ = 2.6)
islesssevereforlower energyphotons, thesoft-cutanalysis isuseful andsoftcuts(assumingΓ=5),andareincludedinTable2.Thelimits
in searching for VHE γ-rays from GRBs which are at cosmological areasobservedonEarth,i.e.theEBLabsorptionfactorwasnottaken
distances. Forexample, thephoton indices of twoblazars PKS2005- intoaccount.ThesystematicerroronaH.E.S.S.integralfluxmeasure-
489(Aharonianetal.,2005)andPG1553+113(Aharonianetal.,2008) mentisestimatedtobe 20%,anditwasnotincludedinthecalculation
weremeasuredtobeΓ>4. oftheupperlimits. ∼
Anexceptiontothis∼analysisschemeisGRB030329.Asthecentral ForthoseGRBswithreported redshifts, theeffect of theEBLon
triggersystemhadyettobeinstalledwhenthisobservationwasmade, theH.E.S.S.limitscanbeestimated.UsingtheEBLmodelP0.45de-
aslightlydifferentanalysistechniquewasused.Thedescriptionofthe scribedinAharonianetal.(2006b),differentialupperlimits(againas-
image and analysis cuts used for the data from GRB 030329 can be sumingΓ=5)attheenergythresholdwerecalculatedfromtheintegral
foundinAharonianetal.(2005).ForGRB030821,onlythestandard- upperlimitsobtainedusingsoft-cutanalysis.Theseupperlimits,aswell
cutanalysis(fortwo-telescopedata)wasperformed(seeSect.5.4). asthosecalculatedwithouttakingtheEBLintoaccount,areshownin
The positional error circle of most GRBs, with the exceptions of Table3.
GRB 030821, GRB 050209, and GRB 070209, is small compared to
theH.E.S.S.pointspreadfunction(PSF).The68%γ-raycontainment
radius,θ ,oftheH.E.S.S.PSFcanbeassmallas 3,depending on
68 ∼ ′
theZ.A.andθ oftheobservations,andtheanalysiscutsapplied.The
offset
68%containment radius,θ ,oftheobservations ofGRB050209and
68
GRB 070209 is about 9 using standard-cut analysis8, slightly larger
′
7 ‘Soft’cutswerecalled‘spectrum’cutsinAharonianetal.(2006a).
8 θ islargerusingsoft-cutanalysis 9 calculatedbyeq.(17)inLi&Ma(1983)
68
Table2.H.E.S.S.observationsofGRBsfromMarch2003toOctober2007.
Standard-cutanalysis Soft-cutanalysis Temporalanalysis
GRBa T Exposure N Z.A. N N α ExcessSigni- E FluxULs N N α ExcessSigni- E FluxULs χ2/d.o.f. P(χ2)
start tel ON OFF th ON OFF th
(min) (min) ( ) ficance(GeV) (cm 2s 1) ficance(GeV) (cm 2s 1)
◦ − − − −
070621 6.5 234.6 4 16 204 2273 0.091 -2.6 -0.18 250 2.8 10 12 731 5903 0.13 -6.9 -0.24 190 5.6 10 12 19.2/28 0.89
− −
050801 15.0 28.2 4 43 13 173 0.091 -2.7 -0.68 400 3.2×10 12 46 442 0.13 -9.3 -1.2 310 1.6×10 11 0.168/3 0.98
− −
070429A 64 28.2 4 23 4 78 0.091 -3.1 -1.2 290 2.4×10 12 20 203 0.13 -5.4 -1.0 220 1.0×10 11 6.39/3 0.094
− −
567.1 14.2 3 64 9 87 0.11 -0.67 -0.21 1850 6.8×10 12 27 236 0.17 -12 -1.9 1360 2.6×10 11
041211Ba (cid:26) 742.3 112.3 4 44 76 1247 0.063 -1.9 -0.21 380 3.7×10−12 317 4353 0.083 -46 -2.4 280 1.8×10−11(cid:27) 14.6/14 0.40
− −
623.3 56.2 4 35 16 272 0.10 -11 -2.2 390 1.0×10 12 97 785 0.14 -15 -1.4 280 1.4×10 11
071003b (cid:26) 691.1 56.2 3 41 25 204 0.10 4.6 0.93 480 5.6×10−12 79 547 0.14 0.86 0.091 340 1.5×10−11(cid:27) 32.3/12 0.0012
− −
041006 626.1 81.9 4 27 80 770 0.10 3 0.32 200 1.1×10 11 302 1974 0.14 20 1.1 150 6.8×10 11 8.89/9 0.45
− −
000767000458120968B 23809460..722 11115226...884 444 234547 294839 1360956198 000...00199011 --1-713.1.58 ---111...433 372108000 322...249×××111000−−−111222 142290129 131077613913 000...111343 ---713.386 ---011...4069 252622000 779...525×××111000−−−111222 111951...889///11622 000...00267042 F.Aharo
007601712110BA 490275.6.78 110132..88 44 4205 5796 988348 00..006933 --21..59 --00..3211 424800 14..43××1100−1122 233174 22667761 00..01833 -1240 0-1.8.09 322000 88..84××1100−1122 145.6.56//1111 00..1965 nian
030329c 16493.5 28.0 2 60 4 26 0.14 0.27 0.13 1360 2.6×10−−12 × − 5.93/3 0.12 eta
000557000726201692B 1792700218...775 111116228...886 444 414088 111000744 111001399160 000...01081931 --2411.82 --021...3169 423842000 447...411×××111000−−−111222 354321385 243622103943 000...111143 -417213 0-2·2.5.·.38·1 213684000 311...455××111000−−−111111 13446.8..737///111282 000...20960665 l.:H.E.S
060403 820.4 52.8 4 39 33 252 0.091 10 1.9 440 4.8×10 12 128 875 0.13 19 1.6 320 1.3×10 11 10.4/6 0.11 .S
060505 1163 111 4 42 99 837 0.091 23 2.4 520 5.6×10−−12 339 2740 0.13 -3.5 -0.18 400 3.9×10−−12 22.1/12 0.036 .ob
050509C 1289 28.2 4 22 31 344 0.083 2.3 0.41 200 1.7×10 11 112 965 0.11 4.8 0.43 150 1.5×10 10 0.301/3 0.96 se
007700772241AA 982973..55 18142..68 44 2330 9703 1742306 00..009519 75..55 00..8588 236200 76..53××1100−−1122 228406 32803472 00..01737 --91.53 --00..8565 226000 11..30××1100−−1111 61.748.3//192 00..8171 rvatio
070209 926.7 56.4 4 41 37 444 0.091 -3.4 -0.51 480 2.3×10−12 185 1442 0.13 4.8 0.33 370 1.1×10−11 5.35/6 0.50 ns
× − × − of
γ
a TheGRBsarelistedintheorderoftherankingschemedescribedinSect.3.3.GRB030821isnotlisted,theresultsofwhicharegiveninSect.5.4. -r
a
b Three-andfour-telescopedataarepresented. y
b
c Aslightlydifferentanalysistechniquewasused,seeSect.4.1.Soft-cutanalysisisnotavailableforthisobservation. ur
s
ts
in
2
0
0
3
–
2
0
0
7
5
6 F.Aharonianetal.:H.E.S.S.observationsofγ-rayburstsin2003–2007
Secondly,combiningthesignificanceoftheresultsfromthreeselected
s 10 subsetsextractedfromthewholesamplewasperformed.Theapriori
ntrie 9 sVeHleEctiflounxcorirtearilaowweerreletvoelcohfooEsBeLthaobsseoGrpRtiBons.wTihtehfaolhloigwhienrgerxepqeucitreed-
e
of 8 mentswereusedtoselectthreesubsets:
er
mb 7 SampleA: thefirst10intherankingdescribedinSect.3.3;
u
N 6 SampleB: allGRBswithameasuredredshiftz<1;
SampleC: all GRBs with a soft-cut energy threshold lower than
5
300 GeV and with either a measured redshift z < 1 or with an
4 unknownredshift.
3
TheresultisshowninTable4.Ascanbeseen,thereisnosignificant
2 evidenceofemissioninanyofthesesubsets.
1
0 5.2.Temporalanalysis
-4 -3 -2 -1 0 1 2 3 4
Significance of individual GRBs
AspossibleVHEradiationfromGRBsisexpectedtovarywithtime,
atemporalanalysistosearchfordeviationfromzeroexcessintheob-
Fig.1.Distributionofthestatisticalsignificance(histogram)as
serveddatawasperformed.Soft-cutanalysiswasusedforallGRBs(ex-
derived from the observationsof 20 GRBs using soft-cutanal-
ceptGRB030329)sincethisanalysishasalowerenergythresholdand
ysis. The mean is 0.4 and the standard deviation is 1.4. Each
abetteracceptance of γ-raysand cosmicraysandtherefore increases
−
entrycorrespondstooneGRB.ThesolidlineisaGaussianfunc- thestatistics.Theγ-likeexcesseventswerebinnedin10-minutetime
tionwithmeanzeroandstandarddeviationunity. intervalsforeachGRBdatasetandwerecomparedtotheassumption
ofnoexcessthroughouttheobservedperiod.Theχ2/d.o.f.valueandthe
correspondingprobabilityareshowninTable2foreachGRB.Within
5.1.Stackinganalysis thewholesample,thelowestprobabilitythatthehypothesisthattheex-
cesswaszerothroughouttheobservationperiodiscorrectis1.2 10 3
−
AlthoughnosignificantexcesswasfoundfromanyindividualGRB,co- (for GRB 071003) and no significant deviation from zero with×in any
addingtheexcesseventsfromtheobservations ofanumber of GRBs oftheGRBtemporaldatawasfound. Standard-cutanalysisproduced
mayrevealasignalthatistooweaktobeseeninthedatafromoneGRB, consistentresults.
providedthatthePSFsoftheH.E.S.S.observationsarebiggerthanthe
errorboxoftheGRBpositions(whichisthecase,seeSect.4.1).Firstly,
stackingofallGRBs(exceptGRB030821,whichhasahighpositional 5.3.GRB070621:ObservationsofaGRBwiththefastest
uncertainty)inthesamplewasperformed.Thisyieldedatotalof 157 reactionandthelongestexposuretime
−
excesseventsandastatisticalsignificanceof 1.98usingthesoft-cut
analysis.Useofstandardcutsproduced asim−ilarresult(seeTable4). GRB070621isthehighest-rankedGRBinthesample(Sect.3.3),i.e.
it has the highest relative expected VHE flux at the start time of the
observations. The duration of the Swift burst was T 33s, thus
90 ∼
Table 3. Differential flux upper limits at the energy thresholds clearly classifying the burst as a long GRB. The fluence in the 15–
150 keV band was 4.3 10 6 erg cm 2. The XRTlight curve isrep-
fromtheH.E.S.S.observationsofGRBswithreportedredshifts. ∼ × − −
resentedbyaninitialrapidly-decayingphaseandashallowphase,with
thetransitionhappeningaroundt +380swheret denotesthetrigger
GRB Redshift E (GeV) F a F a 0 0
th UL corrected time(Sbarufattietal.,2007).Despiteextensiveopticalmonitoring,no
060505 0.0889 400 3.9 10−14 5.8 10−14 fadingopticalcounterpartwasfound.TheH.E.S.S.observationsstarted
003700322099 00..1361847 1336700 71..62××1100−−1153 98..77××1100−−1143 sahtat0llo+w42p0hsasaen.dTlhaessteedofbosrer∼v5atihoonusrsw,elarergbeolythcothinecmidoesnttpwroitmhpthteanXd-rthaye
070724A 0.457 200 2.1×10−13 1.0×10−12 longestamongthosepresented.Figure2showsthe99.9%H.E.S.S.en-
041006 0.716 150 1.8×10−12 2.7×10−11 ergyfluxupperlimitsabove200GeV(usingsoft-cutanalysis),together
061110A 0.758 200 1.7×10−13 1.7×10−11 withtheXRTresults(Evansetal.,2007).Asseen,thelimitsforthispe-
050801 1.56 310 2.1×10−13 ×b riodareatlevelscomparabletotheX-rayenergyfluxduringthesame
071003c 1.604 280 2.0×10 13 b period.Unfortunatelythelackofredshiftinformationforthisburstpre-
−
060526 3.21 220 1.7×10 13 b ventsfurtherinterpretationofthelimits.
−
070721B 3.626 320 1.1×10 13 b
−
×
a Limitsaregiveninunitsofcm 2s 1GeV 1. 5.4.GRB030821:ObservationsofaGRBwithahigh
− − −
b ThelimitscorrectedforEBLabsorptionare>10ordersofmagni- positionaluncertainty
tudelargerthanthatobserved.
SomeGRBs,suchasGRB030821,haveahighuncertaintyinposition;
c Only4-telescopedatawereused. witha relativelylarge camera FoV ( 5 ), the H.E.S.S.telescopes are
◦
∼
abletocoverthewholepositionalerrorboxofsuchGRBs.
Table 4. Combinedsignificance of 3 subsets of GRBs selected Observationsof GRB030821 started18hours aftertheburstand
basedontherequirementslistedinSect.5.1 lasted for a live-time of 55.5 minutes, with a mean Z.A. of 28◦. The
observations were taken when the array was under construction and
only two telescopes were operating, resulting in an energy threshold
Number Soft-cut Standard-cut
of 260 GeV. The GRB has a relatively high uncertainty in position
ofGRBs analysis analysis
as determined from IPN (thethird Interplanetary Network) triangula-
SampleA 10 -2.13 -1.81
tion (Hurleyetal., 2003), and its error box is bigger than the PSF of
SampleB 6 -0.20 1.45
H.E.S.S.However, because of therelativelylargeFoV ofthecamera,
SampleC 11 -0.53 0.48
thewholeerrorbox,andthusthepossibleGRBposition,iswithinthe
allGRBs 21 -1.98 -0.18
H.E.S.S.FoV.Theskyexcessmapoverlaidwiththeerrorboxisshown
F.Aharonianetal.:H.E.S.S.observationsofγ-rayburstsin2003–2007 7
6. Discussion
Theupper limitspresentedinthispaper areamongthemoststringent
-1-2) scm 10-9 10-9-1-2) sm fletevuregxrl)odwaerreipveaertdioledfvr.oeImlns cfVaocHmt,Eptahγrea-br9aley9.9too%bstehcreovnaXfiti-dorenanysceoenfleeGrvgReylBlflsimudxiutsarisn(igonbteshneerevragefyd-
0 GeV (erg 10-10 10-10 0 keV (erg c boseyfvteShrweeliGyft/RaXbBRssoTrabdreeudrlibonycgatthtehedeEasBtamLhieg(thhpiersreipdoosdhssi(fisbtesielia,tnyed.igst.hdFuissicgut.hs2es)ier.dUVbnHellEeoswsfl)u,mxoonsiest
ux >20 n 0.3-1 elexvpeelcstcsodmepteacrtaibolneotof tthhoesperiendiXct-erdayVsHinEsocmomepscoenneanrtiowsi(thDeernmeregryetflaulx.,
E.S.S. Fl10-11 10-11 RT Flux i 22000005O;;nFWatanhneegtoeatthl.ea,rl2.,0h02a8n0)d0.,1;thZehuanngkn&owMne´srezda´rsohsif,ts20o0f1m; Pane’yero&f tWheaxGmRaBns,
H. X in the sample (including GRB 070621, the highest-ranking, which is
10-12102 103 104 10-12 dbiesccauussseedEiBnLSaebcts.o3r.p3t)iocnomatpVlicHaEteetnheerpghieyssiicsasleivnetererpfroertaatiGonRoBfwthiethdazta>,
t - t (s)
0 1.Themeanandmedianredshiftofthe10GRBswithreportedredshifts
Fig.2.The99.9%confidencelevelenergyfluxupperlimits(in is1.3 and 0.7, respectively. If the12 GRBswithout redshift have the
red) at energies>200 GeV derivedfromH.E.S.S. observations sameredshiftdistribution,onewouldexpect 40%ofthem( 5GRBs)
∼ ∼
atthepositionofGRB070621.Theendsofthehorizontallines tohavez< 0.5.Inthiscase,theEBLabsorptionmaynotprecludethe
detectionof thepredicted VHEγ-raysfor theGRBsamplepresented
indicatethestartandendtimesoftheobservationsfromwhich
here11.
theupperlimitswerederived.TheXRTenergyfluxinthe0.3–10
ThereisnoreportedX-rayflareduringtheH.E.S.S.observational
keVbandisshowninblackforcomparison(Evansetal.,2007).
timewindows,thereforenoconclusiononwhetherornotX-rayflares
areaccompanied byVHEflares,aswellastheoriginof X-rayflares,
can be drawn. If UHECRs are generated in nearby GRB sources, as
suggested by some authors, a detectable VHE flux is expected from
nearbyGRBs.Therefore,althoughtheunknown redshiftsofasignifi-
cantfractionofGRBsinoursample(12outof22)andtheuncertainty
inthemodeledVHEtemporalevolutionaresurelyinplay,theresults
presented here do not indicate (but also not exclude) that GRBs are
dominantsourcesofUHECRs.
7. Outlook
The data from our sample of 22 GRBs do not provide any evidence
for a strong VHE γ-ray component from GRBs during the afterglow
phase.EBLabsorptioncanexplainthelackofdetectioninoursample.
However, this does not exclude a population of GRBs that exhibit a
strongVHEcomponent. WhiletheEGRETexperiment didnotdetect
MeV–GeV photons frommost BATSEGRBsinitsFoV, some strong
bursts(e.g.GRB940217)haveprovedtoemitdelayedemission, 1.5
Fig.3.Theγ-likeexcesseventsintheregionoftheGRB030821. hours after the burst, at energies as high as 20 GeV (Hurleye∼tal.,
The errorboxshowsthe positionof the burst localizedby IPN 1994).WithFermi’sobservationsofGRBshav∼ingstartedinmid-2008,
triangulation(Hurleyetal.,2003).Thecolour(grey)scaleisset itislikelythatourknowledgeofthehigh-energyemissionofGRBswill
suchthattheblue/red(black/grey)transitionoccursatthe 1.5σ beimprovedinthenearfuture.
significance level. The sky map was derived using two∼obser- ThefutureprospectsfordetectionatVHEenergiesrelyonthelike-
vationspointingattwodifferentpositions(markedbycrosses), lihoodofobservingaGRBwithlowredshift(e.g.z<0.5)earlyenough.
Inthecaseswherethereisnodetection,sensitiveandearlyupperlimits
resultinginanon-uniformdistributionofeventsinthemap.
on the intrinsic VHE luminosity of these nearby GRBs will still im-
proveourunderstandingoftheradiationmechanismsofGRBs.Theex-
istenceofadistinctpopulationoflow-luminosity(LL)GRBswassug-
gestedbasedonthehighdetectionrateoflow-redshiftLLGRBssuch
as GRB 980425 and GRB 060218 (e.g. Soderbergetal., 2004). Due
inFig.3.Ascanbeseen,thereisnosignificantexcessatanyposition totheir proximity, they are good targetsfor VHE observations. Since
withinthe error box. The skyregion withthelargest number of peak theyaresub-energeticcomparedtootherGRBs,theymaybeaccompa-
excesseventsislocatedinthesouth-easternpartoftheerrorbox.Using niedbyalowerVHEluminosity.Ontheotherhand,ifmostradiation
apoint-source analysis centredat thispeak, afluxupper limit(above areemittedathighenergies,thedetectionprobabilitywouldbemuch
260 GeV) of 1.7 10 11 cm 2 s 1 was derived. Since an upper limit higher.
− − −
derived for an∼y loc×ation in the error box with fewer excess events is Franceschinietal. (2008) claimed a very small γ-rayopacity due
lowerthanthisvalue10,itmayberegardedasaconservativeupperlimit toEBLabsorption.Theopticaldepthisaboutafactorofthreelessthan
oftheVHEfluxassociatedwithGRB030821duringtheperiodofthe theoneweused,dependingontheenergies(Aharonianetal.,2006b).
H.E.S.S.observations. Therefore,on-goingGRBobservationswithH.E.S.S.,aswellasother
ground-basedVHEdetectors,arecrucialtotestthismodel.
10 Alargerexcessimpliesahighervalueoftheupperlimit,sincethe 11 TheopticaldepthofEBLabsorptionfora 100GeVphotonis 3
integratedexposure,thatdependsonZ.A.andθ oftheobservations, atz=1,accordingtotheP0.45modeldemonstr∼atedinAharonianet∼al.
offset
islargelythesameoverthewholeerrorbox. (2006b).
8 F.Aharonianetal.:H.E.S.S.observationsofγ-rayburstsin2003–2007
8. Conclusions
During5yearsofoperation(2003–2007),32GRBswereobserveddur-
ingtheafterglowphaseusingtheH.E.S.S.experiment.Those22GRBs
withhigh-quality datawereanalysed and theresultspresented inthis
paper. Depending on the visibilityand observing conditions, the start
timeoftheobservationsvariedfromminutestohoursaftertheburst.
There isno evidence of VHE emissionfromany individual GRB
duringtheperiodcoveredbytheH.E.S.S.observations,norfromstack-
ing analysisusing the whole sampleand a priori selected sub-sets of
GRBs. Fine-binned temporal data revealed no short-term variability
fromanyobservationandnoindicationofVHEsignalfromanyofthese
timebinswasfound.UpperlimitsofVHEγ-rayfluxduringtheobser-
vations from the GRBs were derived. These 99.9% confidence level
energyfluxupper limitsareatlevelscomparabletothecontemporary
X-rayenergyflux.ForthoseGRBswithreportedredshifts,differential
upperlimitsattheenergythresholdaftercorrectingforEBLabsorption
arepresented.
H.E.S.S.phaseIIwillhaveanenergythresholdof about 30GeV.
WithmuchlessabsorptionbytheEBLatsuchlowenergies,itishoped
that the H.E.S.S. experiment will enable the detection of VHE γ-ray
counterpartsofGRBs.
Acknowledgements. The support of the Namibian authorities and of the
UniversityofNamibiainfacilitatingtheconstructionandoperationofH.E.S.S.
is gratefully acknowledged, as is the support by the German Ministry
for Education and Research (BMBF), the Max Planck Society, the French
MinistryforResearch,theCNRS-IN2P3andtheAstroparticleInterdisciplinary
ProgrammeoftheCNRS,theU.K.ScienceandTechnologyFacilitiesCouncil
(STFC),theIPNPoftheCharlesUniversity,thePolishMinistryofScienceand
HigherEducation,theSouthAfricanDepartmentofScienceandTechnologyand
NationalResearchFoundation,andbytheUniversityofNamibia.Weappreciate
theexcellent workofthetechnical supportstaffinBerlin,Durham,Hamburg,
Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and
operationoftheequipment.P.H.TamacknowledgessupportfromIMPRS-HD.
ThisworkhasmadeuseoftheGCNNoticesandCircularsprovidedbyNASA’s
GoddardSpaceFlightCenter,aswellasdatasuppliedbytheUKSwiftScience
DataCentreattheUniversityofLeicester.
F.Aharonianetal.:H.E.S.S.observationsofγ-rayburstsin2003–2007 9
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