HESS J1640-465 - an exceptionally luminous TeV gamma-ray supernova remnant

0.71 MB·

by  H. E. S. S. Collaboration

AnexceptionallyluminousTeVγ-raySNR 1
HESS J1640−465 – an exceptionally luminous TeV γ-ray supernova
remnant
H.E.S.S.Collaboration,A.Abramowski,1F.Aharonian,2,3,4F.AitBenkhali,2A.G.Akhperjanian,5,4E.Angu¨ner,6G.Anton,7
4
1 S.Balenderan,8A.Balzer,9,10A.Barnacka,11Y.Becherini,12J.BeckerTjus,13K.Bernlo¨hr,2,6E.Birsin,6E.Bissaldi,14J.Biteau,15
0
M.Bo¨ttcher,16C.Boisson,17J.Bolmont,18P.Bordas,19J.Brucker,7F.Brun,2P.Brun,20T.Bulik,21S.Carrigan,2S.Casanova,16,2
2
M.Cerruti,17,22P.M.Chadwick,8R.Chalme-Calvet,18R.C.G.Chaves,20A.Cheesebrough,8M.Chre´tien,18S.Colafrancesco,23
b
e G.Cologna,24J.Conrad,25,26C.Couturier,18Y.Cui,19M.Dalton,27,28M.K.Daniel,8I.D.Davids,16,29B.Degrange,15C.Deil,2
F
P.deWilt,30H.J.Dickinson,25A.Djannati-Ata¨ı,31W.Domainko,2L.O’C.Drury,3G.Dubus,32K.Dutson,33J.Dyks,11M.Dyrda,34
1
1 T.Edwards,2K.Egberts,14P.Eger,2P.Espigat,31C.Farnier,25S.Fegan,15F.Feinstein,35M.V.Fernandes,1D.Fernandez,35A.Fiasson,36

] G.Fontaine,15A.Fo¨rster,2M.Fu¨ßling,10M.Gajdus,6Y.A.Gallant,35T.Garrigoux,18G.Giavitto,9B.Giebels,15J.F.Glicenstein,20
E
H M.-H.Grondin,2,24M.Grudzin´ska,21S.Ha¨ffner,7J.Hahn,2J. Harris,8G.Heinzelmann,1G.Henri,32G.Hermann,2O.Hervet,17
. A.Hillert,2J.A.Hinton,33W.Hofmann,2P.Hofverberg,2M.Holler,10D.Horns,1A.Jacholkowska,18C.Jahn,7M.Jamrozy,37
h
p M.Janiak,11F.Jankowsky,24I.Jung,7M.A.Kastendieck,1K.Katarzyn´ski,38U.Katz,7S.Kaufmann,24B.Khe´lifi,31M.Kieffer,18
-
o S.Klepser,9D.Klochkov,19W.Kluz´niak,11T.Kneiske,1D.Kolitzus,14Nu.Komin,36K.Kosack,20S.Krakau,13F.Krayzel,36
r
st P.P.Kru¨ger,16,2H.Laffon,27G.Lamanna,36J.Lefaucheur,31A.Lemie`re,31M.Lemoine-Goumard,27J.-P.Lenain,18D.Lennarz,2
a
[ T.Lohse,6A.Lopatin,7C.-C.Lu,2V.Marandon,2A.Marcowith,35R.Marx,2G.Maurin,36N.Maxted,30M.Mayer,10T.J.L.McComb,8

2 J.Me´hault,27,28P.J.Meintjes,39U.Menzler,13M.Meyer,25R.Moderski,11M.Mohamed,24E.Moulin,20T.Murach,6C.L.Naumann,18
v M.deNaurois,15J.Niemiec,34S.J.Nolan,8L.Oakes,6S.Ohm,33E.deOn˜aWilhelmi,2B.Opitz,1M.Ostrowski,37I.Oya,6M.Panter,2
8
8 R.D.Parsons,2M.PazArribas,6N.W.Pekeur,16G.Pelletier,32J.Perez,14P.-O.Petrucci,32B.Peyaud,20S.Pita,31H.Poon,2
3
4 G.Pu¨hlhofer,19M.Punch,31A.Quirrenbach,24S.Raab,7M.Raue,1A.Reimer,14O.Reimer,14M.Renaud,35R.delosReyes,2F.Rieger,2
.
1 L.Rob,40C.Romoli,3S.Rosier-Lees,36G.Rowell,30B.Rudak,11C.B.Rulten,17V.Sahakian,5,4D.A.Sanchez,2,36A.Santangelo,19
0
4 R.Schlickeiser,13F.Schu¨ssler,20A.Schulz,9U.Schwanke,6S.Schwarzburg,19S.Schwemmer,24H.Sol,17G.Spengler,6F.Spies,1
1 Ł.Stawarz,37R.Steenkamp,29C.Stegmann,10,9F.Stinzing,7K.Stycz,9I.Sushch,6,16A.Szostek,37J.-P.Tavernet,18T.Tavernier,31
:
v
A.M.Taylor,3R.Terrier,31M.Tluczykont,1C.Trichard,36K.Valerius,7C.vanEldik,7B.vanSoelen,39G.Vasileiadis,35C.Venter,16
i
X
A.Viana,2P.Vincent,18J.Vink,41H.J.Vo¨lk,2F.Volpe,2M.Vorster,16T.Vuillaume,32S.J.Wagner,24P.Wagner,6M.Ward,8
r
a M.Weidinger,13Q.Weitzel,2R.White,33A.Wierzcholska,37P.Willmann,7A.Wo¨rnlein,7D.Wouters,20V.Zabalza,2M.Zacharias,13
A.Zajczyk,11,35A.A.Zdziarski,11A.Zech,17H.-S.Zechlin1
1Universita¨tHamburg,Institutfu¨rExperimentalphysik,LuruperChaussee149,D22761Hamburg,Germany
2Max-Planck-Institutfu¨rKernphysik,P.O.Box103980,D69029Heidelberg,Germany
3DublinInstituteforAdvancedStudies,31FitzwilliamPlace,Dublin2,Ireland
4NationalAcademyofSciencesoftheRepublicofArmenia,Yerevan
5YerevanPhysicsInstitute,2AlikhanianBrothersSt.,375036Yerevan,Armenia
6Institutfu¨rPhysik,Humboldt-Universita¨tzuBerlin,Newtonstr.15,D12489Berlin,Germany
7Universita¨tErlangen-Nu¨rnberg,PhysikalischesInstitut,Erwin-Rommel-Str.1,D91058Erlangen,Germany
8UniversityofDurham,DepartmentofPhysics,SouthRoad,DurhamDH13LE,U.K.
9DESY,D-15738Zeuthen,Germany
10Institutfu¨rPhysikundAstronomie,Universita¨tPotsdam,Karl-Liebknecht-Strasse24/25,D14476Potsdam,Germany
11NicolausCopernicusAstronomicalCenter,ul.Bartycka18,00-716Warsaw,Poland
12DepartmentofPhysicsandElectricalEngineering,LinnaeusUniversity,35195Va¨xjo¨,Sweden
13Institutfu¨rTheoretischePhysik,LehrstuhlIV:WeltraumundAstrophysik,Ruhr-Universita¨tBochum,D44780Bochum,Germany
14Institutfu¨rAstro-undTeilchenphysik,Leopold-Franzens-Universita¨tInnsbruck,A-6020Innsbruck,Austria
15LaboratoireLeprince-Ringuet,EcolePolytechnique,CNRS/IN2P3,F-91128Palaiseau,France
16CentreforSpaceResearch,North-WestUniversity,Potchefstroom2520,SouthAfrica
17LUTH,ObservatoiredeParis,CNRS,Universite´ParisDiderot,5PlaceJulesJanssen,92190Meudon,France
18LPNHE,Universite´PierreetMarieCurieParis6,Universite´DenisDiderotParis7,CNRS/IN2P3,4PlaceJussieu,F-75252,ParisCedex5,France
19Institutfu¨rAstronomieundAstrophysik,Universita¨tTu¨bingen,Sand1,D72076Tu¨bingen,Germany
(cid:13)2c02D0S1M4/RIrAfuS,,CMENARSAacSla0y0,0F,-29–19191Gif-Sur-YvetteCedex,France
21AstronomicalObservatory,TheUniversityofWarsaw,Al.Ujazdowskie4,00-478Warsaw,Poland
22nowatHarvard-SmithsonianCenterforAstrophysics,60gardenStreet,CambridgeMA,02138,USA
23SchoolofPhysics,UniversityoftheWitwatersrand,1JanSmutsAvenue,Braamfontein,Johannesburg,2050SouthAfrica
24Landessternwarte,Universita¨tHeidelberg,Ko¨nigstuhl,D69117Heidelberg,Germany
25OskarKleinCentre,DepartmentofPhysics,StockholmUniversity,AlbanovaUniversityCenter,SE-10691Stockholm,Sweden
26WallenbergAcademyFellow
27Universite´Bordeaux1,CNRS/IN2P3,Centred’E´tudesNucle´airesdeBordeauxGradignan,33175Gradignan,France
28FundedbycontractERC-StG-259391fromtheEuropeanCommunity
29UniversityofNamibia,DepartmentofPhysics,PrivateBag13301,Windhoek,Namibia
30SchoolofChemistry&Physics,UniversityofAdelaide,Adelaide5005,Australia
31APC,AstroParticuleetCosmologie,Universite´ParisDiderot,CNRS/IN2P3,CEA/Irfu,ObservatoiredeParis,SorbonneParisCite´,10,rueAliceDomonetLe´onieDuquet,75205ParisCedex13,France
32UJF-Grenoble1/CNRS-INSU,InstitutdePlane´tologieetd’AstrophysiquedeGrenoble(IPAG)UMR5274,Grenoble,F-38041,France
33DepartmentofPhysicsandAstronomy,TheUniversityofLeicester,UniversityRoad,Leicester,LE17RH,UnitedKingdom
34InstytutFizykiJa¸drowejPAN,ul.Radzikowskiego152,31-342Krako´w,Poland
35LaboratoireUniversetParticulesdeMontpellier,Universite´Montpellier2,CNRS/IN2P3,CC72,PlaceEuge`neBataillon,F-34095MontpellierCedex5,France
36Laboratoired’Annecy-le-VieuxdePhysiquedesParticules,Universite´deSavoie,CNRS/IN2P3,F-74941Annecy-le-Vieux,France
37ObserwatoriumAstronomiczne,UniwersytetJagiellon´ski,ul.Orla171,30-244Krako´w,Poland
38Torun´CentreforAstronomy,NicolausCopernicusUniversity,ul.Gagarina11,87-100Torun´,Poland
39DepartmentofPhysics,UniversityoftheFreeState,POBox339,Bloemfontein9300,SouthAfrica
40CharlesUniversity,FacultyofMathematicsandPhysics,InstituteofParticleandNuclearPhysics,VHolesˇovicˇka´ch2,18000Prague8,CzechRepublic
41AstronomicalInstituteAntonPannekoek,UniversityofAmsterdam,POBox94249,NL-1090GEAmsterdam,theNetherlands
Accepted2014January17.Received2014January17;inoriginalform2013October23
Mon.Not.R.Astron.Soc.000,2–9(2014) Printed12February2014 (MNLATEXstylefilev2.2)
ABSTRACT
Theresultsoffollow-upobservationsoftheTeVγ-raysourceHESSJ1640−465from2004
to 2011 with the High Energy Stereoscopic System (H.E.S.S.) are reported in this work.
The spectrum is well described by an exponential cut-off power law with photon index
Γ = 2.11±0.09 ±0.10 , and a cut-off energy of E = 6.0+2.0TeV. The TeV emis-
stat sys c −1.2
sion is significantly extended and overlaps with the north-western part of the shell of the
SNR G338.3−0.0. The new H.E.S.S. results, a re-analysis of archival XMM-Newton data,
andmulti-wavelengthobservationssuggestthatasignificantpartoftheγ-rayemissionfrom
HESS J1640−465 originates in the SNR shell. In a hadronic scenario, as suggested by the
smoothconnectionoftheGeVandTeVspectra,theproductoftotalprotonenergyandmean
targetdensitycouldbeashighasW n ∼4×1052(d/10kpc)2ergcm−3.
p H
Keywords: radiationmechanisms:non-thermal,ISM:supernovaremnants,ISM:individual
objects:G338.3−0.0
1 INTRODUCTION designated 2FGL 1640.5−4633 in the two-year Fermi-LAT cata-
logue (Nolan et al. 2012). Note that no pulsation has been found
Startingin2004theGalacticPlaneSurvey(Aharonianetal.2006b)
in any wavelength band so far. Due to the large γ-ray to X-ray
performedbytheH.E.S.S.Collaboration,usinganarrayofimaging
ratio luminosity (L /L (cid:39) 30; Funk et al. 2007), Slane et al.
γ X
atmosphericCherenkovtelescopes(IACTs),ledtothediscoveryof
(2010)inferredanevolvedPWNwithalowmagneticfieldandan
nearly70newsourcesinthevery-high-energy(VHE,E>100GeV)
injectionspectrumthatconsistsofaMaxwellianelectronpopula-
γ-rayregime(Carriganetal.2013).Thechallengesincethenhas
tionwithapower-lawtail(ase.g.proposedbySpitkovsky2008)
beentoassociatethesesourceswithastrophysicalobjectsseenin
to reproduce the broadband spectral energy distribution (SED) in
other wavelengths and to identify the underlying radiation mech-
aleptonicPWNscenario.Ahadronicoriginoftheγ-rayemission
anisms. A large fraction of the Galactic VHE γ-ray population
wasconsideredtobeunlikelyasitwouldrequireratherhighambi-
could be associated with regions with recent star-forming activ- entdensities(n (cid:39) 100cm−3),implyingintensethermalradiation
ity and to objects at late stages of stellar evolution such as su-
inX-raysfromtheSNRshellthathassofarnotbeendetected.
pernova remnants (SNRs) and the nebulae produced by powerful
Lemiere et al. (2009) performed a detailed study of the
young pulsars (for a review, see e.g. Hinton & Hofmann 2009).
gaseous environment of G338.3−0.0, and based on the HI ab-
InmanycaseswhereanastrophysicalcounterparttotheVHEγ-
sorption features, derived a distance of (8 − 13)kpc. A recent
ray emission could be identified, however, the nature of the un-
study of the nearby stellar cluster Mercer 81 and the giant HII
derlying particle population remains unclear. Highly energetic γ-
region G338.4+0.1 by Davies et al. (2012) supports this esti-
ray emission could be either produced by relativistic electrons or
mate,whichimpliesthatHESSJ1640−465isthemostluminous
protons(andheaviernuclei).Relativistichadronsundergoinelastic
VHE γ-ray source known in the Galaxy. Throughout this work,
scatteringwithnucleiintheinterstellarmedium(ISM),producing
a distance of 10kpc is assumed. Since the original discovery of
π0-decayγ-rayemission.Ultra-relativisticelectrons,ontheother
HESS J1640−465, the available H.E.S.S. exposure towards this
hand, can up-scatter low-energy photons present at the accelera-
source has quadrupled w.r.t the data used in (Aharonian et al.
tionsiteviatheInverseCompton(IC)process.Inverydensemedia
2006b),andadvancedanalysismethodsarenowavailablethatal-
Bremsstrahlung losses of electrons can significantly contribute to
lowforamuchmoredetailedspectralandmorphologicalstudyof
thegeneratedγ-rayemission.IACTscanplayakeyroleinidenti-
the VHE γ-ray emission. In this work, H.E.S.S. follow-up stud-
fyingtheunderlyingparticlepopulationandstudyingnon-thermal
iesandare-analysisofXMM-Newtondataarepresented.Boththe
processes in γ-ray sources by localising the emission region and
broadbandSEDandtheTeVmorphologyrevealevidenceforpro-
constrainingtheenergyspectrumatveryhighenergies.
tonaccelerationintheSNRshellofG338.3−0.0.
The VHE γ-ray source HESS J1640−465 was discovered
by H.E.S.S. in the Galactic Plane Survey (Aharonian et al.
2006b) and is positionally coincident with the SNR G338.3−0.0
(Whiteoak&Green1996).UsingXMM-NewtonobservationsFunk
2 H.E.S.S.OBSERVATIONSANDRESULTS
et al. (2007) detected a highly absorbed extended X-ray source
(XMMUJ164045.4−463131)closetothegeometriccentreofthe H.E.S.S.isanarrayoffiveimagingatmosphericCherenkovtele-
SNRandwithintheH.E.S.S.sourceregion.TheX-rayandVHE scopes located in Namibia designed to detect VHE γ-rays. The
γ-rayemissioncomponentswereinterpretedassynchrotronandIC fifth telescope started operation in September 2012. All H.E.S.S.
emissionfromrelativisticelectronsinapulsarwindnebula(PWN). datausedtoperformthestudiesdescribedbelowhavebeentaken
ObservationswithChandraconfirmedthepresenceoftheextended between May 2004 and September 2011 with the four-telescope
nebulaandidentifiedapoint-likesourcewhichwassuggestedtobe array(Aharonianetal.2006a).Thetotaldeadtimecorrectedlive
the associated pulsar (Lemiere et al. 2009). Recently, Castelletti timeamountsto63.4hr,comparedto14.3hrintheoriginalpubli-
et al. (2011) analysed new high-resolution multi-frequency radio cation(Aharonianetal.2006b).Observationshavebeenperformed
data of G338.3−0.0 but could only set upper limits on the radio atzenithanglesbetween20◦and65◦withameanvalueof∼33◦.
flux from a potential extended radio nebula. Fermi-LAT observa- The data were recorded with pointing offsets between 0.2◦ and
tions revealed a high-energy (HE, 100MeV<E<100GeV) γ-ray 1.8◦ with a mean value of 1.1◦ from the HESS J1640−465 po-
sourcecoincidentwithHESSJ1640−465(Slaneetal.2010),also sition. Data were analysed using a standard Hillas-type H.E.S.S.
(cid:13)c 2014RAS
AnexceptionallyluminousTeVγ-raySNR 3
analysis1 fortheeventreconstructionandaboosteddecisiontree
basedeventclassificationalgorithmtodiscriminateγ-raysfromthe -46°18'00" H.E.S.S. 120
charged particle background (Ohm et al. 2009). All results were
105
cross-checkedbyanindependentanalysisandcalibrationforcon-
sistency(deNaurois&Rolland2009).
24'00" Mercer 81 90
2.1 Morphology 00) 75
0
2
Thahredscouutsrcaendpuossiintigontheanrdingmboarcpkhgorloougnydheasvtiemabteioennmobettahionded(Bwerigthe nation (J 30'00" 60
etal.2007).Inthissetupaminimumintensityinthecameraim- Decli 45
age of 160p.e. is required, resulting in an energy threshold of 36'00"
Eth =600GeVandapointspreadfunction(PSF)with68%con- 30
tainment radius of r = 0.09◦ for the morphology studies. The
68
fit of a symmetric two-dimensional Gaussian profile, convolved 15
42'00"
with the H.E.S.S. PSF with Sherpa (Freeman et al. 2001) gives
a best-fit position of RA 16h40m41.0s ± 1.0sstat ± 1.3ssys and 0
Dec −46◦32(cid:48)31(cid:48)(cid:48) ± 14(cid:48)(cid:48) ± 20(cid:48)(cid:48) (J2000), consistent with 42m00s 30s 41m00s 30s 40m00s 16h39m30s
stat sys Right Ascension (J2000)
thepreviouslypublishedvalue(Aharonianetal.2006b).Thesys-
tematic error on the best-fit position originates from the pointing Figure1.H.E.S.S.excessmapsmoothedwitha2DGaussianwith0.017◦
precisionoftheH.E.S.S.arrayofabout20(cid:48)(cid:48).Thesourceisintrin- varianceandthebest-fitposition(statisticalerrorsonly)andintrinsicGaus-
sically extended with a Gaussian width of σ = (4.3 ± 0.2)(cid:48). sian width overlaid as blue solid and dashed lines. 610MHz radio con-
S
This extension is 1.6(cid:48) (∼2σ) larger than in the original publica- tours are shown in black (Castelletti et al. 2011). The green circle indi-
catesthepositionofthecandidatePWNXMMUJ164045.4−463131,and
tion, which can be understood as fainter emission belonging to
in gray the best-fit position of the Fermi source 2FGL 1640.5−4633 is
HESS J1640−465 that can now be revealed with the increased
given.ThewhitecircleindicatesthesourceHESSJ1641−463(Oyaetal.
dataset.Figure1showstheH.E.S.S.best-fitpositionandextension 2013)andtheregionofhighradioemissionconnectingHESSJ1640−465
overlaidontheVHEγ-rayexcessmap.TheVHEγ-raysourceen- and HESS J1641−463 is the HII region G338.4+0.1. The progenitor of
closesthenorthernpartoftheSNRshellofG338.3−0.0,thecandi- G338.3−0.0ispotentiallyassociatedwiththemassiveyoungstellarcluster
datePWNXMMUJ164045.4−463131(Funketal.2007)andthe Mercer81(Daviesetal.2012).
Fermi-LATsource2FGL1640.5−4633(Slaneetal.2010;Nolan
etal.2012).Figure1alsoshowssomeindicationforanasymmetric
extensionoftheemissionalongthenorthernpartoftheshelland
2sctoia0owt1inna3rg)wd.tshhTetahhntiestshnueeebxwsttryelaymncstdmiioinsengctroitivshceeGarelsasdououssrsoscieuaeernncmemaoHosddEereleSlsSffirdooJur1maH6l4EthV1Se−HSs4EJk61y3γ6-m4(rO0aa−yypa,4ee6imnt5diasiils--. -1-1-2) s cmN/dE (TeV 111000---111201
d
an oversimplification. The residual emission could indicate some 10-13
emission in between HESS J1640−465 and HESS J1641−463.
10-14
This component is however not detected with high significance,
makingadiscussionofitsorigindifficultinthiscontext.Morpho- 10-15
logical fits in energy bands do not reveal any significant change 10-16
iancbheasntg-fietipnossoituiorcneamndo/roprheoxlotegnysiwonit,hwehniecrhgywo(auslde.hga.vseeeinndiicnattehde s)Residual ( -022 1 10 Energy (Te1V02)
PWNe HESS J1825−137 or HESS J1303−631; Aharonian et al. 1 10 102
Energy (TeV)
2006c;Abramowskietal.2012a).
Figure2.VHEγ-rayspectrumofHESSJ1640−465(top)andfluxresid-
uals(bottom)extractedwithinthe90%containmentradius(seetext).Also
2.2 Spectrum shownisthebest-fitpowerlaw,plusexponentialcut-offmodeland68%
errorband.Allspectralpointshaveaminimumsignificanceof2σ.Thelast
TheVHEγ-rayspectrumisshowninFigure2,andhasbeenex- pointisthedifferentialfluxupperlimitinthisenergybandat95%confi-
tracted using std cuts (60p.e. minimum image intensity, Eth = dencelevel.
260GeV), using the reflected region background method (Berge
etal.2007)andforwardfoldingwithamaximumlikelihoodopti-
andacut-offenergyofE = 6.0+2.0TeV.Thesystematicerrors
misation (Piron et al. 2001) from the 90% containment radius of c −1.2
theVHEγ-rayemissionofHESSJ1640−465of0.18◦aroundthe onfluxnormandindexforthisdatasetarebasedonthedifference
seen between the main and cross-check analysis and are a result
best-fit position. The fit of a power law with exponential cut-off:
dN/dE =Φ ×(E/1TeV)−Γe−E/Ec resultsinaphotonindex of uncertainties in e.g. atmospheric conditions, simulations, bro-
0
kenpixels,analysiscuts,ortherun-selection.Thefitprobabilityp
Γ=2.11±0.09 ±0.10 ,adifferentialfluxnormalisationat
stat sys
1TeVofΦ =(3.3±0.1 ±0.6 )× 10−12TeV−1cm−2s−1 foranexponentialcut-offpowerlawmodelisp ∼ 36%,whereas
0 stat sys
the fit probability for a pure power law model is p ∼ 1%. The
luminosity of HESS J1640−465 above 1TeV at 10kpc distance
1 The software package HAP version 12-03-pl02 with version32 of the is L>1TeV (cid:39) 4.6×1035(d/10kpc)2ergs−1, a factor of ∼ 2.8
lookuptableswasused. higherthanthatoftheCrabnebula.
(cid:13)c 2014RAS,MNRAS000,2–9
4 H.E.S.S.Collaboration
(1.0−2.0)keV,(2.0−4.5)keV,and(4.5−10.0)keV).Events
-1-2) sm 102 HESS J1640-465 RX J1713.7-3946 afrroomunadraelgliosonucrocerrsesdpeotencdtiendgitnoathney9o5f%thceosnetabiannmdsenwterarediuresmoofvtehde
c XMM-Newton PSF at the respective source position in the detec-
g
er tor. The total flux upper limit was derived assuming that the re-
-12 0 maining count-rate from a polygon region enclosing the northern
1
E ( 10 part of the shell is due to background. A power-law model with
d
N / photonindexΓX =−2wasappliedtoconstrainnon-thermallep-
2 dE tonicemission.Twodifferentabsorptioncolumndensitiesasfound
in the literature, N = 6.1×1022cm−2 (Funk et al. 2007) and
H,1
N =1.4×1023cm−2 (Lemiereetal.2009),havebeenconsid-
H,2
1 ered.NodiffuseX-rayemissioncoincidentwiththeSNRshellwas
detectedwiththisdataset.Theresulting99%confidenceupperlim-
itsfortheunabsorbedflux((2−10)keV)areF (N )=4.4×
99 H,1
10−13ergcm−2s−1andF (N )=8.3×10−13ergcm−2s−1.
10-4 10-3 10-2 10-1 1 10 102 99 H,2
Energy (TeV) Thesevalueshavebeenscaledupby11%toaccountforthemiss-
ingareaduetoexcludedpoint-likesources.
Figure 3. Comparison of the HE and VHE γ-ray spectra of
HESSJ1640−465(filledcircles)andRXJ1713.7−3946(opensquares).
Data for RX J1713.7−3946 are from Abdo et al. (2011) and Aharonian
etal.(2011),GeVdataofHESSJ1640−465isfromSlaneetal.(2010). 4 DISCUSSION
Alsoshownisthebest-fitexponentialcut-offpowerlawmodeltothefull
γ-rayspectrum(Table1). The H.E.S.S. source encloses the PWN candidate
XMMU J164045.4−463131 as well as the north-western
half of the incomplete shell of G338.3−0.0. The comprehensive
ThephotonindexasreconstructedwiththenewH.E.S.S.data multi-wavelength data available together with the new H.E.S.S.
at TeV energies is compatible with the photon index as recon- andXMM-Newtonresultsallowforamuchmoredetailedinvesti-
structedintheGeVdomain(Slaneetal.2010;Nolanetal.2012; gationoftheSEDandhencetheunderlyingnon-thermalprocesses
Ackermannetal.2013).Asimultaneousexponentialcut-offpower to be carried out. As the evolutionary state of G338.3−0.0 is
law fit to the GeV data points as derived by Slane et al. (2010), essentialforthediscussion,theageoftheSNRisestimated,and
andnewTeVdatabetween200 MeVand90 TeV(showninFig- theenvironmentinwhichitlikelyexpandedisinvestigated.These
ure3)hasbeenperformed.Theresultofthisfitissummarisedin estimateswillformthebasisforthediscussionoftheoriginofthe
Table1andshowsthatthefluxat1TeV,thephotonindexaswell non-thermalemissioninaPWNandSNRscenario.
asthecut-offenergyareconsistentwiththefittotheH.E.S.S.-only
data.Thefithasaχ2of21for24degreesoffreedom(d.o.f.)with
aprobabilityof63%2andimpliesthatnobreakintheγ-rayspec- 4.1 AgeandEnvironmentofG338.3−0.0
trumbetweentheFermiandH.E.S.S.energyrangeisrequiredin
The age and environment of the SNR have a large influence on
ordertodescribethedata.
theinterpretationandmodelingoftheemissionscenarioandthus
deservediscussioninthiscontext.Previousestimatesputtheageof
theSNRintherangeof(5−8)kyr(Slaneetal.2010),however,as
3 XMM-NEWTONDATAANALYSIS becomesevidentfromthediscussionbelow,itmaybesignificantly
youngerthanthat.
Funk et al. (2007) reported the detection of the candidate PWN
If the X-ray PWN is indeed related to the SNR, then
XMMUJ164045.4−463131withXMM-Newtonandintroducedit
G338.3−0.0 originated from a core-collapse supernova explo-
asapotentialcounterpartofHESSJ1640−465.Asbecomesclear
sionofamassivestar.Suchstarsusuallymodifythesurrounding
fromFig.1theVHEγ-rayemissionregionalsooverlapswiththe
mediumthroughstrongstellarwinds,creatingacavityofrelatively
northern part of the shell of SNR G338.3−0.0. To investigate γ-
lowdensitysurroundedbyahigh-densityshellofswept-upmate-
rayemissionscenariosrelatedtotheSNR,theXMM-Newtondata
rial.(seeWeaveretal.1977;Chevalier1999).Suchawind-blown
(ObsID:0302560201)werere-analysedtoderiveanupperlimitfor
bubblescenariohasneverbeenconsideredforthisobject,butneeds
diffuse X-ray emission originating from the northern part of the
tobeexploredforadetaileddiscussionoftheγ-rayemissionmech-
shell.FortheanalysistheScienceAnalysisSystem(SAS)version
anismspossiblyatworkinHESSJ1640−465.Thesecavitieshave
12.0.1 was used, supported by tools from the FTOOLS package
significant impact on the evolution of the subsequent supernova
andXSPECversion12.5.0(Arnaud1996)forspectralmodelling.
shock front, and such scenarios have been evoked to explain the
Thedataareaffectedbylongperiodsofstrongbackgroundflaring
properties of other SNRs like the Cygnus Loop (e.g. Levenson
activityresultinginnetexposuresofonly5.9ks(PN)and13.5ks
et al. 1998), RCW86 (Vink et al. 1997), and RX J1713.7−3946
(MOS), following the suggested standard criteria for good-time-
(Fukui et al. 2003), all of which have physical diameters simi-
interval filtering. To detect and remove point-like X-ray sources
lar to G338.3−0.0. Chevalier (1999) estimated the size of wind-
the standard XMM-Newton SAS maximum likelihood source de-
blowncavitiesbyrequiringapressureequilibriumbetweenthein-
tectionalgorithmwasusedinfourenergybands((0.5−1.0)keV,
sideofthebubble,whichhasbeenpressurisedbythetotalenergy
ofthewind:1/2M˙v2τ,andthesurroundingmedium.Here,M˙ is
w
2 ThefithasbeenperformedonthebinnedH.E.S.Sspectrumshownin the mean mass-loss rate, vw is the wind speed and τ is the life-
Figure 2 and on the GeV spectrum from Slane et al. (2010) taking into time of the star. With a distance of 10kpc, the radius of the ob-
accountstatisticalerrorsonly. servedshellofG338.3−0.0is10pc,whichisassumedheretobe
(cid:13)c 2014RAS,MNRAS000,2–9
AnexceptionallyluminousTeVγ-raySNR 5
Table1.Best-fitspectrumresultsofthenewH.E.S.S.dataasshowninFigure2,andincombinationwiththeGeVspectrumfromSlaneetal.(2010).
Data Emin Emax Γ Φ0 Ec
10−12cm−2s−1 TeV
H.E.S.S. 260GeV 90TeV 2.11±0.09 3.3±0.1 6.0+2.0
−1.2
H.E.S.S.+Fermi-LAT 200MeV 90TeV 2.23±0.01 3.7±0.2 8.8+2.3
−1.5
comparabletothesizeofthewind-blownbubble.Suchsizescan G338.3−0.0,averageneutralgasdensitiesn¯ lowerthanthatare
H
be achieved by a typical ∼20M O-type star with τ (cid:39)7Myr, alsoplausible.FromtheHIabsorptionmeasurementsandthether-
(cid:12)
M˙ (cid:39) 10−7M yr−1,andv (cid:39) 2600kms−1,evolvinginanHII malradioemission,thehydrogengas(neutralplusionised)inthe
(cid:12) w
regionwithtemperature10kK(Osterbrock1989)andaverageden- region is consistent with densities of n¯ (cid:38) (100−150)cm−3.
H
sityofn∼150cm−3(seebelow,Kudritzki&Puls2000;Muijres Purcelletal.(2012)performedasurveyforhigh-densitygas(n(cid:38)
etal.2012).Thiscorrespondstoatotalmasslossinthemainse- 104cm−3)inNH transitionlinesintheGalacticplane.Withthe
3
quencephaseof0.7M .Anextremecasethatmayprovidealower sensitivityofthissurveyandgiventhatnoemissioninthesetran-
(cid:12)
limittotheageoftheSNRcanbederivedbytheassumptionthat sition lines is seen towards HESS J1640−465 a molecular cloud
theremainingmaterialinsidethecavitysolelyoriginatesfromthe moremassivethan∼8000M isnotsupportedbythedata.How-
(cid:12)
stellar wind. The mean number density then is n ∼ 0.01cm−3 ever,thisdoesnotexcludetheexistenceofsmaller,similarlydense
0
with a total mass swept up by the SNR shock of 0.7M . This clumpsofmaterialintheshellregion(seebelow).Thereisalsono
(cid:12)
means that the SNR shock would evolve freely expanding up to maseremissiondetectedtowardstheTeVemission,whichwould
theradiusofthewind-blownbubble.Assumingaverageshockve- haveindicatedtheinteractionofashockwavewithdensematerial
locitiesbetween(5000−10000)kms−1theageoftheSNRwould (e.g.Walshetal.2011).
be(1−2)kyr,whichisconsiderablyyoungerthantheestimateof
(5−8)kyrbySlaneetal.(2010),owingtothelowerdensity.
4.2 PWNscenario
In addition to the SNR age, also the density of the ISM in
the immediate vicinity of the shock region has major impact on The positional coincidence of HESS J1640−465 and
theinterpretationoftheemissionscenario.Thedensityintheshell 2FGL 1640.5−4633 with the candidate X-ray PWN
surroundingthewind-blownbubblecanbeestimatedwithvarious XMMU J164045.4−463131 is seen as evidence for leptonic
methods,i.e.viathermalradioemission,thermalX-raymeasure- γ-ray emission from a PWN (Funk et al. 2007; Lemiere et al.
mentsandHIabsorptionstudies.Castellettietal.(2011)foundev- 2009; Slane et al. 2010). In these scenarios, electrons are ac-
idence for thermal radioemission in the SNR shell indicating the celerated to energies of hundreds of TeV in the PWN, radiate
presence of dense material. The authors infer electron densities via synchrotron and IC processes and produce the observed
basedonthefree-freeabsorptionfeatureintheradiospectrumof X-ray and HE and/or VHE γ-ray emission. In the following the
ne ∼(100−165)cm−3.NodiffuseX-rayemissionfromtheSNR PWN interpretation will be confronted with the new spectral and
shellhavebeenreportedinFunketal.(2007),andintheprevious morphologicalH.E.S.S.resultsandtheavailablemulti-wavelength
section upper limits have been derived. Slane et al. (2010) argue information.
that therefore high gas densities are not supported. However, the Theγ-rayspectrumofmiddle-agedandoldPWNeischarac-
lackofobservedthermalX-rayemissionmightbeconsistentwith terisedbyabreakintheSEDof∆Γ=0.5attheenergywherethe
theverylargedistanceandhighcolumndensitiesinferredfromthe IC/synchrotronlosstimeoftheparentelectronpopulationissim-
XMM-Newton and Chandra spectra (Lemiere et al. 2009) of the ilar to the age of the source (e.g. Hinton & Hofmann 2009). For
PWN XMMU J164045.4−463131; especially if the plasma tem- youngPWNe(t (cid:39) 1kyr)theγ-rayspectrumfrominteractionsof
peratureisbelow1keV.Onlyforhighertemperatures,ase.g.ob- electronswithmagneticandradiationfieldsiseffectivelyuncooled
servedfromKes32(Vink2004),couldobservablethermalX-rays uptothecut-offenergyasICandsynchrotronlosstimesaremuch
beexpectedfromthissource.Particularly,SNRsevolvingrapidly longerinatypicalPWNenvironment.Thisleadstoapeakinthe
insidelow-densitywind-blowncavitiesarenotexpectedtoproduce ICandsynchrotronspectraatenergiesjustbelowthecut-offenergy
significantthermalX-rayemission.OnlywhentheSNRshockhits intheelectronspectrum.AnICpeak(orspectralbreak)isseenfor
thesurroundingshell,themediumintheshockregionthermalises alloftheGeVandTeVidentifiedPWNe(e.g.Grondinetal.2011;
rapidlyandcoolsextremelyfast,whichmakestheSNRanefficient Aharonianetal.2006c;Abdoetal.2010a;Aharonianetal.2005;
emitterofhardthermalX-rays,butonlyduringashorttime.Later, Abramowski et al. 2012b), but not for HESS J1640−465. To re-
thetemperaturesareexpectedtodropsignificantlybelow1keVdue producetheobservedγ-rayspectralindexΓ (cid:39) 2.2forayoung
γ
tothedecreasedshockspeedsofonlyafew100kms−1 (seee.g. object((cid:46) 2.5kyr),theinjectionspectrumhastobeΓ = 3.4,as
e
Tenorio-Tagleetal.1991).Asoutlinedabove,duetothehighab- Γ =(2Γ −1)–anindexsignificantlysteeperthanpredictedby
e γ
sorptiontowardsG338.3−0.0suchemissionisnotexpectedtobe Fermi acceleration theory. Slane et al. (2010) suggested an addi-
detectable. tionalMaxwellianlow-energyelectroncomponentinordertoex-
Finally,theHIabsorptionfeaturecanbeusedtoinferamaxi- plainthesmoothconnectionoftheHEandVHEγ-rayspectra.As
mum(neutral)gasdensity.AssumingthatalloftheHIgasasstud- showninSection2.2thenewhigh-qualityH.E.S.S.spectrumcon-
iedbyLemiereetal.(2009)between−65kms−1and−55kms−1 nectswiththeGeVspectrumwithoutanydiscernablefeaturesand
is associated with G338.3−0.0 and located in a shell with 4pc thusdoesnotrequiresuchacontribution.Infact,aχ2 testofthe
thickness (as supported by radio observations) at 10kpc, a max- Slane et al. (2010) model on the binned GeV and TeV spectrum
imum density of n (cid:39) 600cm−3 can be derived. How- resultsinaχ2 = 189for25d.o.f.withverylowprobability,not
H,max
ever,sincesomeoftheabsorbinggasmaynotbeassociatedwith supportingasignificantcontributionofsuchaMaxwelliancompo-
(cid:13)c 2014RAS,MNRAS000,2–9
6 H.E.S.S.Collaboration
nent. This can be compared to the exponential cut-off power law 4.3 SNRscenario
modelasshowninTable1,whichhasaχ2 =21for24d.o.f.
Given the spectral and morphological similarity of
From a theoretical point of view, the extent of the PWN
HESS J1640−465 with other Galactic SNRs interacting with
is expected to be smaller than its associated SNR (e.g. Blondin
molecular clouds, an SNR origin of the non-thermal emission is
etal.2001).Thispredictionissupportedbyobservationsofseveral
studiedinthefollowing.Inahadronicγ-rayemissionscenario,a
PWNe,includingMSH15−52(Aharonianetal.2005)andVelaX
high-densityisrequiredtoprovidesufficienttargetmaterialforthe
(Abramowskietal.2012b).TheintrinsicsizeofHESSJ1640−465
relativistic protons to produce neutral pions which subsequently
atTeVenergies,however,islargerthanG338.3−0.0andfeatures
decay into energetic photons (see e.g. Aharonian et al. 1994).
significantoverlapwiththeshelloftheSNR–abehaviourthatis
ThishighdensitymaterialoutsidetheSNRshockcouldeitherbe
notseenforanyotherPWN.
the wind shell surrounding the stellar wind bubble, or the dense
At radio wavelengths, Castelletti et al. (2011) derived up- material known to exist in the vicinity of HESS J1640−465.
per limits on the possible radio emission from the PWN The relatively low ISM density Inside the wind-blown bubble
at various wavelengths, with the most constraining limit of would not be sufficient to account for the bulk of the observed
3.7×10−17ergcm−2s−1at610MHzwithintheX-rayPWN.Due γ-ray emission, and thus the target material must be of different
tothedifferentcoolingtimesoftheunderlyingelectronpopulation, origin. In the environment of G338.3−0.0 there could be at
thePWNisexpectedtohavealargerextentinradiothaninX-rays least two possibilities for the occurrence of sufficiently dense
(e.g.Gaensler&Slane2006).Asnoradioemissionhasbeende- ISM: a) As discussed in Section 4.1 and following Chevalier
tectedattheX-rayPWNlocation,itishardtoestimatethesizeand (1999),wind-blownbubblesaresurroundedbyathindenseshell
hence total flux from a potential radio PWN. The 610MHz map containing the bulk of the material swept-up by the stellar wind.
showsadeficitofemissionattheX-rayPWNlocationandsome IftheexpandingshockofG338.3−0.0isnowclosetothisregion,
enhancementinsidetherestoftheSNR.Thiscouldbeassociated accelerated protons might interact with this dense material and
withprojectedSNRemission,orwitharelicradioPWN.Foryoung subsequentlyproducetheobservedγrays.b)Asecondpossibility
PWNethepeakoftheradioemissionisexpectedtobeclosetothe isthattheSNRshockexpandsintoahighlyinhomogeneousISM
pulsarposition.Sincetheradiosurfacebrightnessaroundtheputa- towardsthenearbyHIIregionfeaturingdenseclumpsofmolecular
tivepulsarismuchlowercomparedtotherestoftheSNRinterior, gassurroundedbyregionsofcomparativelylowdensity.Here,the
thiswouldimplythattheradioexcessisrelatedtoprojectedshell particles could be efficiently accelerated within the inter-clump
emission.Foroldersystems,however,theradioPWNcanverywell medium while energetic protons can penetrate into the dense
fillthefullinterioroftheSNRshell.Asacompromise,thelimitas clumps and produce the observed γ-ray emission. This scenario
givenbyCastellettietal.(2011)isscaledupbyafactorof16to has already been proposed for the young (∼2kyr) VHE γ-ray
covertheinterioroftheSNRshell.Inthiscasetheradiolimitisa emitting SNR RX J1713.7−3946 (see Zirakashvili & Aharonian
factorof∼fivebelowthemodelcurvesinLemiereetal.(2009)and 2010) where dense molecular cloud cores have been detected in
Slaneetal.(2010),andwouldimplyalow-energycut-offoftheun- theshockregion(e.g.Sanoetal.2010).SuchISMconditionsare
derlyingelectronspectrumsignificantlyhigherthanthe50GeVas probably also present in the vicinity of G338.3−0.0, due to its
usedbyLemiereetal.(2009). vicinitytoamassiveanddenseHIIregion,makingthisemission
Insummary,theinterpretationoftheGeVandTeVemission scenarioalsoviableforHESSJ1640−465.
as solely originating from a PWN is very difficult as neither the
Incontrasttomiddle-agedinteractingSNRslikeIC443(Abdo
γ-rayspectrum,northemorphologyortheradiodatasupportsuch
etal.2010c)andW44(Abdoetal.2010b)wheretheγ-rayspectra
apicture.ApossiblesolutionwouldbethattheGeVemissionhas
arestronglypeakedatGeVenergies,RXJ1713.7−3946andother
a different origin than the TeV emission. This, however, requires
youngSNRsemitalargefractionoftheirhigh-energyemissionin
fine-tuningtoexplainthesmoothFermiandH.E.S.S.spectrumand
theTeVregime,eitherduetoadifferentradiationprocessortheir
thepositionalcoincidenceoftheGeVandTeVsources.Alsothe
earlier stage in evolution. Figure 3 shows a comparison between
TeVspectrumalonedoesnotshowanysignificantdeviationfroma
theGeV–TeVspectraofHESSJ1640−465andRXJ1713.7−3946
purepowerlawbelowthecut-offenergy,whichwouldbeexpected
asseenbyFermiandH.E.S.S.Interestingly,theirspectralshapes
forayoungPWN.Infact,theradioupperlimitinCastellettietal.
in the TeV regime are very similar, which could support an age
(2011),theX-raydataandanon-dominantICcomponentintheγ-
younger than (10−20)kyr for G338.3−0.0. However, the GeV
rayregimewouldbeconsistentwithXMMUJ164045.4−463131
spectrum becomes much harder for RX J1713.7−3946 but keeps
being a young PWN (c.f. Fig. 5 in Funk et al. 2007). In general,
thesameslopeforHESSJ1640−465.Leptonicmodelsgivingrise
the featureless γ-ray spectrum over almost six decades in energy
totheobservedshapeoftheγ-rayspectrumofRXJ1713.7−3946
ischallengingforanyleptonicmodelasspectralbreaksandsharp
have been discussed in the literature quite extensively (see e.g.
cut-offsareexpectedintheresultingSEDduetocoolingandKlein-
Abdo et al. 2011; Yuan et al. 2011). However, following Zi-
Nishinaeffects,respectively(e.g.Hinton&Hofmann2009).
rakashvili&Aharonian(2010),thechangeinslopetowardslower
TheTeVemissionalsosignificantlyoverlapswiththenorth- energies for RX J1713.7−3946 could also be explained in a
westernpartoftheshellofG338.3−0.0anditishencequitenat- hadronicscenariobythesmallerpenetrationdepthsintothedense
uraltoexploreanoriginofthenon-thermalemissionintheSNR molecularcloudcoresforprotonswithlowerenergies(seealsoIn-
shell. Especially the spectral characteristics of HESSJ1640−465 oueetal.2012).Theseparticlesthereforecannotinteractwiththe
are similar to that of prominent Galactic SNRs interacting with same amount of material as protons with higher energies, giving
molecularcloudssuchasW28,W51CorIC443(seeOhm2012, risetoanunder-luminousandharderGeVγ-rayspectrum.Thefact
andreferencestherein).Inthefollowingthefocuswillbeonanori- that this feature is not seen for HESS J1640−465 might indicate
ginofthenon-thermalemissionintheSNRshell,bearinginmind an older remnant than e.g. RX J1713.7−3946 (i.e. (cid:38) 2.5kyr) or
thatsomefractionofthetotalTeVemissioncouldplausiblyorigi- differentdiffusionpropertiesofthelocalISMthatallowalsolow-
natefromthePWN. energyprotonstofullypenetratethedensemolecularclumps.An
(cid:13)c 2014RAS,MNRAS000,2–9
AnexceptionallyluminousTeVγ-raySNR 7
-46°24'00" 325 -2-1) sm GMRT/ATCA XMM Fermi-LAT H.E.S.S.
g c 10-10
Mercer 81 300 E (er
d
28'00" 275 dN/ 10-11
2 E
0) 250
00 10-12
2
n (J 32'00" 225
o
eclinati 200 10-13
D
175
36'00" 10-14
10-7 10-5 10-3 10-1 10 103 105 107 109 1011 1013 1015
Energy (eV)
150
125 Figure5.HEandVHEγ-rayspectrumofHESSJ1640−465asgivenin
40'00" Slaneetal.(2010)andshowninFigure2,respectively.TheX-raylimit
100 hasbeenderivedinthenorthernpartoftheradioshellandassumingthe
20s 41m00s 40s 20s 16h40m00s
Right Ascension (J2000) highercolumndensityasderivedbyLemiereetal.(2009)(seeFigure1and
text),andtheradiodataisfromCastellettietal.(2011),scaledbyafac-
Figure4.SpitzerMIPS24µmimageinunitsofMJysr−1 withoverlaid torof0.5,assumingthathalfoftheradioemissioncomesfromthenorth-
contoursfromthesmoothedH.E.S.S.excessmap(white)andcontoursof ernpartoftheshell.Thelong-dashedblueandreddash-dottedcurvesare
thenorth-westernpartoftheSNRshellfromthe610MHzimage,convolved synchrotronandICemissionfromnon-thermalelectrons,respectively.The
withtheH.E.S.S.PSF(magenta,c.f.Fig.1). greendashedcurveistheBremsstrahlungcomponentandthesolidblack
curveishadronicπ0-decayγ-rayemission.
ageof2.5kyrwouldimplysomemixingofthestellar-windmate-
rialandtheISMleadingtoaveragedensitiesinthewindbubbleof beconstrainedbytheobservedsynchrotronspectrumfromradioto
n ∼0.1cm−3(c.f.Section4.1). X-rays.Inthismodelcalculation,amagneticfieldofB =35µG,
0
WhencomparingtheTeVmorphologyofHESSJ1640−465 maximum electron energy of E = 10TeV and electron spec-
c,e
toG338.3−0.0(Fig.1)itbecomesclearthatγ-rayemissiononly tralindexofΓ =2.0isrequiredtoreproducetheradiospectrum
e
showssignificantoverlapwiththenorth-western(NW)partofthe andtonotviolatetheX-raylimit.Thetargetradiationfieldshave
radioshell.Thus,inahadronicscenariothelackofemissionfrom been chosen based on Lemiere et al. (2009), with a dust compo-
thesouth-eastern(SE)shellneedstobeexplained.Insuchamodel nent that has been increased to account for the five times higher
the γ-ray emission is expected to follow the distribution and the radiation field energy density in the northern part of the shell. It
densityofavailabletargetmaterialintheshockregion.Indeed,a is clear from Figure 5 that the predicted IC emission is at least
correlationbetweenthemolecularandatomicgasandtheVHEγ- two orders of magnitude below the observed γ-ray emission for
ray intensity from RX J1713.7−3946 has recently been reported an assumed electron-to-proton (e/p) ratio of 10−2. Furthermore,
byFukuietal.(2012).Thus,ifdensetargetmaterialismuchmore the smooth connection of the HE and VHE γ-ray spectrum can-
abundant in the northern region of G338.3−0.0 compared to the not be explained. A considerably higher e/p ratio of (cid:39) 0.1 (and
south,theobservedTeVmorphologyofHESSJ1640−465iscon- lowermagneticfieldofB (cid:39) 10µG)isrequiredtoreachtheTeV
sistentwithahadronicscenario.Figure4showstheSpitzerMIPS flux. Even in this case, the IC spectral shape and maximum en-
(Riekeetal.2004)24µmimageofthisregion,whichessentially ergyisnotsupportedbytheVHEγ-rayspectrum.Indenseenvi-
tracestheabundanceofinterstellardustanddenseHIIstar-forming ronments,Bremsstrahlungcansignificantlycontributetothenon-
regions.Hereitcanbeseenthatthemeaninfraredintensitytowards thermalemission.Densitiesashighas500cm−3 ande/pratiosof
theNWpartisafactorof∼5higherthantowardstheSEareaof 0.1are,however,requiredtoreachthefluxobservedbyH.E.S.S.
theshell.Therefore,thedifferentdensitiescouldindeedgiverise Inahadronicscenario,atotalenergytransferredintoprotons
totheobservedmorphology.Tofurthertestthehypothesisofthe ofW =2.5×1050erg,maximumprotonenergyE =50TeV
p c,p
NW shell being the origin of the VHE γ-ray emission, only this and spectral index of Γ = 2.2 as well as an average ambient
p
partoftheradioshellwasusedasatemplateandconvolvedwith density n¯ = 150cm−3, is required to reproduce the GeV –
H
the H.E.S.S. PSF. The resulting contours are over-plotted on the TeVspectrum.ThemeasuredTeVfluxcoupledwiththelargeesti-
SpitzerimageinFig.4andshowagoodagreementwiththeVHE mated distance of ∼ 10kpc would imply that HESS J1640−465
γ-rayexcesscontoursfromH.E.S.S. is the most luminous Galactic VHE γ-ray SNR detected so far
Figure5showsthemeasuredSEDofG338.3−0.0alongwith (L (cid:39) 4.6×1035(d/10kpc)2ergs−1). The TeV luminos-
>1TeV
the new H.E.S.S. data and XMM-Newton limits. Also shown is ityisthereforeaboutoneorderofmagnitudehigherthanthatofthe
a single-zone time-dependent model for the continuous injection W51CSNR(Aleksic´etal.2012).Duetotheharderγ-rayspectral
of electrons and protons over an assumed age of G338.3−0.0 of index,HESSJ1640−465hasatotalγ-rayluminositycomparable
2.5kyr (e.g. Funk et al. 2007). High-energy electrons produce to W51C. The product of total energy in interacting protons and
synchrotron and IC γ-ray emission in interactions with magnetic meanambientdensityofW n¯ (cid:39)4×1052(d/10kpc)2ergcm−3
p H
andradiationfields,respectively.High-energyprotonsproduceπ0- requiresaconsiderableamountofSNkineticenergythatistrans-
decayγ-rayemissionininteractionswithmaterialintheSNRshell. ferredtohigh-energyprotonsand/orahighaveragedensityofthe
ThebroadbandSEDcanbeexplainedinthisscenariowitharea- target material as motivated before. With the gas densities esti-
sonable choice of input parameters. The leptonic component can mated above, a very large energy in protons is needed to reach
(cid:13)c 2014RAS,MNRAS000,2–9
8 H.E.S.S.Collaboration
the measured GeV and TeV flux. This implies that either the SN detailedandconstructivecomments,whichsignificantlyimproved
explosion was as energetic as E (cid:39) 4×1051(d/10kpc)2erg thequalityofthepaper.Thisworkisbasedinpartonobservations
SN
(assuming a canonical 10% of SN explosion energy is channeled madewiththeSpitzerSpaceTelescope,whichisoperatedbytheJet
intocosmicrays)and/orthatthefractionofE transferredinto PropulsionLaboratory,CaliforniaInstituteofTechnologyundera
SN
relativistic protons is significantly larger than the canonical 10%, contractwithNASA.
i.e.upto∼40(d/10kpc)2%foratypicalE =1051erg.Note
SN
thatthisestimatecanbeevenhigher,asonlythenorthernhalfof
theSNRshellseemstobeilluminatedbycosmicrays.
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