HESS J1826$-$130: A Very Hard $\gamma$-Ray Spectrum Source in the Galactic Plane

0.98 MB·

by  E. O. Angüner

HESS J1826−130: A Very Hard γ-Ray Spectrum Source in
the Galactic Plane
7
E.O. Angu¨ner1,a), F. Aharonian2,3, P. Bordas2, S. Casanova1,2, C. Hoischen4, I. Oya5
1
0 and A. Ziegler6
2
for the H.E.S.S. Collaboration
n
a
J
1InstytutFizykiJa¸drowejPAN,ul.Radzikowskiego152,31-342Krako´w,Poland
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2Max-Planck-Institutfu¨rKernphysik,P.O.Box103980,D69029Heidelberg,Germany
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3DublinInstituteforAdvancedStudies,31FitzwilliamPlace,Dublin2,Ireland

] 4Institutfu¨rPhysikundAstronomie,Universita¨tPotsdam,Karl-Liebknecht-Strasse24/25,D14476Potsdam,
E Germany
H 5DESY,Platanenallee6,D-15738Zeuthen,Germany
. 6Friedrich-Alexander-Universita¨tErlangen-Nu¨rnberg,ErlangenCentreforAstroparticlePhysics,
h
Erwin-Rommel-Str.1,D91058Erlangen,Germany
p
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o a)Correspondingauthor:[email protected]
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t
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a Abstract.HESSJ1826−130isanunidentifiedhardspectrumsourcediscoveredbyH.E.S.S.alongtheGalacticplane,thespectral
[ index being Γ = 1.6 with an exponential cut-off at about 12 TeV. While thesource does not have aclear counterpart at longer
wavelengths,theveryhardspectrumemissionatTeVenergiesimpliesthatelectronsorprotonsaccelerateduptoseveralhundreds

2 ofTeVareresponsiblefortheemission.Inthehadroniccase,theVHEemissioncanbeproducedbyrunawaycosmic-rayscolliding
v withthedensemolecularcloudsspatiallycoincidentwiththeH.E.S.S.source.
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0
0 INTRODUCTION
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0
Inthepastdecade,observationswiththeHighEnergyStereoscopicSystem(H.E.S.S.)havediscoveredtensofvery
.
1 highenergy(VHE,E >0.1TeV)γ-raysources1byscanningalargefractionoftheGalacticPlane[1].
0 HESSJ1826−130 is an unidentified extended source discovered by the H.E.S.S. telescope array in the multi-
7
TeVdomain.Thesourcehadbeenpreviouslyhiddenintheextendedtailofemissionfromthebrightnearbysource
1
HESSJ1825−137[2].Forsuchsourcesaffectedbysourceconfusion,aninvestigationinenergybandscanprovidean
:
v additionalpowerfultoolfornewdiscoveries[3].HESSJ1826−130showsoneofthehardestspectraeverobservedin
Xi VHEγ-rays,extendinguptoatleastfewtensofTeV.Theshell-typesupernovaremnants(SNRs),G018.1−00.1and
G018.6−00.2[4],arelocatedinthevicinityofthesourcealongwiththeγ-raypulsarPSRJ1826−1256.
r
a Inthiswork,theanalysisof206hofH.E.S.S.datafromtheregionaroundHESSJ1826−130willbepresented.
Finally,possibleVHEγ-rayemissionscenariosrelatedtotheoriginofthesourcewillbebrieflydiscussed.
H.E.S.S. OBSERVATIONSand RESULTS
The H.E.S.S. Telescopes
H.E.S.S.isanarrayoffiveimagingatmosphericCherenkovtelescopeslocatedintheKhomasHighlandofNamibia,
1800 m above sea level. H.E.S.S. in phase I comprised of four 13 m diameter telescopes which have been fully
operationalsince2004.AfifthtelescopewasaddedinthecenterofthearrayandhasbeenoperationalsinceSeptember
1Seehttp://tevcat.uchicago.eduforanupdatedlistofknownVHEγ-raysources.
2012.TheH.E.S.S.phaseIarrayconfigurationissensitivetoγ-rayenergiesbetween100GeVandseveraltensofTeV.
With the additionof the fifth telescope,the energythresholdwas loweredto some tensof GeV. The VHE H.E.S.S.
datapresentedinthispaperweretakenwiththeH.E.S.S.phaseIarrayconfiguration,whichcanmeasureextensiveair
showerswithanangularresolutionbetterthan0.1◦ andanaverageenergyresolutionof15%atanenergyof1TeV
[5].
Detection andMorphological Analysis
TheobservationsofthefieldofviewaroundHESSJ1826−130werecarriedoutbetween2004and2015,correspond-
ingtoanacceptancecorrectedlive-timeof206hofH.E.S.S.phaseIdataaftertheapplicationofthequalityselection
criteria[5].ThedatahavebeenanalyzedwiththeH.E.S.S.analysispackageforshowerreconstructionandthemul-
tivariate analysis technique [6] has been applied for providing an improved discrimination between hadrons and γ
rays. In order to provideimprovedangular resolutionand reduce contaminationfrom the brightand nearby source
HESSJ1825−137,the source position and morphologyhave been obtainedwith a hard cutconfigurationwhich re-
quiresaminimumof160photo-electronsperimage.Thecosmic-raybackgroundlevelwasestimatedusingthering
backgroundmodel[7].Usingthisdataset,HESSJ1826−130isdetectedwithastatisticalsignificanceof21.0σ,deter-
minedbyusingEquation(17)in[8].Figure1showstheacceptancecorrectedandsmoothedwiththeH.E.S.S.point
spreadfunction(PSF)VHEγ-rayexcessmapoftheregionaroundHESSJ1826−130atenergiesgreaterthanE=2
TeV.
FIGURE1.MapofexcesseventswithenergiesE>2TeVfortheregionaroundHESSJ1826−130smoothedwiththeH.E.S.S.
PSF.Thewhitecontoursindicatethesignificanceoftheemissionatthe15σand20σlevel.Thecolorscaleisinunitsofexcess
countspersmoothingGaussianwidth.The68%containmentradiusoftheH.E.S.S.PSFisshownwiththewhitecircleatthelower
leftcorner.ThegreencircleshowstheintegrationregionusedforderivingthesourcespectrumshowninFig.3,whilethegreen
crossindicatesthevalueand1σuncertaintyofthebestfitpositionofthesource.ThenearbySNRs,G018.6−00.2andG018.1−00.1
aremarkedwithyellowcircles,whilethewhitetriangleindicatesthepositionoftheγ-raypulsarPSRJ1826−1256.
Thesourceremainedunnoticedandhiddenforthestandardsourcedetectiontechniquesduetoitsrelativelylow
brightnesswithrespecttothenearbybrightsourceHESSJ1825−137.Duringastudyofapossibleenergy-dependent
morphologyof HESSJ1826−130,a collection of images for events with energies above a set of energy thresholds
(E>1,2,3,4,and5TeV)wascreated(seeFig.2).Asitcanbeseenfromthefigure,HESSJ1826−130becomesmore
visiblewithincreasingenergythreshold.
FIGURE2.EnergydependentVHEγ-rayexcessmapsoftheregionaroundHESSJ1826−130forthesamefieldofviewgivenin
Fig.1.
The positionand extensionof HESSJ1826−130was determinedby fitting a two-dimensionalGaussian model
convolvedwiththeH.E.S.S.PSFtotheexcesseventdistribution.Thecentroidofthe2DGaussiancorrespondingto
thebestfitpositionofthesourceisR.A.(J2000):18h26m0.2s ±7.0s ±1.3s andDec.(J2000):−13◦02’±1.8’ ±
stat sys stat
0.3’ .Theextensionofthesourceisestimatedas0.17◦±0.02◦ ±0.05◦ .
sys stat sys
Spectral Analysis
Acircularregionwitharadiusof0.22◦centeredatthebestfitpositionofHESSJ1826−130(e.g.Fig.1,greencircle)
wasusedasanintegrationregionforextractingthedifferentialVHEγ-rayspectrumofthesource.Thespectrumwas
derived using the forward folding technique [9], and is well described by a power-law with an exponentialcut-off
functiondN/dE=Φ (E/1TeV)−Γexp(−E/E )withΦ =(8.62±0.73 ±1.72 )×1013cm−2s−1TeV−1,Γ=1.61
0 c 0 stat sys
±0.11 ±0.20 andacut-offenergyofE =12.5+4.0TeV.Thisspectralmodelispreferredoverasimplepower-law
stat sys c −2.4
modelatastatisticalsignificancelevelof4.0σfromlog-likelihoodratiotest.Thetimevariabilityoftheemissionof
thesourcewasinvestigatedatdifferenttimescalesbyfittingtheintegralfluxtoaconstant.Inallthetimescales,from
run-wise(∼30min)toyear-wiselightcurves,theχ2fitresultsincompatibilityoftheemissionwithaconstantflux.
TheintegrationregionusedforextractingtheVHE spectrumofHESSJ1826−130isstronglycontaminatedby
HESSJ1825−137,especiallyatlowerenergies(∼40%below1.5TeV and∼20%above1.5TeV),andthespectrum
FIGURE3.VHEγ-rayspectrumofHESSJ1826−130extractedfromthesourceregionshowninFig.1.Theblackdotsshowthe
fluxpointswitherrors.The68%errorbandofthebestfitECPLmodel(blueshadedregion)andthebestfitpower-lawmodel(red
line)arealsoshown.Thedifferentialfluxupperlimitintheshownenergybandisat99%confidencelevel.TheVHEspectrumof
RXJ1713.7−3946isshownforcomparison.
presentedinthissectionisaffectedbythiscontamination.Furtherspectralstudies(simultaneousfittingofbothsource
spectra) are being performed for obtaining the intrinsic spectrum of HESSJ1826−130. Given the relatively softer
spectrumofHESSJ1825−137,theintrinsicspectrumofHESSJ1826−130isexpectedtobeevenharderwithrespect
tothespectralresultspresentedinthissection.
MULTIWAVELENGTH COUNTERPARTS
Two giantmolecularclouds(GMC)with massesoffew105 M at3.7kpcand4.7kpchavebeenfoundalongthe
J
lineofsightoftoHESSJ1826−130whenstudyingNANTENdata[10].TheambientgasdensitiesoftheseGMCsare
estimatedoftheorderof∼600cm−3.
TheSNRG018.1−00.1islocatedbetween4and6.3kpc[4](correspondingtotheprojectedphysicaldistanceof
∼33pcfromthebestfitpositionofHESSJ1826−130),whiletheSNRG018.6−00.2islocatedbetween4and5.2kpc
[4](correspondingtotheprojectedphysicaldistanceof∼18pcfromthebestfitpositionofHESSJ1826−130).
Theγ-raypulsarPSRJ1826−1256isalsolocatedinthevicinityofthesourceasshowninFig.1.Thispulsarhas
aspin-downluminosityofE˙ =3.6×1036 erg/sandacharacteristicageofτ =14.4kyr.Notethatpulsarswithτ <
c c
150kyrandwithE˙/D2 ≥1034ergs−1kpc−2areknowntopowerpulsarwindnebulae(PWNe)thataredetectableat
PSR
veryhighenergies[11].ThispulsarhasbeendetectedbothinX-rayenergies[12]andinhighenergyγ-rays[13].No
distanceestimationforthispulsarisknownfromdispersionmeasure[14]sinceitisaradio-quietpulsar.
DISCUSSION
TheemissionfromHESSJ1826−130ischaracterizedbyaveryhardspectrum(Γ=1.6)withanexponentialcut-off
atabout12TeV.Suchaspectrum,togetherwithaspatialcoincidenceofadensemolecularhydrogenregion,suggests
thattheradiationcouldbeproducedbyseveralhundredTeVprotonscollidingwiththegas.Toexplaintheveryhard
spectrum,one canthinkof a hadronicscenarioin whichrunawayprotonsacceleratedbythe close shell-typeSNRs
G018.1−00.1orG018.6−00.2areemittingTeVphotonswheninteractingwiththedenseambientgasfoundalongthe
lineofsight.TheprotonspectrumshouldextenduptoatleastseveralhundredsofTeV.Theγ-rayluminosity,L ,of
γ
thesourceis8×1033 ergsforadistanceof4kpc.Thistranslatesintoanenergyoutputinacceleratedprotons,Wpp
=Lγ ×tpp,of6×1049(n/1cm−3)−1erg,whileambientgasdensitiesfromNANTENdataanalysisareoftheorder
of600cm−3.Inthehadronicscenario,thehardnessofthephotonspectrumcouldbetheeffectofthehighestenergy
protonsdiffusingindenseclouds,whilethelowerenergyprotonsmightbestillconfinedwithintheacceleratingsource
orefficientlyexcludedfromthecloudsifdiffusioncoefficientinsidethecloudsissuppressed.
A leptonic scenario, where electronsaccelerated by the pulsar PSRJ1826−1256are up-scatteringCMB or IR
photons, can also explain the VHE emission. Such as hard spectrum at H.E.S.S. energies can be produced by an
uncooledelectronpopulationwithspectralindexcloseto-2.0andacutoffataround70TeV.Theenergyoutput(We)
inacceleratedelectronsis2×1047ergsforadistanceof7kpc.Thissourcehasaspectrumverysimilartootherfew
PWNe,inparticular,VelaX[15].HESSJ1826−130couldbeanindicationofadistinctivePWNpopulation,withvery
hardspectraandrelativelyhighcutoffenergies
CONCLUSIONS
TheunidentifiedTeVsource,HESSJ1826−130,standsoutintheH.E.S.S.GalacticPlaneSurveyforitsparticularly
hardspectrum.A protonor electronpopulationextendingup to severalhundredTeV is neededto producethe TeV
emissionfromHESSJ1826−130.Currently,studiestobetterconstrainitsspectrumareunderway.
ACKNOWLEDGMENTS
ThesupportoftheNamibianauthoritiesandoftheUniversityofNamibiainfacilitatingtheconstructionandoperation
ofH.E.S.S.isgratefullyacknowledged,asisthesupportbytheGermanMinistryforEducationandResearch(BMBF),
the Max Planck Society, the German Research Foundation (DFG), the French Ministry for Research, the CNRS-
IN2P3andtheAstroparticleInterdisciplinaryProgrammeoftheCNRS,theU.K.ScienceandTechnologyFacilities
Council(STFC),theIPNPoftheCharlesUniversity,theCzechScienceFoundation,thePolishMinistryofScience
andHigherEducation,theSouthAfricanDepartmentofScienceandTechnologyandNationalResearchFoundation,
the University of Namibia, the Innsbruck University, the Austrian Science Fund (FWF), and the Austrian Federal
MinistryforScience,ResearchandEconomy,andbytheUniversityofAdelaideandtheAustralianResearchCouncil.
We appreciatetheexcellentworkofthetechnicalsupportstaffinBerlin,Durham,Hamburg,Heidelberg,Palaiseau,
Paris,Saclay,andinNamibiaintheconstructionandoperationoftheequipment.Thisworkbenefitedfromservices
providedbythe H.E.S.S. VirtualOrganisation,supportedbythe nationalresourceprovidersofthe EGI Federation.
This research has made use of software providedby the ChandraX-ray Center (CXC) in the applicationpackages
CIAO,ChIPS,andSherpa.ThisresearchhasmadeuseoftheSIMBADdatabase,operatedatCDS,Strasbourg,France.
This research has made use of the ATNF pulsar catalog database (http://www.atnf.csiro.au/research/pulsar/psrcat/).
The NANTEN project is based on the mutual agreement between Nagoya University and the Carnegie Institution
ofWashington.SabrinaCasanovaandEkremOg˘uzhanAngu¨neracknowledgethe supportfromthe Polish National
ScienceCenterundertheOpusGrantUMO-2014/13/B/ST9/00945.
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