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 5 2Max-Planck-Institutfu¨rKernphysik,P.O.Box103980,D69029Heidelberg,Germany 2 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 - o a)Correspondingauthor:[email protected] r t s 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. 2 0 0 INTRODUCTION 7 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. REFERENCES [1] H.E.S.S.Collaboration,H.Abdalla,A.Abramowski,F.Aharonian,F.AitBenkhali,andA.Akhperjanian,The H.E.S.S.GalacticPlaneSurvey,A&Aforthcoming(2016). [2] Aharonian,F.A.etal.,2005,A&A,442,25-29 [3] Oya,I.etal.,2014,TheAstrophysicalJournalLetters,794:L1 [4] Johanson,A.K.andKerton,C.R.,2009,AJ,138,1615 [5] Aharonian,F.A.etal.,2006,A&A,457,899 [6] Ohm,S.,vanEldik,C.andEgberts,K.,2009,AP,31,383 [7] Berge,D.,Funk,S.andHinton,J.,2007,A&A,466,1219 [8] Li,T.-P.andMa,Y.-Q.,1983,ApJ,272,317 [9] Piron,F.,Djannati-Atai,A.andPunch,M.etal.,2001,A&A,374,895 [10] Voisin,F.,Rowell,G.andBurton,M.G.etal.,2016,MNRAS,458,2813-2835 [11] H.E.S.S. Collaboration,H. Abdalla, A. Abramowski,F. Aharonian,F. Ait Benkhali,and A. Akhperjanian, ThePopulationofTeVPulsarWindNebulaeintheH.E.S.S.GalacticPlaneSurvey,submittedtoA&A(2016). [12] SugizakiM.,MitsudaK.andKanedaH.etal.,2001,ApJS,134,77 [13] Abdo,A.A.etal.,2009,Science,325,840 [14] Manchester, R. N., Hobbs G. B. and Teoh A. et al., 2005, AJ, 129, The ATNF Pulsar Catalogue, http://www.atnf.csiro.au/research/pulsar/psrcat [15] Aharonian,F.A.etal.,2006,A&A,448,43