Discovery of a Radio Supernova Remnant and Non-thermal X-ray(2)

=1.83 +2.3×10cm

andΓ+00..37

42.Theunabsorbed uxandluminosityofAXJ1813–178from2–10keVare～7.0×10 12ergscm 2s 1and1.7(d/d4)2×1034ergss 1,respectively.Theseparam-etersareconsistentwithAXJ1813–178beingeitherapul-

sar/PWNoroneofasmallnumberofprimarilynon-thermalX-raySNRssuchasG266.2–1.2andG347.3–0.5.

3.DISCUSSION3.1.DistanceConstraints

Asmentionedpreviously,SNRG12.8–0.0liesnearthelineofsightoftheW33HIIregion/starformationcom-plex.Theradiorecombinationlines,molecularlines(in-cludingmasers),andHIgasassociatedwithW33arisefromLSRvelocitycomponentsat～+30and～+50kms 1(e.g.Sato1977;Biegingetal.1978;Gardneretal.1983).NoHIabsorption 1featuresareevidenttowardW33beyond～+55kms(Radhakrishnanetal. 11972).Sincethetangentpointvelocityis～+170kmsinthisdirection(assumesGalacticcenterdistance8.5kpcandFichetal.1989,rota-tioncurve),W33mustlieataboutthe+55kms 1neardis-tanceof～4.3kpc.SimilartoW33,HIabsorptionspec-trafromtheSouthernGalacticPlaneSurvey(resolution1′,seeMcClure-Grif thsetal.2005)towardG12.8–0.0showab-sorptionto,butnotbeyond～+55kms 1,althoughthesignal-to-noiseislow.Thus,thedistancetoG12.8–0.0islikelytobe 4kpc.

Basedonthederivedradiospectralindex( 0.48)theexpected4m uxdensityis～2.7Jy,suggestingthatSNRG12.8–0.0shouldbereadilydetectable,yetitisnot(Fig.1c,§2.1).Thisdiscrepancycouldbeduetothecloseproxim-ityofW33ifG12.8–0.0liesimmersedinorbehindtheHIIregionduetofree-freeabsorption(seee.gKassim1989).Ininterferometriclowfrequencyimages(ν 100MHz)HIIre-gionscanappearinabsorptionagainsttheresolvedoutdiffuseGalacticplanesynchrotronemissionduetofree-freeabsorp-tion.W33iscoincidentwithsucha4mabsorption’hole’;itsmorphologyissimilartothediffuse90cmand8µmemission(Fig.1b)andextendsasfarwestasG12.8–0.0.Thus,free-freeabsorptionisthemostnaturalexplanationforthenon-detectionofG12.8–0.0at4m,andcon rmstheHIresultthattheSNRmustlieatthedistanceoforbehindW33.

ThehighcolumndensityderivedfromtheASCAdata,N1023cm 2,alsosuggeststhatSNRG12.8–0.0liesbehindH～W33.Alongnearbylinesofsight,theintegratedGalacticHIcolumndensityisonly～2×1022cm 2(Dickey&Lockman1990),suggestingthatasigni cantdiscretesourceofab-sorptionmustbeintheforegroundtotheSNR.COdata(Dameetal.2001)indicatethatdensegasinthedistancerange0–4kpc(muchofwhichisassociatedwithW33)con-tributesahydrogencolumndensity～8×1022cm 2,andcanthusaccountformuchoftheX-rayabsorption.Thisfurtherarguesforadistanceof4kpcorgreaterforSNRG12.8–0.0;weadopta ducialminimumdistanceof4kpc(d4)fortheremainingdiscussion.

3.2.TheProbableYouthofSNRG12.8–0.0

ThesmallangularsizeofSNRG12.8–0.0(2.5′)suggeststhatitisquiteyoung;certainlythehighextinctiontowardthisregionwouldhavepreventeditsdetectionoptically.Thera-diusofSNRG12.8–0.0is1.5(d/d4)pc.IftheSNRisstillfreelyexpandingataspeedvsitsageis～285(d/d4)(vs/v5) 1years 1(v5=5,000kms 1).ShockspeedsofvareexpectedfromSNRswithX-raysynchrotrons=5,000kmsemis-sion(e.g.Aharonian&Atoyan1999).Ifthedistanceis4–10kpc,useofthefree-expansionageisjusti edsincethesweptupmasswouldstillbeasmallfractionoftheejectedmass.AtgreaterdistancestheSNRislikelytohaveentered

We present the discovery of non-thermal radio and X-ray emission positionally coincident with the TeV $\gamma$-ray source HESS J1813--178. We demonstrate that the non-thermal radio emission is due to a young shell-type supernova remnant (SNR) G12.8--0.0, a

3

theSedov-Taylorphase.Forexample,at20kpc,thesweptupmassis～50M⊙(assuminganambientdensityno=1cm 3and10%heliumabundance)andtheSedov-Taylorageis2,520(no)0.5(E/E51) 0.5years(whereEisthekineticen-ergyandE51=1051ergs).Thus,independentofdistance,ifG12.8–0.0isexpandingintoatypicaldensitymediumitisveryyoungwithrepresentativeageestimatesintherange285-2,500years.ExcludingG12.8–0.0,onlysixshelltypeGalacticSNRswithdiameters 3′arecurrentlyknown,un-doubtedlyduetoobservationalselectioneffectssincemanymoreareexpected(Green2004).Ourdiscoveryhighlightstheeffectivenessofhighresolutionandsensitivitylowradiofrequencysurveysin ndingyoungGalacticSNRs.

3.3.X-rayandTeVEmissionfromaYoungPulsar?

Althoughthereisexcellentpositionalagreementbetweentheradio,X-ray,andTeVemission(Fig.2),itiscurrentlyunclearwhetherallthesecomponentsoriginatefromtheSNRshellorifthereisanasyetunidenti edassociatedpulsarorPWN.Nocentralradionebulaorpointsourceisdetectedatourcurrentspatialresolutionandsensitivity,andtherearenoknownradiopulsarswithin16′oftheSNR(Manchesteretal.2005).This eldhasbeensearchedforpulsarsaspartoftheParkesmulti-beampulsarsurvey,toalimiting1.4GHz uxdensityof0.2mJy(Manchesteretal.2001),correspondingtoapseudo-luminosityof～3(d/d4)2mJykpc2.RecentdeeppulsarsearchestowardSNRshavebeenabletoeitherdetectorplaceupperlimitsof～1mJykpc2onthe1.4GHzpseudo-luminosityofcentralpulsars(e.g.Camiloetal.2002).Thus,theParkesmulti-beamsurveyisreasonablyconstraining,butadeepersearchisneeded.

TheX-rayluminosityandphotonindexinferredin§2.2forAXJ1813–178aretypicalofyoungpulsarsandtheirassoci-atedPWN(e.g.Possentietal.2002).InordertosearchforX-2

raypulsationsfromAXJ1813–178weconductedaZnsearch(seeBuccherietal.1983)onthebarycenter-correctedASCAhigh-bitmodeGISdata(with20ksofdata).Nopulsedsignalwasdetectedusing585countsbetween4–8keV.Forperi-odsbetween125msand1000s,we ndanupperlimitonthepulsedfractionof38%(19%)forasinusoidal(sharplypeaked)pulsepro le.Sincemostyoungpulsarsrotatewithperiodsbelowthisrange,andthepulsedsignalmaybeasmallfraction(<10%)ofthetotalpulsarplusPWNX-ray ux,thelackofpulsationsisnotveryconstraining.DeeperX-rayandradioobservationsofthissourceareneededtocon rmorruleoutapulsar/PWNinterpretationforAXJ1813–178.

3.4.X-rayandTeVEmissionfromtheSNRShell?

powerlawenergydistributionmodi edbyanexponentialcut-offdN/dE≈E σexp[ (E/Emax)](whereσistheindexoftheelectrondistribution,andEmaxisthemaximumenergyofacceleratedparticles,seeLazendicetal.2004).SincetheX-rayabsorptioncolumn(andhenceX-rayspectralindex)isnotwellconst …… 此处隐藏：6051字，全部文档内容请下载后查看。喜欢就下载吧 ……