Hydrodynamical simulations of the jet in the symbiotic star(9)

In papers I and II in this series, we presented hydrodynamical simulations of jet models with parameters representative of the symbiotic system MWC 560. These were simulations of a pulsed, initially underdense jet in a high density ambient medium. Since th

models5.WehypothesizethattheX-rayemittingblobsintheNEjetwereejectedlaterthanthoseintheSWjetandthereforehavecooledless.Thishypothesisissup-portedbythefactthattheX-rayemittingcomponentintheNEjetisclosertothecentralcore(about8”)thantheblobsintheSWjet(12–26”,Kelloggetal.2001).AnewSWjetcomponentwithano setofabout1.5”fromthecentralsourcehasrecentlybeenreportedbyNicholset al.1(2007).Assumingajetvelocityofabout600kms(Kelloggetal.2006),i.e.about0.6”peryear,weobtainakinematicageofabout2.5yearsforthiscomponent.EvenifthenewcomponenthadbeenejectedduringtheepochofKelloggetal.(2001)’sobser-vations,its uxwouldhavecontributedtothecentralsource,butnottothatoftheSWjet.Hencethepres-enceofthenewcomponentdoesnotcon ictwithourhypothesis.ThetimeperiodbetweentheemergenceofthenewSWjetcomponentandtheemergenceoftheblobnowlocatedat12”isabout17yearswhichsupportstheinferredperiodfortheejectionofjetpulsesinRAqr.In§3,weshowedthatthetotalX-rayluminosityde-creaseswithtime,probablyduetoadiabaticcoolinginthejet(PaperII).Thise ectprovidesaplausibleex-planationofthedecreaseinX-ray uxinthejetofRAqrfromavalueof5×10 13ergs 1cm 2intheearly1990s(H¨unschetal.1998)to1×10 13ergs 1cm 2in2000(Kelloggetal.2001).Ifthisexplanationholds,weexpecttoseeafurtherdecreasein uxinfutureobser-vations.

6.4.The6.4–6.7keVironlinecomplex

Thisironlinecomplexhasbeenobservedinbothob-jects,CHCygandRAqr.OurmodelspectraalsoshowtheexistenceofthisFelinecomplex(§4).Ezukaetal.(1998) ttedtheobservedspectrumofCHCygwiththreesingle-temperaturecomponents(awarmandahotcom-ponenttoexplainthejetemissionandhardcomponentforthecentralengine)andanadditionalGaussianrep-resenting uorescenceemissionintheFeKαline.This uorescenceoccursclosetothewhitedwarfandtheac-cretiondisk.Sinceitisnotpossibletodisentanglethe uxofthethermalandthe uorescencecomponentsinthislinecomplexandsinceourmodelsdonotincludethee ectof uorescence,wecannotcompareourmodelswiththispartoftheobservedspectrumofCHCyg.InRAqr,theoriginofthehardX-rayemissionismoreambiguous.Kelloggetal.(2001)detected16photonsintherangebetween6.4and6.7keVwhichtheyattributetothecentralsourceduetotheextractionregionstheychose.Thephysicaloriginofthisemission,i.e.ther-malor uorescence,cannotbedecided,sincetherearenotenoughphotonstomodelthespectruminthisen-ergyandcharacterizeitsnature.Inourmodel,we ndsigni canthardradiationincludingcontinuumandironemissionlines,beingemittedbythe rsttwointernalshocksinthejetdownstreamfromthesource(i.e.atadistancelessthan15AU).SinceatRAqr’sdistanceof200pc,15AUcorrespondto75milliarcseconds,whichiswellbelowtheangularresolutionofChandra,theX-rayemissionfromtheseshockscannotbeseparatedfromthe

5

passage,TheeventsinRAqrarethoughttodiskinstabilities.

whilethevariationsinMWC560betriggeredseemtobebyaperiastronresultof9

centralsource.ThemodelX-ray24luminosityinthe6.4–6.7keV1rangeisbetween5.1×10ergs 1and7.3×1028ergs formodeli’andbetween2.8×1028ergs 1and5.1×1029ergs 1formodeliv’,respectively,depend-ingonwhetherthejetisinitsminimumormaximumstate.Atadistanceof200pc,this2correspondsto uxesbetween1.1×10 18ergs 1cm and1.1×10 13ergs 1cm 2 .14SinceKelloggetal.(2001)measureda uxof4.9×10ergs 1cm 2at6.41keV,wesuggestthatthemeasuredironline uxmaybeemittedbythejetitself,andanadditional uorescencecomponentisnotneeded,inRAqr.However,newsimulationswiththesamejetvelocityasinRAqrareneededinordertotestthissuggestion.

7.CONCLUSIONS

Wehaveusedourmodelsofpulsed,radiativejetsinsymbioticstarsinordertoinvestigatetheirX-rayprop-ertiesindetail.Thesemodelsshowthatthewell-studiedpole-onjetinMWC560shouldbeeasilydetectedbytoday’sX-raytelescopessuchasChandraandXMM-Newton,sinceourmodel uxand13itstimevariationforthissourceareoftheorderof10 ergs 1cm 2.

We ndminimaandmaximaintheX-rayemissionL(computedbyintegratingovertheenergyrange0.15–15XkeV)whichareconnectedwiththeperiodicemergenceofjetpulses.ThemaximaofthetotalX-rayluminos-ityoccur2–3daysaftertheemergenceofnewjetpulses,whichareejectedevery7days.Thesizeofthe uctua-tionsis50%andmoreoftheaverageemission,makingsuchX-ray ashingjetsdetectablewithChandraandXMM-Newton.

TheX-rayspectraofourmodeljetsarerichinemissionlinefeatures,themostprominentofwhichcorrespondtoobservedfeaturesinthespectraofCHCyg.

Byusinglowandhighenergytemperatureproxiesde-rivedfromthespectra,wecanshowthattheemissioncanbeadequatelycharacterizedwithahotandawarmoptically-thinplasmacomponent.Thehotcomponenthastemperaturevaluesofabout0.7keV(1.6keV)duringtheminima(maxima)ofthetotalX-rayluminosityandthewarmcomponenthastemperaturevaluesofabout0.14keV(0.33keV)duringtheminima(maxima).

Whilemodeliv’isappropriateforMWC560,wehaveshownthatmodeli’,whichhasalowerjetpulsedensitythanmodeliv’,ismoreappropriateforthejetinCHCygintermsofexplainingthelowerX-ray ux.Otherpossi-bilitiestoreducethe uxarealongertimescalebetweenthepulsesandasmallervelocitydi erencebetweenthesteadyjetandthejetpulses.Whichoftheabovesce-nariosisthemostlikelyonehastobetestedinfuturesimulationswhichhavetobetunedtothejetinCHCyg.Ourmodelsalsoo eraplausibleexplanationforthedi erencesinluminositiesandtemperaturesintheNEandtheSWjetofRAqr.Weassumethattheejectionofthejetpulsesonbothsidesareoutofphasewitheachother.WehypothesizethattheX-rayemittingblobsintheNEjetwereejectedlaterthanthoseintheSWjetandthereforehavecooledless.

We ndtheexistenceofironlineemissioninthe6.4–6.7keVrangeinourmodelswhichisalsoobservedinboth,CHCygandRAqr.OurmodelscannotbedirectlyappliedtoCHCyg,becauseofanadditional uorescencecomponentfromthecentralsourceandaccretiondiskin