Adv. SpaceRes.Vol. 13, No. 12, pp.(12)103—(12)l11, 1993 0273—1177/93$6.00+ 0.00Printed in Great Britain. Mi rights reserved. Copyright ~ 1993 COSPAR

IMPLICATIONS OFROSATOBSERVATIONSFORTHE LOCAL HOTBUBBLE

S. L. Snowden

PhysicsDepartment,UniversityofWisconsin,Madison,WI 53706,USAandMax-Planck-Institutftir ExtrarerresirischePhysik,D-8046Garching,Germany

ABSTRACT

The ROSATall-sky surveyand GuestObserverpointedobservationshavebrought a wealthofhigh-qualitydatato the studyof the soft X-ray diffusebackground.Analysis of the spatialstruc-tureof the 1/4keV flux, including observationsof shadowscastby discretecloudsin theinterstellarmedium,haveallowedthe separationof the observedflux into foregroundandbackgroundcom-ponentssupportingaspatiallyvaryinglocal componentconsistentwith previousideasof thelocalhot bubble,ahighly variablegalactichalo component,andan extragalacticbackground.

INTRODUCTION

Local Hot Bubble

Thereis much evidencein the literaturesupportingthe existenceof an irregularly shapedlocalcavity in the cooler, mostly neutralmaterialof the galaxy,surroundingthe Sun, with a radialextent (from the Sun) of from ‘-~ 30 pc to ~ 200 pc. Interstellarabsorption-linestudiesusingnearbystars,particularly with H Lyman a, are definitive in this respect (cf., /1,2/), allowinga direct measurementof the foregroundinterstellarmediumcolumn density. The direction of/3 CMa provides an extremeexampleof the cavity extent. Lying nearthe galacticplane, I226°,b —14°,with a distanceof 210 pc, the foregroundcohmmdensityof neutralhydrogenis ~ 2 x 1018 cm2 /3/. This yieldsan averageforegroundspacedensityof 0.003H I cm3,over two ordersof magnitudeless than theaveragegalacticplanevalueof “~ 0.5H I cm3 /4/.

Direct 21-cmobservationsof galacticH I alsoexhibit evidenceof a local deficiencyin interstellarneutralmaterialconsistentwith theexistenceof alocal cavity /5,6/. While the averagehalfthick-nessof the galacticdisk in HI is ‘~3.2x102°cm2/4,6/, themeasuredHI columndensityat highgalacticlatitude is as low as0.5x102°cm2/7/. Clearly, thelocal interstellarmediumis unusualwhen comparedto the averageconditionsof the galacticdisk. Absorptionline measurements,whicharestronglyweightedtowardslow galacticlatitude due to the distribution of usefulstars,show distancesto the edgeof the cavity from 30 Pc to > 200 Pc /2/ while 21-cmobservationsareconsistentwith acavity boundaryof 200 Pcor greaterat highgalacticlatitudes/8/.

Being tidy in one’shousekeeping,onemight ask thequestion: “What fills thelocal cavity?” In atleastonedirection,/3 CMa, asignificantpathlength(40—90pc) is filled withwarm(T~25000K)ionizedgas/3/. Diffuse, partly ionizedclouds,suchas that which surroundsthe Sun/9/, mayalso be “floating” in the cavity. Evenso, mostof the cavity is so far left unoccupied/10/. Theobservationof a nonzeroX-ray flux below 0.28 keV in all directions,specifically including thegalacticplane, coupledwith the short meanfree path of theseX-rays in the neutralinterstellarmedium~ 1O~~H I cm2), requiresthe existenceof local X-ray emission. This requirementis

(12)103

(12)104 S. L. Snowden

madeevenstrongerby sounding-rocketobservationsin theBe band(-..‘ 0.07—0.111keV) /10,11/.While theintensityin this bandis tightly correlatedover a factorof threein dynamicrangewiththehigherenergyB band(“.~0.13— 0.188keV) /10/,the band-averagedmeanfreepathfor the BeBand is 1019 H I cm2, 15% that of the B band.. Sinceasignificant fraction of the galacticH I is diffuse (not dumpedinto compactoptically thick clouds) /12/, the observedX-ray flux,which is consistentwith emissionfrom a 106 K plasma,mustoriginateon the near side of anysignificant column densityof H I. Since wehavearelatively unfilled local cavity in the galacticH I andthe requirementof a local regionof 106 K plasma,the obvious solution is to put thelatterin the former. Thisapproachhasbeenrathersuccessfulin modeffingthe soft X-ray diffusebackgroundandis the observationalbasisfor thelocal hotbubble,LHB /8,13,14,andreferencestherein!.

Diffuse X-ray Background

The soft X-ray diffuse background,SXRB, at 1/4 keV shows extensivespatial variationoverthe sky with the most noticeablefeaturebeing the galacticplane-to-polevariation. Figure 1displaysan Aitoff equalareaprojectionof the cleaned1/4keV ROSATall-sky surveydata, zerocentered,in galacticcoordinates.The X-ray intensityat high galactic latitudesis typically (withlarge variations)a factor of threegreaterthan nearthe plane. This spatialstructureproducesan obvious global negativecorrelationwith the column densityof galacticneutralhydrogen,acorrelationknown from the first observationsof the 1/4keY SXRB /15,16/. Figure 2 displaysthe best exampleof this negativecorrelationin a scatterplot of ROSATsurveydatafrom thenortherngalactichemispherein the longituderange 40°< 1 < 180°.The H I dataare fromDickey andLockman/17/. While this negativecorrelationis very suggestiveof asoft X-ray fluxof extragalacticor galactichaloorigin absorbedby thegalacticH I/e.g., 15/,otherconsiderationssuchas the lack of observedshadowingby discreteH I clouds,the nonzeroflux in the plane,andenergyindependenceof the observedspatialvariation,providedgraveproblemsfor suchamodel/18/.

.1 c~.

~~y)~’ ~ •\ . . ~ ~I: •.“,.: :\. ~7 7~7~çi~~Y~4

t j-.. i c~ 4t~ i ~ ix~u c ~-. r~ ~:.f~..i J4~~~4O~

~ VU o~ c& L 1 ~ ‘~) J~J

\ \ j-~----~~W ~ /\~~V

Fig. 1. ROSATall-sky surveyimageof the1/4keV SXRB. Theprojectionisin galacticcoordinateswith north up and the galacticcenterat the center,longitudeincreasesto the left. The countratesarefield-of-view (diameter 2°)integratedwith theminimum contourat 2 counts~ anda 1.5 countss~contourspacing.

TheLocal Hot Bubble (12)105

10+

J~t+

~6

+

2

20 - 40 60 80 100

H I COLUMN DENSITYFig. 2. Scatterplot of ROSAT1/4 keV count rateversusgalacticH I column density/17/ forthe region‘~ 30°<1 < 180°,b>00. The X-ray count ratesare the sameasin Fig. 1 andthe H Icolumndensitiesare in units of 1019 H I cm2.

With the origin of the 1/4keV diffusebackgroundmore-or-lessconfinedto thegalacticdisk, thequestionthenariseswhetherthelocal hotbubbleis typical of the interstellarmedium,i.e., whatis the galacticfilling factor of hot (~.#106 K) plasma.As an alternativeto adiscreteLHB asanorigin for the SXB.B, thehighly dumpedinterstellarmediumof McKee andOstriker/19/ wasadaptedto form a model wherethe X-ray emitting plasmahasa largefilling factor forming amatrix with embeddedcold douds/20/. However,21-cmobservationsof neutralhydrogenfailedto find the requiredhigh degreeof dumping Id., 7 andreferencestherein/ andthemodel wasfurthershownto be inconsistentwith the X-ray data/21/.

However,theshortmeanfreepathof 1/4keV X-rays(~1.5 x 1020 cm2,r = 1) andthesignificantfractionof diffuseH 1/12/,ruleout theuseof theseX-raysasaprobeof thelargescaleinterstellarmediumand generalgalacticstructure.Giventhe averagespacedensityof ‘~ 0.5 H I cm~3anda diffuse fraction of 0.5 for the H I, we can at best “see” out to only ~ 300 Pc. Thus,whetherthe localhotbubbleis anisolatedandunusualartifact in thegalaxy or anexampleof acommonfeatureis difficult, if not impossible,to determineby observationsofthe 1/4keV SXR.B alone. Ontheotherhand,thereare severalotherwell-definedregionsof 1/4keV emissionvisibleon theX-rayskywhichare qualitativelysimilar to the LHB, e.g., theLoop I, Eridanus,andMonoceros/Geminienhancements/22/.

Figure 3 showsamap of the ROSATsurvey3/4 keV SXRB. The spatialstructureis completelydifferentfrom that at 1/4keY witha few discreteextendedfeaturessuchasLoop I, the VelaSNR,and the CygnusSuperbubbleandLoop superposedon arelatively isotropicbackground.Whilediscussionsof X-ray emissionin the LHB are to a largeextentrestrictedto the 1/4 keV band,somemodelsof theSXRB havesuggestedthat a largefraction of the observed3/4 keV bandfluxoriginatesin the LHB aswell /e.g.,23,24/.The differencein morphologyof the SXRB in the twobandsplacessignificant constraintson theemissionmechanisms,however.

(12)106 5. L. Snowden

•0~

/: 7ç~j~/~ ~‘ 6~

(c~/~~ ~ ~g ~ $JW~)~ A~\!Ydl-~7

~ I~J. /°.~l/

0

Fig. 3. ROSATall-sky surveyimage of the 3/4 keV SXRB. The projectionis the sameas inFigure 1. The minimumcontouris at 0.75 counts ~andthe contourspacingis 0.5 countss1.

DisplacementModel

The displacementmodel /8,25,26/statesthat thebulk of theobserved1/4keV flux (morespecif-ically, the observedB bandflux /8/), exclusiveof discreteextendedfeaturessuchas the NorthPolar Spur/Loop I complex,Monoceros/Geminiand Eridanusenhancementswhich haveothermore-or-lesswell understoodorigins, originatesas thermalemissiondistributedthroughthe inte-rior of theLHB. The spatialvariationin theX-ray intensity,andnegativecorrelationwithgalacticH I, is due to variationsin the radialextentof thecavity: in directionsof higher intensity/lowercolumn density,the cavity is moreextendedandthe hot plasmahas “displaced” the coolerñeu-tral material. The displacementmodelreproducesthe observednegativecorrelationbetweenthediffuse backgroundand H I column densityand is successfulin describingthe generalfeaturesof the diffuse background. It is also consistentwith most otherrelevant observationssuch asthe interstellarabsorptionline and 21-cmmeasurements,the energyindependenceof the X-rayintensityvs. H I column densitynegativecorrelation,andthe observedlackof H I clumping/8/.

However,while the Be andB bandintensitiestrackwell together,as expectedfrom the displace-ment model,the C band(r-~0.16— 0.284keV) to B bandratio showsconsiderablescatter.Someof thisscatterwasexplainedby theobservationof acolorgradientin thebackgroundnearly alongthe galacticcenter/anticenteraxis /27/. The residualscattercould be causedby eitheremissionor absorptionvariationswithin the local hot bubble or the contributionof emissionexternalto

the cavity.

ROSAT

Dataprovidedby the ROSAT/28/ all-sky survey /29/ and GuestObserver(GO) pointedob-servationsoffer greathopetowardsanunderstandingof the origin of the 1/4keY SXRB anditsconnectionwith the interstellarmediumand LHB. The X-ray Telescope(XRT) /30/ with thePositionSensitiveProportionalCounter(PSPC) /31/ is the bestsatellite-borneinstrumentforthe study of the SXRB ever flown. The effective areaat 1/4 keV, fast optics, spatialresolu-tion, exceptionallylow instrumentalbackground,andstability combineto makeit an exceptional

TheLocal Hot Bubble (12)107

observatory.

The ROSATall-sky surveywas completedin half a year andhas nearly completecoverageofthe entire sky. While the exposurevariedconsiderablyover the sky, the exposuresaretypicallysufficient to studythe1/4keV SXRB on 20’ x 20’ scaleswith theadditionalbenefitof excludingpoint-sourceson a 3’ angularscale.The longerexposuresachievedby GO observationsallow theusefulpixel size to be reducedto afew arc minutesat 1/4 keV andallow the studyof the SXRBat higherenergies(0.5 — 1.5 keY) with 10’ resolutionor better. The two possibilitiestogetherallow a flexible approachto analysiswith largeareascoveredat lower resolutionandthe abilityto studylimitedregionsat highspatialresolution.While not as definiteasobservingthe B andCbandswith separatefilters, thereare also sufficientcounts,spectralresolution,andgain stabilityto divide the 1/4keY bandin half, providing additionalspectralinformation /29/.

RESULTS

ShadowObservations

By far the most spectacularresultsfrom ROSAT,in termsof the 1/4 keY SXRB, hasbeentheobservationof shadowscastby discreteH I andmolecularcloudsin bothsurveydata/32/ andGO data/33/. Shadowssuchas thesehadbeensearchedfor sincethe early1970swith singularlackof success.The quality of ROSATdatawas demonstratedin that the shadowswere clearlyvisible with only the standarddataprocessing.

The observationof shadowshasthe greatestimplicationfor the existenceof galactichalo X-rayemissionanda flux of extragalacticorigin /34/. The presentX-ray halo is looking very patchywith large variationsin emissionmeasureon angularscalesof 5°— 10°/35/. Theimplied distantemissionmeasurecanbe considerablylargerthan that of thelocal hotbubble /32/.

While shadowobservationsallow anunambiguousdeterminationof the flux of distantorigin (i.e.,beyondthe object casting the shadow),they can also with proper considerationof noncosmicbackgroundcomponents/36,37,38/allow the determinationof the foregroundand likely local-hot-bubbleemission. To date,few GO observationshavebeenreducedwith reliableresults forthe foregroundfraction at 1/4keY. This is due to acontaminationcomponentof uncertainoriginwith a relatively long period (—.‘ 1 d) that affects essentiallyall observationswith count ratesoccasionallycomparableto the cosmicbackground/37/. However,this backgroundcomponentcanbe easily identified in the all-sky surveydataand all time-variablecontaminationcan beremoved. GO observationscanbe comparedto the surveydataandthe level of contaminationdetermined.

On the otherhand,many of the GO shadowingobservationsof high-latitudemolecularcloudshaveshownno evidencefor shadowingat 1/4 keY. In thesecases,all of the observedX-ray fluxoriginatesin front of the cloud andthe cleanedsurveydatacanbe integratedover a largerareato accuratelydeterminethe foregroundemission. The clouds without shadows,e.g.,MBM 12,are likely embeddedin muchmoreextendedoptically thick cloudsor sheetsof H I /39/ which insomecases(likely for MBM 12) form the boundaryfor thelocal cavity.

The cloudswhichdid not showshadowsat 1/4keY quiteoftenshowshadowsin the3/4keV band.MBM 12, the prototypical exampleof high-latitudemolecularclouds,hasa very well specifieddistanceof 65 Pc /40/. The 3/4 keY shadowthat it castsis quite deepat ~ 70%, implying a 2oupperbruit to theforegroundfractionof 30% /41/.

JASR 13:12—I

(12)108 S. L. Snowden

All-Sky Survey

The large-scalestructureof the ROSAT1/4 keY diffuse background(Fig. 1) is quite similar tothat of previousall-sky surveystakenat lowerresolution/cf., 18/. What also becomesapparentwhen the map is comparedto an H I map of the galaxy, is the wealth of detailed negativecorrelationbetweenthe two. This is seenboth with discreteH I enhancementscorrelatingwithX-ray minima(e.g., 1 45°,b -~-45°)andlocalizedH I minima correlatingwith X-ray maxima(e.g., 1 ~ 315°,b 20°).While bothnotedfeaturesare fairly largein scale(‘-.-~ 20°),thereareotherexamplesof bothtypeson degreescalesor less.

Table1: ForegroundCountRatesForSelectedDirections/Objects

1 b Foreground Dist 1/4keY 3/4 keY1/4 keY Rate° pc Shadow Shadow

Chamaeleon 298 -15 240 140 nob yesGP1 170 0 260 noGP2 100 0 290 noGP3 20 0 310 noMBM 20 210 -35 350 < 125 no yesOphiuchus 350 20 370 150 no yesMBM 36 4 36 380 no yesCepheus 114 16 390 450 flQb

GP4 230 0 410 noMBM 12 155 -35 410 65 no yes~ CMa 226 -15 410 noMBM 40 38 45 475 70 fl

0b

Draco 90 40 510 > 300 yesUMa 150 50 560 yes

aUthtsof 10_6 countss~arcmin2

1’Basedon preliminaryanalysis,D.N. Burrows/privatecommunication/

Table 1 lists the foreground(LHB) intensityderivedfrom the cleanedall-sky surveydatafor anumberof directions.Forthegalacticplane(GP)andall-sky minimumdata,locationswerechosenwhichhadno evidencefor structurecausedby a contributingflux of distantorigin. Nevertheless,thesevaluescanbebest interpretedasupperlimits to the flux originatingin the LHB. GO datafrom observationsof the high latitude molecularcloudsMBM 12, MBM 20, andMBM 36 andsurveydatafrom the Ophiuchusregionshow no evidencefor 1/4 keY shadows. Therefore,theentireobservedflux canbe assumedto originatein front of the cloudsandthereforelikely withinthe LHB. Survey data for the Draco andUMa regionsshow strong shadowsat the -‘~ 50% levelandthe foregroundfractionsweredeterminedby modelling the negativecorrelationbetweenthe1/4 keY intensityand cloud column density. The foregroundintensitiesshow a factor of> 2variation in the foregroundratesover the sampledregionswith higher count ratestypically athighergalactic latitudes.

Geminga

The observationof thepulsar Gemingahassignificant implicationsfor the local hot bubble. Itsprecursorsupernovamay prove to be the power sourcefor the presentthermal energyof theplasma.Geminga,which is at 1 ‘-~~195°,b 4°,is perhapsas nearas 38 Pc from the Sun/42/. Ifso, it is locatedwell within the local cavity. The ageof 3 x iO~years/42/ is notunreasonablefor thereheatingof thecavity. As a sidenote,it could alsoexplaintheobserved26Al excesswhichsuggestsa recent (—‘ iO~years/43/) nearbysupernova.

TheLocal Hot Bubble (12)109

DISCUSSION

The observationof shadowsin the soft X-ray diffuse backgroundat 1/4 keV with ROSATisperhapsthe mostsignificant advancefor our understandingof the origin of theseX-rays sincethey were first discovered.With the datapreviouslyavailable,thebest that could be attemptedwas to model the generalfeaturesof the global structure. With the ROSATsurvey dataand

GO observations,the possibility now exists to map out in three dimensionsthe distribution ofemissionresponsiblefor the observed1/4keV flux. Thebestexampleis againthe galacticX-rayhalo. Previousmodelsof haloemissioncontributingto the diffuse backgroundtendedto assumesphericalor planeparalleldistributionsabovethe disk. Thus fitting suchmodel distributionstothe true patchyhalo producedfits which hadlow statisticalsignificanceandimplied little or nohalo or extragalacticcontributionto the observed1/4keV flux.

Examinationof Figure 1 and the foregroundcount ratesand existenceof shadowsin Table 1alreadygives significant insight into the 1/4 keY SXRB andX-ray emissionby the LHB. Asidefromahandfulof relativelysmallanddistinctenhancements(e.g.,CygnusLoop,VelaSNR,Mono-ceros/GeminiEnhancement),the generallack of structureat lower galactic latitudesreinforcesthe model of a local origin for the bulk of the observed1/4 keV flux. The lack of any significantenhancementnear/3 CMa is also relevant.If the LHB is only thelocal representationof hotgaswhich is commonin the galacticplane,this is the most-likely direction to observeevidencefor alargefilling factor due to the longpath length virtually free of X-ray-absorbingneutralmaterial.However,the SXRB count ratein thedirectionof ,3 CMa (distance-~210 pc) is the sameas thecount ratetowardsMBM 12, knownto be only -..~ 65 pc from the Sun.

At higher galacticlatitudes,the situation is different with considerablestructureand as muchas 50% or moreof the observedflux originatingin the halo or beyond.Nevertheless,the simplepictureof the displacementmodel andLHB still remainsmore-or-lessintact.1) The LHB displaysover a factorof two variationin X-ray intensityoverthe sky with thehighercountratestendingto be at highergalacticlatitudes.2) At low galacticlatitudesawayfrom obviousdiscreteemissionfeatures,the LHB accountsfornearlyall, if not all, of the observed1/4 keY flux.3) While thereexistotherbubblesof hot gasin the solarneighborhood(e.g.,Monoceros/GeminiandEridanusEnhancements,Loop I), they donot form anobviousinterconnectingtunnel system.

With the Gemingasupernovaperhapsprovidingapowersourcefor presentthermalenergystoredin theLHB (3—11x i0~°ergs /8/), thepictureof theLHB and1/4keY SXRB is nearlycomplete.The LHB becomesan exampleof relatively isolatedcavitiesin the H I of the galacticplane. Itmay havebeenrecentlyreheated(althoughthat is probablyunnecessary/8/) and is now at arelativelystabletemperatureof 106 K. Thereis at leastonepartiallyionizedcloudin theinteriorofthe cavityandasignificantpathlengthof warmionizedgasexistsin at leastonedirectionwhichprobablyboundsthe X-ray-emittingplasmaof theLHB. At highergalacticlatitudes,theflux fromthe LHB is augmentedby extragalacticandhighly non-uniformgalactichaloemissionabsorbedby galacticH I, occasionallycomparableto theLHB in intensity. In anumberof directionsnearthe galacticplane,the 1/4keY SXRB exhibits evidencefor otherseeminglyisolatedhotbubbles,perhapssimilar to the LHB.

While thegreatestemphasishasbeenplacedonthe 1/4keV emissionin the LHB, GO observationswill also provide significant insight into the distributionof emissionresponsiblefor the 3/4 keYSXRB. Forinstance,the GO observationof MBM 12 demonstratesthat ~ 30% of the observed3/4keY flux in that directioncanarisein theLHB. MBM 12 is apparentlyembeddedin thelargesolid angleTauruscloud which canbe seenin Figure 3 as a regionof slightly depressedsurfacebrightnessin the galacticanticenterregion. This puts considerableconstraintson LHB modelswhich producesizablefractionsof the observed3/4 keY SXRB. On the otherhand,the upper

(12)110 5. L. Snowden

limit is significantly greaterthan the flux expectedfrom the thermalemissionresponsiblefor theLHB flux at 1/4keV. Nonequilibriurnemissionmodelswithmorerealistictreatmentsof evolutionandcavity edgeeffects canperhapsproducethis flux, if it is truly required.

Thisresearchwassupportedinpartby theNationalAeronauticsandSpaceAdministrationundergrantNAG 5-629 andtheMax PlanckInstitutefor ExtraterrestrialPhysics.

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