Astron. Nachr./AN332,No.2,147–152(2011)/DOI10.1002/asna.201111516 First detection of a magnetic field in the fast rotating runaway Oe star ζ Ophiuchi⋆ S.Hubrig1,⋆⋆,L.M.Oskinova2,andM.Scho¨ller3 1 AstrophysikalischesInstitutPotsdam,AnderSternwarte16,14482Potsdam,Germany 2 Universita¨tPotsdam,Institutfu¨rPhysikundAstronomie,14476Potsdam,Germany 3 EuropeanSouthernObservatory,Karl-Schwarzschild-Str.2,85748Garching,Germany 1 1 0 Received2011Jan21,accepted2011Jan26 2 Publishedonline2011Feb15 n a Keywords stars:mass-loss–stars:early-type–stars:magneticfield–stars:kinematicsanddynamics–X-rays:stars J –stars:individual:ζOphiuchi 8 ThestarζOphiuchiisoneofthebrightestmassivestarsinthenorthernhemisphereandwasintensivelystudiedinvarious 2 wavelengthdomains.Thecurrentlyavailableobservationalmaterialsuggeststhatcertainobservedphenomenaarerelated to the presence of a magnetic field. We acquired spectropolarimetric observations of ζOph with FORS1 mounted on ] R the8-mKueyentelescopeoftheVLTtoinvestigateifamagneticfieldisindeedpresentinthisstar.Usingallavailable S absorptionlines,wedetect ameanlongitudinal magneticfieldhBziall = 141±45G,confirmingthemagnetic nature ofthisstar.WereviewtheX-raypropertiesofζOphwiththeaimtounderstandwhethertheX-rayemissionofζOphis . h dominatedbymagneticorbywindinstabilityprocesses. p - (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim o r t s 1 Introduction tometricobservations,althoughBalona&Kambe(1999)fa- a [ voredaperiodintheregionof1cycle/day. During the last years a gradually increasing number of O, 1 ζOph is also well-known for its variability in the X- earlyB-type,andWRstarshavebeeninvestigatedformag- v ray band. Oskinova etal. (2001) studied the ASCA obser- neticfields,andasaresult,aboutadozenmagneticO-type 0 vations of ζOph that covered just more than the expected 0 starsarepresentlyknown(e.g.,Hubrigetal.2008;Martins 5 et al. 2010; Hubrig et al. 2011a). The recent detections of rotation period of the star. A clearly detected periodic X- 5 magnetic fields in massive stars generate a strong motiva- rayfluxvariabilitywith∼20%amplitudeintheASCApass- . band(0.5-10keV)wasreported.Aperiodof0.77dwasde- 1 tiontostudythecorrelationsbetweenevolutionarystate,ro- 0 tationvelocity,andsurfacecomposition,andtounderstand tected and a possible connection with the recurrence time 1 (0.875d±0.167d)oftheDACsinUVspectraofthestarwas theoriginandtheroleofmagneticfieldsinmassivestars. 1 discussed.TheDACsinthespectraofOstarsarecommonly The star ζOphiuchi (=HD149757) of spectral type : v O9.5V is a well-knownrapidly rotating runaway star with explainedbylarge-scalestructuresinthestellarwind,mod- i ulated by rotation and possibly related to a surface mag- X extremelyinterestingcharacteristics.It undergoesepisodic netic field (Cranmer & Owocki 1996). Waldron etal. (in r mass loss seen as emission in Hα, and it is possible that a it rotates with almost break-up velocity with vsini = preparation, private communication) found that SUZAKU 400kms−1 (Kambe et al. 1993). Various studies indicate data on ζOph suggest a period of ∼0.98d that is consis- tentbutslightlylargerthantheX-rayperiodicitiesfoundin differenttypesof spectraland photometricvariability.The ASCAdata(Oskinovaetal.2001)andinChandraHETGS UVresonancelinesshowmultiplediscreteabsorptioncom- data(Waldron2005).Inaddition,theHETGSdataappearto ponents (DAC) in the UV (e.g. Howarth et al. 1984) and indicate an additionalcyclic period of ∼0.33d in the hard strong line profile variations in optical spectra reconciled X-rayband(>1.2keV). with travelingsectorialmodesof high degree(e.g. Reid et al. 1993). Highly precise MOST (Microvariabilityand Os- Theresultsofourpreviousstudiesseemtoindicatethat cillationsofStars)satellitephotometryin2004hasyielded thepresenceofamagneticfieldismorefrequentlydetected atleasta dozensignificantoscillationfrequenciesbetween incandidaterunawaystarsthaninstarsbelongingtoclusters 1 and 10 cycles/day, hinting at a behaviour similar to or associations (Hubrig et al. 2011b; Hubrig et al. 2011a). β Cephei-typestars(Walkeretal.2005).Nounambiguous Thecurrentlybestavailableastrometric,spectroscopic,and rotationperiodcouldbeidentifiedinspectroscopicandpho- photometricdatawereusedtocalculatethekinematicalsta- ⋆ Basedonobservations obtained attheEuropeanSouthern Observa- tusofmagneticO-typestarswithpreviouslyunknownspace tory,Paranal,Chile(ESOprogramme081.C-0410(A)). velocities.TheresultssuggestthatmostofthemagneticO- ⋆⋆ Correspondingauthor:[email protected] typestarscanbeconsideredascandidaterunawaystars. (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim 148 S.Hubrig,L.M.Oskinova&M.Scho¨ller:MagneticfieldintherunawayOestarζOph TheavailableobservationalmaterialsuggeststhatζOph isamainsequencesinglestarinthefieldwithrunawaychar- acteristics.Usually,toexplaintheoriginofmassivestarsin thefield,twomechanismsarediscussedintheliterature.In onescenario,closemultibodyinteractionsinadensecluster causeoneormorestarstobescatteredoutoftheregion(e.g. Leonard & Duncan 1990). For this mechanism, runaways are ejected in dynamical three- or four-body interactions. Analternativemechanisminvolvesasupernova(SN)explo- sionwithinaclosebinary,ejectingthesecondaryduetothe conservation of momentum (Zwicky 1957; Blaauw 1961). Blaauw (1952) suggested the origin of ζOph in the Scor- piusOB2associationduetoitspropermotionvector,which pointsawayfromtheassociation.Morerecently,Hoogerw- erfetal.(2001)suggestedthatthestargaineditspaceveloc- ity of ∼30kms−1 in a supernovaexplosionwithin a close binary in Upper Scorpius about 1–2Myr ago. The authors identified PSR B1929+10 as an associated pulsar with a characteristicageof∼3Myr,consistentwiththekinematic ageofζOphwithintheuncertainties.Tetzlaffetal.(2010) Fig.1 (onlinecolourat: www.an-journal.org)Combined reinvestigatedthescenarioofabinarySNinUpperScorpius IR Spitzer IRAC (3.6µm blue, 4.5µm green, 8.0µm red) involvingζOph and PSR B1929+10and concludedthatit image of the bow shock around the runaway star ζOph. isverylikelythatbothobjectswereejectedduringthesame Archival data have been used. Galactic coordinates are supernova event. In their work, the considered association shown.Theimagesizeis∼36′×31′. age range implies that the progenitor star of the produced neutron star had a spectral type between O6/O7 and O9 emission.Ontheotherhand,Fullertonetal.deriveamuch with a mass rangefrom18to 37M⊙. The X-rayemission smaller mass-loss rate from fitting the UV PV resonance ofthepulsarseemstobedominatedbynon-thermalradia- doublet,theproductofthemass-lossrateandtheionfrac- tionprocesses(e.g.Beckeretal.2006).Anarc-likenebula surrounding PSR B1929+10 and extending up to 10′′ was tion of P+4 being only M˙ q(P+4)<∼1.3 × 10−10M⊙yr−1 with q(P+4)<1. The mass-loss rates derived from fitting identified in Chandra data and interpreted as a bow-shock ∼ thewindprofilesofUVresonancelinesdependlinearlyon nebula(Hui&Becker2008).Theestimatedmagneticfield thedensity.Toresolvethisdiscordanceinmass-lossdeter- strengthintheshockedregionaccountsfor∼75µG,while minations based on ρ2- and ρ-diagnostics, Fullerton etal. the typical magnetic field strength in the ISM is about 2– suggestthatthewindsarestronglyclumpedwithavolume 6µG. filling factor of ∼10−3–10−5. Marcolino etal. (2009) an- The presenceof a bow-shocknebula hasalso been de- alyzed optical and UV spectra of ζOph among their sam- tectedforζOph.Figure1showsanimagebasedonarchival ple of O-type dwarfs. They derive an upper limit on the SpitzerIRACmaps(AOR17774848).Recently,Kobulnicky mass-lossrateofζOphas1.6×10−9M⊙yr−1 ifthewind etal.(2010)analyzedasampleofbowshocksaroundmas- wassmooth.ThisvalueagreeswiththeM˙ q(P+4)valueob- sive stars in Cygnus-X. They used the analytical descrip- tainedbyFullertonetal.(2006). tion of momentum-drivenbow shocks and dust/polycyclic Using the example of the O-type supergiant ζPuppis, aromatic hydrocarbon emission models to estimate stellar Oskinovaetal.(2007)demonstratedthatthediscordanceof mass loss rates from the observed properties of the bow mass-loss rates found by Fullerton etal. can be overcome shocks. It was found that mass-loss rates in the range be- byaccountingforstellarwindporosity(seealsoSundqvist tween 10−7M⊙yr−1 and a few times 10−6M⊙yr−1 are etal.2010).ItwasfoundfortheO5IastarζPuppisthatonly required to generate the bow shocksaround typicalB2V - amoderatereductionofthemass-lossratebyafactorof2–3 O5Vtypestars. (comparedtothesmoothwindmodels)isrequiredtorepro- The mass-loss rate M˙ of this star was empirically ob- ducebothHαandPVlines.Ifthisresultholdsalsofornon- tained from different diagnostics by a number of authors. supergiantOtypestars,themass-lossrateofζOphisonly Repolust etal. (2004) fitted the Hα photospheric absorp- a few times lower compared to the radio-based mass-loss tionlineandderivedtheupperlimitontheζOphmass-loss determinedbyFullertonetal.,i.e.∼10−7M⊙yr−1.Impor- rate as 1.8×10−7M⊙yr−1. Fullerton etal. (2006) deter- tantly, this mass-loss rate is in agreement with values that mined the radio-based mass-loss rate of ζOph as 1.1 × arerequiredtoproducebowshocksaroundOstars(Kobul- 10−7M⊙yr−1. The mass-loss rates determined from ra- nickyetal.2010). dio depend on the square of the density since the physical An additional aspect, which may hint at the presence mechanism responsible for the radio emission is free-free of a magnetic field in runaway stars, is that a number of (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org Astron.Nachr./AN(2011) 149 individual abundance studies indicate nitrogen enrichment in the atmospheres of runaway stars (e.g. Boyajian et al. 1.00 2005). Nitrogen enrichment was found in ζOph by Villa- x mariz & Herrero (2005). Recent NLTE abundance analy- Flu 0.95 d ses(e.g.,Moreletal.2008;Hunteretal.2008)suggestthat e 0.90 z slowrotatorshavepeculiarchemicalenrichmentsuchasni- mali 0.85 trogenexcessorborondepletion,andthesepeculiaritiesare r o N 0.80 linkedtothepresenceofamagneticfield.Ontheotherhand, 0.75 Hubrig et al. (2011c) showed that some magnetic massive starspreviouslyassumedtobeslowrotators,areinfactfast rotators,butareviewedclosetotheirrotationpoles. %] 0.2 [ Totestthemagneticnatureofthisparticularlyinterest- V/I 0.0 ing rapidly rotating runaway star, we acquired spectropo- s e llaarriimmeettreircFoObsReSrv1ataiotntshewVithLTth.eInlotwhi-srewsoolruktiownesrpeepcotrrtopthoe- Stok -0.2 firstdetectionofamagneticfieldinthisstar. 4840 4850 4860 4870 4880 4890 Wavelength [A˚] 2 Magnetic fieldmeasurements Spectropolarimetric observations with FORS1 (Appen- 1.00 zeller et al. 1998) were obtained on 2008 May 23 x u 0.98 (MJD54609.34)using grism 600Band a slit width of 0′.′4 Fl d 0.96 to achieve a spectral resolving power of R ≈ 2000. The e z use of the mosaic detector made of blue optimized E2V ali 0.94 m 0.92 chipsandapixelsizeof15µmallowedustocoveralarge r o spectral range, from 3250 to 6215A˚, which includes all N 0.90 0.88 hydrogen Balmer lines from Hβ to the Balmer jump. Six continuousseriesoftwoexposureswithdurationsbetween 0.3 and 3sec were taken at two retarder waveplate setups %] 0.2 [ (α = +45◦ and−45◦). For allbutthe first exposurepairs V/I 0.0 weachievedasignal-to-noiseratiobetween1000and1500. s e Mbeofroeudnedtaeillsseownhethree(oeb.gse.,rvHinugbrtiegchentiaqlu.e20w0i4tha,F2O0R04Sb1, acnand Stok -0.2 references therein). The mean longitudinal magnetic field, hB i,wasderivedusing 5870 5880 5890 5900 5910 5920 z Wavelength [A˚] V g eλ2 1 dI eff =− hB i, (1) I 4πm c2 I dλ z e Fig.2 Stokes I and V spectra of ζOph in the spectral where V is the Stokes parameter that measures the circu- regionsaroundHβ (top)andtheNaIdoublet(bottom). lar polarisation, I is the intensity in the unpolarised spec- trum, g is the effective Lande´ factor, e is the electron eff charge, λ is the wavelength, m the electron mass, c the InFig.3wepresenttimeseriesofStokesI spectracor- e speedoflight,dI/dλisthederivativeofStokesI,andhB i respondingtoourdatasetofsixsub-exposuresintheregion z is the mean longitudinal magnetic field. The longitudinal around He II 4686A˚ and He I 4713A˚. Although the time magnetic field was measured in two ways: using only the lapsebetweentheobservationsofthefirstpairandthelast absorptionhydrogenBalmerlinesorusingtheentirespec- pairisonly13minutes,somesmalllineprofilevariationsin trumincludingallavailableabsorptionlines.Weobtainfor theHeIlinearealreadydetectableatsuchshorttimescales. themeanlongitudinalmagneticfieldusingallavailableab- The only other measurements of the magnetic field in sorptionlineshB i = 141±45Gandforthemeanlon- ζOph have been presented by Schnerr et al. (2008) who z all gitudinal magnetic field using the hydrogen Balmer lines used the MuSiCoS spectropolarimeter to derive Stokes I hB i =123±54G.Ourdetectionusingtheentirespec- andV spectrawiththeLeastSquareDeconvolutionmethod. z hyd trum has a significance of 3.1σ, determined from the for- No longitudinalmagnetic field was detected in this star at mal uncertainties we derive. In the Stokes V spectra dis- a levelmorethan 3σ. However,the measurementerrorsin tinct Zeeman signatures are well visible at the position of their observations were in the range from 700G to more hydrogenandmetallines.InFig.2wedisplayStokesI and than3kG. V spectra in the spectral regions around Hβ and the Na I Massivestarsusuallyendtheirevolutionwithafinalsu- doublet. pernovaexplosion,producingneutronstarsor blackholes. www.an-journal.org (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim 150 S.Hubrig,L.M.Oskinova&M.Scho¨ller:MagneticfieldintherunawayOestarζOph theaimtounderstandwhethertheX-rayemissionofζOph 1.25 isdominatedbymagneticorwindinstabilityprocesses. 1.20 Babel& Montmerle(1997) studiedthe case ofa rotat- ing star with a dipole magnetic field sufficiently strong to x 1.15 Flu confine stellar wind. The magnetic field locally dominates ed 1.10 the bulk motion of stellar wind, when the ratio of mag- z mali netic to kinetic energy density, B2/µ0ρv2 > 1, where v or 1.05 isthesupersonicflowspeed.Acollisionbetweenthewind N 1.00 streams from the two hemispheres in the closed magneto- sphereleadstoastrongshockandX-rayemission. 0.95 MHD simulations in the framework of this magnet- 4680 4690 4700 4710 4720 4730 ically confined wind shock (MCWS) model were per- Wavelength [A˚] formed by ud-Doula & Owocki (2002) and Gagne´ etal. (2005). Using their notation, the wind is confined when Fig.3 Six Stokes I spectra corresponding to six sub- η∗ ≡(R∗2B2)(M˙ v∞)−1 >1. New observations are re- exposures in the region around He II 4686A˚ and He I quired to establish whether the magnetic field of ζOph is 4713A˚.SmalllineprofilevariationsintheHeIlineareal- a dipole. However, for the purpose of this discussion, let us assume that the field has an average strength of 150G. readydetectableonthetimescaleofonly13min. Using the stellar parameters of ζOph as inferred by Mar- colinoetal. (2009),we estimateη∗(ζOph) ∼ 103,i.e. the The initial massesof these stars rangefrom∼8–10M⊙ to magnetic field should dominatethe wind motionup to the 100M⊙ or more, which correspond to spectral types ear- Alfve´nradiusthatislocatedat<∼10R∗.Inthiscase,theX- lier than about B2. Contrary to the case of Sun-like stars, rayemissionshouldmainlyoriginatefromtheMCWS. the magnetic fields of stars on the upper main sequence The MCWS model predicts that the X-ray emitting (Ap/Bp stars), white dwarfs, and neutron stars are domi- plasmashouldbelocatedatafewR∗fromthephotosphere; nated by large spatial scales and do not change on yearly thattheX-rayemissionlinesshouldbenarrow;thattheX- time scales. In each of these classes there is a wide distri- ray luminosity should be higher and the spectrum harder bution of magnetic field strengths, but the distribution of than in non-magnetic stars; that in oblique magnetic rota- magnetic fluxes appears to be similar in each class, with tors the X-ray emission should be modulated periodically maxima of Φmax = πR2B ∼ 1027−28Gcm2 (Reiseneg- asaconsequenceoftheoccultationofthehotplasmabya ger2001;Ferrario&Wickramasinghe2005),arguingfora cooltorusofmatter,orbytheopaquestellarcore. fossil field whose flux is conserved along the path of stel- ThelinesofHe-likeionsobservedinX-rayspectraare lar evolution. According to Reisenegger (2009) the mag- usefultoderivethelocationofthelineformationregionin neticfluxeshavepossiblybeengeneratedonorevenbefore hot stars because forbidden line emission is depressed by themain-sequencestageandtheninheritedbythecompact ultraviolet pumping. The latter depends on the distance to remnants. the stellar photosphere (Gabriel & Jordan 1969; Blumen- ThemagneticfieldstrengthofthepulsarPSRB1929+10 thal etal. 1972). The SiXIII line observed in the Chan- is 0.5129×1012Gauss. Assuming simple conservation of dra HETGS/MEG spectrumis shown in Fig. 4. prominent magneticfluxweobtainafieldstrengthofjustafewGauss forbidden line can easily be distinguished in this fugure, forthe moremassive pulsarprogenitors.ForζOph,which while normally forbidden lines are strongly suppressed in issupposedtobeformedina binarySN,sharingthesame the spectra of OB-type stars. The presence of the forbid- environment with the SN progenitor, the expected field denlineimpliesthatthelineformationregionislocatedfar strength would be of the order of 10G. This value is no- from the photosphere,so that the radiative excitation does tablylowerthanourcurrentmeasurement,possiblyindicat- notleadtothedepopulationofthecorrespondingmetastable ingthateitherthemagneticfieldofthismiddle-agedpulsar energy levels. Waldron & Cassinelli (2008) found that the hassignificantlydecayedduringthefewMyrsaftertheSN SiXIIIlineisformedat1.8±0.7R∗inζOphandthatother explosionorothermechanismsplayaroleinthegeneration He-like lines are formed even further out in the wind. In- ofmagneticfieldsinO-typestars. terestingly,thestrongforbiddenSiXIIIlineisalsoobserved intheChandraspectrumofthemagneticstarτSco.Cohen 3 Discussion etal.(2005)deriveaSiXIIIlineformaitonradiusforτSco intherangebetween1.1R∗ and1.5R∗.Theseradiiofline ζOph has been extremely well studied in all wavelength formation are smaller than those found in the prototypical ranges, from the X-ray by all major X-ray satellites (with MCWS model object θ1OriC, 3.4 ± 0.8R∗ (Waldron & theexceptionofXMM-Newton)totheinfraredregionwith Cassinelli2008). Spitzer.InviewofthedetectionofamagneticfieldonζOph Oskinovaetal.(2006)studiedtheChandraspectrumof reported in this work, we review its X-ray properties with ζOphamongotherO-typestars.TheyfoundthattheX-ray (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org Astron.Nachr./AN(2011) 151 0.06 0.06 SiXIII FeXVII 0.05 m)0.05 m) o o gstr gstr0.04 An0.04 An c/ c/ e e nts/s0.03 nts/s0.03 u u o o C C0.02 x (0.02 x ( u u Fl Fl 0.01 0.01 λR λI λF λ0=15.01 Ao 0.00 0.00 6.60 6.65 6.70 6.75 6.80 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Wavelength (Ao) v/v 8 Fig.4 TheSiXIIIlineobservedinthespectrumofζOph 0.10 NeX (co-addedMEG±1).Verticaldashedlinesindicatetherest- frame wavelength: λR – resonant line, λI – sum of in- m) twearcvoemlebnigntahtsioanrelcinoerrse,cλteFd–fofrotrhbeidraddeinallivneel.ocTihtyetraeksetn-frfaromme gstro0.08 n Hoogerwerfetal.(2001). c/A0.06 e s s/ nt u o0.04 emissionlinesinthisstararenarrowandthatthesignatures C ofwindabsorptiononlineprofilesareweak.Figure5shows ux ( Fl theFeXVIIandNeXlinesasmeasuredbyChandraplotted 0.02 overunitsofthewindterminalvelocity,v∞=1550kms−1. λ =12.14 Ao 0 Thelinesareonlyslightlybroadened,ifatall. 0.00 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 TheX-rayluminosityofζOph,LX =1.2×1031ergs−1 v/v and the ratioL /L = 4×10−8 arequite usualamong 8 X bol Fig.5 FeXVII(upperpanel)andNeX(lowerpanel)lines late type OV stars (Oskinova etal. 2006). Adopting the mass-loss rate from ζOph as <∼1.8× 10−7M⊙yr−1, Os- observed in the spectrum of ζOph (co-added MEG ±1). Vertical dashed lines indicate the rest-frame wavelength, kinova etal. (2006) noticed that in ζOph the ratio of X-ray to wind mechanical luminosity Lmech (M˙ v∞2/2), correctedfortheradialvelocity. L /L >8.5×10−5,isafewtimeshigherthaninother X mech∼ single O-type stars. Thismay be related to the lower wind Acknowledgements. WewouldliketothankY.Naze´ fordrawing opacityinζOph,oritmayhintatsomeadditionalmecha- our attention to this interesting star and the anonymous referee nismofX-raygenerationbesidestheintrinsicwindshocks. forvaluablecomments.LMOacknowledgesthefinancialsupport FromtheiranalysisofChandraspectra,Zhekov&Palla from grant number FKZ 50 OR 1101. This work used archival (2007)derivedthedifferentialemissionmeasure(DEM)for dataobtainedwiththeSpitzerSpaceTelescope,whichisoperated ζOph among other OB stars in their sample. They found bytheJetPropulsionLaboratory,CaliforniaInstituteofTechnol- thatinζOphtheDEMsharplypeaksatabout6MK.While ogy,underacontractwithNASA.Wealsouseddataobtainedfrom this is a significantlylowertemperaturethanfoundforthe theChandraDataArchiveandsoftwareprovidedbytheChandra DEM peak in case of θ1OriC (50MK), it is higher than X-rayCenter(CXC). found for other stars of similar spectral types (∼3MK). Thus, considering the X-ray temperature of ζOph, it is References not straightforward to attribute its X-ray emission to the MCWS.Ontheotherhand,recentstudiesofOstarswithde- Appenzeller,I.,Fricke,W.,Fu¨rtig,W.,etal.:1998,TheESOMes- tectedmagneticfields(e.g.,HD191612,HD108)showthat senger94,1 their X-ray propertiesare diverse (Naze´ etal. 2004, 2010) Babel,J.,Montmerle,T.:1997,A&A323,121 andmay bedifficultto fullyreconcilewith the predictions Balona,L.A.,Kambe,E.:1999,MNRAS308,1117 oftheMCWSmodel. 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