A 150MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721

We report the detection of Zeeman-split Lalpha absorption pi and sigma+ lines in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum of the magnetic cataclysmic variable RX J1554.2+2721. Fitting the STIS data with magne

DRAFTVERSIONFEBRUARY2,2008

APreprinttypesetusingLTEXstyleemulateapjv.6/22/04

A150MGMAGNETICWHITEDWARFINTHECATACLYSMICVARIABLERXJ1554.2+27211

B.T.GÄNSICKE2,S.JORDAN3,K.BEUERMANN4,D.DEMARTINO5,

P.SZKODY6,T.MARSH2,J.THORSTENSEN7

DraftversionFebruary2,2008

arXiv:astro-ph/0408301v1 17 Aug 2004

ABSTRACT

WereportthedetectionofZeeman-splitLyαabsorptionπandσ+linesinthefar-ultravioletHub-bleSpaceTelescope/SpaceTelescopeImagingSpectrographspectrumofthemagneticcataclysmicvari-ableRXJ1554.2+2721.FittingtheSTISdatawithmagneticwhitedwarfmodelspectra,wederivea eldstrengthofB 144MGandaneffectivetemperatureof17000K Teff 23000K.ThismeasurementmakesRXJ1554.2+2721onlythethirdcataclysmicvariablecontainingawhitedwarfwitha eldexceeding100MG.Similartotheotherhigh- eldpolarARUMa,RXJ1554.2+2721isoftenfoundinastateoffeeblemasstransfer,whichsuggeststhataconsiderablenumberofhigh- eldpolarsmaystillremainundiscovered.

Subjectheadings:stars:individual(RXJ1554.2+2721)–novae,cataclysmicvariablesline:formation–stars:

magnetic elds–whitedwarfs

1.INTRODUCTION

Inpolars,asub-classofcataclysmicvariables(CV),thestrongmagnetic eldofthewhitedwarfsuppressesthefor-mationofanaccretiondisk,andsynchronizesthewhitedwarfspinwiththeorbitalperiodofthebinary.Whereassinglemagneticwhitedwarfsshowa elddistributionrangingfromafewtenskGto 1000MG2000;Jordan2001;Gänsickeetal.2002;Schmidtetal.2003)therehasbeen(andstillis)adiscomfortinglackofCVscontainingwhitedwarfswithmagnetic eldsinexcessof100MG.The rsthigh- eldpolartobeidenti edwasARUMa,wherea eldstrengthofB 230MGwasesti-matedfromtheZeeman-splittingoftheLyασ+componentdetectedinafar-ultraviolet(FUV)spectrumobtainedwiththeIUE(Schmidtetal.1996).UsingdetailedHST/STISdata,Gänsickeetal.(2001)re nedthe eldstrengthmeasure-menttoB 200MG.Schmidtetal.reportedasec-ondhigh- eldpolar,V884Her,anddeterminedB 150MGfrommatchinganumberofnarrowcircularlypolarizedab-sorptionfeaturesintheopticalspectrumwithZeeman-splitBalmerlinecomponents.

Here,wereporttheidenti cationofathirdhigh- eldpo-lar,RXJ1554.2+2721,henceforthRXJ1554.RXJ1554wasidenti edasaCVonthebasisofitsemissionlinespectrumintheHamburgQuasarSurvey(Jiangetal.2000).Itsmag-neticnaturewassuggestedbyTovmassianetal.(2001),whoalsoprovideda rstestimateoftheorbitalperiod,placingthesysteminthe2 3hperiodgap.Thorstensen&Fenton

BasedonobservationsmadewiththeNASA/ESAHubbleSpaceTele-scope,obtainedattheSpaceTelescopeScienceInstitute,whichisoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.,underNASAcontractNAS5-26555.

2DepartmentofPhysics,UniversityofWarwick,CoventryCV47AL,UK,boris.gaensicke@warwick.ac.uk

3InstitutfürAstronomieundAstrophysik,UniversitätTübingen,Sand1,72076Tübingen,Germany,jordan@ari.uni-heidelberg.de

4Universitäts-Sternwarte,Geismarlandstr.11,37083Göttingen,Ger-many,beuermann@uni-sw.gwdg.de

6OsservatoriodiCapodimonte,ViaMoiariello16,I-80131Napoli,Italy,demartin@na.astro.it

6AstronomyDepartment,UniversityofWashington,Seattle,WA98195,USA,szkody@astro.washington.edu

7DepartmentofPhysicsandAstronomy,DartmouthCollege,6127WilderLaboratory,Hanover,NH03755-3528,USA,j.thorstensen@dartmouth.edu

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improvedtheperioddeterminationtoPorb=151.865±0.009min.Basedontheidenti cationofbroadbumpsintheiropticalspectraascyclotronharmonics,tentativelysuggesteda eldstrengthofB=31MGforRXJ1554.

2.FUVSPECTROSCOPY

Medium-resolutionFUVspectroscopyofRXJ1554wasobtainedwithHST/STISonFebruary27,2003,aspartofourongoingsnapshotsurveyofcataclysmicvariablesetal.WeusedtheG140Lgratingincon-junctionwiththe52′′×0.2′′aperture,providingaspectralresolutionof 1.2Åandaspectralcoverageof1150 1710Å.TheSTISspectrumofRXJ1554(Fig.1)con-tainsabluecontinuumsuperimposedwithnarrowemis-sionlinesofCIIIλ1176,SiIIIλ1206,NVλ1240,CIIλ1335,SiIVλλ1393,1402,CIVλ1550,andHeIIλ1640.Aweakbroademissionbumpcenteredon～1300ÅmayberelatedtotheSiIIIλλ1294–1309multiplet,theSiIIλλ1260,65dou-bletisnotdetected.Amostunusualfeatureisthebroad(～75Å)absorptionlinecenteredon～1280Å.Itfallsclearly

+

blue-wardofthe1400Åquasi-molecularH2absorptionob-servedincoolwhitedwarfs.Consideringthemagneticna-tureofRXJ1554,atantalisingpossibleidenti cationforthisfeatureistheσ+componentofLyαZeeman-splitina eldB>100MG,analogoustothatobservedintheFUVspectrumofARUMaGänsickeetal.WemeasuredaF28×50LPmagnitudeof 16.8(roughlyequiv-alenttotheRband)fromtheSTISCCDacquisitionimagetakenpriortotheFUVspectroscopy.Thesystemwashenceinastateofrelativelylowaccretionactivity3.SPECTRALANALYSIS

WehaveanalysedtheSTISspectrumofRXJ1554usingmagneticwhitedwarfmodelspectracomputedwiththecodeofanda ttingroutineverysimilartotheonedescribedbyEuchneretal.Inbrief,our tinvolvesthreefreeparameters:themagnetic eldstrengthB(wherewerestrictedourmodelstocentereddipole elds),thean-glebetweentheline-of-sightandthemagneticaxisΨ,andtheeffectivetemperatureofthewhitedwarfTeff.Fluxandcircularpolarizationspectraarecalculatedforalargenum-

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