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Polarisation studies of the prompt gamma-ray emission from GRB 041219a using the Spectromet

The spectrometer aboard INTEGRAL, SPI, has the capability to detect the signature of polarised emission from a bright gamma-ray source. GRB 041219a is the most intense burst localised by INTEGRAL and is an ideal candidate for such a study. Polarisation can

Astronomy&Astrophysicsmanuscriptno.0702738(DOI:willbeinsertedbyhandlater)

February5,2008

Polarisationstudiesofthepromptgamma-rayemissionfromGRB041219ausingtheSpectrometeraboardINTEGRAL

S.McGlynn1,D.J.Clark2,A.J.Dean2,L.Hanlon1,S.McBreen3,D.R.Willis3,B.McBreen1,A.J.Bird2,andS.

Foley1

arXiv:astro-ph/0702738v2 12 Mar 2007

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SchoolofPhysics,UniversityCollegeDublin,Dublin4,Ireland

SchoolofPhysicsandAstronomy,UniversityofSouthampton,Southampton,SO171BJ,UKMax-Planck-Institutf¨urextraterrestrischePhysik,D-85741Garching,Germany

Received/Accepted

Abstract.Linearpolarisationingamma-rayburstpromptemissionisanimportantdiagnosticwiththepotentialtosigni cantly

constrainmodels.ThespectrometeraboardINTEGRAL,SPI,hasthecapabilitytodetectthesignatureofpolarisedemissionfromabrightγ–raysource.GRB041219aisthemostintenseburstlocalisedbyINTEGRALwitha uenceof5.7×10 4ergscm 2overtheenergyrange20keV–8MeVandisanidealcandidateforsuchastudy.PolarisationcanbemeasuredusingmultipleeventsscatteredintoadjacentdetectorsbecausetheComptonscatterangledependsonthepolarisationoftheincomingphoton.Asearchforlinearpolarisationinthemostintensepulseofduration66secondsandinthebrightest12secondsofGRB041219awasperformedinthe100–350keV,100–500keVand100keV–1MeVenergyranges.Itwaspossibletodividetheeventsintosixdirectionsintheenergyrangesof100–350keVand100–500keVusingthekinematicsoftheComptonscatterinteractions.ThemultipleeventdatafromthespectrometerwasanalysedandcomparedwiththepredictedinstrumentresponseobtainedfromMonte–CarlosimulationsusingtheGEANT4INTEGRALmassmodel.Theχ2distributionbetweentherealandsimulateddataasafunctionofthepercentagepolarisationandpolarisationanglewascalculatedforallthreeenergyranges.Thedegreeandangleofpolarisationwereobtainedfromthebest– tvalueofχ2.Aweaksignalconsistentwithpolarisationwas

31

foundthroughouttheanalyses.Thedegreeoflinearpolarisationinthebrightestpulseofduration66swasfoundtobe63+ 30%

14

atanangleof70+ 11degreesinthe100–350keVenergyrange.Thedegreeofpolarisationwasalsoconstrainedinthebrightest

39+12

12softheGRBandapolarisationfractionof96+ 40%atanangleof60 14degreeswasdeterminedoverthesameenergyrange.However,despiteextensiveanalysisandsimulations,asystematice ectthatcouldmimictheweakpolarisationsignalcouldnotbede nitivelyexcluded.Ourresultsoverseveralenergyrangesandtimeintervalsareconsistentwithapolarisationsignalofabout60%butatalowlevelofsigni cance(~2σ).ThepolarisationresultsarecomparedwithpredictionsfromthesynchrotronandComptondragprocesses.ThespectrumofthisGRBcanalsobewell tbyacombinedblackbodyandpowerlawmodelwhichcouldarisefromacombinationoftheComptonandsynchrotronprocesses,withdi erentdegreesofpolarisation.Wethereforeconcludethattheproceduredescribedheredemonstratesthee ectivenessofusingSPIasapolarimeter,andisaviablemethodofmeasuringpolarisationlevelsinintensegamma–raybursts.

Keywords.gamma–rays:bursts–gamma–rays:observations–polarisation

1.Introduction

Polarisationisapowerfultoolforinvestigatingemissionpro-cessesinlonggamma–raybursts(GRBs).Thelinkbetweentheγ-rayproductionmechanismandthedegreeoflinearpolarisa-tioncanbeexploitedtoconstrainmodels.

Longgammarayburstsarelinkedtothecollapseofamas-sivestarwhichformsarapidlyrotatingblackhole.Forarecent

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