Context : Gamma-rayburstsarethoughttobeproducedbyshort-lived , supercriticalaccretionontoanewborncompactobject.Someprocessisbelievedtotapenergyfromthecompactobject , ortheaccretiondisk , poweringthelaunchofarelativisticjet.Forthefirsttime , thankstogravitationalwaveobservationsoftheGW170817binaryneutronstarmergerbytheLIGOandVirgointerferometers , andtoaglobalefforttowardsalong-term , high-cadence , multi-wavelengthmonitoringoftheafterglowoftheassociatedshortgamma-rayburstGRB170817A , wecanconstructindependentestimatesoftheGRBjetenergyandofthemassintheaccretiondiskinitscentralengine .

Aims : Weestimatetheaccretion-to-jetenergyconversionefficiencyinGW170817 , thatis , theratioofthejettotalenergytotheaccretiondiskrestmassenergy.WecomparethisquantitywiththeoreticalexpectationsfromtheBlandford-Znajekandneutrino-antineutrinoannihilationjet-launchingmechanismsinbinaryneutronstarmergers .

Methods : Basedonpreviouslypublishedmulti-wavelengthmodellingoftheGRB170817Ajetafterglow , thatincludesinformationfromtheVLBIcentroidmotion , weconstructtheposteriorprobabilitydensitydistributiononthetotalenergyinthebipolarjetslaunchedbytheGW170817mergerremnant.Byapplyinganewnumerical-relativity-informedfittingformulafortheaccretiondiskmass , weconstructtheposteriorprobabilitydensitydistributionoftheGW170817remnantdiskmass.Bycombiningthetwo , weestimatetheaccretion-to-jetenergyconversionefficiencyinthissystem , carefullyaccountingforuncertainties .

Results : Theaccretion-to-jetenergyconversionefficiencyinGW170817is \eta \sim 10 ^ { -3 } withanuncertaintyofslightlylessthantwoordersofmagnitude.Thislowefficiencyisingoodagreementwithexpectationsfromthe \nu \bar { \nu } mechanism , whichthereforecannotbeexcludedbythismeasurementalone.SuchanefficiencyalsoagreeswiththatanticipatedfortheBlandford-Znajekmechanism , providedthatthemagneticfieldinthediskrightafterthemergerispredominantlytoroidal ( whichisexpectedasaresultofthemergerdynamics ) .

Conclusions : Weestimated , forthefirsttime , theaccretion-to-jetenergyconversionefficiencyinagamma-rayburstbycombiningindependentestimatesofthejetenergyandaccretiondiskmass.TheestimatedvalueislowcomparedtooptimalefficienciesexpectedfromtheBlandford-Znajekmechanism , butthisisexpectedinthesesystemsduetothemagneticfieldconfigurationinthediskandtotherelativelyshortaccretiontimescale.Italsoagreeswiththatexpectedforthe \nu \bar { \nu } mechanism , andcannotthereforebeusedtodiscriminateamongthetwomechanisms.Futureapplicationsofthismethodtoalargernumberofsystemswillreducetheuncertaintiesintheefficiencyandrevealwhetheritisuniversalornot.This , inturn , willprovidenewinsightsonthejet-launchingconditionsinneutronstarmergers .