Colliding neutron stars Gravitational waves, neutr

CollidingneutronstarsGravitationalwaves,neutrinoemission,andgamma-rayburstsM.Ruert1?andH.-Th.Janka2??1InstituteofAstronomy,MadingleyRoad,CambridgeCB30HA,UnitedKingdom2Max-Planck-InstitutfurAstrophysik,Postfach1523,85740Garching,GermanythedateofreceiptandacceptanceshouldbeinsertedlaterAbstract.Three-dimensionalhydrodynamicalsimulationsarepresentedforthedirecthead-onoro-centercolli-sionoftwoneutronstars,employingabasicallyNew-tonianPPMcodebutincludingtheemissionofgravi-tationalwavesandtheirback-reactiononthehydrody-namicalow.Aphysicalnuclearequationofstateisusedthatallowsustofollowthethermodynamicalevolutionofthestellarmatterandtocomputetheemissionofneu-trinos.Predictedgravitationalwavesignals,luminositiesandwaveforms,arepresented.Themodelsareevaluatedfortheirimplicationsforgamma-rayburstscenarios.Wendanextremelyluminousoutburstofneutrinoswithapeakluminosityofmorethan41054erg=sforseveralmil-liseconds.Thisleadstoaneciencyofabout1%fortheannihilationofneutrinoswithantineutrinos,correspond-ingtoanaverageenergydepositionrateofmorethan1052erg=sandatotalenergyofabout1050ergdepositedinelectron-positronpairsaroundthecollisionsitewithin10ms.Althoughthesenumbersseemveryfavorableforgamma-rayburstscenarios,thepollutionoftheepair-plasmacloudwithnearly101Mofdynamicallyejectedbaryonsis5ordersofmagnitudetoolarge.Thereforetheformationofarelativisticallyexpandingreballthatleadstoagamma-rayburstpoweredbyneutrinoemissionfromcollidingneutronstarsisdenitelyruledout.Keywords:gammarays:bursts{gravitation:waves{elementaryparticles:neutrinos{stars:neutron{binaries:close{hydrodynamics1.IntroductionThemergingoftwoneutronstarsthatmakeupabinarysystemisofinterestbothbecauseitisapowerfulsourceofgravitationalwavesandbecauseitmightbethecentralSendoprintrequeststo:H.-Th.Janka?e-mail:[e-mail]??e-mail:[e-mail]diedintenselyduringthepastfewyears,sincetheoc-currencerateishighenough(e.g.Narayanetal.1991,Phinney1991,Tutukovetal.1992,Tutukov&Yungelson1993,Lipunovetal.1995,vandenHeuvel&Lorimer1996,Lipunovetal.1997,Prokhorovetal.1997,Bethe&Brown1998)thatonemightbeabletomeasuretheobservableconsequences.However,directcollisionsoftwoneutronstarsaremuchrarer[therateis1010peryearpergalaxy(Centrella&McMillan1993)ascomparedtothebinarymergerrateof106to105peryearpergalaxy]andthushavenotreceivedmuchattention.Twopreviouslypublishedsetsofhydrodynamicalmod-elsofcollidingneutronstarswerecomputedbyCentrella&McMillan(1993)andbyRasio&Shapiro(1992).BothusedanSPHcodetosimulatethedynamicsoftwopoly-tropicneutronstarsofequalmasseswithadiabaticindex=2andstartedataninitialdistanceofaboutfourneu-tronstarradiionparabolicorbits.Usingarelativelysmallnumberofparticles(2048),Centrella&McMillan(1993)wereabletoproduceacatalogofgravitationalwavelumi-nositiesandgravitationalwaveformsfordierentimpactparameters.Rasio&Shapiro(1992)restrictedthemselvestofewersimulationswithhigherresolution(16000parti-cles)ofneutronstarcoalescenceaswellashead-oncolli-sions.Theyfoundthatupto5%ofthetotalmasscanes-capefromthesystemsandthatstrongshocksoccur.Theirresultsforthegravitationalwaveformsandluminositiesrevealasmallerrstmaximum(causedbytheinitialfreefallofthetwostars),followedbythemainpeakassociatedwiththerapiddecelerationofthecollidingmatterduringthepropagationoftherecoilshocks.Neutronstarcollisionshaverepeatedlybeensuggestedintheliteratureaspossiblesourcesofgamma-raybursts(e.g.,Katz&Canel1996,Dokuchaev&Eroshenko1996;alsoDokuchaevetal.1998),poweredeitherbyneutrino-antineutrinoannihilationwhichproducesanepair-pho-tonreball,orbyhighlyrelativisticshockswhichareformedduringthecollision(orcoalescence)andejectmatteratrelativisticvelocities(Shaviv&Dar1995).Katz&Canel2M.Ruert&H.-Th.Janka:Collidingneutronstars(1996),inparticular,developedtheideathatlonggamma-rayburstsmightbeexplainedbyaccretioninducedcol-lapseofbaredegeneratewhitedwarfs,whileshortburstsmightoriginatefromneutronstarcollisions.Theyesti-matedthepost-collisiontemperaturetobekT100MeV,andexpectedaneutrinopulsewithawidthof2.5ms,andatotalnumberofescapingneutrinosof1058(referingtoDaretal.1992).Withameanneutrinoenergyofabout6MeVthiscorrespondstoatotalenergyof1053ergemit-tedinneutrinos.Theysketchedthepicturethatshortlyafterthecollision,denselumpsofhotmatterexpandandbecomeopticallythintoneutrinoswhicharereleasedinapowerfuloutburstthatisluminousenoughtoproducethedesiredgamma-rayburstenergybyneutrino-antineutrinoannihilationifoneassumesaneciencyof1%.Inordertoobtaintheobservedgamma-rayburstratestheyhadtoinvokeatotalof109hypotheticalclusterswithinz1with108neutronstarseach.Theystatedthat\wecannotexcludethepossibilitythatsuchclustersarecommonlyfoundatthecentersofgalaxies.Neutronstarcollisionsmightalsobeconsideredasin-teresting,becausetheycouldbeviewedatasmoreviolentvariantsofthemergingprocesswithamoreextremedy-namicalevolution.Thismightsupporttheformationofrelativisticshocksandpossiblyleadtoalargerburstofgravitationalwavesandneutrinos.Moreviolenceofthemergingproc