Gamma-ray bursts and density evolution of neutron star binar(4)

The evolution of the comoving cosmic merger-rate density of neutron star binaries n_c(z) is calculated using a distribution of their merging times provided by population-synthesis computations of binary stars. We adopt an exponential law for the star forma

4P.Bagotetal.:γ

-bursts

Fig.3.CumulativecountsofburstsN(>P)oftheBATSE3Bcatalog(histogram)andmodelcurvescomputedforparame-tersshowninthe gure(fortgal(0)=12.16

Gyr)

Fig.4.Redshiftz0.4necessarytoachieveagreementwiththe

BATSE3BlogN logPdistribution.Thesolidlinecorre-spondstoset1andthedottedlineisforset1b.Inbothcases,thecurrentgalacticageusedistgal(0)=12.16Gyr.

(Totani1997).They,however,predictadi erentbehaviorifstarburstsoccurathighz.

Theabsolutevalueofthe(ns,ns)mergerrateisfoundtobe～100timeslargerthantheGRBsfrequency.Es-capefromthisconundrumisobtainediftheopeningangleoftheobservedphenomenonisafewdegrees,whichiscon-sistentwith re-ballmodelswhereleptonsareconvertedintobulkbarionicmotion(M´esz´aros&Rees1992).

Ifγ-rayburstsindeedoriginatefrom(ns,ns)mergersandourmodelforstarformationiscorrect,interestingimplicationsfollow.TheprogenitorsofbrightGRBsorburstsatlowredshift(z<～1.5)mostlikelybelongedtoearly-typespiralgalaxieswhereastheprogenitorsofthedimmestburstsandthoseathighredshift(z>～2)werelocatedinellipticalgalaxies.Themajorityoftheburstsofgravitationalwavesinthismodelareexpectedtoorig-inatefromearly-typespiralgalaxies.IfmoststarsintheUniverseformedindwarfstar-burstgalaxies,asubstan-tialfractionoftheparentalpopulationmayoriginatefromthem.Suggestedbycurrentobservationaldatathedecline

inthestarformationratebeyondz≈1.2(e.g.Connollyetal.1997),ifreal,willshow-upasaturnoverinthelogN logPdistributionforGRBs.

AsnoticedbyTutukov&Yungelson(1994)andcon- rmedbyPZY98(theirFigs.6and8),spacevelocitiesof(ns,ns)binariesmaywellexceedescapevelocitiesofdwarfaswellasgiantgalaxiesandtheymaytravelupto～1Mpcbeforecoalescence.Thus,asigni cantfractionofthesitesofGRBsmaynotbedirectlyassociatedwithstarformingregions.

Acknowledgements.WethankJanvanParadijsforreadingthemanuscript.ThisworkwaspartiallysupportedthroughNWOSpinozagrant08-0toE.P.J.vandenHeuvelandRFBRGrant96-02-16351.L.R.YacknowledgeshospitalityoftheAs-tronomicalInstitute“AntonPannekoek”andMeudonObser-vatory.

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