Volume105,Number1,January–February2000 Journal of Research of the National Institute of Standards and Technology [J.Res.Natl.Inst.Stand.Technol.105,97(2000)] Current Topics in Gamma-Ray Astrophysics Volume 105 Number 1 January–February 2000 Grant J. Mathews and Thispaperreportsonrecentprogressto- whichproduces(cid:1)1051ergin(cid:2)raysof P. Maronetti wardunravelingtheoriginofgamma-ray thecorrectspectralandtemporalproper- bursts.Itisconcludedthatneutron-star ties. University of Notre Dame, binariesareoneofthefewremaining Department of Physics 225 candidates.Amodelisproposedbased upongeneralrelativistichydrodynamic Nieuwland Science Hall, studieswhichindicateanewphysical Keywords: gamma-raybursts;neutrino Notre Dame, IN 46556 processbywhichtopoweragamma-ray bursts;supernovae. burst.Relativisticallydrivencompression, and heating,andcollapseoftheindividual Jay Salmonson and neutronstarscanoccurmanysecondsbe- foreinspiralandmerger.Thiscompres- J. R. Wilson Accepted: July22,1999 sionmayproduceaneutrinoburstof(cid:1)1053 ergslastingseveralseconds.Theassoci- Lawrence Livermore atedthermalneutrinoemissionproducesan National Laboratory, e+–e–pairplasmaby(cid:1)–(cid:1)annihilation.We Livermore, CA 94550 showfirstresultsofasimulatedburst Availableonline:http://www.nist.gov/jres 1. Introduction There are numerous aspects of modern gamma-ray appear to strike earth daily from an isotropic distribu- astrophysics which are intimately connected with high tioninthesky[2].Theyexhibitnoconfirmedgamma- precisiongamma-rayastronomy.Highresolutionobser- ray lines in a spectrum that is not obviously thermal. vations of galactic gamma-ray lines have provided pro- Attempts to identify their source have not met with found insight into the internal workings of supernovae success—at least until recently. as well as their recent history in the Galaxy. Since this The problem has certainly not been one of limited topic is considered in another paper from this confer- theories. Well over 100 papers have been written at- ence,weconcentratehereontheotherexcitingtopicin tempting to explain GRB origin [3]. The real problem currentgamma-rayastrophysics—gamma-raybursts.In- hasbeenafailuretofindanopticalcounterparttoiden- deed, the papers from this conference are inextricably tify the source location. This was due to the limited immersedineffortstodetectemittersofgammaradia- angular resolution of GRB detectors which permitted tion.Forthatreason,discussionofgamma-raydetection thousands of possible sources within any given error oughttoincludeadiscussionoftheultimateemitterof box.TheNASAComptonGamma-RayObservatory,for gamma-radiationintheuniverse—themysteriousorigi- example,thoughidentifyingthousandsofbursts[2],has nator of gamma-ray bursts (GRBs). The puzzle of the beenabletolocateonlyahandfulofbursts[4]tobetter originofgamma-rayburstshasbeenwithussincethey than 1(cid:3) precision, and none to the minute resolution were first discovered [1] in the 1970s. Although they required to find an optical counterpart. 97 Volume105,Number1,January–February2000 Journal of Research of the National Institute of Standards and Technology This uncertainty has led to two proposed sites for Ithasbeenspeculatedforsometimethatinspiraling GRB origin: 1) galactic ((cid:1)10kpcto300kpc) sources neutron stars could provide a power source for cosmo withanimplied4(cid:4)gamma-rayburstenergyof1037erg logical gamma-ray bursts. The rate of neutron star to 1039erg; or 2) a cosmological (>100Mpc) source mergers (when integrated over the number of galaxies with an energy of (cid:1)1051erg to 1052erg. Opinion as to out to high redshift) could account for the observed whichsourceismostimpliedbythedatahasremained GRB event rate. Previous, Newtonian and post divided into two camps, until recently. Newtonian studies [9] of the direct merger of two Thefieldhaschangedinthepastyear,however,due neutronstarshavefoundthattheneutrinoemissiontime to high-resolution burst detections from the BeppoSax scales are so short that it would be difficult to drive X-Ray satellite as well as observations from ASCA, a gamma-ray burst from this source. However, our RXTE, and ROSAT. For the first time arcminute numerical studies of the strong field relativistic hydro- gamma-ray burst locations have been determined dynamics of close neutron star binaries in three spatial quicklyenoughtoallowfollow-upsearchesforoptical dimensions[10,11,12,13]haveshownthatneutronstars orradiocounterparts.Thesesearcheshaverevealedthat in a close binary can experience relativistic compres- at least some (cid:2)-ray bursts involve weak x-ray, optical, sion and heating over a period of seconds. This effect or radio transients, and are of cosmological origin [5]. cancauseeachofthestarstocollapsetotwoblackholes TheMgIabsorptionand[OII]emissionlinesalongthe priortomerger.Duringthecompressionphaseasmuch line of sight from the GRB970508 optical transient as 1053erg in neutrinos can be emitted before the stars imply[6]aredshiftZ(cid:5)0.835.Theimplieddistanceof collapse [12]. This effect may provide a new mecha- greaterthana10billionlightyearsmeansthatthisburst nismtopowercosmologicalgamma-rayburstsandtheir must entail a release of (cid:1)1051erg in (cid:2)rays on a time x-rayandopticalcounterparts.Here,wereportonpre- scale of seconds. This requirement has been rendered liminary efforts to better quantify this release of even more demanding by another recent event [7] in neutrino energy around the binary and numerically which a GRB appears centered on a galaxy at redshift exploreitsconsequencesforthedevelopmentofae+–e– 3.42. This implies that the energy of a 4(cid:4) burst would plasma and associated GRB. have to be as much as 3(cid:6)1053erg, comparable to the In previous work [12] we computed properties of visiblelightoutputof(cid:2)109galaxies,roughlytheentire equal-mass neutron star binaries as a function of mass visible universe!! and EOS. From these we deduced that compression, Clearly, it is a challenge to explain such an un- heating and collapse can occur at times from a few fathomable burst of energy, and even more perplexing secondstoafewhoursbeforebinarymerger.Ourcalcu- to put essentially all of that energy in nothing but lation of the rates of released binding energy and (cid:2)rays!! Based upon the accumulated observations we neutronstarcoolingsuggeststhatinteriortemperatures can probably conclude that the following four features as hot as 70MeV are possible. This leads to several characterize the source environment: 1) The ratio of seconds of high neutrino luminosity, L (cid:2)1053ergs-1. (cid:1) burst energy to the beam solid angle 4(cid:4)E/(cid:7) is This much neutrino luminosity would convert to an (cid:1)1053erg, e.g., with 1% beaming the burst energy is e+–e–pairplasmaabovethestarsasisalsoobservedin (cid:2)1051erg; 2) The multiple peak temporal structure of supernova simulations[14]. This plasma is a viable mostburstsprobablyrequiresmultiplecollidingshocks candidate source for cosmological gamma-ray bursts. [8]; 3) The observed afterglows imply some surround- We have studied the transport of this neutrino flux ing material on a scale of light hours; and 4) the pres- above the neutron star using a modified version of the enceofOIIemissionlinessuggeststhattheburstsoccur supernova code of [14]. We find entropies as high as in a young stellar population. S/k(cid:2)106 (i.e., few baryons) in the pair plasma above Possible sources consistent with these conditions thestars.Wehavealsomadeasphericalcalculationof probably involve some kind of catastrophic collapse/ the hydrodynamic evolution of the pair plasma based accretion in an environment somewhat depleted in uponourcalculatedneutrinoemissionandanefficiency baryons. Some proposed sites include accretion onto (1% to 10%) for the conversion of neutrinos to e+–e– supermassive black holes, AGNs, relativistic stellar pairs. The results are quite encouraging. We inject the collisions, hypernovae, and binary neutron star coales- pairplasmaintoasphericalgridatarateconsistentwith cence. Each of these possibilities, however, remains the compression-induced thermal neutrino emission speculative until definitive multi-dimensional models which itself is determined by the gravitational wave can be constructed for their evolution. For the remain- emission time [12]. The plasma is evolved hydro- derofthispaperweshalldiscussapreliminaryattempt dynamically until it becomes optically thin and the to construct such a model for relativistically driven escapeof(cid:2)raysiscalculated.Bythistimetheaverage GRBs from neutron-star binaries. temperature is about 10eV, but the special relativistic 98 Volume105,Number1,January–February2000 Journal of Research of the National Institute of Standards and Technology gamma-factor is (cid:2)3(cid:6)104. This produces an integrated magneticfieldlinestothefluidmotionmayalsohavean photon energy spectrum which is quite typical of ob- effect. We are currently investigating both possibilities. servedbursts.Itpeaksataround200keVandextendsto Regarding the magnetic field, we have noted [13] that afewMeV.Nearlyallenergydepositedintoe+–e–pairs theneutron-starfluidseemstorelaxtoanearlyspinless ends up as (cid:2) rays. state prior to collapse. This spinless state, however ac- Figure1showsacalculationof(cid:2)-rayburstluminosity quires net fluid motions and reconnecting field lines as a function of time compared with a typical “single- relative to the corotating local inertial frame of the burst” from the BATSE catalog. The integrated energy binary. These reconnections can cause the magnetic in gamma-rays from the calculated burst is (cid:1)1051erg. field to grow within the stars. The similarity between the observed and calculated Wehavemadeapreliminarysimulationofthegrowth burst is remarkable considering that there has been no rate of the magnetic field by introducing an electro- parameter fitting in these calculations. Single-burst magnetic vector potential into the evolution equations. durationsinthemodelvaryfrom(cid:2)1sto10s.Wealso We followed the evolution of the vector potential for find that if the masses of the stars differ by more than 10ms (about one orbit) assuming that the fluid is a (cid:1)5%thatthe(cid:2)-rayemissionseparatesintotwobursts perfect conductor. The magnetic field energy grows spaced a number of seconds apart. Indeed, there are a exponentially with an e-folding time of about 1ms. numerous bursts in the BATSE catalog consistent with Thus,thefieldcouldbuildupveryquicklytoamagni- this morphology. The bimodal character of the burst tude such that reconnection and back reaction of the durations(theso-calledt90distribution)arisesnaturally fluid inhibits further growth. The limiting fields could in this model from the likelihood that for many bursts approachanequipartitionlimitashighas1017gaussjust only one collapse is observed. before collapse. As the magneitic field grows it should Themultipleshockmechanismnecessarytoaccount bubble from the surface. We speculate that interactions forthetypicalmultiplepeakstructureobservedinmany of these magnetic bubbles the surrounding pair plasma burstsmayresultfromthecouplingoftheplasma(and might lead to the multiple peak structure observed in jet) evolution to the orbit dynamics. The coupling of many GRBs. Of some relevance to this workshop is the fact that during the neutrino emission phase a small fraction of thesurfacematerialisablatedfromthestarsinabaryon wind not unlike that expected to occur in supernovae [15]. We have estimated that in that case some unique gamma-raylineswillbeemitted.Thereasonisthatthe baryonicmaterialislikelytoevolvefromadissociated neutron-rich gas into heavy nuclei far on the neutron rich side of stability. The material is likely to become optically thin when it is still composed of nuclei far from stability. The decay back to stability should includespectrallinesofneutron-richnucleiforseveral minutesaftertheburst.Thelevelofthisactivity,how- ever,isasmallfractionofthebulkoftheburst.Hence, the ultimate confirmation of this GRB paradigm will ultimatelyrequiretheapplicationofextremehigh-reso- lution gamma-ray spectroscopy to test this prediction. Acknowledgments WorkatUniversityofNotreDamesupportedinpartby DOE Nuclear Theory DE-FG02-95ER40934, NSF PHY-97-22086, and by NASA CGRO NAG5-3818. WorkperformedinpartundertheauspicesoftheU.S. Department of Energy by the Lawrence Livermore Fig.1. Calculatedgamma-rayburstluminosity(lowercurve)com- National Laboratory under contract W-7405-ENG-48 paredwithasimilarsingleburstfromtheBATSEcatalog.Thetotal and NSF grant PHY-9401636. releasedenergyfromthecalculatedburstis(cid:2)1051erg. 99 Volume105,Number1,January–February2000 Journal of Research of the National Institute of Standards and Technology 2. 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[13] G.J.Mathews,P.Marronetti,andJ.R.Wilson,Phys.Rev.D58, 043003(1998). [14] J. R. Wilson and R.W. Mayle, 1993, Phys. Reports 227, 97 (1993). [15] S.E.Woosleyetal.,Astrophys.J.433,229(1994). About the authors: Grant J. Mathews and P. Maronetti are physicists at the University of Notre Dame and Jay Salmonson and J. R. Wilson are physicistsatLawrenceLivermoreNationalLaboratory. 100