Supernova 1987A-Ten Years After

arXiv:hep-ph/9707501v1 29 Jul 1997

SU P E R N O V A 1987A - T E N Y E A R S A F T E R

A rnon D ar D epartm ent ofPhysics and Space R esearch Institute Technion -IsraelInstitute ofTechnology H aifa 32000),Israel

A bstract Supernova 1987A becam e a m ilestone in physics and astronom y. T he m ost im portant things that have been learned from it, the m ost im portant problem s yet to be solved and the prospects for learning im portant new physics from future observations ofnearby supernova explosions are shortly sum m arized.

1

Introduction

SN 1987A ,the supernova explosion on February 23,1987 in the nearby Large M agellanic C loud only about 50 kpc away,was the brightest supernova seen since the invention of the telescope. It is the rst supernova w hich has been visible to the unaided eye since K epler saw SN 1604,the last Supernova seen in our M ilky W ay galaxy. It has o ered a unique opportunity to observe for the rst tim e a supernova explosion from a relatively close distance w ithin the range of various detection techniques. T he rst signals that were recorded on Earth were neutrino signals in the M ont Blanc (A glietta et al. 1987), K am iokande (H irata et al. 1987), IM B (Bionta et al. 1987) and Baksan (A lexeyev et al. 1988) underground detectors and an uncon rm ed gravitational wave signal in the R om e detector (A m aldiet al. 1987). T hey were followed by a spectacular optical ash that began a few hours later, but was the rst signalfrom 1987A that had been noticed (M cN aught 1987). O bservations of SN 1987A have continued since then, from the ground (opticaltelescopes,radio telescopes,gravitationalwave antenas,high energy ray C erenkov telescopes and extensive air shower arrays) from underground (neutrino telescopes),from high in the air(detectorsaboard high altitude planesand balloons)and from space (H ubble Space Telescope,X -ray telescopes and -ray telescopes). T hey have yielded rich inform ation w hich is offundam entalim portance for astrophysics as wellas for other branches ofphysics and w hich is docum ented in hundreds ofpapers and m any excellent review s that have been published in the scienti c literature. Iw illnot attem pt to review this vast literature but rather focus on w hat I think are the m ost im portant consequences ofSN 1987A ,the m ost im portant things that we have learned from it,the m ost im portant problem s yet to be solved and the prospects for learning im portant new physics from future observations ofnearby supernova explosions.

2

T he B irth of E xtrasolar N eutrino A stronom y

Perhaps the m ost im portant consequence of SN 1987A is the birth of extrasolar neutrino astronom y: W hen the rst large underground water C erenkov detectors,IM B and K am iokande,were constructed for looking for proton decays,it was suggested that they can also perform asneutrino telescopes(e.g.,D ar1983 and referencestherein)w hich m ay detect neutrino bursts from galactic supernova explosions and the di use cosm ological neutrino background from stellar evolution and past supernovae (e.g.,D ar 1985). T his was dram atically dem onstrated w hen the K am iokande and IM B telescopes detected the

neutrino burst from SN 1987A .T his m onum entalsuccess has probably convinced physicistsand funding agenciesthatgalactic and extragalactic neutrino astronom y are notjust a dream butareim portantachievablescienti cgoals.T hishasalready resulted in theconstruction ofSuperkam iokande,an am azing galactic and near galactic neutrino telescope. Togetherw ith the pioneering studiesofthe D U M A N D project,SN 1987A perhapsalso led to the construction ofthe A M A N D A experim ent under the south pole,the Baikalexperim ent under lake Baikaland to the planned N EST O R and A N TA R ES deep sea projects in the M editerranean sea o shore Pylos in G reece and o shore France,respectively. T he U niverse isopaque to very high energy gam m a raysbecause ofelectron-positron pairproduction on intergalactic background photons. Itis,however,transparent to neutrinos. It is anticipated that w hen the above experim ents w illbe scaled up to a 1 km 3,they m ay detect very high energy neutrinos from A ctive G alactic N ucleiat cosm ologicaldistances, from the m ysteriousG am m a R ay Burstersand from otherunexpected sources. T hey also m ay point at the nature and identity of the cosm ic accelerators and help solve the 85 years m ystery ofthe origin ofhigh energy cosm ic rays. T hese,to m y m ind,m ay be the m ost im portant consequences ofSN 1987A ...

3

Supernova T heory

A lready before SN 1987A ,the theory oftype IIsupernova explosions (SN eII) was able to explain m any ofthe observed properties ofSN eIIthatoccur atcosm ologicaldistances at a rate ofabout1 persecond perU niverse,butwasnotable to explain the exactexplosion m echanism (see,e.g.,Shapiro and Teukolsky 1983 and references therein,Bruenn 1987 and references therein). T his has not been changed by SN 1987A in spite ofcontinuous theoreticalprogress,im pressive num ericale orts and m any im portant re nem ents in the theory of SN eII as a result of the detailed observations of both SN 1987A and other nearby SN eII.Itisnow generally believed thatsphericalsym m etric one-dim ensional(1-D ) codesw ith the bestavailable physics (im proved progenitorpro les,im proved equation of state,im proved opacities and neutrino transport and generalrelativistic e ects) cannot reproduce SN eII. Let m e rst sum m arize the SN eII theory prior to SN 1987A ,its spectacular success and its serious problem s. Standard stellar evolution theory predicts that m assive stars 8M M 20M 7 evolvefor 10 y by thetherm onuclearburning ofheavierand heavierfuelsand term inate in anion like red supergiant w ith a w hite dwarflike centralcore consisting prim arily

of iron group nuclei and supported prim arily by electron degeneracy pressure. W hen the core m ass exceeds the C handrasekhar m ass ofabout 1:4M ,gravity overcom es the degeneracy pressure and collapse begins (e.g., A rnett 1977; Barkat 1977). T he central density of the core increases quickly and reaches a value w here electrons from the top of the Ferm i sea can be captured and convert protons, free and in bound nuclei, into neutrons via e + p ! n + e. T he capture ofelectrons results in a short neutronization burst (m s) w hich stops because ofPauliblocking by neutrinos w hich are trapped in the core (because ofneutralcurrentelastic scattering from nuclei). Electron capture from the top ofthe Ferm isea by free protons and iron group nucleireduces degeneracy pressure and accelerates the collapse. T he collapse becom es essentially a free fall w ith a tim e p scale t 1= G 50 m s. W hen the centraldensity ofthe core reaches supranuclear density the repulsive Q C D forcesbetween nucleon constituents(quarksand gluons)ofthe sam e colorstop the collapse,the core bouncesand drivesa strong shock wave thatclim bs outside through the infalling layers. T he strong shock supported by energy transport through convection and neutrinos is believed som ehow to reverse the infall velocity of the layers, to overcom e their gravitationalbinding and to propelthem to the observed expansion velocities ofm ore than 10000 km s 1 w hich am ounts to a totalkinetic energy ofabout1051 erg.T he shock isbelieved to produce the spectacularlightdisplay ofSN eII (G rassberg, Im shennik and N adyozhin 1971): W ith a velocity w hich is a considerable fraction ofthe velocity oflight it takes the shock a few hours to reach the atm osphere of thesupergiant(typicalradiusofabout1013cm ).W hen itreachestheatm osphereitheatsit up to a high tem perature w hich producesa U V ash. H owever,the integrated lum inosity ofSN eII ( 1049erg) and the totalkinetic energy ofthe ejected shell(1051erg) are only a tiny fraction of the released energy. M ost of the gravitationalbinding energy of the collapsed core ( G M 2=R a few 1053 erg)w hich isreleased in the collapse isconverted into therm al energy of a protoneutron star, w hich cools slow ly ( 10 s) by radiating neutrinos from its surface (C olgate and W hite 1966;W ilson et al. 1986 and references therein, M ayle et al1987 and references therein). T he protoneutron star is essentially opaque to neutrinos w hich are therm ally produced m ainly via e+ e ! in the hotcore (centraltem perature 30 M eV ) and di use slow ly to the surface oflast scattering (the \neutrinosphere") w here they are em itted w ith a m uch sm aller tem perature, typically 3 4 M eV for electron neutrinos and 7 8 M eV for and neutrinos,w hich can be predicted from quite generalconsiderations (e.g.,D ar1987). SN 1987A provided a dram aticcon rm ation ofthesepredictionsofthetheory ofSN eII. SN 1987A was caused by the violent death ofa m assive star ( 20M ). T he integrated light em ission ( 1049erg) and the kinetic energy of the expanding shell ( 1051erg)

consisted only ofa tiny fraction ofthe energy released by SN 1987A .M ostofthe energy (a few 1053erg) was radiated in neutrinos,w hich indeed were detected by the M ont Blanc, K am iokande,IM B and Baksan underground detectors. A s expected, neutrino em ission preceded the rst U V light ash by a few hours. T he average energy ofthe e’s was 13 M eV (tem peratureofabout4 M eV )and theduration oftheneutrino burstwas 10 s. T his energy is consistent w ith the gravitationalbinding energy released in the form ation ofa neutron starin stellarcore collapse. T he U V ash and itsspectralevolution waswell tted by a shock wave reaching the surface ofthe supergiant star and heating it. T he detection of -ray lines and infrared em ission lines con rm ed that the exponentialdecay ofthe supernova light curve was because the rem nant was being heated by radioactivity from isotopes m ade in the explosion,0:07M of56C o and 0:003M of57C o. H owever,som e m ajor predictions ofSN eIItheory were not very successfuland m any puzzles rem ain. T hey include: a. W hy was the progenitor ofSN 1987A a blue supergiant and not a red supergiant? b. H ow were the triple rings around the rem nant ofSN 1987A (Fig 1.) form ed? c. W hy was the explosion aspherical,as evident from the debris ofSN 1987A ? d. W hat is the explosion m echanism ofSN eII? e. D id SN 1987A produce a neutron star and w hen w illit becom e visible? f. D id SN 1987A produce a black hole ? g. D id SN 1987A bang tw ice? h. D id SN 1987A em it signi cant gravitationalradiation? a. T he P rogenitor: For the rst tim e the progenitor of a SN eII has been clearly identi ed. A fterthe fading ofthe optical ash from SN 1987A ,carefulm easurem entshave show n that a type B3 blue supergiant, entry num ber 202 in the declination band 690 south ofthe equator in a catalog ofLM C giants com piled by N .Sanduleak,w hich was at the exact position of SN 1987A , disappeared in the explosion, w hereas its two blue neighborstars(Star1 and Star2 atrespectively 2.90 and 1.66 arcseconds away)survived the explosion. A stronom ers were astonished to nd that the progenitor ofSN 1987A was a blue supergiant and not a red supergiant as thought to be the case for m ost SN eII. T wo alternative explanations have been proposed: Perhaps a 20M blue star on the m ain sequence swelled up to becom e a red supergiant,lost m ass through a stellar w ind then contracted and reheated to becom e a blue supergiant. A nother explanation that leads to a blue supergiant is that the progenitor form ed from the m erger oftwo stars in a binary system . T he prior history ofSanduleak -690 202 is probably im printed in the

circum stellar nebulae around SN 1987A and w illbe able to test the two m odels. b. T he R ings: T he gas surrounding SN 1987A was expected to be illum inated by EU V and X -rays(C hevalier1988)em itted w hen the explosion shock wave reached the envelope ofthe pre-supernova star. Early im agestaken by the H ubble Space Telescope,w hich was launched in A pril1990,unexpectedly have show n (W am pler et al1990; Jakobsen et al 1991)thatthe lightem ission from the circum stellar gasaround the rem nant ofSN 1987A islocalized in threering likeform salong a com m on axisw hich passesthrough therem nant ofSN 1987A (see Fig. 1) and is tilted at roughly 450. T he inner ring is centered on the 17 rem nant,hasan approxim ate radiusofR 6:1 10 cm (0:65 ly),a m assabout0.2 to 0.4 1 M and a radialvelocity vr 10 km s .T he ring isalso extraordinarily sym m etric and highly localized in both space ( R =R 10% ) and velocity. V LB radio observations and recentH ST observationshave show n thatthe radiusofthe glow ing debrisfrom SN 1987A is now about 0.1 arcseconds (about 15% ofthe distance to the ring) and the expansion speed has been nearly constant,over the past 10 year history,i.e., 0:01 arcsecond per yearorvr 2500 km s 1.T hisism uch slowerthan the speediest m aterialobserved back in 1987,w hich reached 30000 km s 1,butprobably wasofa sm allm assw hich wasslowed dow n by the circum stellar gas. T hus,itseem s thatthe ringswere there before SN 1987A . Various m odels have been proposed for the origin ofthe rings. T he sam e basic structure isseen w ith H ST in the H ourglassN ebula,suggesting thatsom e com m on aspectsofm ass loss were at work both in this planetary nebula and in SN 1987A .C onsequently, it was suggested that the SN 1987A rings form ed by the illum ination of a pre-supernova red giant w ind that was m uch thicker at the waist than the poles resulting in an expected hour-glassshape (Luo and M cC ray 1991;W ang and M azzali1992;Blondin and Lunqvist 1993; M artin and A rnett 1995). It was suggested that the glow of the rings is form ed by recom bination ofelectrons and atom s that were ionized by the EU V and X -ray ash from SN 1987A in the case ofthe inner ring,and by the EU V and X -ray em issions from a relativistic conicaljets in the case ofthe externalrings. O ther m odels assum e that the innerring isa relicfrom an accretion disk (M cC ay and Lin 1994)orfrom an excretion disk from w hich the presupernova star was born (C hen and C olgate 1996). It is also possible that the inner and outer rings are thin ash ionized layers at the inner surfaces ofm uch greater m ass ofcircum stellar as yet unseen. c. A sphericalE xplosion ? Jets? R ecent high resolution V LB radio im ages(G aensler et al1997)and H ST opticalim ages (Pun 1997)ofSN 1987A and its inner ring show that the glow ing debris ofthe supernova itselfis elongated along the axis ofthe rings. It was pointed outthata a m ergeroftwo starsin a binary system (Podsiadlow ski1992)leadsto a blue supergiantprogenitorand can explain an equatorialout ow ofseveralsolarm asses

of gas during a m erger of the two stars som e 20.000 years before the explosion. Such a m erger would probably yield a progenitor that is highly attened by rotation. If so, the explosion would naturally blow out preferentially along the polar axis,perhaps even jetting the ejecta. A lthough such a m odelm ay be plausible,it is not yet welldeveloped, certainly not universally accepted. If supernovae explode aspherically it is im printed upon the ejecta and has additionalsignatures such as signi cant gravitationalradiation (M onchm eyeretal.1991),natalkicksto nascentneutron stars(Burrow sand H ayes1996; W oosley 1987),m ixing ofiron-peak and r-processnucleosynthetic products,generation of pulsar m agnetic elds and perhaps jetting ofthe debris. d. T he explosion M echanism ? In spite of im pressive theoretical and num erical e ortsduring the pastten years,we stilldo notknow how type IIsupernovae explode and convert 1% oftheirgravitationalenergy release into kinetic energy ofdebris. Since the neutrino observations ofSN 1987A provided strong support forthe basic picture ofSN eII it is w idely believed that neutrinos coupled w ith convection transport su cient energy from the core to the m antle to blow it o . Because the observed kinetic energy in SN eII isso steady,m any investigators have hoped (and som e stilldo)thatim provem ents in the inputm icroscopic and m acroscopic physics in one-dim ensional(1-D )sphericalsym m etric calculationsw illlead to the solution. T he im provem ents in m icrophysics included the use ofim proved neutrino opacitiesathigh densities,the inclusion ofthe neutrino annihilation ! e+ e m echanism (G oodm an,D arand N ussinov 1987)and neutrino bream strahlung nn ! nn + (Suzuki 1993) in energy transport and the use of im proved equation ofstate at high densities. T he im portant im provem ents in m acrophysics and num erics included the use ofim proved progenitor structure,the inclusion ofconvection,the use of im proved neutrino transportalgorithm (m ulti-group, ux lim ited,fulltransport,di usion) and the inclusion of generalrelativistic e ects. O ther authors believe that the correct explosion m echanism can only be dem onstrated through m ultidim ension (2-D or a full 3-D ) calculations. In fact, the recent V LB radio observations and H ST observations of SN 1987A suggest that SN 1987A and perhaps m any SN eII explode aspherically and perhaps w ith jetting oftheir debris. T he natalkicks to new born neutron stars m ay also be a result ofasphericalexplosion. N um ericalcalculations ofsuch asphericalexplosions require m ulti-D codes. A lthough such m ulti-D codes have been developed and applied to study core collapse SN eII (e.g.,H erant et al. 1994;Burrow s,H ayes and Fryxell1995; M ezzacappa etal.1996;Janka and M uller1996),they stilldo notinclude allthe relevant physics:N oneisa full3-D ,noneincorporatesgeneralrelativity,nonehascorrectly treated allknow n neutrino processes in the core,none adequately handles transportin either the angular or radialdirection.

N eutrinos alone, in 1-D codes, do not seem to be able to revive the stalled shock. A variety ofhydrodynam ic instabilities have been invoked by theorists over the years to help explode supernovae. N eutrino driven instabilities between the neutrinospheres and the stalled shock are generic feature of core-collapse supernovae (Bethe 1990; H erant, Benz and C olgate 1992;H erantetal.1994;Burrow s,H ayes,and Fryxell1995;Janka and M uller1996;M ezzacappa et al1996).T hough it is generally accepted that pre-explosion cores ofm assive stars are hydrodynam ically unstable,the role ofconvective m otions in driving supernova explosions is not yet clear. C ore overturn driven by negative entropy and lepton gradientsduring the deleptonization and cooling of the protoneutron star m ay boost the driving neutrino lum inosities (Burrow s 1987;K eil,Janka and M uller 1996). O nly after a fullneutrino transport w ill be incorporated in m ulti-dim ensionalcalculations it w illbecom e clear w hether neutrinos can drive supernova explosions. e. Is T here a N eutron Star? T he 12 seconds neutrino burst from SN 1987A suggests theform ation ofa neutron star,atleasttransiently. Besidesthe neutrino burstthere isno other evidence that the SN 1987A rem nant contains a centralneutron star. Independent observationshave failed to con rm reported observation (M iddleditch)ofa 2.1 m soptical pulsar. A t present, the em ission observed from SN 1987A is com pletely accounted for by radioactive energy sources (m ainly 44T iw ith half life of 78 years) in the debris, so the energy input from a pulsar or any other source m ust be sm all. To have escaped detection, the centralcom pact object m ust have a lum inosity less than a few hundred tim es that of the sun and far less than that of the 943 years old pulsar in the C rab N ebula. H owever,the average colum n density ofthe expanding shell(assum ing spherical sym m etry) is 20M =4 R 2 0:5 g cm 2 for an average expansion velocity around 2500km s 1. T he debris now is quite cold throughout (a few hundred K only) and probably blocks the light from the centralsource for decades,or longer ifthe debris are clum ped and the source happens to lie behind a cloud. H owever,the expanding shellis not opaque to energetic gam m a rays. f.g. A C entralB lack H ole ? D ouble B ang? Itwassuggested thatlatetim eaccretion (Brow n, Bruenn and W heeler 1992) m ay have induced collapse of the nascent neutron star into a black hole. Such a scenario m ay lead to two neutrino bursts (\double bang") wellseparated in tim e,and m ay explain the M ontBlanc early signal(A glietta etal1987). But the M ont blanc early signalim plies unrealisticly large binding energy release in the rst bang w hich has not been detected by the K am iokande,IM B and Baksan detectors. T he ngerprint ofa centralstellar black hole are di cult to detect. T here is a chance to

\detect" the centralblack hole only ifit is orbitted by a close com panion w hich survived the explosion. h. N atalK ick and G ravitationalR adiation Pulsarlocations(e.g.,Taylorand C ordes 1993)and properm otion data (e.g.,H arrison,Lyne and A nderson 1993)im ply thatradio pulsars are a high-speed population. M ean three-dim ensionalgalactic speeds of450 90 km s 1 have been estim ated (Lyne and Lorim er 1994),w ith m easured transverse speeds ofindividualpulsars reaching up to 1500 km s 1. Im pulsive m ass loss in a spherical supernova explosion that occurs in a binary can im part to the nascent neutron star a substantialkick (G ott,G unn,and O striker 1970). H owever,theoreticalstudies ofbinary evolution through the supernova phase have di culty reproducing the observed velocity distributions (Fryer,Burrow s and Benz 1997).T his im plies thatneutron starsreceive an extra kick atbirth.A nisotropicneutrino radiation (C hugai1984;W oosley 1987)havebeen invoked to accelerate neutron stars. A 1% dipole asym m etry in the neutrino radiation of a neutron star’sbinding energy issu cientto accelerate itto 300 km s 1.Jetting ofthe ejecta along the polar axis and im balance between the m om enta ofthe two opposite jets can also be the origin ofthe natalkick. A sphericalexplosion,perhapseven jetting ofthe debris are already evident in the high resolution V LB radio im ages (G aensler et al1997) and in the recent H ST im ages (Pun et al1997). M erger scenarios and non axisym m etric collapse can lead to very signi cant gravitationalwave em ission at typicalfrequencies of c=2 R a few kilo H ertz. H owever a reliable estim ate ofthe gravitationalwave signal (wave form and light curve) probably w illhave to wait untilthe explosion m echanism becom es clearer. Perhaps the C altech-M IT Laser Interferom etric G ravitational W ave O bservatory (LIG O ) w ill detect gravitational wave signals from SN eII, before reliable theoreticalestim ates becom e possible ?

4

Lim its O n P article P roperties and Interactions

A strophysics and cosm ology provide test grounds for the standard m odel of particle physics and extensions ofthe m odelover distances,tim e scales and other conditions not accessible to laboratory experim ents. Lim its on neutrino propertiesfrom SN eII(lifetim e, m ixing,decay m odes m agnetic m om ents) were derived long before SN 1987A .SN 1987A provided a new test ground w hich attracted the attention ofm any m ore physicists. N ew lim itswere derived notonly forstandard particlesand m inim alextensionsofthe standard m odel,but for allkinds ofhypotheticalparticles and interactions. H ere,I w illlim it m y sum m ary to standard particles and wellm otivated m inim alextensions of the standard

m odelofparticle physics. I w illquote only lim its w hich were derived from observations and generalconsiderationsand eitherdo notdepend on ,orare insensitive to the detailed m odeling ofSN eII.Iw illfocus m ainly on im provem ents since 1988 ofthe lim its on neutrino propertiesw hich were included in Table 1 ofm y talk atLa T huile one yearafterSN 1987A (D ar 1988). Table I,to m y judgem ent,sum m arizes the m ost im portant lim its. M ass Lim its O n Standard N eutrinos. T he traveltim e of relativistic neutrinos of m ass m and energy E from a distance D to Earth is given approxim ately by t= (D =c)[1 + (1=2)(m c2=E )2]:

(1)

T he observed energies and the dispersion in arrivaltim esofthe neutrinos from SN 1987A were used to estim ate upper lim its on the m ass ofthe e. A lthough the lim its are m odel dependent,they are not very sensitive to the m odels. A lim it ofabout m e < 15 eV was obtained both from sim ple m odels and from m ore \sophisticated" m odels (w hich are not necessarily m ore reliable). T he particle data group (Barnett et al. 1966) do not quote a laboratory lim it since \unexplained e ects have resulted in signi cantly negative m 2e in the new precise tritium beta decay experim ents". T he cosm ologicalbound (e.g.,C ow sik 1977 ), m < 94 h 2 eV ,yields m < 15 eV for stable neutrinos,for the currently best m easured values of the cosm ologicalparam eters, 0:3 and h 0:7. T his lim it on m e m ay be im proved by one order of m agnitude by the m ore sensitive detectors like Superkam iokande and SN O if they w ill detect a therm al neutrino burst from a m ore distant SN eIIor a neutronization burst from a galactic SN eII(e.g.,D ar 1988). Ifneutrinos are D irac particles w ith nonzero m ass they can ip their helicity in collisions w ith neutrons in the protoneutron star. R ight (left) handed neutrinos (antineutrinos)w hich have no standard electroweak interactionsescape im m ediately and coolthe hotprotoneutron star(PN S).Since the neutrino helicity ip cross section isproportional to m 2 (standard electroweak Z 0 exchange yields flip G2F m 2 = ),the observed cooling rate ofSN 1987A was used to obtain the lim it m < 15 keV for D irac neutrinos (R a elt and Seckel1988;G ri olsand M asso 1990;D ar1990).T hislim itism uch weaker than the cosm ologicallim it,butapplies also to unstable neutrinos w ith > R P N S = c> 10 7s !It is m uch stronger than the laboratory lim its,m < 170 keV and m < 24 M eV . T he Supernova m ass lim its on m uon and tau neutrinos m ay be im proved by about two to three orders of m agnitude by future m easurem ents of the tim e structure of the neutrino burstsfrom galactic SN eIIw ith neutrino telescopes like SN O ,Superkam iokande and H ELLA Z w hich are sensitive to allneutrino avors (e.g.,D ar 1988). Finally, m uch stronger neutrino m ass lim its can be obtained from SN eII neutrino bursts ifneutrinos are m ixed and oscillate.

N eutrino O scillations. T he num berofeventsw hich were detected by K am iokande and IM B,their angular distribution,and their energy distribution and the m axim albinding energy release in gravitational core collapse suggest that they are m ostly ep ! ne+ events. H owever,ifthe e were obtained by neutrino oscillation from ’sor ’sinto e’s in vacuum w ith a large m ixing angle,their tem perature should have been m uch higher, T 7 M eV . T he observed tem perature T 4 M eV ofthe K am iokande and IM B 19 eventspractically excludesthelargeanglevacuum oscillation solution to thesolarneutrino problem . N eutrino Lifetim e. Ifthe neutrinosw hich were detected arethose thatwere em itted by SN 1987A (no m ixings,no \conspiracy schem es"),then their m ean life tim e m ust satisfy 12 ( e)> D =c 5 10 s,since they arrived w ith theexpected num ber(see,e.g.,Bahcall, D arand Piran 1987). A xion M ass. Various extensions of the standard m odelpredict the existence of light neutralpseudoscalars,like the axion proposed by Pecceiand Q uinn (PQ )in orderto solve the problem ofC P conservation in strong interactions. T he originalaxion associated w ith the breaking ofthe PQ sym m etry at the weak scale (fw ) is excluded experim entally,but notthe invisible axion ifthe breaking scale ism uch larger(fa fw m a fa).T he axion lifetim eisvery long buta strong m agnetic eld can enhance itsdecay via a ! v .Lim its on the m assofthe invisible axion were derived from laboratory experim ents,astrophysics and cosm ology. T he absence ofa ray signalfrom SN 1987A has lim ited its m ass to the narrow w indow 10 6eV ma 10 3eV (e.g.,R a elt 1990). W hat to E xpect? D etectors like Superkam iokande and SN O w illallow for the rst tim e good energy,tim e and avorspectroscopy ofboth the neutronization burstand the therm al burst from galactic SN eII. In particular, the early phase of core collapse that precedes SN eII is better understood than the explosion. N eutrinos from this phase are em itted m ainly due to e captures on free protons and iron group nuclei. U p to core densities of ’ 3 1011 g cm 3 (neutrino trapping density) these e escape freely from the overlying stellar m atter w ithout any interaction that changes their energy. T he total num berofneutrinosem itted from a 1.4 M stellarcore asitevolvesfrom a initialdensity 9 of 4 10 g cm 3 to a neutrino-trapping density of’ 3 1011g cm 3 is ’ 1056. T he duration ofthis e burst is a few m s. T he charge-current and neutral-current reactions, eD ! ppe and x D ! pn x ,respectively,on deuterium nucleiin SN O and the x e ! 0 x e scattering in them orem assiveSuperkam iokandewaterdetector,can beused to detect the neutronization burst from galactic SN eII,to identify its avor content and m easure the neutrino energies . T hese m ay yield new im portant inform ation on the physicaland

the nuclear con guration of the collapsing stellar core and on neutrino properties, in particular on neutrino m asses, avor m ixing and m atter oscillations (D ar 1988).

5

M ore To E xpect

T hestory ofSN 1987A isnotoveryet.Forastrophysicistsperhapsthem ostexciting future developm entsw illbe the collision ofthe debrisw ith the circum stellargasand ringsw hich w illshed m ore light on the nature ofthe explosion and on the history ofthe progenitor before its supernova phase,the em ergence ofa neutron star and the birth ofa pulsar: Future F irew orks. T he blast wave from SN 1987A w illstrike the inner ring som e six to ten years from now (C hevalier and D warkadas 1995;Borkow ski,Blondin and M cC ray 1997)and thering ispredicted to brighten by a factor 103 in allbandsoftheelectrom agnetic spectrum . Shock acceleration w illprobably begin to produce relativistic particles and ray em ission from the inner ring. Shining the P ast. W hen the blastwave w illcontinue to propogate into the interstellar m edium it w illlit m ore rings and shells w hich m ay have been ejected by the progenitor in its presupernova phase. N eutron Star E m ergence and P ulsar B irth. T hedebrisw ill rstbecom etransparent to -raysand X -rays. Ifthe hotneutron staristhere,itw illglow in therm alX -rays. Ifit has begun pulsed em ission over the w hole electrom agnetic spectrum and ifwe happen to lie w ithin the pulsar beam ing cones,we w illstart to see pulsed em ission ofradio waves, X -rays,and perhaps rays. It w illtake m uch longer (halfa century or m ore) before the debris w illbecom e transparent to opticalphotons.

6

C oncluding R em arks

Perhapsthem ostim portantconsequencesofSN 1987A arethebirth ofextrasolarneutrino astronom y,theconstruction ofgalacticand extragalacticneutrino telescopesand thepush to the construction ofgravitationalwave detectors. A llthese w illhelp solve som e ofthe m ost interesting puzzles in astronom y and test interactions and particle properties over physicaldom ains not accessible to laboratory experim ents. A cknow ledgem ent:T he authorwould like to thank M .G reco and G .Belletinifortheir generousity and for their friendship w hich has been extended to him over m any years.

Table I:Expected lim itson neutrino propertiesfrom nearby SN eIIcom pared w ith the corresponding lim its from terrestrialexperim ents,from SN 1987A and from cosm ology. P roperty M asses m e m m Lifetim e ( e) ( ) ( ) M ixing


Electric C harge q( e) q( ) q( ) M agnetic M om ent ( e) ( ) ( ) R adiative D ecay B 1 =m Flavors

TerrestrialE xp

SN 1987A

170 keV 24 M eV (A tm ospheric ’s) > 4 10 2 s > 4 10 2 s Excl uded R egion ( m 2 > 0:1eV 2 sin22 > 0:01

< 15 eV < 1 eV < 15 keV (ifD irac) < 100 eV < 15 keV (ifD irac) < 100 eV

< 10 13e < 10 6e < 10 2e

< 2 -

1:8 7:4 5:4

10 10 B 10 10 B 10 7 B

> 20 s/eV 3

> 5 -

1012 s

Large A ngle M ixing E xcluded

< 10 < 10 < 10

1017e

12 B 12 B 14 B

> 2 1016 s/eV (m > 20 eV ) 5

N earby SN eII (E xpected)

C osm ology

< 15 eV < 15 eV < 15 eV

> 1014 s > 1012 s > 1012 s

> 103 s > 103 s > 103 s

Sm allA ngles A lso E xcluded? < 1 < 2 < 2 < 10 < 10 < 10

10 18e 10 17e 10 17e 14 B 14 B 14 B

> 1017 s/eV (m > 20 eV ) 3

3

Fig.1: A n im age of the triple ring structure around the rem nant of SN 1987A taken in early 1997 by the W ide Field and Planetary C am era 2 ofthe H ubble Space Telescope.

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