Short Duration Gamma-Ray Bursts with Extended Emission from Proto-Magnetar Spin-Down

0.5 MB·English

by  B. D. Metzger

Mon.Not.R.Astron.Soc.000,1–6(????) Printed2February2008 (MNLATEXstylefilev2.2)
Short Duration Gamma-Ray Bursts with Extended
Emission from Proto-Magnetar Spin-Down
B. D. Metzger1,2⋆, E. Quataert1, T. A. Thompson3
1Astronomy Department and Theoretical Astrophysics Center,University of California, Berkeley, 601 Campbell Hall, Berkeley CA, 94720
2Department of Physics, University of California, Berkeley, Le Conte Hall, Berkeley, CA 94720
3Department of Astronomy and Centerfor Cosmology & Astro-Particle Physics, The Ohio State University,140 W. 18th Ave.,Columbus, OH, 43210
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Accepted .Received;inoriginalform
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0
2
ABSTRACT

n Evidence is growing for a class of gamma-ray bursts (GRBs) characterized by an ini-
a tial ∼ 0.1−1 s spike of hard radiation followed, after a ∼ 3−10 s lull in emission,
J by a softer period of extended emission lasting ∼ 10−100 s. In a few well-studied

8 cases,these“shortGRBswithextendedemission”shownoevidenceforabrightasso-

ciated supernova (SN). We propose that these events are produced by the formation
]
h and early evolution of a highly magnetized, rapidly rotating neutron star (a “proto-
p magnetar”) which is formed from the accretion-induced collapse (AIC) of a white
- dwarf(WD), the mergerandcollapseofa WD-WD binary,or,perhaps,the mergerof
o
a double neutron star binary. The initial emission spike is powered by accretion onto
r
t the proto-magnetarfrom a small disk that is formedduring the AIC or merger event.
s
The extended emission is produced by a relativistic wind that extracts the rotational
a
[ energy of the proto-magnetar on a timescale ∼10−100 s. The ∼10 s delay between
the prompt and extended emission is the time required for the newly-formed proto-

2 magnetar to cool sufficiently that the neutrino-heated wind from its surface becomes
v ultra-relativistic.Becauseaproto-magnetarejectslittleorno56Ni(<10−3M⊙),these
3
eventsshouldnotproduceabrightSN-liketransient.Wemodeltheextendedemission
3
from GRB060614 using spin-down calculations of a cooling proto-magnetar, finding
2
reasonableagreementwithobservationsforamagnetarwithaninitialrotationperiod
1
. of ∼ 1 ms and a surface dipole field of ∼ 3×1015 G. If GRBs are indeed produced
2
by AIC or WD-WD mergers, they should occur within a mixture of both early and
1
late-type galaxiesand should not produce strong gravitationalwaveemission. An ad-
7
ditional consequence of our model is the existence of X-ray flashes unaccompanied by
0
a bright SN and not associated with massive star formation.
:
v
i Key words: Stars: neutron; stars: winds, outflows; gamma rays: bursts; MHD
X
r
a
1 INTRODUCTION brightsupernova(SN)despiteresidinginsideastar-forming
Swift and HETE-2 have demonstrated that long and short region(Ofeketal.2007;Fynboetal.2007),suggestingthat
duration gamma-ray bursts (GRBs) originate from distinct it may result from a “failed-SN” collapsar (Woosley 1993).
stellar progenitors. Long GRBs track ongoing star forma- AnotherparticularlystrikingexampleisGRB060614,which,
tion and result from the death of massive stars (Stanek et although officially classified as a long-duration burst based
al.2003).Ontheotherhand,shortGRBshavenowbeenlo- solely on its 102 s T90 duration, more closely resembles a
calizedtobothearlyandlate-typehostgalaxiesofmoderate standardshortGRBinotherways.ThisSwift burstshowed
redshift(Barthelmyetal.2005;Bergeretal.2005;Bloomet no energy-dependent time lag (Gehrels et al. 2006) and,
al. 2006), indicating a more evolved progenitor population. like GRB060615, no SN was detected down to an optical
Even among the classes of “long” and “short” GRBs, brightness∼100timesfainterthanSN1998bw(Gal-Yamet
however,diversity isemerging. Oneexampleis GRB060505 al. 2007). Additional clues to thenatureof GRB060614 are
detected by the Burst Alert Telescope (BAT) on-board revealed through the evolution of its prompt emission. The
Swift. This long duration burst showed no evidence for a BATlightcurvebeginswithaninitialspikeofhardemission
(lasting∼5s)whichisfollowed,afteralullinemission(last-
ing∼5s), by asofter ‘hump’of extendedemission (lasting
⋆ E-mail:[email protected]
(cid:13)c ????RAS
2 B.D. Metzger, E. Quataert, T.A. Thompson
∼100 s). This is followed by a remarkably standard X-ray, which is formed from the accretion-induced collapse (AIC)
optical, and ultra-violet afterglow (Mangano et al. 2007). ofawhitedwarf(WD)(Nomoto&Kondo1991),themerger
The hybrid long/short properties of GRB060614 led andcollapseofaWD-WDbinary(Kingetal.2001),or,per-
Gal-Yam et al. (2006) and Gehrels et al. (2006) to pro- haps,themergerofadoubleneutron(NS)starbinary(Gao
pose that it forms the prototype for a new class of GRBs, &Fan2006).Theinitialspikeofemission ispoweredbyac-
which we call “short GRBs with extended emission” (or cretion onto the proto-magnetar from a small disk formed
SGRBEEs) (see, however, Fynbo et al. 2006). Roughly a during the AIC or merger (§2). The EE is produced by a
quarter of the short bursts detected by Swift (including relativistic wind that extracts the rotational energy of the
GRBs 050709 and 050724) show evidence for high energy magnetaronatimescale∼10−100 s(§3),apicturesimilar
extended emission (EE) distinct from the standard after- to that originally proposed by Usov (1992). The ∼3−10 s
glow andlate-timeflares;thissuggests thatSGRBEEsmay delay between the prompt and EE is the time required for
actually befairly common. Indeed,Norris&Bonnell (2006; the proto-magnetar to cool sufficiently that the neutrino-
NB06)foundahandfulofshortGRBsintheBATSEcatalog heatedwindfromitssurfacebecomesultra-relativistic.In§4
qualitativelysimilar toGRB060614. AlthoughNB06’ssam- wesummarizeourresultsand thepredictionsof ourmodel.
plerepresentsonly ∼1% of theBATSEshort bursts,asoft For concreteness we focus our discussion on AIC and WD-
tail of EE would generally not have been detectable. NB06 WD merger channels of isolated magnetar birth.
also find that the dynamic range in the ratio of prompt to 2 ACCRETION PHASE
extendedflux(andfluence)ofSGRBEEsappearstobevery IneitherAIC(Nomoto&Kondo1991)oraWD-WDmerger
large, ∼104.Thislarge burst-to-burstvariation in therela- (e.g., Yoon et al. 2007), the WD (or merged WD binary)
tiveenergyreleasedduringthepromptandextendedphases will be rapidly rotating prior to collapse and must eject a
suggests that these components are physically decoupled. sizablefractionofitsmassintoadiskduringthecollapsein
One explanation for EE from an otherwise short GRB ordertoconserveangularmomentum.Indeed,the2DMHD
is the interaction of the relativistic outflow with the cir- AIC calculations performed by Dessart et al. (2007; D07)
cumburstenvironment.However,whentheEEissufficiently show that a quasi-Keplerian ∼ 0.1−0.5M⊙ accretion disk
brighttobeaccuratelysampled,itstimeevolutionishighly forms around the newly-formed proto-neutron star (PNS),
variableandcannotbesmoothlyextrapolatedbackfromthe extending from the PNS surface at RNS ∼ 30 km to large
onset of the X-ray afterglow (Nakar 2007). A multi-peaked radii (with a half-mass radius of a few RNS).
lightcurveisalsodifficulttoproducefromtheshockheating We propose that the prompt emission in SGRBEEs is
of a binary companion (MacFadyen et al. 2005). Based on powered by the accretion of this disk onto the PNS. This
itssimilaritytopromptemission,theEEinSGRBEEsmost scenarioisnotunlikemostotherCOMmodelswiththeim-
likely results from late-time central engine activity. portant exception that the accreting object is a NS rather
A popular model for the central engine of short GRBs than aBH. Thecharacteristic timescale for accretion is the
is accretion onto a black hole (BH) formed from a compact viscous timescale tvisc =R2/αΩKH2, given by
object merger (COM) (Paczynski 1986). SGRBEEs pose a
challenge to COM scenarios because their long durations tvisc≈1s M −1/2 0.1 R0 3/2 H/R0 −2, (1)
and two-component nature are difficult to produce in mod- (cid:18)M⊙(cid:19) (cid:16) α (cid:17)(cid:16)4RNS(cid:17) (cid:18) 0.2 (cid:19)
els powered purely by accretion. The accretion timescale of
whereαistheviscosityparameterandM isthePNSmass;
tchome pcoamrapbalecttoditshkepdroudrautcieodnforfomtheaimnietriaglerspeivkeen,tc,aanlnthootuegxh- R0, H, and ΩK ≡ (GM/R03)1/2 are the disk’s radius, scale
height, and Keplerian rotation rate, respectively. For H ≈
plain the long duration of the EE, especially in cases when
0.2R0, as expected for a neutrino-cooled disk accreting at
the latter produces the bulk of the observed fluence. For M˙ ∼ 0.1−1M⊙ s−1 (Chen & Beloborodov 2007), tvisc ∼
BH-NS mergers, the fall-back of matter ejected into highly
0.1−1 s, comparable to theduration of theprompt spike.
eccentricorbitsduringtheNS’stidaldisruptionmaybesuf-
At early times, the accretion ram pressure PR ≃
ficient to power the EE (Rosswog 2007), but whether the ρv2/2 ≈M˙v /8πR2 (where v =(GM/R)1/2) exceeds the
regular delay between the prompt and EE phases, and the ff ff ff
magnetic pressure PM =B2/8π at the PNS surface:
largevariationinthefluxofeach,canbereproducedremains
to beTdheeteNrmSinkeicdks(sreeeq,uhiorwedevteor,pFraobdeurceetaal.co2m00p6a)c.t binary PPMR ≈10(cid:18)0.1MM⊙˙ s−1(cid:19)(cid:18)MM⊙(cid:19)1/2(cid:16)1027φGBcm2(cid:17)−2(cid:16)3R0NkSm(cid:17)3/2.(2)
and the potentially long delay until merger imply that a
significant fraction of COMs should have large offsets from HereφB =1027(B(RNS)/1015G)(RNS/10km)2 Gcm2 isthe
theirhostgalaxies. Althoughtheoffsetdistributionofshort NS’s conserved magnetic flux. Thus, although we postulate
GRBsasawholeappearsmarginallyconsistentwithcurrent that the NS possesses a surface field strength ∼ 1015 G
COM population synthesismodels (Belczynski et al. 2006), once contracting to its final radius RNS ∼ 10 km, the field
well-localized SGRBEEs thus far appear exclusively inside shouldnotsignificantlyaltertheearly-timedynamicsofthe
ornearthestarlight of theirhost galaxies; indeed,theirav- accretion-powered phase (Ghosh & Lamb 1978).
erage offset from host center is only ∼ 2.5 kpc (Troja et The total energy released when a ∼ 0.1−0.5M⊙ disk
al.2007).ThehighincidenceofSGRBEEswithdetectedop- accretes onto a NS (∼ 1052 −1053 ergs) is more than suf-
tical afterglows (∼90%) also suggests that these events re- ficient to explain the isotropic γ-ray energy of the prompt
sideinsidethediskoftheirhostgalaxies(Trojaetal.2007). spike of GRB060614 (≃ 1.8×1050 ergs). However, as we
InthisLetter,weproposethatSGRBEEsareproduced discussfurtherin§3,amajorobstacletodrivingarelativis-
by the formation and early evolution of a strongly magne- tic wind from the vicinity of a newly-formed PNS is the
tized, rapidly rotating neutron star (a “proto-magnetar”) baryon-rich wind from the hot PNS’s surface. Because the
(cid:13)c ????RAS,MNRAS000,1–6
Extended Emission from Proto-Magnetar Spin-Down 3
mass loss from a rapidly rotating, highly magnetized PNS and its already substantial neutrino-driven mass-loss rate
is augmented by centrifugal effects (Thompson et al. 2004; is enhanced by centrifugal slinging. However, as the PNS
hereafterTCQ04),D07arguethatanearly-timerelativistic coolsandspinsdown,Figure1showsthatσLC risesrapidly,
outflow from the PNS is unlikely. Although we agree with exceeding∼10byt∼3−10s.Becauseanultra-relativistic
D07’s conclusion for moderately low latitudes, the centrifu- outflow is necessary to produce nonthermal GRB emission,
galenhancementofmasslossalongtherotation axisisneg- the∼3−10stimescalerequiredforthewindtoreachlarge
ligible;hence,thetotalmasslosspersolidanglenearthero- σLCcorrespondstothedelaybetweentheaccretion-powered
tation axisis approximately given by itsnon-rotating value promptspikeandthespin-down-poweredEEinourmodel.
of MΩ ∼ 10−5M⊙ str−1 (Thompson et al. 2001; hereafter 3.1 Extended Emission Light Curve Model
T01). Thus, if the energy deposited per solid angle above In an attempt to directly connect central engine physics
thepoleexceedsEΩ ∼1051 ergs str−1 theLorentzfactor of to observed GRB properties, we have modeled the emis-
the outflow may reach Γ ∼ EΩ/MΩc2 ∼ 100, sufficient to sionproducedbyaspinning-downproto-magnetarusingthe
overcome compactness constraints which can be placed on wind evolution calculations (Fig. 1) and an internal shock
short GRBs (Nakar2007). emission model. Byinvokingstrongshocks,weassumethat
Onepossibilityforeffectivelybaryon-freeenergydeposi- the Poynting-flux of the wind is efficiently converted into
tionisneutrino-antineutrinoannihilationalongtherotation kinetic energy somewhere between the light cylinder radius
axis.Forinstance,Setiawanetal.(2006)findthat∼2×1050 (∼ 107 cm) and the internal shock radii (∼ 1013 − 1015
ergs is released by annihilations from a ∼ 0.1M⊙ disk ac- cm). This assumption is motivated by observations of the
cretingat M˙ ∼0.3M⊙ s−1, aswould beexpected following CrabNebulaandotherpulsarwindnebulae,wheredetailed
AIC.AnMHDjetisperhapsamorepromisingsourceofthe modelingrequiresefficientconversionofmagnetictokinetic
relativisticmaterialthatproducesthepromptemission.Al- energy (Kennel & Coroniti 1984). Although we do not ex-
thoughjets from NSX-raybinaries arein general less pow- clude the possibility that the wind remains Poynting-flux
erful than their BH counterparts (Migliari & Fender 2006), dominatedatlargeradii,weleavetheanalogous lightcurve
theNSX-raybinaryCircinusX-1producesoneofthemost calculation in a magnetic dissipation model to future work.
relativistic microquasar jets known (Fenderet al. 2004). In the internal shock model, a GRB’s emission is pow-
3 SPIN-DOWN PHASE ered by “dissipation” (i.e., electron acceleration and radia-
Whether produced by the core collapse of a massive star tion) of therelative kineticenergy between distinct compo-
or the AIC of a WD, a PNS must radiate its gravitational nents of a relativistic wind. Since we do not have a quanti-
binding energy via optically-thick neutrino emission during tative model for the (potentially stochastic) processes that
thefirsttKH ∼40sofitslife(Burrows&Lattimer1987).A settheshorttimescalevariabilityinaproto-magnetarwind,
smallfractionofthisneutrinofluxisreabsorbedbybaryons we discretize the relativistic outflow into N shells released
inthePNS’satmosphere,drivingawindfromitssurface.In at constant intervals dt in time. A shell released at time t
thepresenceofasufficientlystrongmagneticfieldandrapid is given Lorentz factor Γ=σLC(t), energy E =E˙(t)dt, and
rotation,magneticstressestapintothePNS’srotationalen- mass M = E/Γc2, where σLC(t) and E˙(t) are taken from
ergy, enhancing the energy-loss in the wind (TCQ04). The Figure1.The“shellaveraged”lightcurvethatwepresentin
proto-magnetarisunlikelytohaveasignificanteffectonthe Figure2istakeninthelimitthatN →∞anddt→0andis
accretion-powered phase for t ∼< tvisc ∼ 1 s (see eq. [2]). insensitive to the shell discretization prescription adopted.
Onsomewhatlongertimescales,however,thediskmassand We begin releasing shells when σLC > 10 because the 2D
accretion rate will decrease, and the PNS will be spun up MHD simulations of Bucciantini et al. (2006) show that
through accretion and byitsKelvin-Helmholtzcontraction. above σLC ∼ 10 the outflow transitions from being hoop-
Thus, as M˙ decreases from its peak value, the disk will be stresscollimated along therotation axistoexpandingmore
cleared away and the proto-magnetar wind will expand rel- isotropically;hence,wedonotexpectthematerialejectedat
atively freely into space. σLC∼>10tointeractwithmuchofthematerialejectedwhen
Inapreviouswork,wecalculatedthepropertiesofPNS σLC ∼< 10. We stop releasing shells when neutrino heating
winds with magnetic fields and rotation during the first effectively ceases at t=tKH because we do not have a reli-
tKH ∼ 40 s after core bounce (Metzger et al. 2007; M07a). able model for σLC(t) after this point. This is a reasonable
Theimportanceofthemagneticfieldinacceleratingawind approximationifE˙ orthedissipationefficiencydropssignif-
is quantified by the magnetization σ ≡ B2/4πρc2 evalu- icantly once the outflow transitions from a modest-σ wind
ated at the light cylinder radius RL ≡ c/Ω, where Ω is the to a very high-σ, pulsar-like wind at t= tKH (as suggested
PNS’s rotation rate. If the magnetic energy is fully con- by,e.g., Thompson 1994 and TCQ04).
vertedintothekineticenergyofbulkmotion,eitherdirectly Upon release, each shell propagates forward in radius
or through thermalization and subsequent thermal or mag- with constant velocity until it collides with another shell.
netic pressure-driven expansion (e.g., Drenkhahn & Spruit From the properties of the collision, we calculate (1) the
2002), then at sufficiently large radii Γ∼σ(RL)≡σLC (for “thermal” energy released by dissipation of the shells’ rela-
σLC >1). Hence, for a PNS to produce an ultra-relativistic tivekineticenergy,(2)theobservedspikeofradiation(using
outflow, σLC must be ≫1. thetechniquesummarizedin§2ofGenetetal.2007),and(3)
Figure 1 shows our calculation of σLC and the wind thefinalmassandenergyofthecompositeshell,whichthen
energy loss rate E˙ as a function of time t after core bounce continues to propagate forward. We assume that a fraction
for a PNS with initial rotation period P0 = 1 ms for three ǫeoftheenergyreleasedbyeachcollisiongoesintorelativis-
surfacedipolefieldstrengths:B0 =1015 G,3×1015 G,and ticelectrons,whichradiatetheirenergythroughsynchrotron
1016 G. In all models, the wind is nonrelativistic (σLC <1) emission. Efficient synchrotron cooling is justified if even a
for t < 1 − 10 s because at early times the PNS is hot modest fraction of the magnetic flux at the light cylinder
∼
(cid:13)c ????RAS,MNRAS000,1–6
4 B.D. Metzger, E. Quataert, T.A. Thompson
103 103
1.4
σ
LC
102 102 -1s) 1.2
g
er
σLC 110 . 110.51-1E (10 erg s) 49c Luminosity (10 001...680
E metri 0.4
10-1 10-1 olo
B
0.2
10-2 10-2 0.0
1 10 10 100
Time Since Core Bounce (s) Observer Time (s)
Figure 1. Magnetization at the light cylinder σLC (Left Axis) Figure2.Luminosityofinternalshockemissionfromtheproto-
andenergylossrateE˙ (RightAxis)asafunctionoftimesincecore magnetarwindsinFigure1;electronaccelerationefficiency ǫe=
bounce fora proto-magnetar with initialrotation period P0 =1 0.5is assumed. Note the lack of emissionat earlytimes because
ms and three surfacedipolemagnetic field strengths: B0 =1015 the outflow is non-relativistic. The gradual onset of the emis-
G(dashedline),3×1015 G(dottedline),and1016 G(solidline). sion once σLC > 10 is due to the large Thomson optical depth,
whichdecreases astheoutflow expands.Thelate-timedeclinein
emission is the onset of curvature emission from the last shock,
producedbytheshellreleasedattKH=40s.Thelate-timeBAT
light curve from GRB060614, shown with a light solid line and
scaled to the physical isotropic luminosity, is reproduced in a
time-averagedsensebytheB0=3×1015 Gmodel.
is preserved to large radii. Thomson scattering of the non-
4 DISCUSSION
thermal radiation is taken into account, but photospheric
emission is not calculated. ShortGRBswithextendedemissionchallengetheparadigm
Figure 2 shows our calculation of the EE light curve thatshortGRBsresultexclusivelyfromCOMs.Thecentral
for the wind solutions given in Figure 1. We find that the engineinthesesystemsmayinsteadbeanewly-formedmag-
efficiency for converting the relative kinetic energy of the netar. The timeline of ourmodel is summarized as follows:
outflowtothermalized energyis∼10−20%.Providedthat
• AIC or WD-WD merger produces a proto-magnetar
ǫ >0.1,theseefficienciesareconsistentwiththosetypically
inef∼erredforshortGRBs(e.g.,Nakar2007). Proto-magnetar and a disk of mass ∼0.1M⊙ (t∼tdyn ∼100 ms)
• Disk accretes onto the proto-magnetar, generating the
winds possess a significant reservoir of “free energy” and
achievehighefficiencybecauseΓ(t)increasesmonotonically, prompt emission spike(t∼tvisc ∼0.1−1 s; see eq. [1])
• Free proto-magnetar wind transitions from non-
allowing faster material ejected at later times to catch up
relativistic to ultra-relativistic (t∼3−10 s; see Fig. 1)
with theslower material ejected earlier.
• Proto-magnetar spinsdown,generatingX-rayemission
To first order, our simplified model produces light
on observed longer timescale (t∼10−100 s; see Fig. 2)
curvessimilartotheEEobservedfromSGRBEEs.Thepeak
fluxislargerformorerapidlyrotating,stronglymagnetized A model similar to the one described here was proposed
PNSsandthetimetopeakfluxissmaller.InFigure2wealso by Gao & Fan (2006); in their model, late-time flares from
showthelate-timeBATlightcurvefrom GRB060614 (from short GRBs are powered by dipole spin-down of a super-
Butler & Kocevski 2007) for comparison with our models. massive, transiently-stable magnetar formed by a NS-NS
We find reasonable agreement between the data and the merger.However,currentevidencesuggeststhat SGRBEEs
modelwithB0 =3×1015 G,suggestingthattheprogenitor form a distinct population with only modest offsets from
of GRB060614 possesses a surface field strength somewhat their host galaxies (Troja et al. 2007). If transiently-stable
larger than those of Galactic magnetars. If synchrotron in- magnetarsfrom NS-NSmergers indeedproducemost SGR-
ternal shock emission is indeed the correct model for the BEEs,anequalnumberwouldbeexpectedwithlargeoffsets.
radiation from a proto-magnetar wind, the softening of the A more promising channel of isolated magnetar birth
EE can also bequalitatively understood. Dueto themono- may be the AIC of a WD or the merger and collapse of a
tonic rise of Γ(t), the Lorentz factor of the aggregate shell WD-WDbinary.Therateoftheseeventsisdifficulttocon-
increases with time; however,thefield strength in thewind strain directly because the Ni mass synthesized in a PNS
B ∼ B(RL)(r/RL)−1 declines as the internal shock radius wind is less than ∼10−3M⊙ (Metzger et al. 2007b; M07b)
increases. In our model, these effects combine to decrease and is therefore unlikely to produce a bright optical tran-
the synchrotron peak energy Epeak ∝ ΓB by a factor of sient.Thereis,however,indirectevidencethatisolatedmag-
∼ 10 during the period of observable emission. This pre- netar birth occurs in nature. The rapidly rotating, highly
dicteddegreeofspectralsofteningisstrongerthanthefactor magnetic WD RE J0317-853 has a mass M =1.35M⊙ and
of ∼2 decrease in Epeak inferred for GRB060614 by Zhang was likely produced from a WD-WD merger; if RE J0317-
et al. (2007); indeed, the observed constancy of E is a
peak
problem generic to most internal shock models. (cid:13)c ????RAS,MNRAS000,1–6
Extended Emission from Proto-Magnetar Spin-Down 5
853’s progenitor binary had been slightly more massive, it WD-WD merger prior to collapse (e.g., Yoon et al. 2007)
wouldprobablyhavecollapsedtoarapidlyrotatingmagne- may accrete onto the NS at later times, powering a bipolar
tar(Kingetal.2001).Isolated NSbirthviaAICisalsoone outflow similar to that produced during the prompt accre-
of the only Galactic r-process sites consistent with current tionepisode;inthiscase,thedelayuntilEEreflectstheac-
observations of elemental abundances in metal-poor halo cretion timescale at the WD radius (∼109 cm), which D07
stars (Qian & Wasserburg 2007). Although unmagnetized estimate is tvisc ∼ 100 s for α ∼ 0.1. Late-time accretion
PNSwindsfailtoproducesuccessfulr-process(T01),proto- and spin-down powered EE can be distinguished based on
magnetarwindsmaybesufficientlyneutron-richtoproduce thepresenceorabsence,respectively,ofajet breakand the
∼ 0.1M⊙ in r-process elements (D07; M07b). For AIC or observed ratio of SGRBEEs to off-axis, purely-EE XRFs.
WD-WD mergers to produce the entire Galactic r-process Assuming that jets from the prompt and delayed accretion
yieldrequiresarate∼10−5−10−6 yr−1,comparabletothe episodesaresimilarlycollimated,SGRBEEswithaccretion-
observedlocalshortGRBrate(Nakar2007).Finally,Levan powered EE should not be visible off-axis; by contrast, if
et al. (2006) argue that the correlation found by Tanvir et theEEispoweredbymagnetarspin-downatleast asmany
al. (2005) between a subset of short GRBs and local large- off-axis XRFsare expected as standard SGRBEEs.
scalestructureisevidenceforachannelofisolatedmagnetar Finally,itisimportanttodistinguishtheobservablesig-
birth,if these burstsare produced by SGR-likeflares. natureofamagnetarproducedbyanAIC,WD-WDmerger,
AtheoryforSGRBEEsmustexplainthelargeburst-to- orNS-NSmergerfromthatproducedbythecorecollapseof
burstvariationintheratiooftheflux/fluenceoftheprompt amassivestar,whichmayinsteadproduceatraditionallong
and EE components (NB06). The angular momentum of duration GRB (e.g., M07a). The magnetic fields and rota-
AIC and WD-WD mergers may vary between events, re- tionratesofthemagnetarsproducedviathesechannelsmay
sulting in a wide distribution in both the properties of the differ,whichwouldmodifyE˙ andσofthewind(Fig.1)and
accretion disk formed (which influences the prompt emis- henceitsobservableproperties.Althoughisolatedmagnetar
sion) and the rotation rate of the proto-magnetar (which spin-down may becomparatively simple because theproto-
determines theEE). Event-to-eventvariability may also re- magnetar wind expands relatively freely into space, a free
sultfromtheviewingangleθ oftheobserverwithrespect magnetar wind isnearly isotropic and soits emission is rel-
obs
to the rotation axis. The spin-down power of the magnetar atively weak and difficult to detect. By contrast, the wind
variesasE˙ ∝sin2(θobs)forσLC ≫1;hence,thelightcurves from a magnetar produced via core collapse is collimated
inFigure2remainreasonablyaccurateformoderatelylarge intoabipolarjet bytheoverlyingstellar mantle(Uzdensky
θ ,butaviewerlookingdowntherotationaxis(θ ∼0o) &MacFadyen2006;Komissarov&Barkov2007;Bucciantini
obs obs
may observe little or no EE. Conversely, equatorial view- etal.2007)andtheobservedemissioncanbemuchbrighter
ers would observe EE but no prompt spike; therefore, this dueto thejet’s modest opening solid angle.
modelpredictsaclassoflongdurationX-rayflashes(XRFs)
not associated with very massive star formation or accom- ACKNOWLEDGMENTS
panied by a SN. Such an event may have already been ob-
We thank J. Bloom, N. Bucciantini, N. Butler, D. Perley,
served. GRB060428b is an XRF with a light curve similar
A. Spitkovsky, and E. Troja for helpful discussions and in-
totheEEfromGRB060614whichwaslocalizedinsideared
formation. EQ and BDM were supported bytheDavid and
galaxy at redshift z = 0.347. Assuming this galaxy is the
LucilePackardFoundationandaNASAGSRPFellowship.
host, GRB060428b released an isotropic energy ∼ 2×1050
ergs, comparable to EE of GRB060614, and showed no SN
REFERENCES
componentatonemonthdowntoanopticalbrightness∼20
times fainter than SN1998bw (Perley et al., in prep). Barthelmy, S.D., et al. 2005, Nature, 438, 994
Because the EE flow is symmetric in azimuth, equato- Belczynski, K., et al. 2006, ApJ, 648, 1110
rialviewersshouldnotobserveaclassic jet-break(although Berger, E., et al. 2005, Nature,438, 988
amoreshallowbreakispossible;Thompson2005).Further- Bloom, J. S., et al. 2006, ApJ,638, 354
more, although only Γ∼>10 material contributes to the EE Bucciantini, N., et al. 2006, MNRAS,368, 1717
in Figure 2, the magnetar releases ∼ 1052 ergs in earlier, Bucciantini, N., et al. 2007, ArXiv e-prints, 705,
mildly relativistic material (Γ ∼ 1−10; see Fig. 1) that arXiv:0705.1742
is hoop-stress collimated along the PNS rotation axis. This Burrows, A., & Lattimer, J. M. 1986, ApJ, 307, 178
material maybecomevisibleasaradiotransient asit slows Butler, N. R., & Kocevski, D. 2007, ApJ, 663, 407
downandbecomesnon-relativisticonatimescaleofmonths Chen, W.-X., & Beloborodov, A. M. 2007, ApJ,657, 383
to years. If AIC or WD-WD mergers indeed produce SGR- Dessart, L., et al. 2007, ApJ,669, 585 (D07)
BEEs, these events should be bound to both early and late Drenkhahn,G., & Spruit,H. C. 2002, A&A,391, 1141
typegalaxies;indeed,thewell-knownSGRBEEs050709and Faber, J. A.,et al. 2006, ApJL, 641, L93
050724 werelocalized toastarforming galaxyandanellip- Fender,R., et al. 2004, Nature, 427, 222
tical galaxy, respectively (Villasenor et al. 2005; Berger et Fynbo,J. P. U.,et al. 2006, Nature, 444, 1047
al. 2005). Unlike COMs, magnetar birth from AIC or WD- Gal-Yam, A., et al. 2006, Nature,444, 1053
WD mergers should not produce strong gravitational wave Gao, W.-H., & Fan, Y.-Z.2006, ChJAA,6, 513
emission, and because the magnetar will not collapse to a Gehrels, N., et al. 2006, Nature,444, 1044
BH,itsmagneticenergycouldpowerlate-timeX-rayflares. Genet, F., Daigne, F., & Mochkovitch, R. 2007, MNRAS,
Although we have concentrated on spin-down-powered 381, 732
EE, isolated NSbirth may producetheEE of SGRBEEsin Ghosh, P., & Lamb, F. K.1978, ApJL,223, L83
other ways. Specifically, the accretion disk produced by a Kennel, C. F., & Coroniti, F. V. 1984, ApJ, 283, 694
(cid:13)c ????RAS,MNRAS000,1–6
6 B.D. Metzger, E. Quataert, T.A. Thompson
King,A.R.,Pringle,J.E.,&Wickramasinghe,D.T.2001,
MNRAS,320, L45
Komissarov, S. S., & Barkov, M. V. 2007, MNRAS, 382,
1029
Levan, A.J., et al. 2006, MNRAS,368, L1
MacFadyen, A. I., Ramirez-Ruiz, E., & Zhang, W. 2005,
ArXivAstrophysics e-prints,arXiv:astro-ph/0510192
Mangano, V.,et al. 2007, A&A,470, 105
Metzger, B. D., Thompson, T. A., & Quataert, E. 2007,
ApJ,659, 561 (M07a)
Metzger, B. D., Thompson, T. A., & Quataert, E. 2007,
ArXive-prints, 708, arXiv:0708.3395 (M07b)
Migliari, S., & Fender,R. P. 2006, MNRAS,366, 79
Nakar, E. 2007, PhR,442, 166
Nomoto, K., & Kondo, Y.1991, ApJL, 367, L19
Norris, J. P., & Bonnell, J. T. 2006, ApJ, 643, 266
Ofek, E. O., et al. 2007, ApJ, 662, 1129
Paczynski, B. 1986, ApJL, 308, L43
Qian, Y.-Z., & Wasserburg, G. J. 2007, PhR,442, 237
Rosswog, S.2007, MNRAS,376, L48
Setiawan,S.,Ruffert,M.,&Janka,H.-T.2006,A&A,458,
553
Stanek, K.Z., et al. 2003, ApJL, 591, L17
Tanvir, N. R.,et al. 2005, Nature, 438, 991
Thompson, C. 1994, MNRAS,270, 480
Thompson,T.A.,Burrows,A.,&Meyer,B.S.2001,ApJ,
562, 887 (T01)
Thompson, T. A., Chang, P., & Quataert, E. 2004, ApJ,
611, 380 (TCQ04)
Thompson, T. A. 2005, Nuovo Cimento C Geophysics
SpacePhysics C, 28, 583
Troja,E.,etal.2007,ArXive-prints,711,arXiv:0711.3034
Usov, V.V. 1992, Nature, 357, 472
Uzdensky,D.A.,&MacFadyen,A.I.2006,ApJ,647,1192
Villasenor, J. S., et al. 2005, Nature, 437, 855
Woosley, S.E. 1993, ApJ, 405, 273
Yoon, S.-C., Podsiadlowski, P., & Rosswog, S. 2007, MN-
RAS,380, 933
Zhang, B., et al. 2007, ApJL,655, L25
(cid:13)c ????RAS,MNRAS000,1–6