Formation Scenario of Magnetars: The Puzzle of Isolation

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by  Nirvikar Prasad

Formation Scenario of Magnetars: The Puzzle of Isolation
Nirvikar Prasad
RamanResearchInstitute,Bangalore560080,INDIA
Abstract. Magnetars(SGRsandAXPs)areoneofthemostevolutionarypathsofaneutronstar.Theseobjectshaveanultra-
strongmagneticfieldB∼1015GattheirsurfaceandshowpersistentX-raypulsationsandtransientbursts.Tilldatethereare
8 14magnetarsknown:5SGRs(4confirmed,1candidate)and9AXPs(7confirmed,2candidates).Itisanopenpuzzlethatall
0 theseobjectsareisolatedandnonehavebeenfoundinbinaries.Wediscusstheformationscenariowhichcanleadtosucha
0 situation.
2
Keywords: magnetars,binaryevolution

n PACS: 97.60.Bw,97.60.Gb,97.60.Jd
a
J
INTRODUCTION Duetophotodisintegrationandneutrinolossestheshock
9
stalls a few thousand kms from the PNS which mean-

] Magnetars (SGRs and AXPs) are neutron stars having while continues to accrete. In the post bounce phase
h
a surface magnetic field, B ∼ 1015 G. SGRs are high- thePNSdeleptonisesintheKelvin-Helmholtztimescale
p
energytransientburstsourcesandinquiescentphaseare t ∼30semitting∼1053ergs.ThePNScontracts,spins
- KH
o X-ray pulsars whereas the AXPs are a quieter version upandaneutronstarofradius10kmisborn.Theexact
r andshowonlypersistentX-raypulsations[1].The spin mechanism which leads to the reversalof core-collapse
t
s periodsof magnetarslie ina narrowrangeof(5–12s). into a supernova explosion is not yet fully understood
a
Most of them lie close to the galactic plane implying [2].
[
that magnetars are young objects. All of them are iso- The basic question is that what could be happening

1
lated. This fact can be attributed to the small statistics duringthesupernovawindow,i.e.differentstageswhich
v
available. But if this is not true and magnetars discov- areinvolvedinthesupernovaexplosion,whichcanlead
5
eredinthe futurearealso foundtobe isolatedthenthis to disruption? It has been suggested that magnetar pro-
6
3 becomesoneofthemostinterestingpuzzles,answersto genitorsaremassive[3].Assumingthistobethecasea
1 whichwillshedlightontheirformationscenario.Ifthe magnetarbinarywillhavealargemassratio,q≡M /M .
1 2
. progenitor of a magnetar has a binary companion then The simplest case for disruption is when the explosion
1
0 something must be happening during the formation or leads to the loss of more than half of the mass of the
8 thesubsequentevolutionstagewhichleadstothedisrup- system(M1+M2)/2,thenthebinarywilldisrupt,since
0 tionofthebinary.Hereweassumethatsuchisthecase theorbitalvelocitywillexceedtheescapevelocityofthe
: and qualitatively speculate on the possible scenarios in companion[4].
v
i whichthiscanoccur.Wefirstconsiderthepossibilityof Core-collapse is one of the stages in the supernova
X
binarydisruptionduringthe differentstages which lead window where understandingaboutall the variablesin-
r toasupernovaexplosionandaprotomagnetar.Thenwe volved is yet to happen. Specifically, there has been a
a
discussamagnetarinabinaryandlookatitsevolution. recentrevivalof interest in the possible role of rotation
andmagneticfieldintheexplosionmechanismofcore-
collapsesupernovae.LeBlancandWilson[5]foundthat
MAGNETAR FORMATIONSCENARIO whenastrongmagneticfieldiscombinedwithrotation,
the corecollapseproducesanaxialjet. Recentobserva-
We begin by exploringthe case of disruption of the bi- tional studies of the polarization of supernovae and re-
nary due to the supernova explosion of the magnetar latedissueshasleadtothefirmconclusionthatcorecol-
progenitor. The successful birth of a neutron star in- lapse supernovaearealwaysasymmetricandfrequently
volves a number of stages. It starts with core-collapse bi-polar [6]. It is quite possible that progenitor rotation
ofabout1.5M⊙ ofiron-groupelementsofradius∼few and magnetic field along with the progenitor mass and
thousandkmswhichduetothecollapsebecomesaneu- metallicity play an important part in the core collapse
tronrichsphereofradiusabout50kms,aproto-neutron andsubsequentexplosion.Intheprocessofcore-collapse
star (PNS). The abrupt halt of the collapse of the inner strong toroidal magnetic fields will be generated which
core due to repulsive nuclear forces generates a shock can produce magnetocentrifugal jets [7]. Also the oc-
wave which passes through the outer half of the core. currence of magnetorotational instabilities in the core-
collapse phase has been investigated [8]. How does the separationdecreasesslowly.Weknowfromthetheoryof
magnetocentrifugaleffecttranslate forthe case of mag- stellarstructurethatasastarevolvesthefractionofstel-
netarsandwhatspecialcircumstanceslead totheir pro- larmatternearthesurfaceincreases.Thusmoreevolved
duction and accompanyingenergies and kicks are open stars can exert greater gravitationaltorques on the neu-
questions and areas of current research, see for eg. [9]. tron star. The orbital decay timescale, which decreases
One possibility,for example is that in the core-collapse as the gravitationaltorquesincreases, is thus dependent
supernova of a magnetar progenitor, the magnetic field ontheevolutionarystateofthecompanionstar.Formore
androtationinconjunctionwithothervariablesmayend evolved giant configuration the orbital decay timescale
upimpartingahighkickvelocitytothemagnetarinvari- becomesshorterand the spiral-inis faster. Eventuallya
ablywhichmaydisruptthebinary. tightlyboundspiralresultsastheorbitalvelocitiesofthe
Another important interval where huge (∼1053ergs) twocoresexceedtherotationalvelocityoftheenvelope.
amounts of energy is emitted is the Kelvin-Helmholtz Thisrapiddecayacceleratesthegasneartheneutronstar
timescale tKH ∼ GM2/Ln R where Ln is the total neu- andthecoreofthegiantwhichleadstosupersonicveloc-
trinoluminosity.ThisiswherethePNSdeleptonisesand ities andthe generationofshock.These shockstransfer
thecompactstarisborn.Inthecontextofmagnetarsthis orbitalangularmomentumtothespinoftheCEandthe
is the interval when the strong poloidal magnetic fields CEisspunup.
areestablishedviathea –W dynamo[10].Theprotomag- As a result the matter near the two cores is spun up
netarwhichis bornrapidlyrotatinglosetheir rotational andthisleadstotheonsetofmasslossfromthesystem.
energy,∼1052ergs,efficientlytotheco-rotatingmagne- Thishappensbecause,dueto the spin of matter,the ef-
tizedwind[11].Thespindowntime-scale,t ∼w˙/w ∼ fectivegravitydecreaseswhichleadstounbalancedpres-
J
(2/5)(M˙/M)(rNS/rA)2 whererNS and rA are the neutron sure gradients. Mass loss rates may reach ∼ 1M⊙yr−1
star radiusand the Alfvén radius,is of the orderof few [13]. As the mass near the two cores clear up gravita-
seconds.Thevelocityofthiswindisnearcanditdrives tionaltorquesdecreasewhichleadstotheincreaseinthe
anenergeticshockintothe slowersupernovashockand orbitaldecaytimescale. Thusthe CE ejectiontimescale
ahyperenergeticsupernovaresults.Ifthissomehowtrig- becomescomparableorevenshorterthantheorbitalde-
gersextrememasslossthendisruptionofthebinarywill cay timescale.Thisgiveenoughtimeto the CE to eject
occur. Of course a great deal of work needs to be done fromthesystemleavingbehindtwotightlyboundcores.
beforeonecanascertainthecorrectchannelofmagnetar Thusthespiralinginofthetwocoresandcoalescenceis
formationandfindthepossiblelinktoitsisolation. prevented.Forlessevolvedstars,duetothepresenceof
largeamountofmassnearthecorethemasslosscontin-
ueswhichleadstotheirmerger[12].
MAGNETARBINARY EVOLUTION Now let’s look at a magnetar in a binary system un-
dergoingCEevolution.Asthemagnetarapproachesthe
Considerthe generalevolutionof a massive binarysys- giantcompaniontheusualprocessesasexplainedabove
tem. If it is a wide binary then disruptionprobabilityis willtakeplace.Itwillformatightlyboundspiralwiththe
highwhentheprimaryexplodesin a supernova.On the coreofthegiant.TheCEwillspinupandmasslossfrom
otherhandif itis a close binarythenmass transferwill the system will ensue.As the matternear the two cores
occur from the primary as it overflows its Roche-lobe. clears up somewhat, gravitationaltorques will decrease
ThebinaryevolutionisclassifiedasCaseA,B,orCde- andhencetheorbitaldecaytimewillincrease.However
pendingontheprimarystateofevolutionattheonsetof sincethesystemistightlyboundnow,themagneticfield
Roche-lobe overflow. The mass transfer will affect the ofthemagnetarwillthreadthroughtheCEandstartaf-
orbital dynamics of the binary and the evolution of the fectingit.Themagneticfieldswillexertabrakingforce
primaryandthesecondarystars. onmotionoftheCE.Thiswillleadtoanincreaseofthe
Generally three modes of mass transfer in the evo- ejectiontimescaleoftheCE.Ifthisejectiontimescalebe-
lution of the binary are possible, viz., conservative, comesgreaterthanorbitaldecaytimescale,thenthetwo
quasi-conservativeandthecommonenvelopeorthenon- cores will spiral together and coalesce. The outcome is
conservativeevolution.Sincethecomponentsdiffersig- thatoneendsupwitheitheramagnetar(orneutronstar)
nificantlyinbothmassandsizeacommonenvelope(CE) whichsurvivesthemergerorablackhole.Inanycasean
evolutionatsomestageisverylikely[12].We consider isolatedobjectresults.
a magnetar in an evolved binary where we assume that
theCEhasformedanddiscussthepossiblewaysitwill
evolve. DISCUSSIONS
LetusfirstconsiderageneralpictureofCEevolution
[12].Duetogravitationaltorques,asthecompactstarap- We have tried to discuss the possible ways a magnetar
proachesthesurfaceofitslargercompanion,theorbital may lose its companion if it originally was in a binary.
Iffurtherobservationalevidenceaccumulatesthatmag-
netars are isolated objects then it becomes importantto
findthemechanismbywhichthisishappening.Fromthe
observationalpointofviewitisimportanttolookatthe
nearby regions of an active magnetar. This may give a
clue to the possible channel of disruption or merger. In
the parameterspace the windowwhich leadsto the for-
mation of magnetar instead of a normal neutron star is
difficulttoquantify.However,thesuccessfulworkingof
a -W dynamo together with the constraint of disruption
which results in isolation, will lead to the tightening of
requiredconditions.Wehavepresentedaqualitativedis-
cussionwhichrequiresa lotofwork/simulationsbefore
anythingconcrete can be said aboutthe differentpossi-
bilities.Adetailedanalysiswillappearelsewhere.
ACKNOWLEDGMENTS
We thankA.DeshpandeandB. Paulforhelpfuldiscus-
sions.
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