The Latitude of Type I X-Ray Burst Ignition on Rapidly Rotating Neutron Stars

0.1 MB·English

by  Randall L. Cooper

ACCEPTEDBYTHEASTROPHYSICALJOURNALLETTERS
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
THELATITUDEOFTYPEIX-RAYBURSTIGNITIONONRAPIDLYROTATINGNEUTRONSTARS
RANDALLL.COOPERANDRAMESHNARAYAN
Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138
AcceptedbyTHEASTROPHYSICALJOURNALLETTERS
ABSTRACT
We investigate the latitude at which type I X-ray bursts are ignited on rapidly rotating accreting neutron
˙
stars. Wefindthat,forawiderangeofaccretionratesM,ignitionoccurspreferentiallyattheequator,inaccord
˙ ˙
withtheworkofSpitkovskyetal. However,forarangeofM belowthecriticalM abovewhichburstscease,
˙
ignition occurs preferentially at higher latitudes. The range of M over which nonequatorial ignition occurs
7
is an increasingfunctionof the neutronstar spin frequency. These findingshave significantimplicationsfor
0
thermonuclearflamepropagation,andtheymayexplainwhyoscillationsduringtheburstrisearedetectedpre-
0
2 dominantlywhentheaccretionrateishigh. TheyalsosupportthesuggestionofBhattacharyya&Strohmayer
thatnon-photosphericradiusexpansiondouble-peakedburstsandtheunusualharmoniccontentofoscillations
n
duringtheriseofsomeburstsresultfromignitionatorneararotationalpole.
a
J Subjectheadings:accretion,accretiondisks—stars: neutron—X-rays:binaries—X-rays:bursts

4
2
1. INTRODUCTION (Fryxell&Woosley 1982; Bildsten 1995; Spitkovskyetal.

2002,hereafterSLU02).Time-resolvedspectroscopyandob-
1 Type I X-ray bursts are thermonuclear explosions
servations of large amplitude oscillations during the rise of
v that occur on the surfaces of accreting neutron stars in
some type I X-ray bursts from rapidly rotatingneutron stars
8 low-mass X-ray binaries (LMXBs) (Babushkinaetal.
0 1975; Grindlay&Heise 1975; Grindlayetal. 1976; supportthisconclusion(Strohmayeretal.1997,1998).
7 Belianetal. 1976; Woosley&Taam 1976; Joss 1977; SLU02 investigated the dynamics of localized ignition on
1 Maraschi&Cavaliere1977;Lamb&Lamb1977,1978),and a rapidly rotating neutron star. They found that the latitude
0 at which localized ignition occurs is of great importance in
they are triggered by unstable hydrogen or helium burning
7 determiningthe burningfrontpropagationspeed. Itcanalso
(for reviews, see Cumming 2004; Strohmayer&Bildsten
0 affect the stability and nature of zonal flows during the de-
2006). The physics of type I X-ray bursts is generally well
/ cay phase of bursts (Cumming 2005), the harmonic content
h understood, and detailed time-dependent one-dimensional
ofburstriseoscillations(Bhattacharyya&Strohmayer2005),
p models (e.g., Woosleyetal. 2004; Fiskeretal. 2006) have
- beenrathersuccessfulatreproducingthegrosscharacteristics and the light curves of non-photospheric radius expansion
o (PRE)bursts(Bhattacharyya&Strohmayer2006a,b).SLU02
of bursts, such as their fast rise times of ∼1 s, durationsof
r assert that, due to the reduction of the effective gravita-
t ∼10–100s,andrecurrencetimesofafewhourstodays.
s tional acceleration, ignition is likely to occur at the equator
Nearly all type I X-ray burst models are one-dimensional
a
of a rapidly rotating neutron star. However, recent observa-
: and thus implicitly assume that matter accretes spherically
v tionssuggestotherwise. Bhattacharyya&Strohmayer(2005,
onto a nonrotating neutron star. Therefore, ignition occurs
i 2006a,b)arguethattheharmoniccontentofburstriseoscilla-
X simultaneouslyovertheentirestellarsurface. However,these
tionsandnon-PREdouble-peakedburstsfromtheLMXB4U
assumptions are clearly inapplicable to the vast majority of
r 1636–536,whichharborsa neutronstar with spin frequency
a accreting neutron stars. The Rossi X-Ray Timing Explorer
ν=581Hz,requireignitiontooftentimesoccuratornearthe
hasdetectedhighlysinusoidaloscillationswithfrequenciesof
rotationalpole,atleastinthissource.
45–1122Hzduringburstsin17LMXBs(Strohmayer&Bild-
InthisLetter,weevaluatetheoreticallythelatitudeatwhich
sten 2006 and references therein; Bhattacharyyaetal. 2006;
typeIX-rayburstsonrapidlyrotatingneutronstarsaremost
Bhattacharyya 2006; Kaaretetal. 2006). It is thought that
probably ignited. In §2 we outline the physics that governs
theburstoscillationfrequencycorrespondstotheneutronstar
the latitude of type I X-ray burst ignition and determine the
spin frequency (e.g. Strohmayer&Markwardt 1999), which
ignitionlatitudeasafunctionoftheglobalaccretionrate. We
implies that many neutron stars that exhibit bursts are in
discussourresultsandconcludein§3.
fact rapidly rotating. Indeed, Munoetal. (2001, 2004) have
shownthatthepropertiesofburstsdependsensitivelyonthe
2. IGNITIONLATITUDE
neutron star rotation rate, and so rotation must be consid-
Inthissection,wefirstoutlinethebasicphysicsthatdeter-
eredintheoreticalburstmodels. Furthermore,sincethetime
minesthe latitudeoftypeI X-rayburstignitionona rapidly
required to accumulate a sufficient amount of fuel to trig-
rotatingneutronstar using a simple model. We then use the
ger a burst is much greater than the duration of the result-
global linear stability analysis of Cooper&Narayan (2005)
ing burst, it is highly unlikely that all of the accreted fuel
tocarryoutmoredetailedcalculations.
overtheentireneutronstarsurfacewillignitesimultaneously
(Joss 1978; Shara 1982). Thus, contrary to the assumptions
2.1. BasicPhysics
ofmosttheoreticalmodels,ignitionalmostsurelyoccursata
point,andtheresultingthermonuclearflamesubsequentlyen- Usingtheone-zoneburstignitionmodelofBildsten(1998),
gulfs the whole stellar surface in ∼1 s, the burst rise time we expand upon the work of SLU02 to determine the most
probablelatitudeofburstignitiononthesurfaceofarapidly
Electronicaddress:[email protected],[email protected] rotating neutron star. The reader is encouraged to refer to
2 Cooper&Narayan
these worksforfurtherdetails. We neglectgeneralrelativis-
p /2
tic correctionsthroughoutfor clarity. FollowingSLU02, we
assumethatatalltimespriortoignitiontheaccretedplasma
is in hydrostatic equilibrium and is at rest in the corotating
frame(e.g.Inogamov&Sunyaev1999).Wepresumethatthe
accretedmatterspreadsinsuchawayastominimizethegrav- n = 600 Hz
itational potential energy of the accreted layer. The base of n p /4 n = 300 Hz
g
theaccretedlayeristhereforeanequipotentialsurface,andso i
the pressure at the base of the accreted layer is the same at l
every latitude (e.g., Clayton 1983). Hydrostatic equilibrium
thus implies that P =Σ (λ)g (λ) is independent of lati-
layer eff
tude,wherePisthepressureatthebaseoftheaccretedlayer,
Σ isthecolumndepth,g istheeffectivegravitationalac- 0.0
layer eff
celeration,andλ≡π/2- θisthelatitude.Thus,Σ ∝g- 1,
layer eff 0.0 0.2 0.4 0.6 0.8 1.0
andso . .
Σ˙ ∝g- 1, (1) M/M (p /2)
eff crit
whereΣ˙ =Σ˙(λ)isthelocalmassaccretionrateperunitarea. FIG.1.—Plotshowsλign,thelatitudeatwhichtypeIX-rayburstsigniteon
arapidlyrotatingneutronstar,derivedfromaone-zoneburstmodelfortwo
Adecpctohrdatinwghtiochehqeulaiutimonig(2n0it)esoΣf iBgnilvdasrteiens(a1s998), the column sgtleolblaarlraoctcarteiotinonfreraqtueesnMc˙i,esigνn.itMion=o1c.c4uMrs⊙aatnthdeReq=u1a0tokrm(λ.F=o0r)a.wHiodweeravnegr,efoofr
˙
Σign∝Σ˙- 1/5ge- f2f/5. (2) aatrhainggheeroflaMtitundeeasr.tThehecrriatnicgaeloraftMe˙aobvoevrewwhhicichhnbounresqtsuacteoarsiea,liiggnniittiioonnooccccuurrss
is∝ν2.
Combiningequations(1)and(2)givestheignitiontime asa
functionoflatitude where ν, M, and R are the spin frequency, mass, and radius
t (λ)≡Σ /Σ˙ ∝g4/5. (3) oftheneutronstar,respectively.Thus,foraneutronstarwith
ign ign eff spin frequencyν =600Hz, type I X-ray bursts will ignite at
˙ ˙
Thustign,thetimeittakestoaccreteacriticalamountoffuel theequatorforM/Mcrit(π/2).0.89andoffoftheequatorfor
to trigger a type I X-ray burst, is an increasing function of ˙ ˙
0.89.M/M (π/2)<1.Withinthelatterrangeofaccretion
crit
g . Duetocentrifugalacceleration,g islowestattheequa-
eff eff rates,thelatitudeatwhichignitionoccursisgivenby
torofarapidlyrotatingneutronstar,whichmeansthatt (λ)
ign ˙ ˙
is a minimum at the equator. Therefore, ignition will occur 0, M<M (0),
crit
preferentiallyattheequator(SLU02).
neHutorownevsetra,rnduocelesarnobturanlwinagysontritghgeesrurafabcuersotf. anBoatchcrtehteinog- λign= arccos"ννKr1- M˙M˙crcitr(itπ(λ/)2) 2/3#, M˙crit(0)<M˙ <M˙crit(π/2),
(cid:16) (cid:17)
retical models (e.g., Fujimotoetal. 1981; Paczyn´ski 1983; (8)
Bildsten 1998) and observations(e.g. Cornelisseetal. 2003; whereνK is the Keplerian frequency. Figure 1 shows a plot
˙
Gallowayetal.2006)suggestthatthereisalocalcriticalac- ofλign asafunctionofM. WeseethattypeIX-rayburstig-
cretion rate Σ˙ above which nuclear burning is stable, and nitiononarapidlyrotatingneutronstaroccurspreferentially
thus bursts docrnitot occur. By equation (1), Σ˙ is related to neartheequatorforawiderangeofM˙. However,forasmall
theglobalcriticalaccretionrateM˙ suchthat crit rangeof M˙ near M˙crit, ignition occursat higherlatitudes be-
cause nuclear burning becomes stable near the equator. As
Σ˙ ∝g- 1M˙ , (4) equation(7)illustrates,therangeofaccretionratesoverwhich
crit eff crit
˙ ˙ nonequatorialignitionoccursincreasesdramaticallywithν.
whereM =M (λ)isdefinedtobethecriticalglobalaccre-
crit crit
tion rate abovewhichnuclearburningisstable atlatitude λ.
2.2. GlobalLinearStabilityAnalysis
Accordingtoequation(24)ofBildsten(1998),
We now use the global linear stability analysis of
Σ˙ ∝g1/2, (5) Cooper&Narayan (2005), which is an expanded and im-
crit eff
provedversionofthemodelofNarayan&Heyl(2003),tode-
andso
M˙ ∝g3/2. (6) terminethe latitude of type I X-ray burstignitionon rapidly
crit eff rotating neutron stars. We assume that matter accretes at a
˙
This implies that, since g is lowest at the equator and in- global rate M onto a rapidly rotating neutron star of mass
eff
creases towards the rotational pole, there exists a range of M =1.4M⊙, radius R=10 km, and spin frequency ν =650
accretion rates for which nuclear burning is stable near the Hz,anditspreadsoverthestellarsurfaceinthesamemanner
equatorandunstablenearthepole. Consequently,foragiven asdescribedin§2.1. We setthecompositionoftheaccreted
˙
λ andcorrespondingcriticalaccretionrateM (λ ),nuclear mattertobethatoftheSun,suchthatattheneutronstarsur-
0 crit 0
burning is stable for all λ<λ and unstable for all λ>λ . face the hydrogen mass fraction X =0.7, helium mass frac-
0 0
Clearly, at these accretion rates, bursts will be triggered off tionY =0.28, CNO mass fraction ZCNO =0.016, and heavy
the equator. For M˙ >M˙ (π/2), no bursts are triggered at elementfractionZ=0.004,whereZ referstoallmetalsother
crit
anylatitude. Takingthefunctionalderivativeofequation(6) thanCNO.
˙
gives We plottheignitiontimetign asa functionofM forλ=0,
˙ ˙
π/4, and π/2 in Figure 2. For M .0.16M , t (λ) is a
δM˙crit ≈0.11 ν 2 1.4M⊙ R 3, (7) minimum at the equator, and so bursts igniteEdpdrefigenrentially
M˙ (π/2) 600Hz M 10km near the equator, in agreement with both SLU02 and equa-
crit (cid:16) (cid:17) (cid:18) (cid:19)(cid:18) (cid:19)
BurstIgnitionLatitude 3
delayed mixed bursts, see Cooper&Narayan 2006). In ef-
0.8
fect,thedelayedmixedburstregimesignificantlyextendsthe
˙
0.7 range of M over which nonequatorial ignition occurs. The
global linear stability analysis and the two-zone model of
0.6 l = p /2 Cooper&Narayan(2006)agreebetterwithobservationsthan
s) l = p /4 allothercurrentburstmodels, butthisagreementholdsonly
ay 0.5 l = 0 inatime-averagedsense(e.g.Gallowayetal.2006). Bythis
d
(n 0.4 wemeanthat,whileobservationsimplyt˙hatthe˙meanign˙ition
g timeht iisanincreasingfunctionofM forM&0.15M ,
ti ign Edd
0.3 which is in accord with the results shown in Figure 2, the
ignitiontimet measuredbetween pairs of bursts observed
ign
0.2
in natureexhibitssignificantdeviationsrelativeto the mean.
Ourmodelscannotaccountforthis behavior. Since the pre-
0.1
dictedburstignitionlatitudeismodel-dependent,andnocur-
0.10 0.15 0.20 0.25 0.30 0.35
. . rentmodelcansuccessfullyreproducethechaoticbehaviorof
M/M bursts, we have little confidence in the accuracy of our pre-
Edd
FIG. 2.— Plot showstign(λ), the type I X-ray burst ignition timescale, dictedignitionlatitudesλign. However,alltheoreticalmodels
atthreedifferentlatitudesλforaneutronstarwithspinfrequencyν=650 wouldpredictthatburstsignitepreferentiallyneartheequator
˙ ˙ ˙ ˙
Hz, M=1.4M⊙, andR=10km. Foraccretion rates M.0.16MEdd,tign at low valuesof M and off of the equatorat higherM. This
islowestattheequator, whichmeansthatburstsigniteattheequator. For resultisveryrobust.
˙ ˙ ˙
0.16.M/MEdd .0.19, the burstignition latitude increases withM. For Recent observations have generated a renewed interest in
0.19.M˙/M˙Edd.0.3,althoughnuclearburningisunstableatallλ˙,ig˙nition the latitude of type I X-ray burst ignition. First, non-PRE
occurspreferentiallynearthepolewheretignislowest.For0.3.M/MEdd. double peaked bursts have been observed in a few LMXBs
0.34,nuclearburningbecomesstableatlowlatitudes,andignitionstilloccurs
preferentiallynearthepole. (e.g. Sztajnoetal. 1985; Penninxetal. 1989; Kuulkersetal.
tion (3). Here, M˙ =8.3×1017 gs- 1 denotes the mass ac- 2002). Bhattacharyya&Strohmayer(2006a,b)arguethatig-
Edd nition near a rotational pole can explain the non-PRE dou-
cretionrate at whichthe accretionluminosityis equalto the
˙ ˙ ble peakedburstsobservedin 4U 1636–536,whichcontains
Eddingtonlimit. For 0.16.M/M .0.19, λ gradually
Edd ign a neutron star with ν = 581 Hz. Their simple model of
˙
increaseswithincreasingM, againinaccordwiththeresults nonequatorial ignition and subsequent thermonuclear flame
of §2.1. However, Figure 2 shows a new regime of bursts propagation and temporary stalling near the equator quali-
˙ ˙
for M & 0.19M , the regime of “delayed mixed bursts,” tatively reproduces both the light curves and spectral pro-
Edd
which was first identified by Narayan&Heyl (2003). De- files of such bursts. Second, although burst oscillations de-
layedmixedburstsaremixedhydrogenandheliumburststrig- tectedinboththeriseanddecayareusuallyquitesinusoidal,
gered by unstable helium burning, and they are precededby Bhattacharyya&Strohmayer(2005)reportevidenceforsub-
a long period of stable nuclear burning. Using their global stantialharmoniccontentintheoscillationsduringtheriseof
linear stability analysis, Narayan & Heyl found that, within aburstfrom4U1636–536.Theyagainsuggestthatnonequa-
this range of accretion rates, (1) the ignition timescale is an torialignitionandsubsequentflamepropagationcan explain
˙
increasing function of M (which is consistent with obser- theobservations. Theseauthorsacknowledgedthatnonequa-
vations, e.g., vanParadijsetal. 1979, 1988; Cornelisseetal. torialignitionisatoddswiththeworkofSLU02,whoargued
˙
2003; Remillardetal. 2006; Gallowayetal. 2006), and (2) forequatorialignitionatallM. Ourresultthatnonequatorial
the criticalglobalaccretionrate abovewhich delayedmixed ignition is more likely when the accretion rate is high natu-
burstsoccurisanincreasingfunctionofthegravitationalac- rallyreconcilesthisdiscrepancy.
˙ ˙ ˙
celeration g . Thus, for the range 0.19. M/M .0.3, That nonequatorial ignition is preferred at higher M may
eff Edd
althoughnuclear burningis unstableat all latitudes, ignition explain why oscillations in the rising phase of some bursts
occurs preferentially near the pole where t is lowest. For onrapidlyrotatingneutronstarsoccurpredominantlyonthe
ign
0.3.M˙/M˙ .0.34, nuclear burningbecomescompletely banana branch, i.e. when the inferred accretion rate is high
Edd
stableatlowlatitudes,andignitionagainoccurspreferentially (Strohmayer&Bildsten2006).Thiscanbeunderstoodasfol-
nearthepole. ForM˙/M˙ &0.34,nuclearburningisstable lows. SLU02 find thatburningfrontspropagatemuchfaster
Edd ˙
atalllatitudes,andnoburstsoccur. neartheequatorthannearthepoles. IfM islowandignition
occursneartheequator,theflamewillspreadinlongitudeona
3. DISCUSSIONANDCONCLUSIONS timescalethatismuchshorterthantheburstrisetimescaleand
Theresultsofboththesimpleone-zonemodelofBildsten quickly produce an axisymmetric belt. An observer would
(1998) and the more detailed global linear stability analysis detectnooscillationsduringtherisingphasebecausethereis
˙
ofCooper&Narayan(2005)suggestthatburstsigniteonthe no longitudinalasymmetry. On the other hand, if M is high
equatoratlowaccretionratesandoffoftheequatorathigher and ignition occurs close to, but not directly at, a rotational
accretion rates. The global linear stability analysis predicts pole, the flame willspread in the longitudinaldirectionon a
that nonequatorial ignition occurs over a much larger range timescalethatiscomparabletotheburstrisetimescale. This
of M˙ – nearly 50% for ν =650 Hz versus only about 10% slowlongitudinalspreadwouldcreateanon-axisymmetrichot
in the one-zonemodel– and that ignition should occur near spot and hence oscillations. We note that oscillations in the
the pole for most of this range. The differences in the re- decay phase of some bursts are again observed only when a
sults of the two models are due to the delayed mixed burst systemisonthebananabranch.However,burstoscillationsin
regime,whichispredictedinthegloballinearstabilityanal- thedecayphasearenotwellunderstoodtheoretically,andwe
ysis but not in the one-zone model (for an explanation of arehesitanttosuggestaconnectionbetweentheseoscillations
4 Cooper&Narayan
andnonequatorialignition. ignitionismorelikelytooccurwithinthisconfinementregion
We stress that our results apply only to rapidly rotating, than at a specific latitude. Although some bursts observed
weakly-magnetic neutron stars. Rapid rotation induces a from the accreting millisecond pulsar SAX J1808.4–3658
stronglatitudinaldependenceontheeffectivegravitationalac- exhibit timing features that Bhattacharyya&Strohmayer
celerationwhichrestrictsburstignitiontoapreferredlatitude. (2006c) suggest originate from midlatitudinal ignition, we
Foranonrotating(orslowlyrotating)neutronstar, allphysi- speculatethatitisduetomagneticconfinement.
calquantitiesareindependentofλ(ornearlyso),andignition
couldoccuratanylatitude.Thus,oscillationsduringtheburst
˙
risecouldoccuratanyMforslowlyrotatingneutronstars. WethankJoshGrindlayfordiscussionsthatmotivatedthis
Strong magnetic fields may channel and confine accreted investigationandtherefereeforusefulcomments. Thiswork
mattertosomeregionoftheneutronstarsurface.Inthiscase, wassupportedbyNASAgrantNNG04GL38G.
REFERENCES
Babushkina, O. P., Bratolyubova-Tsulukidze, L. S., Kudryavtsev, M. I., Kuulkers,E.,Homan,J.,vanderKlis,M.,Lewin,W.H.G.,&Méndez,M.
Melioranskiy, A. S., Savenko, I. A., & Yushkov, B. Y. 1975, Soviet 2002,A&A,382,947
AstronomyLetters,1,32 Lamb,D.Q.&Lamb,F.K.1977,inEightTexasSymposiumonRelativistic
Belian,R.D.,Conner,J.P.,&Evans,W.D.1976,ApJ,206,L135 Astrophysics,ed.M.D.Papagiannis(NewYork:NewYorkAcademyof
Bhattacharyya,S.2006,MNRAS,submitted(astro-ph/0605510) Sciences),261
Bhattacharyya,S.&Strohmayer,T.E.2005,ApJ,634,L157 Lamb,D.Q.&Lamb,F.K.1978,ApJ,220,291
—.2006a,ApJ,636,L121 Maraschi,L.&Cavaliere,A.1977,HighlightsofAstronomy,4,127
—.2006b,ApJ,641,L53 Muno,M.P.,Chakrabarty,D.,Galloway,D.K.,&Savov,P.2001,ApJ,553,
—.2006c,ApJ,642,L161 L157
Bhattacharyya,S.,Strohmayer,T.E.,Markwardt,C.B.,&Swank,J.H.2006, Muno,M.P.,Galloway,D.K.,&Chakrabarty,D.2004,ApJ,608,930
ApJ,639,L31 Narayan,R.&Heyl,J.S.2003,ApJ,599,419
Bildsten,L.1995,ApJ,438,852 Paczyn´ski,B.1983,ApJ,264,282
Bildsten,L.1998,inTheManyFacesofNeutronStars,ed.R.Buccheri,J. Penninx,W.,Damen,E.,vanParadijs,J.,Tan,J.,&Lewin,W.H.G.1989,
vanParadijs,&M.A.Alpar(NATOASISer.C,515;Dordrecht:Kluwer), A&A,208,146
419 Remillard,R.A.,Lin,D.,Cooper,R.L.,&Narayan,R.2006,ApJ,646,407
Clayton, D. D. 1983, Principles of stellar evolution and nucleosynthesis Shara,M.M.1982,ApJ,261,649
(Chicago:UniversityofChicagoPress,1983) Spitkovsky,A.,Levin,Y.,&Ushomirsky,G.2002,ApJ,566,1018(SLU02)
Cooper,R.L.&Narayan,R.2005,ApJ,629,422 Strohmayer,T.&Bildsten,L.2006,inCompactStellarX-RaySources,ed.
—.2006,ApJ,652,584 W.H.G.LewinandM.vanderKlis(Cambridge:CambridgeUniv.Press),
Cornelisse, R.,in’tZand,J.J.M.,Verbunt,F.,Kuulkers,E.,Heise,J.,den 113
Hartog,P.R.,Cocchi,M.,Natalucci,L.,Bazzano,A.,&Ubertini,P.2003, Strohmayer,T.E.&Markwardt,C.B.1999,ApJ,516,L81
A&A,405,1033 Strohmayer,T.E.,Zhang,W.,&Swank,J.H.1997,ApJ,487,L77
Cumming,A.2004,NuclearPhysicsBProceedingsSupplements,132,435 Strohmayer,T.E.,Zhang,W.,Swank,J.H.,White,N.E.,&Lapidus,I.1998,
—.2005,ApJ,630,441 ApJ,498,L135
Fisker,J.L.,Görres,J.,Wiescher,M.,&Davids,B.2006,ApJ,650,332 Sztajno, M.,van Paradijs, J.,Lewin, W.H. G.,Trumper,J.,Stollman, G.,
Fryxell,B.A.&Woosley,S.E.1982,ApJ,261,332 Pietsch,W.,&vanderKlis,M.1985,ApJ,299,487
Fujimoto,M.Y.,Hanawa,T.,&Miyaji,S.1981,ApJ,247,267 vanParadijs,J.,Cominsky,L.,Lewin,W.H.G.,&Joss,P.C.1979,Nature,
Galloway, D. K., Muno, M. P., Hartman, J. M., Savov, P., Psaltis, D., & 280,375
Chakrabarty,D.2006,ApJS,submitted(astro-ph/0608259) vanParadijs,J.,Penninx,W.,&Lewin,W.H.G.1988,MNRAS,233,437
Grindlay, J., Gursky, H., Schnopper, H., Parsignault, D. R., Heise, J., Woosley,S.E.,Heger,A.,Cumming,A.,Hoffman,R.D.,Pruet,J.,Rauscher,
Brinkman,A.C.,&Schrijver,J.1976,ApJ,205,L127 T.,Fisker,J.L.,Schatz,H.,Brown,B.A.,&Wiescher,M.2004,ApJS,
Grindlay,J.&Heise,J.1975,IAUCirc.,2879 151,75
Inogamov,N.A.&Sunyaev,R.A.1999,AstronomyLetters,25,269 Woosley,S.E.&Taam,R.E.1976,Nature,263,101
Joss,P.C.1977,Nature,270,310
—.1978,ApJ,225,L123
Kaaret,P.,Prieskorn,Z.,in’tZand,J.J.M.,Brandt,S.,Lund,N.,Mereghetti,
S., Gotz, D., Kuulkers, E., & Tomsick, J. A. 2006, ApJL, submitted
(astro-ph/0611716)