The pre-outburst flare of the A 0535+26 August/September 2005 outburst

0.17 MB·

by  I. Caballero

Astronomy&Astrophysicsmanuscriptno.a0535 (cid:13)c ESO2008
February2,2008
LE
The pre-outburst flare of the
A0535+26 August/September 2005 outburst
I.Caballero1,A.Santangelo1,P.Kretschmar2,R.Staubert1,K.Postnov1,3,D.Klochkov1,A.Camero-Arranz4,
M.H.Finger5,I.Kreykenbohm1,6,K.Pottschmidt7,8,R.E.Rothschild9,S.Suchy9,J.Wilms10,andC.A.Wilson5
8
0 1 Institutfu¨rAstronomieundAstrophysik,Sand1,72076Tu¨bingen,Germany
0 2 ISOC,EuropeanSpaceAstronomyCentre(ESAC),P.O.Box78,28691VillanuevadelaCan˜ada(Madrid),Spain
2 3 SternbergAstronomicalInstitute,119999,Moscow,Russia
4 GACE,InstitutodeCienciasdelosMateriales,UniversidaddeValencia,POBox20085,46071Valencia,Spain
n
5 NationalSpaceScienceandTechnologyCenter,320SparkmanDriveNW,Huntsville,AL,35805,USA
a
J 6 ISDC,16Ch.d’E´cogia,1290Versoix,Switzerland
7 CRESST,UniversityofMarylandBaltimoreCounty,1000HilltopCircle,Baltimore,MD21250,USA
1
8 NASAGoddardSpaceFlightCenter,AstrophysicsScienceDivision,Code661,Greenbelt,MD20771,USA
2
9 CASS,UniversityofCaliforniaatSanDiego,LaJolla,CA92093-0424,USA

10 Dr.Remeis-Sternwarte,AstronomischesInstitutderUniversita¨tErlangen-Nu¨rnberg,Sternwartstr.7,96049Bamberg,Germany
]
h
Received<date>;Accepted<date>
p
-
o ABSTRACT
r
t Aims.WestudythespectralandtemporalbehavioroftheHighMassX-rayBinaryA0535+26duringa‘pre-outburstflare’which
s
a tookplace∼5dbeforethepeakofanormal(typeI)outburstinAugust/September2005.Wecomparethestudiedbehaviorwiththat
[ observedduringtheoutburst.
Methods.WeanalyseRXTEobservationsthatmonitoredA0535+26duringtheoutburst.Wecompletespectralandtiminganalyses
1 ofthedata.Westudytheevolutionofthepulseperiod,presentenergy-dependent pulseprofilesbothattheinitialpre-outburstflare
v andclosetooutburstmaximum,andmeasurehowthecyclotronresonance-scatteringfeature(hereafterCRSF)evolves.
7 Results.Wepresentthreemainresults:aconstantperiodP=103.3960(5)sismeasureduntilperiastronpassage,followedbyaspin-up
6 withadecreasingperiodderivativeofP˙=(−1.69±0.04)×10−8ss−1atMJD53618,andPremainsconstantagainattheendofthemain
1 outburst.Thespin-upprovidesevidencefortheexistenceofanaccretiondiskduringthenormaloutburst.WemeasureaCRSFenergy
3 ofE ∼50keVduringthepre-outburstflare,andE ∼46keVduringthemainoutburst.Thepulseshape,whichvariessignificantly
cyc cyc
1. duringbothpre-outburstflareandmainoutburst,evolvesstronglywithphotonenergy.
0
Keywords.X-rays:binaries-stars:magneticfields–stars:individual:A0535+26
8
0
:
v 1. Introduction (Fingeretal. 1994). The estimated distance of the system
i
X is 2kpc (Steeleetal. 1998). The measured pulse period of
The Be/X-ray binary A0535+26 1 is a transient source,
the neutron star at MJD 53614.5137 is P=103.39315(5)s
r characterized by quiescent states with X-ray luminosity
a (Caballeroetal.2007).Duringquiescenceaspin-downtrendof
L .1036ergs−1, interrupted by normal (type I) outbursts, as-
X theneutronstarhasbeenreported(Fingeretal.1996;Hilletal.
sociated in general with periaston passage, when a luminos-
2007), and during giant outbursts the neutron star shows a
ity L ∼1036−37ergs−1 is reached, and giant (type II) out-
X strong spin-up. In the June 1983 giant outburst, a spin-up of
bursts for which LX>1037ergs−1. A0535+26 was discovered ν˙∼0.6 × 10−11Hzs−1 was measured (Sembayetal. 1990). In
by Rosenbergetal. (1975) during a giant outburst of luminos-
the February 1994 giant outburst the spin-up reached ν˙∼1.2×
ity level of L(3−7keV)∼ 1.2×1037ergs−1. Since then, five giant 10−11Hzs−1 and quasi periodic oscillations were present, con-
outbursts have been detected in October 1980 (Nagaseetal.
firmingthepresenceofanaccretiondisk(Fingeretal.1996).
1982), June 1983 (Sembayetal. 1990), March/April 1989
(Makinoetal. 1989), February 1994 (Fingeretal. 1994), and
May/June 2005 (Tuelleretal. 2005). Following the giant out- The X-ray spectrum of A0535+26 is typically described
by a phenomenological model consisting of a power law with
burst in May/June 2005, two normal outbursts occurred in
August/September 2005 (Finger 2005b) and December 2005 an exponentialcutoff. Two absorptionlike features, interpreted
as cyclotron resonance scattering features CRSFs (fundamen-
(Finger2005a).
tal and first harmonic), have been observed in the spectrum
ThesourceA0535+26isinabinaryorbit,withanO9.7IIIe
at ∼45keV and ∼100keV (Kendziorraetal. 1994; Groveetal.
opticalcompanionHDE245770(Giangrandeetal.1980),ofor-
bitalperiodP =110.3±0.3daysandeccentricitye=0.47±0.02 1995). Recent observationsduringthe August/September2005
orb
normaloutburstwithINTEGRALandRXTE(Kretschmaretal.
Sendoffprintrequeststo:I.Caballero, 2005;Wilson&Finger2005;Caballeroetal.2007)andSuzaku
e-mail:[email protected] (Teradaetal.2006)haveconfirmedthisresult.Thecentroiden-
1 Referredtoas1A0535+262inSIMBAD ergyoftheCRSFwasmeasuredduringthemainoutburstatdif-
2 I.Caballeroetal.:Pre-outburstflareofA0535+26
2005 August 2005 September Table 1. Formalfunctiondecribingthe pulse periodduringthe
29 1 8 15 22 29 outburst.ThereferenceforthethirdorderpolinomialfitisMJD
1000 103.405
53618.FromMJD53629theperiodisessentiallyconstant.
103.400
800 MJD MJD
−1U) 53608.70−53613.02 53613.11−53629
PC 103.395 P(s) 103.3960(5) 103.3883(5)
−1ounts s 600 103.390P(s) PP˙¨((ssss−−21)) -- (−1(.699±±3)0.×041)0×−1150−8
c
e ( 400 d3P/dt3(ss−3) - (2.5±0.9)×10−20
at
A r 103.385
C
P
200 barycentric-correctedPCA and HEXTE light curves and cor-
103.380
rected the photonarrivaltimes for the orbitalmotion.We used
Periastron
the orbital ephemeris from Fingeretal. (1994) with the peri-
0 103.375
53610 53620 53630 53640 astron epoch updated to MJD 53613.0±1.3(Fingeretal. 2006,
MJD 2007). We then produced pulse profiles using a constant, trial
Fig.1. Left axis, dotted line: ∼3–30keV PCA mean count rate period obtained using INTEGRAL observations of the same
for each available observation during the August/September outburst (Caballeroetal. 2007). For both PCA and HEXTE
2005outburst.EachdiamondindicatesoneRXTEobservation. we determined a reference time in the pulse profiles, and per-
The mean was obtained averaging each individual light curve. formedaphase-connectionanalysisaspreviouslycompletedin
Theerrorsshownarethesigma ofthedistributionforeachob- Caballeroetal.2007(seealsoFerrignoetal.2007).Asaphase
servation. Right axis, solid line: Pulse period evolution during reference we selected the midpoint of a sharp edge present in
theoutburst.Theverticallineindicatesthetimeofperiastron. thepulseprofiles,anapparentlystablefeature.Thepulseperiod
is approximately constant during the initial pre-outburst flare,
and a significant spin-up is measured after periastron. The pe-
ferentluminositylevelsanditwasnotfoundtovarywithX-ray
riodevolutionafterperiastroncanbedescribedbyathird-order
luminosity.
polynomial. The results of the fit are provided in Table 1. The
In this Letter we focuson the August/September2005nor-
period at any time during the outburst is well-described using
maloutburst.Attheonsetofthisoutburst,asharp,pre-outburst
thosefunctions,and±5msisaconservativeestimateoftheun-
flareisobservedsuperimposedonthegradualincreasetowards
certainty. The function describing the period evolution is plot-
the peak flux in the main outburst. This pre-outburst flare de-
ted in Fig. 1. It is consistent with the period measured using
velops on a time scale of 1 day, short compared to the three-
INTEGRALdatawithinuncertainties(Caballeroetal.2007).
week duration of the main outburst. As can be seen from the
light curve measured bySwift/BAT, the flare is one of several
suchflaressuperimposedontherisingedgeofthemainoutburst 3.2.Pulseprofileevolutionduringtheoutburst
(see ?).The pre-outburstflare studied here reachesa PCA flux
of810countss−1PCU−1,andthemaximumfluxduringthemain Applying the measured period and its derivatives to fold light
outburstis 1005countss−1PCU−1. The spectral and timing be- curves, we studied changes in the pulse profiles between the
pre-outburstflare and the main outburst. Strong changesin the
havioursofthesourceduringthepre-outburstflareappeartobe
differentfromtheonesobservedduringthemainoutburst. pulse-profile shape, with photon energy, are observed for both
the main outburst and pre-outburst flare but in quite different
ways. Fig. 2 shows pulse profiles from observations (a), dur-
2. InstrumentsandObservations ingthepre-outburstflare,and(b),closetothemaximumofthe
main outburst. Observation (b) was chosen because of its high
RXTE (Bradtetal. 1993) observed A0535+26 between 2005
dataqualityandthepulseprofilesarerepresentativeofdataac-
August 28 and 2005 September 24 in a target of opportu-
quiredduringtheoutburstpeak.Thelow-energypulseprofilesin
nity observation completed as part of a campaign studying ac-
bothcasesshow a complexpattern,butdifferentstructuresand
creting pulsars. We present observationsfrom the Proportional
evolution.Athigherenergiesbothpulseprofilesshowasimpler
CounterArrayPCA(2–60keV,Jahodaetal.1996)andfromthe
twopeakedshape.Duringthemainoutburst,oneofthepeaksis
High Energy X-ray Timing ExperimentHEXTE (20–200keV,
stronglyreducedatthecyclotronenergy.Duringthepre-outburst
Rothschildetal.1998).Atotalof44pointedobservationswere
flarethereisasmoothevolutiontowardshigherenergieswithout
completedtomonitortheoutburstwithatotalexposuretimeof
achangecrossingthecyclotronenergy.Energy-dependentpulse
∼140ks.Fig.1showsthePCAlightcurveduringtheoutburstin
profilesforalltheobservationsduringtheoutburstareshownin
the∼3–30keVrange,obtainedbyaveragingtheindividuallight
Camero-Arranz(2007)andadetailedstudywillbepresentedin
curvesof all the observations.The analysisofRXTEdata was
aforthcomingpaper(Camero-Arranzetal.2007).
performedwithFTOOLS6.3.1andthespectralanalysiswiththe
X-RaySpectralFittingPackageXSPECv11(Arnaud1996).
4. Spectralanalysis
3. Timinganalysis To model the phase-averaged,X-ray continuum, we have used
a phenomenologicalmodel, a power law times an exponential
3.1.Evolutionofpulseperiodduringtheoutburst
cut-off(XSPECmodelcutoffpl),whichisthesimplestcapa-
We measuredthepulseperiodofthepulsarwithhighaccuracy bleofreproducingthedata.ItisdescribedbyF(E)∝E−αe−E/Efold,
duringtheoutburst.Foreachavailableobservation,weextracted where α is the photonindex and E the foldingenergy.One,
fold
I.Caballeroetal.:Pre-outburstflareofA0535+26 3
−1U) 53610 53615 53620 M J D 53625 53630 53635 tion(b)weincludedaharmonic,cyclotronlinewiththeenergy
−1A rate (counts s PC1024680000000000 a) b) 9cfih0xa%endTgacaebotlinEenfic2edycnce=eonr1ncg0etya)2i.i.n5sWssktetehaVectiosb(nteiitccslitaus-lfildmytevesfaiargslounumeirfiesdctfhaoaenrttct.Ehoecnyttcho=ru1er0ep2ol.ob5ts+−se43r..t53vhkaaettViothnaest
PC a) b) during the pre-outburstflare and for the sum of those observa-
45 50 tions,aswellasthebest-fitvaluesforthemainoutburst(obser-
35 40 vation(b)inFig.2).
25 PCA 1.75−2.98keV 30 PCA 1.75−2.98keV
125 140
100 110
75 PCA 2.98−4.63keV 90 PCA 2.98−4.63keV 19.5 12
120 120 19.0 11
105 100
counts/s 111 956024000000 PPCCAA 46..6238−−68..2786kkeeVV 111 259878050005 PPCCAA 46..6238−−68..2786kkeeVV E(keV)fold111788...505 σ(keV) 1089
7
40 PCA 8.76−10.42keV 55 PCA 8.76−10.42keV 17.0
100 275 44 46 48 50 52 44 46 48 50 52
E (keV) E (keV)
80 225 cyc cyc
60 PCA 10.42−20.45keV 175 PCA 10.42−20.45keV Fig.3.Efold vsEcyc (left)andσvsEcyc (right)contourplotsfor
70 100 oneobservationnearthemaximum(dottedlines,observation(b)
50 80 in Fig. 2) and for the sum of the three available observations
30 HEXTE A 20.15−45.47keV 60 HEXTE A 20.15−45.47keV duringthepre-outburstflare(solidlines).Thecontoursindicate
12 20 χ2 +2.30(68%),4.61(90%),9.21(99%)levels.
6 min
10
0 HEXTE A 45.47−91.19keV 0 HEXTE A 45.47−91.19keV
0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0
Phase Phase
Fig.2.PCAandHEXTEbackgroundsubstractedpulseprofiles
mfLr(oa5−mx5i0mkoeunVme)∼oo0bf.4steh5re×voa1uti0tob3n7uerdrsgutr(sib−n)1g,,watnhitdehfpLror(e5m−-p5o0eknreiVea)so∼tbr0os.en7r5vfl×aatri1oe0n3(7ane)er,agrwst−iht1he. −1−1nts s PCU) 1680000000 53610 5361M5JD 53620 53625
Two pulse cycles are shown for clarity. In the upper panel the ou 400
PCAlightcurveisshown.Theverticallinesindicatethecorre- e (c 200
spondingobservations.Theexposuretimesare2.2ksin(a)and A rat 60
C 50
12.35ksin(b). P 55
45
Ecyc50 53610 53611
andinsomeobservationstwo,absorptionfeatureswereincluded 45
in the model to enable an accurate description of the data. We 40
modeledthemwithaGaussianopticaldepthprofile,whichmod- 2.0
ifies the continuum in the following way: F′(E)=F(E)e−τ(E), 1.5
α 1.0
where τ(E)=τe−(E−Ecyc)2/(2σ2). A Gaussian emission line was 0.5
addedto the modelto accountforthe Fe Kα fluorescenceline, 0.0
withenergyfixedat6.4keVandwidthto0.5keV.Duetoafea- V) 30
e
etudrgee,inseteheRorethsisdcuhaillds eatroaul.n2d0∼06)4,.7wkeeeVxc(liundsetrdumdaetnatfaolrXeenneorgnieLs E(kfold 1200
below5keVinouranalysis.
29 1 8
We performed a spectral analysis of all observations avail-
2005 August 2005 September
able and studied the evolutionof the cyclotron-lineenergyand
Fig.4. Firstpanel:PCAlightcurve.Secondpanel:fundamental
continuumparametersduringtheoutburst.Wemeasuredacon-
CRSFenergyduringtheourbusrt.Insetshowsazoomonthepre-
stant value for the centroid energy of the fundamental CRSF
outburstflare.Thirdandforthpanels:photonindexandfolding
duringthemainoutburst,asreportedbyCaballeroetal.(2007).
energyevolution.Errorsare90%confidenceforoneparameter
During the pre-outburst flare, the cyclotron-centroid energy is
ofinterest(χ2 +2.7).
measured at a higher value, reaching E =52.0+1.6keV, com- min
cyc −1.4
pared to E =46.1+0.5keV during the main outburst (at 90%
cyc −0.5
confidence). To study the significance of the change, we have Fig. 4 shows the evolution of the CRSF energy during the
producedχ2 contourplots for the observationsduringthe flare outburst, as well as the evolution of the continuum parameters
and during the main outburst. As an example,in Fig.3 contour photonindexαandfoldingenergyE fortheobservationsin
fold
plotsareshownfortheobservationclosetothemaximumofthe whichthecyclotronlinewas detected.Thecontinuumparame-
mainoutburstlabeled(b)inFig.2,andforthesumofthethree tersarequitevariableatthebeginningoftheoutburst.Thespec-
availableobservationsduringthepre-outburstflare.Inobserva- trumatthebeginningoftheflareappearstobesofter,withpho-
4 I.Caballeroetal.:Pre-outburstflareofA0535+26
tonindexα∼1.2,andthenbecomesharderintheflare.Therising note first of all that the timescale of the flare in their simula-
ofthemainoutburstpresentsaharderspectrum,withα∼0.6.The tionsappearstobetoolong(10%oftheorbitalperiod)compared
decayoftheoutburstshowsasmoothsofteningofthespectrum. totheflaredurationobserved.TheinspectionoftheSwift-BAT
light-curveofA0535+26duringtheoutburststudiedhereshows
that RXTE actually observed only one of a collection of short
5. SummaryandDiscussion flares in the rising part of the outburst (?). Such flares are not
reproducedbytheabovesimulations.Thismodeldoesnottake
In this paper, we present evidence for significant changes in
intoaccountthedisk-magnetosphericinteraction.
the spectral and timing parameters of the Be/X-ray binary
Amoreplausibleinterpretationisthatthepre-outburstflares
A0535+26, during its normal outburst in August/September
couldbecausedbymagnetosphericinstabilitiesbetweentheac-
2005.Ourthreemainresultsarethefollowing:
cretiondisk andthe neutron-starmagnetosphereattheonsetof
1. The pulse periodof the neutronstar appearsto be constant accretion. The instability causes the plasma that has accumu-
duringthepre-outburstflare,P=103.3960(5)s,andaspin-up lated in the disk-magnetosphere,boundary-layerto rapidly fall
startsatperiastron,P˙=(−1.69 ±0.04) ×10−8ss−1measured onto the neutron star surface close to the magnetic poles, but
at MJD 53618. The pulse period falls exponentially at the alongdifferentfieldlinesthan(quasi)stationaryaccretion(see?
endoftheoutburst. fordetails).
2. Theenergy-dependent,pulseprofilesduringthepre-outburst
flare are significantlydifferentfromthose measuredfor the Acknowledgements. WethankISSI(Bern),fortheirhospitalityduringtheteam
meetingsheldthere.ICthanksESAC(ESA)inMadridfortheirhospitality.This
mainoutburst.
researchissupportedbytheBundesministeriumfu¨rWirtschaftundTechnologie
3. During the pre-outburst flare, the fundamental cyclotron- throughtheGermanSpaceAgency(DLR)undercontractno.50OR0302.
lineenergycentroidreachesE =52.0+1.6keV,significantly
cyc −1.4
higherthanforthemainoutburst,E =46.1+0.5keV. References
cyc −0.5
This is the first observation of a normal outburst that mea- Arnaud,K.A.1996,inAstronomicalSocietyofthePacificConferenceSeries,
Vol. 101, Astronomical Data Analysis Software and Systems V, ed. G. H.
suresa spin-uprate forA0535+26,providingevidencethatan
Jacoby&J.Barnes,17
accretion disk is being detected during a Type I outburst. The Bradt,H.V.,Rothschild,R.E.,&Swank,J.H.1993,A&AS,97,355
measuredspin-upP˙=(−1.69 ±0.04) ×10−8ss−1,orν˙=(1.58 ± Caballero,I.,Kretschmar,P.,Santangelo,A.,etal.2007,A&A,465,L21
0.04) ×10−12Hzs−1,issmallerthanthespin-upmeasureddur- Camero-Arranz,A.2007,PhDthesis,,UniversidaddeValencia
inggiantoutburstsinthepast,e.g.,ν˙∼0.6×10−11Hzs−1 inJune Camero-Arranz,A.,Wilson,C.A.,Connell,P.,etal.2005,A&A,441,261
Camero-Arranz,A.etal.2007,inpreparation
1983(Sembayetal.1990),orν˙∼1.2×10−11Hzs−1 inFebruary
Ferrigno,C.,Segreto,A.,Santangelo,A.,etal.2007,A&A,462,995
1994(Fingeretal.1996). Finger,M.H.2005a,ATel,676
The pulse profiles measured during the main outburst are Finger,M.H.2005b,ATel,595
consistent with those during a giant outburst in March/April Finger,M.H.,Bildsten,L.,Chakrabarty,D.,etal.1999,ApJ,517,449
Finger,M.H.,Camero-Arranz,A.,Kretschmar,P.,Wilson,C.,&Patel,S.2006,
1989 albeit at a different luminosity level, L ∼1.26 ×
(23−53keV) inBulletin oftheAmerican Astronomical Society, Vol.38,Bulletin ofthe
1037ergs−1 (Kendziorraetal. 1994). A comparison between AmericanAstronomicalSociety,359
thesepulseprofilesandthoseobtainedforthepre-outburstflare Finger,M.H.,Cominsky,L.R.,Wilson,R.B.,Harmon,B.A.,&Fishman,G.J.
in this paper suggest that a different mode of accretion takes 1994,inAIPConf.Proc.308:TheEvolutionofX-rayBinariese,ed.S.Holt
&C.S.Day,459
placeduringtheflare(see?).
Finger,M.H.,Wilson,R.B.,&Harmon,B.A.1996,ApJ,459,288
The energy of the CRSF during the main outburst is Finger,M.H.etal.2007,inpreparation
consistent with the energy measured with INTEGRAL close Giangrande,A.,Giovannelli,F.,Bartolini,C.,Guarnieri,A.,&Piccioni,A.1980,
to the maximum of the main outburst, E =45.9 ± 0.3keV A&AS,40,289
cyc
Grove,J.E.,Strickman,M.S.,Johnson,W.N.,etal.1995,ApJ,438,L25
(Caballeroetal. 2007) and with the energy measured with
Hayasaki,K.&Okazaki,A.T.2006,MNRAS,372,1140
Suzakuat the end of the main outburst, E =46.3+1.5keV (us-
cyc −1.3 Hill,A.B.,Bird,A.J.,Dean,A.J.,etal.2007,MNRAS,381,1275
ing the same Gaussian model for the line, Teradaetal. 2006). Jahoda,K.,Swank,J.H.,Giles,A.B.,etal.1996,inProc.SPIEVol.2808,p.
From the observed cyclotron energy, the estimated, magnetic 59-70, EUV,X-Ray,andGamma-Ray Instrumentation forAstronomy VII,
fieldatthesiteoftheX-rayemissionduringthemainoutburst, OswaldH.Siegmund;MarkA.Gummin;Eds.,ed.O.H.Siegmund&M.A.
Gummin,59–70
assuming a redshift of z=0.3, is B∼5.2 × 1012G, and during
Kendziorra,E.,Kretschmar,P.,Pan,H.C.,etal.1994,A&A,291,L31
the pre-outburst flare B∼5.8×1012G. During the main part of Kretschmar,P.,Kreykenbohm,I.,Pottschmidt,K.,etal.2005,ATel,601
the outburst the cyclotron-line energy remains constant within Makino,F.,Cook,W.,Grunsfeld,J.,etal.1989,IAUCirc.,4769
the errors, in spite of the changes in the luminosity. This sug- Mowlavi,N.,Kreykenbohm,I.,Shaw,S.E.,etal.2006,A&A,451,187
Nagase,F.,Hayakawa,S.,Kunieda,H.,etal.1982,ApJ,263,814
geststhatthestructureoftheaccretioncolumnisdifferentfrom
Postnov,K.etal.2007,inpreparation
that observed in both V 0332 +53 (Mowlavietal. 2006) and
Rosenberg,F.D.,Eyles,C.J.,Skinner,G.K.,&Willmore,A.P.1975,Nature,
4U 0115+63 (Tsygankovetal. 2006). The source is likely to 256,628
be in the sub-Eddingtonregime,as for Her X-1 (Staubertetal. Rothschild,R.E.,Blanco,P.R.,Gruber,D.E.,etal.1998,ApJ,496,538
2007)forwhichitisbelievednoshockhasformedintheaccre- Rothschild,R.E.,Wilms,J.,Tomsick,J.,etal.2006,ApJ,641,801
Sembay,S.,Schwartz,R.A.,Orwig,L.E.,Dennis,B.R.,&Davies,S.R.1990,
tioncolumn.
ApJ,351,675
Pre-outburst flares have been observed in other accret- Staubert,R.,Shakura,N.I.,Postnov,K.,etal.2007,A&A,465,L25
ing pulsars, such as 2S1845-024 (Fingeretal. 1999) and Steele,I.A.,Negueruela,I.,Coe,M.J.,&Roche,P.1998,MNRAS,297,L5
EXO2030+375(Camero-Arranzetal.2005). Terada,Y.,Mihara,T.,Nakajima,M.,etal.2006,ApJ,648,L139
Tsygankov,S.S.,Lutovinov,A.A.,Churazov,E.M.,&Sunyaev,R.A.2006,
Hayasaki&Okazaki (2006) have modeled accretion disks
MNRAS,371,19
around neutron stars in Be/X-ray binaries. Their SPH simula-
Tueller,J.,Ajello,M.,Barthelmy,S.,etal.2005,ATel,504
tionsreproducea seriesof normaloutburstsfromthe accretion Wilson,C.A.&Finger,M.H.2005,ATel,605
diskformedabouttheneutronstaratperiastron.Insomecases,
single flares are found preceding the outburst maximum. We
I.Caballeroetal.:Pre-outburstflareofA0535+26 5
Table2.Best-fitvaluesfortheavailableobservationsduringthepre-outburstflare,wheretheCRSFenergyismeasuredatahigher
position,forthesumoftheobservationsduringtheflare,andforoneobservationclosetothemaximumofthemainoutburst.Lis
theluminosityinunitsof1037ergs−1inthe5−50keVrange.Errorsare90%confidenceforoneparameterofinterest(χ2 +2.7).
min
Obs.ID E (keV) σ(keV) τ E (keV) α MJDstart Exp.(ks) L χ2 /dof
cyc fold red
91086-01-03-01 48.3+1.1 6.4+1.0 0.27+0.03 18.2+0.4 0.73+0.02 53610.599 1.152 0.85 1.19/170
−1.0 −1.0 −0.03 −0.4 −0.02
91086-01-03-02 50.2+1.8 8.7+1.4 0.57+0.08 18.8+1.0 0.91+0.04 53610.665 2.208 0.45 0.88/170
−1.6 −1.3 −0.08 −1.0 −0.04
91086-01-03-03 52.0+1.6 10.8+1.3 0.53+0.06 19.2+0.8 0.83+0.03 53610.729 2.560 0.57 0.90/170
−1.4 −1.1 −0.06 −0.7 −0.03
sum 49.9+0.8 8.6+0.7 0.40+0.03 18.3+0.4 0.77+0.02 53610.599 5.920 0.66 1.18/220
−0.8 −0.7 −0.03 −0.4 −0.02
91085-01-01-03 46.1+0.5 10.1+0.5 0.47+0.03 18.0+0.4 0.59+0.02 53614.7 12.352 0.75 1.19/214
−0.5 −0.5 −0.02 −0.3 −0.02
102.5 8.4+2.2 1.0+0.4(1stharmonic)
−1.9 −0.2