Accreting X-ray pulsars observed with Integral

0.31 MB·English

by  Ingo Kreykenbohm

ACCRETINGX-RAY PULSARS OBSERVED WITHINTEGRAL
IngoKreykenbohm1,2,SonjaFritz1,NamiMowlavi2,Jo¨rnWilms3,PeterKretschmar4,Ru¨digerStaubert1,and
AndreaSantangelo1
1Institutfu¨rAstronomieundAstrophysik–Abt. Astronomie,Sand1,72076Tu¨bingen,Germany
2IntegralScienceDataCentre,Chemind’Ecogia16,1290Versoix,Switzerland
3Dr. RemeisSternwarte,Universita¨tErlangen-Nu¨rnberg,Sternwartstr. 7,96049Bamberg,Germany
4EuropeanSpaceAstronomyCentre(ESA),Madrid,Spain
ABSTRACT mechanismisonlyroughlyunderstood,asadetailedun-
7
derstandingwouldrequireafullmagneto-hydrodynamic
0
0 treatment,incorporatingthepresenceofamagneticfield
2 In this paper, we review the observational properties of∼1012G,andafullhandlingofgeneralrelativity. Es-
of two accreting X-ray pulsars, the persistent source pecially the behavior and shape of cyclotron resonant
n 4U1907+09andthetransientV0332+53.AccretingX- scattering features (CRSFs or cyclotron lines) are not
a
raypulsarsareamongthebrightestsourcesintheX-ray verywellunderstoodandrequiredetailedstudying. But
J
skyandarefrequentlyobservedbyIntegralandotherX- also only a very general picture of the overall accretion
0
ray missions. Neverthelessthey are still very enigmatic geometryandtheaccretionprocessitselfexists. Analyz-
3
sources as fundamental questions on the X-ray produc- ingthetemporalbehaviorofaccretingneutronstarsalong

1 tion mechanism still remain largely unanswered. These with thespectralpropertiesallowstostudytheseimpor-
v questionsareaddressedbyperformingdetailedtemporal tantquestions.
9 and spectral studies on several objects over a long time
6 range. INTEGRAL is an excellent instrument to study these
8 sources asINTEGRALfrequentlyobservesthe galactic
1 Ofvitalimportanceisthestudyofcyclotronlinesasthey centerandthegalacticplaneandduetoitslargefieldof
0 provide the only direct link to the magnetic field of the view. Asaresult,anenormousamountofdataonaccret-
7 pulsar. While some objects show cyclotron lines which ing X-ray pulsars is obtained. In this paper we present
0 are extremely stable with time and with pulse phase, in some recent results on the analysis of two accreting X-
h/ otherobjectsthelinesstronglydependonthepulsephase, ray pulsars: the detection of a complete torque rever-
p theluminosityofthesource,orboth. salin4U1907+09[9] andtheevolutionofthetransient
- V0332+53overitsrecentoutburstandespeciallythebe-
o Ofspecialinterestinanycasearetransientsourceswhere
haviorofthecyclotronline[27,33].
r it is possibleto studythe evolutionofthe cyclotronline
t
s andthesourceingeneralfromtheonsetuntiltheendof
a
theoutburstthroughawiderangeofdifferentluminosity
: 2. 4U1907+09
v states.
i
X
We present an observationalreview of a transient and a
r persistentaccretingX-raypulsarobservedwithIntegral. The wind-accreting High Mass X-ray Binary system
a 4U1907+09 [13] consists of a neutron star in an ec-
centric (e = 0.28)8.3753d orbitaroundits companion.
Keywords: X-rays: stars–stars: magneticfields–stars: Basedonalowerlimitforthedistanceof5kpc,theX-ray
flare – stars: pulsars: individual: 4U 1907+09 – stars: luminosityisapproximately21036ergs−1[15].Thestel-
pulsars:individual:V0332+53. lar companionhas been classified as a O8–O9 Ia super-
giantwith an effectivetemperatureof 30500K, a radius
of26R ,aluminosityof5105L ,andamasslossrate
⊙ ⊙
1. INTRODUCTION of710−6M⊙yr−1[6].
Similar to other accreting neutron stars, the X-ray con-
Accreting X-ray pulsars are among the most prominent tinuumof4U1907+09canbedescribedbyapower-law
sources in the X-ray sky. Since accreting X-ray pul- spectrumwith anexponentialturnoverat∼13keV [25].
sars are usually young objects, they are clustered along Thespectrumismodifiedbystrongphotoelectricabsorp-
the galactic plane. Although already discovered in the tion with a columnN = 1.5–5.71022cm−2 [7] which
H
first days of X-ray astronomy, many fundamentalques- isduetothedensestellar windandisthereforestrongly
tions remainunansweredto date. The X-rayproduction variable over the orbit. The column density is maximal
1.8 MJD
1.6
−1ec] 1.4 53138.4 53139.0 53139.6 53140.2
s
unts [ 1.2 30 53138.12 53M13 J8D.20 53138.28
Co 1.0 50
−
M
−1ASec] 00..684 −1ounts s] 20 −1Count rate [counts s] 12340000
Counts [s 3 nt rate [c −100 3 4 5 6 7
BIS− 2 Cou 10 2004 May 13
I
1
0.0 0.5 1.0 0.5
Orbital−Phase (T = 0) 0
90
Figure1. Lightcurveof4U1907+09,foldedwiththeor-
bitalperiodof8.3753days[from9]. UpperPanel:Rossi 0 6 12 18 0 6 12 18 0 6 12
X-rayTimingExplorerAllSkyMonitor(ASM)2–12keV. 2004 May 13 2004 May 14 2004 May 15
LowerPanel: IBIS20–40keVlightcurve.
Figure 2. IBIS(ISGRI)20–40keV lightcurve ofrevolu-
tion193[from9]. Thebintimeis200s. Theinsetshows
betweentheendoftheprimaryX-rayflareandthestartof
a close up of the flare, the bin time in this case is 40s.
thesecondaryflare(Fig.1)oftheorbitallightcurve[31].
X-axisnumbersarehours.
Athigherenergies,thespectrumexhibitscyclotronreso-
nantscatteringfeatures(CRSF)at∼19keVand∼40keV MJD
[24,25,7].
46000 49000 52000
442
Withapulseperiodof∼ 440s,4U1907+09isaslowly
rotating neutron star [23]. The neutron star has ex-
hibited a steady linear spin down with an average of
P˙ = +0.225syr−1 from P = 437.5s in 1983
pulse pulse
˙
to 440.76s in 1998 [16]. Recently, a decrease in P
pulse
to∼0.115syr−1, abouthalfthelongtermvaluewasre- 440
ported[2].
c]
e
s
d [ MJD
o 52800 53100 53400
2.1. TimingAnalysis Peri 441.4
441.3
TpuhlesaXtio-rnasywliigthht acuprveerioodf 4oUf 1∼940471+s0.9 clAeadrdliytiosnhaolwlys, 438 Period [sec] 441.2
4U1907+09also exhibitsflareswitha typicallengthof 441.1
severalhours[23,16]. Fritzetal. [9]observedfoursuch
441.0
flareswithcountrateincreasesofatleast5σoveritsnor- 2003 2004 2005
mallevel(Fig.2).Threeofthem,inrevolutions187,193, 436
and305,areassociatedwiththemainpeakintheorbital 1985 1990 1995 2000 2005
lightcurve(Fig.1)whiletheflareobservedinrevolution
188islinkedtothesecondarypeak. Figure3.Evolutionofthe4U1907+09pulseperiodover
thelast20years[asshownby9]. Diamondsindicatethe
4U1907+09showedasteadyspindownrateofP˙ = resultsobtainedinthiswork,othersymbolsindicatedata
pulse
+0.225syr−1 [16]. Baykal et al. [2] usingRXTE-data from[23,plus-signs],[5,asterixes],[25,triangles],[16,
determined a spin down rate 0.5 times lower than the squares], and [1, 2, crosses]. Note the clear deviation
longtermvalue. Fritzetal. [9]usingINTEGRALdata, from the long term trend leading to a complete spin re-
extend the tracking of the pulse period evolution to the versal. Theinsetshowsacloseupoftheperiodsfoundby
years2003–2005.Thereforetheseauthorsfirstdetermine INTEGRAL[9].
the pulse period by first using epoch folding [22] to get
3 Rev. 0048
a starting period and then performing pulse phasing to
2
get a higher accuracy. Fig. 3 shows the long-time his- 1
tory of the period evolution based on all available data. 0 a
NotethattherecentperiodsobtainedfromRXTE-data[2]
4 Rev. 0059
showacleardeviationfromthehistoricspindowntrend 3
of P˙ = +0.225syr−1 from [15]. The periods ob- 2
tainepdulsbeyFritz etal. [9] fromINTEGRAL-data(shown 1 b
as diamonds) not only confirm this change of the trend, 3 Rev. 0069
2
butshowacompletetrendreversalfromthehistoriclong 1
termspindowntrendtoaspinuptrendfromMJD53131 0 c
onwards(seeFig.3).
3 Rev. 0135
2
Usingthederivedperiods,Fritzetal. [9]obtainedpulse 1
profiles for all revolutions (Fig. 4) by folding the light 0 d

curveswiththebestrespectiveperiods.Nochangeinthe 6 Rev. 0188
shapeofthepulseprofileisobvious,especiallynochange
4
˙
intheshapebefore,during,andafterthereversalofP can 2
beseen. e
6 Rev. 0193
4
2
2.2. Discussion f

6 Rev. 0246
IntheINTEGRALdata,aspinupphaseof4U1907+09 4
iabsercianltegeacorlofynPds˙piesuttleseecnt=tedw+iatfh0te.P2r¨22=50sy0yersa−−r1s1,.owfInistph2i0nt0hd2eoaawvnnadi(l2Fa0big0le.33dfi)aratsatt −1ounts sec] 224 Rev. 0248 g
iwnedriecadteiotencstefodr[a2]d.eAcrsesahsoewinntihneSmecatg.n2i.t1u,dtehoefnsepwinINdoTwEn- ate [c 0 h
GRAL data show a torque reversal from ∼MJD53131 nt R 23 Rev. 0249
onwards,withthesourcenowexhibitingaspinupwitha ou 1
rate of P˙pulse = −0.158syr−1. So far, theINTEGRAL C 0 i
resultsappeartobeconsistentwithP¨ =0s−1(the1σup- 3 Rev. 0250
perlimitforP¨ duringthespinupis−410−5s−1yr−2). 2
1
The conventional interpretation of spin up in accreting 0 j
X-raypulsarswithastronglymagnetized,disk-accreting 4 Rev. 0255
neutronstar isthattheaccretiondiskistruncatedbythe 2

magnetic field of the neutron star [see, e.g., 10, 11, 12, 0 k
andreferencestherein]. Thetransferofangularmomen-
3 Rev. 0295
tum from the accreted matter onto the neutron star will 2
lead to a torque onto the neutron star resulting in spin 1
up. The general expectation of these models is that the 0 l

Alfve´nradiusrm,wheretheaccretionflowcouplestothe 6 Rev. 0305
magneticfield, is assumedto be close to the co-rotation 4
radius,r . 2
co m

Thissimplemodel,however,isunabletoexplainthelong 6 Rev. 0361
episodes of constant P˙. For 4U1907+09the magneto- 4
spheric radiusinferredfromthe cyclotronline measure- 2 n
ments is r ∼ 2400km, while r ∼ 12000km [16].
m co 3 Rev. 0379
Moving rm out to rco would require a magnetic field of 2
∼1014Gasforamagnetar,whichistwoordersofmag- 1
nitude larger than the magnetic field deduced from the 0 o
observedcyclotronlines. 0. 0 0. 5 1. 0 0. 5
Pulse Phase
Fritzetal.[9],however,thendiscussthattheamodelpre-
Figure 4. Pulse profiles for different revolutions in the
sentedrecentlybyPernaetal.[28]isabletoexplainthese
20–40keVband[from9]. Panelsa–eshowpulseprofiles
observations. ThismodelisbasedonthefactthatX-ray
obtainedduringtheperiodofspindown,andpanelsf–o
pulsarsare obliquerotators, i.e., the neutronstar’s mag-
showspinuppulseprofiles.
neticfieldistiltedbyanangleχwithrespecttotheaxis
of rotation of the neutron star. Assuming the accretion
diskissituatedintheneutronstar’sequatorialplane,for
a ringofmatter in the accretiondisk, the magneticfield s)
strength then depends on the azimuthal angle and thus crab 1 20-30 keV V 0332+53
theboundaryofthemagnetosphereisasymmetric. Such ux ( 30-40 keV
aconfigurationcanleadtoregionsinthediskwherethe Fl
propeller effect is locally at work, while accretion from
other regions is possible. This then results in a nonlin-
˙
ear dependenceof the accretion luminosity from the M 40-60 keV
throughtheouterpartsofthedisk. Pernaetal. [28]point
out that a nontrivialconsequenceis that for values of χ 10-1
greaterthanacriticalvalue,χ ,limitcyclesarepresent,
crit
where cyclic torquereversalepisodesare possible with-
˙ a. IBIS/ISGRI
out a change in M. For typical parameters. χ is be-
crit
tween ∼25◦ and ∼45◦. Perna et al. [28] and Fritz et
al. [9] stress that the existence of the limit cycles de- s)
pendsonlyonχandonthepulsar’spolarmagneticfield, crab 10-15 keV
B, and no variation of external parameters is required x ( 5-10 keV
u
to trigger torque reversals, while the torque reversal of Fl 1
4U1907+09 with no associated drop in luminosity and 3-5 keV
nochangein theshapeofthepulseprofileisdifficultto RXTE/ASM
explaininconventionalmodels.
10-1
3. V0332+53
b. JEM-X
53350 53360 53370 53380 53390 53400 53410 53420
TherecurrentX-raytransientV0332+53experiencedan t (MJD)
X-ray outburst from December 2004 to February 2005
[19,29]. Theoutburstwaspredictedalmostoneyearbe- Figure5. a LightcurveofV0332+53overthedecayof
foretheactualoutburst[14]duetotheopticalbrightening the outburst in different energy bands in Crab units [as
oftheopticalcompanionBQCam,aO8-O9estar. shownby27]. Thesolidcurvesarefitstotheexponential
(fromMJD=53376to53412withτ=30days)andlinear
TheobservationofthisoutburstofV0332+53byRXTE (fromMJD=53412to53422)bSameasa,butforJEM-X
revealedthepresenceofthreecyclotronlines[4,19,29]. fluxesandwithanexponentialdecayofτ=20days. The
ThefundamentalCRSFhasanenergyof28keV,andthe 2–12keVRXTE/ASMlightcurveisalsoshown,usinga
firstandsecondharmonicsareat50keVand71keV,re- normalizationfactorof77cpsfor1Crab.
spectively. Thiscorrespondstoamagneticfieldstrength
of2.7×1012G[19,29]. Thespectrumisotherwisede-
scribedbyapowerlawmodifiedbyahigh-energycut-off
athigherenergies.
0.5
PCU2 RXTE
0.45
3.1. Fluxes
0.4
ThelightcurveofV0332+53inISGRIandJEM-Xdur-
C0.35
ing the outburstis shownin Fig. 5. Mowlavietal. [27] H F
describethedeclinephasebyanexponentialdecayupto E
0.3
MJD≃53412, followedby a linear decrease. The expo- D
nential decay is less rapid at the ISGRI energiesthan at C
0.25
theJEM-Xenergies.Fromasimpleestimation,theexpo- B
A
nentialdecaytime isabout30 daysabove20keV (solid
0.2
curvesinFig.5a)andabout20daysbelow15keV(solid
curves in Fig. 5b). The exponential decay is also con- 0.4 0.45 0.5 0.55 0.6
SC
firmedbythe2–12keVlightcurveasobservedbytheAll
SkyMonitor(ASM)onboardofRXTE(seeFig.5b).
Figure 6. Color-Color Diagram of the outburst of
V0332+53[from30]usingRXTE-data. TheX-raycol-
Reig et al. [30] discovered a very interesting behavior
ors are defined as soft color SC=7.5-10 keV/4-7.5 keV
ofthecolor-colordiagramovertheoutburst(seeFig.6).
andhardcolorHC=15-30keV/10-15keV.
They derived background-subtractedlight curves corre-
sponding to the energyranges c1 = 4−7.5 keV, c2 =
30
a E (keV)
29.5 cyc,1
29
28.5 a)
V0332+53
28
V
27.5 V 0332+53 e
k
c/1.000
4.9 e
4.8 s/s
4.7 nt
4.6 u
4.5 o0.100
4444....2341 b s 1 (keV) alized c
2.15 rm0.010
2.1 c t o
2.05 1 n
2
1.95
1.9
1.85 0.001
1.8 60
1.75
40 b)
53380 53385 53390 53395 53400 53405 53410 53415 53420
20
t (MJD)
0
c −20
Figure 7. Evolutionofthe parametersof the fundamen- −40
−60
talcyclotronline[from27]. alineenergy,bwidth,andc
depth.Theverticalbarsrepresent90%uncertainty,while 10 c)
5
the horizontalbars indicate the duration of each obser-
0
vation. c −5
−10
6
7.5−10keV,c3 = 10−15keVandc4 = 15−30keV 4 d)
andthendefinedthesoftcolor(SC)astheratioc2/c1and 2
thehardcolor(HC)astheratiobetweenc4/c3. c −02
−4
Theseauthorsobservedthatthesourcewasinasoftstate −6
duringthepeakoftheoutburstandasthecountratede- 10 e)
creasedthesourcemovedupandrightinthecolorcolor 5
diagram,i.e. itbecameharder. Attheendoftheoutburst 0
(regionsEandF)thesourcemovedagaintotheleft,i.e., c −5
lowervaluesofSC,whileHCremainedmostlyconstant. −10
Therefore two spectral states or branches can be distin-
10 f)
guished:asoftstatewhenthesourceisbrightandahard 5
statewhenthesourcegetsdimmer. 0
c −5
ThepatterninthecolorcolordiagramofV0332+53re- −10
semblesthatoflowmassX-raybinaryZsource[seee.g
10 100
34]. Inthispicture,thesoftbranchwouldcorrespondto
Channel Energy [keV]
the normalbranchandthe hardbranchto the horizontal
branchin a Z source, however,as Reig et al. [30] point
out, the flaring branch observed in Z source is missing
Figure 8. a Spectrum and folded model for JEM-X
in V0332+53. During the decline of the outburst, the
(left) and ISGRI (right) using data of revolution 284
source moveswithoutjumpsthroughthe Z shape in the [from 27]; b residuals of the model with Γ=−0.41 and
colorcolordiagram,takingabout80daysfromthesoftest
Efold=7.4keV,andwithoutanycyclotronlinemodeled;c
tohardestpoint[30].
one Gaussianis includedat29.3keV;d a secondGaus-
sian is added at 51keV to model the first harmonic; e
as before, but with the energy of the first line fixed at
3.2. Spectrum 27.5keV,thevaluefoundinrevolution273;fsameasd,
butwithΓfixedtothevalueof−0.17foundinrevolution
273. Thestrongresidualsinthetwobottompanelsshow
The spectra of accreting X-ray pulsars are a superposi- thattheparametersofthefundamentalcyclotronlinedo
tion of contributionsfromthe hotspotsat bothmagnetic indeedchange.
polesandthelowerpartsoftheaccretioncolumns.Since
no self consistent model is available, various empirical
modelshavetobeused[18]
dependenceresultsinthefollowingrelation:
Ecyc,1 =−0.10L37+28.97keV,
where L37 is the luminosity in units of 1037erg s−1.
Assuming that for low luminosities the emission comes
practically from the neutron star surface, Tsygankov et
al. [33]estimatethemagneticfieldonthesurfaceofthe
neutron star for canonical values for the radius and the
massoftheneutronstartoB =3.01012G.
3.3. Discussion
V0332+53isahighlymagnetizedpulsarwithamagnetic
fieldofabout3.0×1012G[19,29,33],asderivedfrom
theenergyofthefundamentalcyclotronline. TheX-ray
outburstsfromthissystemareobservedwithatimerecur-
renceofabout10years.Theyarebelievedtobetriggered
by a massive mass ejection from the optical companion
star, resulting in the optical brightening of the system
in 2004 [14]. The ejected mass feeds an accretion disk
Figure9. Dependenceoftheenergyoftheenergyofthe
fundamentalcyclotronlineofV0332+53[from33]. Tri- aroundthepulsar,fromwhichitisfunneledbythemag-
neticfieldtowardsthepolarcapsandfallsonthesurface
angles represent results from INTEGRAL, and squares
oftheneutronstar.
fromRXTE.
The analysis of the 2005outburstprovidesnew insights
towards a better understanding of V0332+53 and simi-
Mowlavi et al. [27] use the simple “cutoffpl” model to lartransientsourceswithinthisbasicpicture. TheX-ray
fit the combined ISGRI and JEM-X data, to which two lightcurve(Sect.3.1)displaysacharacteristicexponen-
cyclotronlines with a Gaussian opticaldepth profile[3] tialdecaywithatimescaleof20to30days,followedby
havebeenadded. Amultiplicativeconstantisappliedto alineardecrease.Thisremarkablechangeoccursapprox-
allow for the differentnormalizationsof the two instru- imatelyatthesame momentwhenthe trendinthe color
ments. Furthermorea systematic errorof 2% is applied colordiagramchanges.Thismayprovidesomeinforma-
to accountfor the uncertaintiesin the responsematrices tionontheaccretionandespeciallythegeometry.
ofJEM-XandISGRI.Nophotoelectricabsorptionisre-
AsdiscussedbyMowlavietal. [27],anexponentialde-
quiredat low energies, compatiblewith the fact thatthe
cayofthefluxisobservedalsoindwarfnovaeandinsoft
JEM-X data are analyzed only above 4keV. The same
X-ray transients on time scales from a few days for the
modelisusedforallobservationstokeepconsistencybe-
former to several weeks for the latter [21, 32]. King &
tweentheparametersfromoneobservationtothenext.
Ritter [17] show how illuminated disks in low-mass X-
ray binary stars can produce such an exponential decay
The cyclotron line parameters for the fundamental line
onthetimescalesobservedforthesoftX-raytransients.
[27] are displayed in Fig. 7 as a function of time. The
Subsequently,numericalcalculationsbyDubusetal. [8]
spectrumwiththefitsandtheresidualsforamodelwith
showed,thatindeedirradiatedaccretiondisksproducean
no, one, two, andthreecyclotronlinesusing thedata of
exponentialdecayoftheX-rayluminosity.
revolution284isshowninFig.8. Theenergyofthefun-
damentalcyclotronlineincreasesby7%fromthefirstto
AsMowlavietal. [27] pointout,itistemptingtomake
the last INTEGRAL observations, and the depth of the the parallel with the results observed in V0332+53.
line increases by 16% while at the same time the width
The exponentially decreasing flux followed by a linear
decreasesby12%.Thesechangesaresignificant;accord-
decrease is very similar to what is observed in disk-
ingtoMowlavietal. [27],theattempttofitthespectrum
illuminated systems. Such a behavior as observed by
inrevolution284withalineat27.5keVresultsinstrong
Mowlavi et al. [27] has never before been observed in
residualsaroundtheline(Fig.8e), butalsovariationsof
a high mass accreting X-ray binary pulsar system. The
continuumcanberuledouttoexplainthechangesinthe
X-rayfluxisproportionaltothemassaccretionrate,and
lineparameters(Fig.8f).
therefore the accretion rate would then also be propor-
tional to the mass of the disk. An unknown change in
Tsygankovetal. [33]thenwentonestepfurtherandin- the disk would then trigger the switch to the linear de-
stead plotting the energy of the fundamental cyclotron cay phase. Furthermore, Mowlavi et al. [27] suggest
line versus time, these authors show the dependence of thatthetwodifferentdecaytimesobservedinV0332+53
thecyclotronenergyontheluminosityofthesource,thus areduetothepresenceofatleasttworegionscontribut-
combining Figs. 7 and 5 into Fig. 9. A linear fit to this ing to the X-ray continuum: one time scale governsthe
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heightswithintheaccretioncolumn[27].
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ACKNOWLEDGMENTS
RayAstronomy,Proc,XXVIIIYamadaConf.,Fron-
tiersScienceSeries,2,23–32
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