DTIC ADA587381: No Anticorrelation between Cyclotron Line Energy and X-ray Flux in 4U 0115+634

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by  Defense Technical Information Center

A&A551,A6(2013) Astronomy
DOI:10.1051/0004-6361/201220359 &
(cid:2)c ESO2013 Astrophysics
No anticorrelation between cyclotron line energy and X-ray flux
in 4U 0115+634
S.Müller1,C.Ferrigno2,M.Kühnel1,G.Schönherr3,P.A.Becker4,M.T.Wolff5,D.Hertel1,F.-W.Schwarm1,
V.Grinberg1,M.Obst1,I.Caballero6,K.Pottschmidt7,F.Fürst8,I.Kreykenbohm1,R.E.Rothschild9,P.Hemphill9,
S.M.Núñez10,J.M.Torrejón10,D.Klochkov11,R.Staubert11,andJ.Wilms1
1 Dr.KarlRemeis-Observatory&ECAP,UniversitätErlangen-Nürnberg,Sternwartstr.7,96049Bamberg,Germany
email:[email protected]
2 ISDCDataCenterforAstrophysics,UniversityofGeneva,16chemind’Écogia,1290Versoix,Switzerland
3 Leibniz-InstitutfürAstrophysikPotsdam,AnderSternwarte16,14482Potsdam,Germany
4 SchoolofPhysics,Astronomy,andComputationalSciences,MS5C3,GeorgeMasonUniversity,4400UniversityDrive,Fairfax,
VA22030,USA
5 SpaceScienceDivision,NavalResearchLaboratory,Code7655,4555OverlookAve.,S.W.,Washington,DC20375,USA
6 CEASaclay,DSM/IRFU/SAp-UMRAIM(7158)CNRS/CEA/UniversitéParis7,Diderot,91191Gif-sur-Yvette,France
7 CRESSTandNASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA,andCenterforSpaceScienceandTechnology,
UMBC,Baltimore,MD21250,USA
8 SpaceRadiationLaboratory,290-17Cahill,Caltech,1200EastCaliforniaBlvd.,Pasadena,CA91125,USA
9 CenterforAstrophysicsandSpaceSciences,UniversityofCalifornia,SanDiego,LaJolla,CA92093,USA
10 InstitutoUniversitariodeFísicaAplicadaalasCienciasylasTecnologías, UniversityofAlicante,POBox99,03080 Alicante,
Spain
11 InstitutfürAstronomieundAstrophysik,UniversitätTübingen,Sand1,72076Tübingen,Germany
Received10September2012/Accepted23November2012
ABSTRACT
WereportonanoutburstofthehighmassX-raybinary4U0115+634withapulseperiodof3.6sin2008March/Aprilasobserved
withRXTEand INTEGRAL.Duringthe outburst theneutron star’sluminosity variedby afactor of 10 inthe3–50 keV band. In
agreementwithearlierworkwefindevidenceoffivecyclotronresonancescatteringfeaturesat∼10.7,21.8,35.5,46.7,and59.7keV.
Previous workhad found ananticorrelation between the fundamental cyclotron lineenergy and theX-rayflux. Weshow that this
apparentanticorrelationisprobablyduetotheunphysicalinterplayofparametersofthecyclotronlinewiththecontinuummodels
usedpreviously,e.g.,thenegativeandpositiveexponentpowerlaw(NPEX).Forthismodel,weshowthatcyclotronlinemodeling
erroneously leadstodescribingpartof theexponential cutoff andthecontinuum variability,andnot thecyclotron lines.Whenthe
X-raycontinuumismodeledwithasimpleexponentiallycutoffpowerlawmodifiedbyaGaussianemissionfeaturearound10keV,
thecorrelation between theline energy and thefluxvanishes, and thelineparameters remain virtuallyconstant over theoutburst.
Wethereforeconclude thatthepreviouslyreportedanticorrelationisanartifactoftheassumptionsadopted inthemodelingofthe
continuum.
Keywords.X-rays:binaries–pulsars:individual4U0115+634–magneticfields
1. Introduction abovethesurfaceoftheneutronstar.SeeBeckeretal.(2012)for
adiscussionofthedifferentmodesofaccretionintheaccretion
InallaccretingX-raypulsarstheaccretiongeometryisstrongly column.
affectedbythemagneticfieldoftheneutronstar,whichhasval- Owing to the strong magnetic field of accreting X-ray pul-
ues of a few 1012G at the magnetic poles. The accretedmatter sars, cyclotron resonance scattering features (cyclotron lines;
is forcedto follow the magneticfield linesfromthe Alfvénra- CRSFs) are observable in many X-ray pulsars. These features
dius inward to the accretion columns located at the magnetic have been reported for about 20 X-ray pulsars (Caballero &
poles of the neutron star. The shape of these columns remains Wilms2011).CRSFsareproducedbyphotonsgeneratedinthe
anopenquestion.Thesimplestmodelsassumecylinderorslab- accretion column of the neutron star interacting with electrons
like geometries, although more complicated shapes have been (Schönherretal.2007;Mészáros1992,andreferencestherein).
discussed (Mészáros 1984; Becker et al. 2012). On each mag- The motion of these electrons perpendicular to the B-field is
neticpoleoftheneutronstar,ahotspotiscreated,emittingther- quantized into Landau levels with energy differences of about
malphotonsintotheaccretioncolumn(Davidson1973).These (cid:2) (cid:3)
photons are upscattered in energy by inverse Compton scatter- ΔE ≈12keV B · (1)
ing within the accretion column, forming the hard X-ray spec- 1012G
trum (Becker & Wolff 2007; Ferrigno et al. 2009). Depending
on the source luminosity,and thusmass accretion rate, M˙, and Photons inelastically scattering off these electrons will imprint
decelerationmechanism,aradiation-dominatedshockcanform absorption-linelikefeaturesontothespectralcontinuum.
ArticlepublishedbyEDPSciences A6,page1of10
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1. REPORT DATE 3. DATES COVERED
2013 2. REPORT TYPE 00-00-2013 to 00-00-2013
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
No anticorrelation between cyclotron line energy and X-ray flux in 4U
5b. GRANT NUMBER
0115+634
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
California Institute of Technology,Space Radiation Laboratory,1200 E. REPORT NUMBER
California Blvd,Pasadena,CA,91125
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13. SUPPLEMENTARY NOTES
14. ABSTRACT
We report on an outburst of the high mass X-ray binary 4U 0115+634 with a pulse period of 3.6 s in 2008
March/April as observed with RXTE and INTEGRAL. During the outburst the neutron star?s luminosity
varied by a factor of 10 in the 3?50 keV band. In agreement with earlier work we find evidence of five
cyclotron resonance scattering features at ∼10.7, 21.8, 35.5, 46.7, and 59.7 keV. Previous work had
found an anticorrelation between the fundamental cyclotron line energy and the X-ray flux. We show that
this apparent anticorrelation is probably due to the unphysical interplay of parameters of the cyclotron
line with the continuum models used previously, e.g., the negative and positive exponent power law
(NPEX). For this model, we show that cyclotron line modeling erroneously leads to describing part of the
exponential cutoff and the continuum variability, and not the cyclotron lines. When the X-ray continuum is
modeled with a simple exponentially cutoff power law modified by a Gaussian emission feature around 10
keV the correlation between the line energy and the flux vanishes, and the line parameters remain virtually
constant over the outburst. We therefore conclude that the previously reported anticorrelation is an
artifact of the assumptions adopted in the modeling of the continuum.
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE Same as 10
unclassified unclassified unclassified Report (SAR)
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
A&A551,A6(2013)
MeasuringtheenergyoftheCRSFyieldsinformationabout relationbetweensourcefluxF andfundamentalcyclotronline
X
the B-fieldstrengthinthelineformingregion.SincetheB-field energyE .WesummarizethispaperinSect.5.
0
ofaneutronstarcanbeassumedtobedipole-liketofirstorder,
in many models the centroid energy of CRSFs depends on the
characteristicemissionheight,i.e.,thealtitudeofthelineform- 2. Observationsanddatareduction
ingregion(Basko&Sunyaev1976;Burnardetal.1991;Mihara The outburst of 4U 0115+634 started 2008 March 12, after a
et al. 2004). Since this altitude depends on the complex rela- phase of almost four years of quiescence, when Swift/BAT de-
tion between the deceleration of the accreted material and the
tected a significant increase of the X-ray flux (Krimm et al.
mass accretion rate, it is expected to depend on the source lu-
2008). The outburst lasted about 40days and exceeded a flux
minosity. Such a dependencehas indeed been observed. Three of∼200mCrabinthe2–12keVRXTE/ASMband(Fig.1).The
types of behavior have been observed. One type shows a pos-
wholeoutburstwasmonitoredwithpointedobservationsbythe
itive correlation between the X-ray luminosity L and the cy-
X Rossi X-ray Timing Explorer (RXTE, Bradt et al. 1993) and
clotron line energy E (e.g., Her X-1, Staubert et al. 2007).
cyc severalpointedobservationsbytheINTErnationalGamma-Ray
TheothertypeshowsanegativecorrelationbetweenL andE
X cyc Astrophysics Laboratory (INTEGRAL, Winkler et al. 2003).
(e.g.,V0332+53,Nakajimaetal.2010,andreferencestherein).
Here, we use data from RXTE’s Proportional Counter Array
Finally,therearealso cyclotronsourcesknownwith a constant
(PCA, Jahodaet al. 2006)and the High EnergyX-Ray Timing
cyclotronlineenergy(e.g.,A0535+26,Caballeroetal.2007).
Experiment (HEXTE, Rothschild et al. 1998), as well as the
As suggested by Staubertet al. (2007) and Klochkovet al.
INTEGRAL Soft Gamma-Ray Imager (ISGRI, Lebrun et al.
(2011)andthenquantifiedbyBeckeretal.(2012),thesediffer-
2003).Data were reducedwith ourstandardanalysispipelines,
entrelationsbetween L and E couldbedueto the different
X cyc based on HEASOFT (v. 6.11) and INTEGRAL OSA v. 9.0.
decelerationmechanismsfortheaccretedmaterial.Aboveacer-
Data modeling was performed with the newest version of
taincriticalluminosity,L ,theaccretedmaterialisdecelerated
crit the Interactive Spectral Interpretation System (ISIS, Houck &
mainlybytheradiationpressure.Inthissituation,anincreasein
Denicola2000).
luminositystopsthematerialearlier,leadingtothelineforming
The PCA consisted of five proportional counter units
region being in a region of lower B-field strength and yielding
(PCUs), which were sensitive between 2 and 60 keV. Since
anegativecorrelationbetweenE andL .Forverylowlumi-
cyc X PCU 2 is known to have been the best calibrated one (Jahoda
nosities, the deceleration is dominated by the gas pressure. In
etal.2006),weonlyextractedspectrafromthetoplayerofthis
thisregimehighermassaccretionrateswillcompresstheaccre-
PCU inthestandard2fmode.ThehardX-rayspectrumfrom
tion moundand thus lead to a positive correlationbetween the
15–250keVwasmonitoredwithHEXTE.HEXTEconsistedof
luminosityandthecyclotronlineenergy.Finally,atintermediate
twoindependentclusters,AandB,botharraysoffourphoswich
luminosities one expects a regime where the accretion column
scintillationdetectors.The“rockingmechanism”ofthese clus-
is fairly stable and the deceleration is dominated by Coulomb
ters allowed a near realtime measurement of the background.
interactions.
Since this mechanism failed for cluster A before our observa-
4U0115+634(Giacconietal.1972)isoneoftheX-raypul-
tions, we only used data from cluster B. The other instrument
sars for which CRSFs have been studied in great detail (see,
forthehardX-rayswasthepixelizedCdTe detectorISGRIbe-
e.g.,Wheatonetal.1979;Whiteetal.1983;Nagaseetal.1991;
hindaCodedMaskonboardtheINTEGRALsatellite.Itcovers
Heindl et al. 1999; Santangelo et al. 1999;Mihara et al. 2004; the energy range between ∼18 keV and 1MeV. We extracted
Nakajima et al. 2006; Tsygankov et al. 2007; Ferrigno et al. all available ISGRI spectra, for which 4U 0115+634 was less
2009, 2011).The source shows 3.61s pulsation and has an or- than10◦ off-axis.Table1 containsan observationlog (see also
bitalperiodof ∼24.3d (Cominskyetal. 1978;Rappaportet al.
Fig.1).
1978).TheopticalcounterpartistheO9estarV635Cas(Johns
Forthespectralanalysisweuseddatabetween3and50keV
etal.1978;Ungeretal.1998).
for PCA, and between 20 and 100 keV for HEXTE and for
In previous outbursts, CRSFs have been detected up to the
ISGRI. A systematic error has been added in quadrature to
fifth harmonic (Heindl et al. 2000). This is the largest number
the PCA and ISGRI data, using the canonical values of 0.5%
of detected CRSFs in an accreting X-ray pulsar. In later work, and2.0%(Jahodaetal.2006,andIBISAnalysisUserManual1),
a negative correlation between L and E was found (e.g.,
X cyc respectively.Toimprovethesignaltonoiseratioofthespectra,
Nakajimaetal.2006;Tsygankovetal.2007;Mülleretal.2010),
we averaged the data over 14 data blocks in time. These data
meaningthatthesourceshouldbelocatedinthesupercriticallu-
blocksare definedin the fifth column of Table 1 and shown in
minosityregime.Otherauthorsevenfoundadiscontinuousbe-
Fig. 1. We omitted one ISGRI dataset from our analysis (rev-
havior of the fundamental line energy and explained this by a
olution number 677), because no simultaneous RXTE data are
suddenchangeoftheaccretionconditions(Lietal.2012). availableatthattimeandthesourcewasalmostin theoffstate
In this work we revisit the behavior of the CRSFs in duringthatobservation.
4U0115+634.Focusingondatatakenduringagiantoutburstin
2008March/April(Fig.1).Weshowthatthepreviouslyclaimed
relation between the source flux F and E strongly depends 3. Spectralanalysis
X cyc
onthechoiceoftheunderlyingcontinuummodel.InSect.2we
summarize the observations. In Sect. 3 we describe the differ- 3.1.Pulsarcontinuummodels
entcontinuummodelsandthefluxandtime dependenceofthe
A large problem when analyzing X-ray spectra from X-ray bi-
continuumandcyclotronlineparameters.Wearguethatthecy-
narypulsarsistofindagooddescriptionofthebroadbandcon-
clotronline is only modeledcorrectly whenusing an exponen-
tinuum. In most cases, semi-physical models, which are some
tially cutoff power law continuum with a strong emission fea- variant of a power law with a high energy exponential cutoff
ture around 10 keV. Furthermore, alternative continua models,
whichhavebeenusedinsomeearlierinvestigations,resultinin- 1 http://www.isdc.unige.ch/integral/analysis#
correctcyclotronline parameterswhich cancause theapparent Documentation
A6,page2of10
S.Mülleretal.:NoanticorrelationbetweencyclotronlineenergyandX-rayfluxin4U0115+634
Table1.Logofobservationsforthe2008outburstof4U0115+634. .1998.2000.2002.2004.2006.2008.2010
1997.1999.2001.2003.2005.2007.2009.
50 650
IDa Startdate MJD t b[s] ec V) March 2008 April 2008
01-01-00 2008Mar.20 54545.27–54545.43 83e3x6p/2413 1 ke 45 RXTE 300 600
01-01-01 2008Mar.20 54545.47–54545.49 1648/382 1 12 40 20 INTEGRAL 250 550
–
01-02-04 2008Mar.21 54546.97–54547.20 12448/3938 2 2 200 500
( )
01-02-00 2008Mar.23 54548.07–54548.25 9904/3082 2 1] 35 10 150 450 eV
0011--0022--0012 22000088MMaarr..2244 5544554499..1997––5544554590..3104 68383362//13914386 33 −ctss 30 1 3 5 7 911 13 51000 400 –12k
01-02-03 2008Mar.25 54550.24–54550.28 2384/934 3 e[ 25 0 epoch 2 4 6 8 1012 14 0 350 (2
01-02-05 2008Mar.26 54551.08–54551.17 4096/1679 4 at . 54560 54580 300 b]
r a
01-02-06 2008Mar.26 54551.28–54551.31 752/625 4 nt 20 250 Cr
01-03-02 2008Mar.29 54554.23–54554.38 576/2112 5 u m
01-03-01 2008Mar.31 54556.14–54556.16 768/473 6 co 15 200 e[
01-03-00 2008Mar.31 54556.25–54556.37 768/1558 6 M 150 at
S 10 r
01-03-03 2008Apr.02 54558.93–54559.10 5456/3104 7 A 100
01-04-00 2008Apr.04 54560.04–54560.22 4240/2998 7 E 5
01-04-01 2008Apr.06 54562.01–54562.18 5888/2908 8 XT 50
01-04-02 2008Apr.06 54562.95–54563.09 5552/2317 8 R 0 0
01-04-03 2008Apr.07 54563.99–54564.07 3856/1548 9 51000 52000 53000 54000 55000
01-04-04 2008Apr.08 54564.82–54564.98 7808/2853 9 MJD
01-04-05 2008Apr.09 54565.86–54566.10 10896/4101 10 Fig.1.RXTE/ASMlightcurveof4U0115+634(2–12keV).Theinset
01-04-06 2008Apr.10 54566.98–54567.00 1648/699 10
shows a close-up view of this lightcurve, showing the 2008 outburst
01-05-00 2008Apr.11 54567.00–54567.01 1056/381 11 indetail.ArrowsindicatethetimesofpointedRXTEandINTEGRAL
01-05-01 2008Apr.11 54567.96–54568.13 9632/3176 11 observations.Numbersindicatetheobservationepochsusedinthedata
16-01-00 2008Apr.13 54569.98–54570.16 10192/3043 12 analysis(seealsoTable1).
16-01-01 2008Apr.15 54571.94–54572.12 10368/3162 12
16-02-00 2008Apr.18 54574.04–54574.15 7184/1924 13
16-02-01 2008Apr.22 54578.36–54578.51 6480/1679 14 wherefortypicalX-raypulsarsEbreak ∼ 10keVandwhichhas
anunphysical“kink”atE ,whichcanresultinline-likeresid-
664 2008Mar.21 54546.48–54547.68 55981 2 break
ualsaroundthisenergy(Kretschmaretal. 1997;Kreykenbohm
667 2008Mar.30 54555.46–54556.04 28279 6
etal.1999).ForthisreasonKlochkovetal.(2008)andFerrigno
668 2008Apr.02 54558.45–54559.05 27655 7
et al. (2009) smooth the continuum around this break, using a
669 2008Apr.07 54563.38–54564.02 28495 9
modeloftheform
670 2008Apr.10 54566.41–54567.01 30990 10
673 2008Apr.17 54573.90–54574.50 31360 13 ⎧
675 2008Apr.24 54580.62–54581.97 70902 14 ⎪⎪⎪⎪⎨E−Γ forE ≤ Ebreak−ΔE
677 2008Apr.29 54585.69–54586.91 63662 – PLINT(E)∝⎪⎪⎪⎪⎩EAE−Γ3e+xpB(E−2E+/ECE+) D ffoorr|EE≥−EEbreak|+<ΔΔEE
fold break
Notes.ThefirstpartofthetablecontainstheRXTEobservations, the
second part the ISGRI data. (a) For RXTE, the first column contains (4)
the number of the Obs-ID after “93032-”. For ISGRI, it contains the
revolution number. (b) For RXTE, the two numbers correspond to the where the coefficients A, B,C, and D are chosen such that the
PCAandHEXTEexposures,respectively.(c)Epochfordatagrouping, continuumanditsfirstderivativearecontinuousatE = Ebreak±
seetextfordetails. ΔE andwhereΔE (cid:2)0butsufficientlysmall.
Other commonchoicesto avoid the breakare the so called
negative and positive power-law exponential (NPEX, see, e.g.,
(Kreykenbohmet al. 2002,and referencestherein),are used to
Makishima et al. 1999, and references therein), a continuum
describethespectra.Thesemodelscanbejustifiedbyconsider-
modelwithfivefreeparametersoftheform
ingthe Comptonizationof softphotonsin an accretioncolumn
(eBxpeockneernt&ialWlyocluffto2ff00p7o,waenrdlarewfemreondceels,cthaellreedinC)u.tTohfefmPLositnbIaSsIiSc NPEX(E)∝(cid:9)E−Γ1 +αE+Γ2(cid:10)exp(−E/Efold), (5)
andXSPEC(Arnaud1996),hastheform
with photon indices Γ1,2 ≥ 02, and the “Fermi Dirac cutoff”
CutoffPL(E)∝ E−Γexp(−E/E ), (2) (FDCUT,seeTanaka1986)
fold
wScithhönthheerprhoettoanl.in2d0e0x7,)Γ.,Hanodwtehveerf,oltdhiinsgseimneprlgey,mEofdoledl(siseeo,fet.egn.,, FDCUT(E)∝ E−Γ 1+exp((E−1E )/E )· (6)
butnotalways,insufficienttodescribethecontinuaofobserved break fold
X-raypulsars,asthecurvatureinthecontinuumisoftenseento Common to all of these empirical continua is that despite the
startatdifferentenergies.Toobtainagoodspectralfit,therefore, smoothertransitionaroundE theyoftenneedtobemodified
break
modelswithmorefreefitparametersandmorecomplexshapes furtherbyabroadexcessordepressionaround10keV.Thisso-
aresometimesrequired.Themostsimpleofthesemodelsisthe called“10keVfeature”hasbeenobservedinmanyX-raypulsar
powerlawwithhighenergycutoff spectra, either in absorptionor in emission, although its origin
(cid:4) isstillnotclear(Coburn2001).Thisfeatureisusuallymodeled
E−Γ forE ≤ E
PLCUT(E)∝ break (3)
E−Γexp((E −E)/E ) forE > E 2 OftenonefixesΓ =2,see,e.g.,Nakajimaetal.(2006).
break fold break 2
A6,page3of10
A&A551,A6(2013)
as an additive Gaussian line, with the centroid energy EG, the 3 1
widthσ ,andthefluxA . 0
G G
-3
In addition to those empirical models, based on the the-
3 2
oretical description of bulk and thermal Comptonization in
0
magnetizedaccretioncolumnsofBecker&Wolff(2007,andref-
-3
erencestherein),Ferrignoetal.(2009)havedevelopedacontin- 3 3
uum model depending on physical parameters like the column 0
radius, the accretion rate, the electron temperature, the B-field -3
strength.Ahybridmodelwhichcombinesthebroadbandcontin- 3 4
uumwithCRSFsiscurrentlyinpreparation(Schwarmetal.,in 0
-3
prep.).Thesephysics-basedcontinuummodelsarestillinactive
developmentand,because of the large numberof free parame- 3 5
0
ters,onlysuitedforveryhighsignaltonoisedata.Forthisreason
-3
mostobservationalworkusesoneofthe continuadescribedby 3 6
Eqs.(2)–(6). 0
For sources with cyclotron lines, the continuum model is -3
modified with a description of the cyclotron line. Since phys- 3 7
ically self consistent line models such as those described by 0
Schönherr et al. (2007) are still not available for all sources, σ] -3
empirical line models are generally used. The two most com- χ[ 3 8
0
mon CRSF models are those using a Gaussian or a pseudo-
-3
Lorentzian profile (see discussion by Nakajima et al. 2010). 3 9
Here, we model the CRSFs with a pseudo-Lorentzian optical 0
depth profile (Miharaet al. 1990),i.e., we multiplythe contin- -3
uummodelwith 3 10
(cid:11) (cid:12) 0
τ(WE/E )2 -3
CYCLABS(E)=exp −(E−E )2cy+cW2 , (7) 3 11
cyc 0
-3
withthe centroidenergy,Ecyc, the widthofthefeature,W, and 3 12
the optical depth of the line, τ. This approach allows the com- 0
parisonofthelineparameterswithresultsfrompreviouspapers, -3
inwhichalsotheCYCLABSmodelhasbeenused(e.g.,Nakajima 3 13
0
etal.2006;Tsygankovetal.2007;Lietal.2012).Intheremain-
-3
derofthepaper,welabelthecyclotronlineparameterswiththe
3 14
numberofrespectiveharmonic,where0correspondstothefun-
0
damentalline.AscommonforCRSFs,Wandτarestronglycor- -3
related(Coburn2001).Wethereforesetalowerlimitof0.5keV 10 20 30 40 50
forthewidth,whichiscomparabletothetypicalPCAresolution. energy[keV]
Inordertodescribethedataof4U0115+634,inadditionto
Fig.2.DeviationsbetweenthedataandthebestfitwithCutoffPLand
several of the continuum models discussed above, we take in- 10 keV feature for the PCA (circles), HEXTE (crosses), and ISGRI
terstellar absorption into account, modeling it with an updated (filled diamonds) in units of standard deviations σ. For these fits, no
versionofTBabs3,usingabundancesbyWilmsetal.(2000)and cyclotronlineshavebeentakenintoaccount.Thenumbersontheright
crosssectionsbyVerner&Yakovlev(1995).Wefurthermoreac- sideindicatetherespectiveepoch.
countforanintrinsicFeKαfluorescenceline,whichwedescribe
withanadditivenarrowGaussianemissionlinewithfrozencen-
troidenergyat6.4keV,awidthof10−4 keV,andafluxAFe and 3.2.Fittingstrategy
equivalent width W . Assuming a narrow line is justified be-
Fe
causeinsystemslike4U0115+63FeKαisexpectedtohavea Since the CutoffPL is the easiest continuum model for X-ray
widthontheorderof(cid:2)0.5keV(see,e.g.,Torrejónetal.2010), pulsars,weusedthismodelforourfits.Furthermore,the10keV
whichcannotberesolvedbyPCA. feature in emission is requiredto get a good descriptionof the
Inordertotakeintoaccountthatthefluxnormalizationofthe data. For each epoch, the fits have been simultaneously per-
differentinstrumentsused is notperfectly knownwe introduce formedforallavailableinstruments.Figure2showstheresidu-
cross calibration constants c and c as free fit param- alsofthebestfitusingthiscontinuumwithouttakinganyCRSFs
HEXTE ISGRI
eters. These constants also account for flux differences of the into account. These residuals clearly indicate the need of the
source between the observationswhich were not fully simulta- cyclotronlines in this model at ∼11, 22, and sometimes above
neous. The PCA backgroundwas estimated using the standard 30keV.Asthequalityofindividualdatasetsisstronglyvariable
models provided by GSFC. These estimates often show a flux overtheoutburst,inaninitialfitrunavariablenumberofCRSFs
deviationfromtheproperbackgroundbyafewpercent.Thisde- were addedto individualfits untilthe reducedχ2, χ2 ∼ 1. To
red
viation was accounted for by multiplying the background flux avoidcontinuummodelingwiththeabsorptionfeatures,wefixed
withanotherfitparameter,c (seealsoRothschildetal.2011). thewidthsoftheCRSFsofthesecondandhigherharmonicsto
b
typicalvalues,i.e.,4keV(Ferrignoetal.2009).
3 See http://pulsar.sternwarte.uni-erlangen.de/wilms/ Inordertocheckwhetherthenumberofdifferentcyclotron
research/tbabs/ features in the model affects the fit results of the continuum
A6,page4of10
S.Mülleretal.:NoanticorrelationbetweencyclotronlineenergyandX-rayfluxin4U0115+634
NH [1022cm−2] Efold [keV] a χ2red≈1.1
0 2 4 10 12 100
13
a bNH=! 1.7×1022cm−2 0.6 −1V] 10
V] 12 0.5 ke
e −1 1
[k 0.4Γ tss
Efold 11 0.3 ate[c 0.1
r
10 0.2 0.01 FeKα 10keV
0.1 line feature
10-3
c d 2 b
0.6 0.6 σ[] 0
χ
0.5 0.5 -2
5 10 20 50 100
Γ0.4 0.4Γ energy[keV]
0.3 0.3 Fig.4. a) Folded spectrum from epoch 6 together with best fit (red)
andallmodelcomponents,i.e.,CutoffPL,10keVfeature,FeKαflu-
0.2 0.2
orescenceline,andfivecyclotronfeatures.b)Residualsofthebestfit,
0.1 0.1 shownasthedifferencebetweenthedataandthemodelnormalizedto
0 2 4 10 11 12 13
theuncertaintyofeachdatapoint.Darkbluedatapointscorrespondto
NH [1022cm−2] Efold [keV] PCA,cyanpointstoHEXTE,andorangepointstoISGRI.
Fig.3.a)c)andd)Confidencecontoursforthefoldingenergy,thepho-
tonindexandthehydrogencolumndensityforepoch6.b)Confidence
contours between the E and Γ with N frozen to 1.7×1022cm−2. the first fit run with the CutoffPLcontinuumand 10 keV fea-
fold H
Contourlinescorrespondtothe68.3%,90%,and99%level.Colorindi- ture,andfixedNH (seealsoTable2).Asalreadynotedinprevi-
catesΔχ2withrespecttothebestfitvalue,withthecolorscalerunning ous analyses for 4U 0115+634, the continuum varies over the
fromorange(lowΔχ2)todarkblue(largeΔχ2). outburst (e.g., Li et al. 2012). During the brightest phases of
theoutburstthespectrumisthehardestone.AsshowninFig.3,
thevariationofΓissignificantlylargerthantheveryslightcor-
parametersand the fundamentaland first harmonicCRSF, in a relationseenbetweenΓand E .Asexpected,thesource’sin-
second run all spectral fits included only the fundamental line trinsic iron Kα line is significfoalndtly present during all epochs.
and its first and second harmonics.For epochs,where only the
As also observedin other X-ray transient pulsars(Inoue 1985)
first harmonic was necessary to obtain a good fit in our initial
and expected for a line originating from fluorescence, the line
run, we added the second harmonicwith fixed parametersdur-
flux is positively correlated to the X-ray flux from the source.
ingthesecondrun,holdingitsparametersatthemeanvaluesof
Theequivalentwidthstaysconstantwithintheuncertaintiesover
theresultsfromtheotherepochs.
theoutburst.The10keVfeaturevariesinboth,thecentroiden-
SinceinitialfitswiththeCutoffPLcontinuumshowedlarge
ergyE anditsfluxA .Itsenergyseemstobeslightlyanticor-
uncertaintiesforthehydrogencolumndensity,N ,withoutasig- G G
H relatedwiththesourceflux,whiletherelativefluxinthefeature,
nificantvariabilitybetweendifferentepochs,weheldN fixedat
H i.e.,theratiobetweenthefluxinthe10keVfeatureandthetotal
themeanvalueNH =1.7×1022cm−2toavoidunphysicalcorre- flux,remainsconstant.Itswidth,σ ,remainsrelativelyconstant
lationsbetweenthesefitparameters(Fig.3). G
between3.0keVand3.5keV.
Thetwofitrunsshowthatalmostallfitparametersareequal Verydifferentfromvirtuallyallpreviousanalyses,however,
withintheirrespectiveuncertaintiesforbothruns.OnlyE and
fold thecyclotronlineisseentobeonlyslightlyvaryingovertheout-
thelineparametersofthesecondharmonicareslightlydifferent
burst.ThecoloreddatapointsinFigs.6and7showthecentroid
inepoch6and7.Thesedifferencescanbeexplainedbythefact energy,E ,thewidth,W ,andtheopticaldepth,τ ,ofthefun-
that for these two cases the second run has to account for the 0 0 0
damentalcyclotronlineagainsttimeandfluxfortheCutoffPL
equivalent of the four harmonics required in the first run. The
continuum,modified with the 10 keV feature. In these fits, the
differences affect mainly the higher energies and are compen-
widthW remainsconstantthroughouttheoutburst,andthereis
satedbyachangeinthefoldingenergyandtheclosestcyclotron onlyasli0ght(∼3σ)indicationthattheopticaldepthτ wassome-
line, i.e., the second harmonic. The remainder of this paper is 0
whatshallower duringthe initial phasesof the outburst,before
basedontheresultsfromthefirstfitrun. ∼MJD54552,althoughitcannotbeexcludedthatsomeofthis
An examplefor the bestfit using the CutoffPLcontinuum effect is due to a correlation with the “10 keV feature”, which
with 10 keV feature and five cyclotron features (first fit run,
during this time of the outburst peaks close to the cyclotron
epoch6)isshowntogetherwithallmodelcomponentsinFig.4.
line at around 8 keV. Most importantly, however, as shown in
Thisspectrumwasrecordedbyallthreeinstrumentsduringthe
Fig.6theenergyofthefundamentalcyclotronlineisonlyvery
maximumoftheoutburst,i.e.,epoch6.Allfitresultsfromruns
slightly variable: during early phases of the outburst the line
usingtheCutoffPLcontinuumaresummarizedinTable2.
is around11 keV, movingtowards 10 keV as the outburstpro-
gressed.Thisresultiscontrarytoallearlierstudiesofoutbursts
4. Spectralevolution:Noanticorrelation of4U0115+634,wherestrongchangesinthelineenergywere
seen(e.g.,Lietal.2012;Tsygankovetal.2007;Nakajimaetal.
ofthecyclotronlineenergywithflux
2006;Miharaetal.2004).
Figures5–7showtheevolutionofthecontinuumandcyclotron Where does this difference in line behavior come from?
lineparametersaswellastheparametersfortheFeKαlinefor As discussed in Sect. 3.1 a large variety of continuum models
A6,page5of10
A&A551,A6(2013)
Table2.Continuumparametersofthetimeresolvedspectralanalysis.
epa L b Γ E A c W E A c σ c c c χ2 /d.o.f.
X fold Fe Fe G G G b HEXTE ISGRI red
[keV] [10−3] [eV] [keV] [keV]
1 4.38+0.02 0.59+0.04 10.7+0.5 2.5+0.6 55+12 8.3+0.3 0.18+0.02 3.4+0.1 1.00+0.02 0.73+0.02 – 1.13/50
−0.02 −0.05 −0.5 −0.6 −12 −0.2 −0.02 −0.3 −0.02 −0.02
2 5.35+0.02 0.56+0.03 11.5+0.4 2.9+0.7 53+10 8.0+0.2 0.19+0.02 3.3+0.1 0.96+0.01 0.74+0.01 0.87+0.02 1.12/77
−0.02 −0.03 −0.4 −0.7 −10 −0.2 −0.02 −0.1 −0.01 −0.01 −0.02
3 7.48+0.02 0.54+0.03 11.6+0.3 3.7+0.9 46+10 7.3+0.2 0.34+0.02 3.6+0.1 0.94+0.02 0.75+0.01 – 1.32/48
−0.02 −0.03 −0.3 −0.9 −10 −0.2 −0.01 −0.1 −0.02 −0.01
4 8.66+0.03 0.45+0.03 10.8+0.3 4.4+1.0 46+10 7.1+0.3 0.37+0.03 3.5+0.1 0.94+0.02 0.78+0.01 – 0.88/48
−0.03 −0.03 −0.3 −1.0 −10 −0.2 −0.03 −0.1 −0.02 −0.01
5 9.24+0.05 0.42+0.05 10.7+0.4 5.4+1.7 52+17 7.0+0.8 0.44+0.08 3.6+0.4 0.92+0.05 0.94+0.02 – 0.79/48
−0.06 −0.08 −0.6 −1.7 −17 −0.5 −0.07 −0.3 −0.05 −0.02
6 10.18+0.04 0.36+0.04 11.3+0.5 5.8+1.6 51+13 7.2+0.6 0.31+0.07 3.0+0.4 0.93+0.04 0.88+0.02 1.04+0.02 1.09/75
−0.04 −0.07 −0.6 −1.5 −13 −0.3 −0.05 −0.3 −0.04 −0.01 −0.02
7 9.43+0.03 0.36+0.04 10.8+0.4 6.4+1.2 61+10 7.7+0.7 0.30+0.04 3.1+0.2 0.94+0.02 0.84+0.01 1.05+0.02 1.18/75
−0.03 −0.07 −0.5 −1.2 −10 −0.4 −0.04 −0.2 −0.02 −0.01 −0.02
8 8.70+0.03 0.38+0.04 10.2+0.3 5.9+1.0 61+10 7.7+0.4 0.32+0.04 3.3+0.2 0.93+0.02 0.78+0.01 – 1.27/48
−0.03 −0.04 −0.3 −1.0 −10 −0.3 −0.04 −0.2 −0.02 −0.01
9 7.84+0.03 0.50+0.03 11.5+0.4 5.1+1.0 60+10 7.7+0.4 0.32+0.03 3.3+0.2 0.93+0.02 0.75+0.01 0.96+0.02 0.99/77
−0.02 −0.04 −0.3 −1.0 −10 −0.3 −0.03 −0.2 −0.02 −0.01 −0.02
10 7.17+0.03 0.52+0.04 11.6+0.4 4.8+0.9 61+12 7.8+0.5 0.27+0.03 3.1+0.2 0.95+0.02 0.75+0.01 0.87+0.02 1.11/77
−0.02 −0.07 −0.5 −1.0 −12 −0.3 −0.03 −0.2 −0.02 −0.01 −0.02
11 5.76+0.02 0.52+0.04 10.7+0.4 3.9+0.8 61+12 8.0+0.4 0.21+0.02 3.0+0.2 0.97+0.02 0.76+0.01 – 1.17/48
−0.03 −0.07 −0.5 −0.8 −12 −0.3 −0.02 −0.2 −0.02 −0.01
12 4.33+0.02 0.56+0.06 10.5+0.6 2.6+0.6 53+12 8.2+0.5 0.18+0.02 3.0+0.2 0.96+0.01 0.73+0.01 – 1.10/48
−0.02 −0.10 −0.7 −0.6 −12 −0.3 −0.02 −0.2 −0.01 −0.01
13 2.84+0.02 0.59+0.06 9.6+0.5 1.9+0.5 55+13 7.9+0.6 0.13+0.02 3.1+0.2 0.95+0.02 0.72+0.02 0.86+0.03 0.96/81
−0.02 −0.16 −0.9 −0.5 −14 −0.3 −0.03 −0.3 −0.02 −0.02 −0.03
14 1.40+0.02 0.41+0.22 8.3+1.2 0.6+0.5 35+27 8.5+0.5 0.04+0.03 3.0+0.5 0.98+0.02 0.69+0.04 0.44+0.03 0.86/81
−0.02 −0.28 −1.2 −0.3 −17 −0.7 −0.02 −0.7 −0.02 −0.04 −0.03
epa E W τ E W τ E τ E τ E τ
0 0 0 1 1 1 2 2 3 3 4 4
[keV] [keV] [keV] [keV] [keV] [keV] [keV]
1 11.0+0.4 0.9+3.9 0.04+0.02 21.6+0.3 0.9+1.0 0.24+0.18 – – – – – –
−0.8 −0.4 −0.02 −0.3 −0.4 −0.07
2 11.2+0.5 0.8+2.4 0.03+0.02 22.1+0.3 2.6+1.0 0.24+0.03 32.7+1.3 0.25+0.07 42.8+2.5 0.26+0.09 – –
−0.6 −0.3 −0.02 −0.3 −0.9 −0.03 −1.3 −0.07 −2.2 −0.08
3 11.1+0.5 0.5+1.3 0.03+0.02 22.0+0.2 0.5+0.8 0.26+0.05 37.5+1.0 0.20+0.05 – – – –
−0.5 −0.0 −0.02 −0.2 −0.0 −0.12 −1.0 −0.05
4 11.0+0.5 0.8+2.4 0.03+0.02 21.8+0.3 1.1+0.9 0.19+0.16 36.4+1.0 0.22+0.07 – – – –
−0.7 −0.3 −0.02 −0.3 −0.6 −0.05 −1.2 −0.07
5 11.0+0.6 2.5+1.5 0.12+0.15 22.1+0.6 1.4+2.3 0.18+0.24 36.2+1.8 0.18+0.09 – – – –
−0.7 −1.5 −0.07 −0.6 −0.9 −0.07 −2.0 −0.08
6 10.7+0.3 1.8+1.0 0.11+0.14 21.8+0.4 4.8+0.9 0.46+0.05 35.2+0.7 0.48+0.07 46.7+1.2 0.44+0.10 59.7+2.0 0.52+0.24
−0.4 −1.2 −0.07 −0.4 −0.9 −0.05 −1.0 −0.08 −2.0 −0.13 −3.2 −0.25
7 10.7+0.2 2.3+0.9 0.18+0.14 22.1+0.2 3.8+0.6 0.45+0.05 33.3+0.9 0.35+0.06 41.6+1.4 0.34+0.08 52.8+1.8 0.42+0.13
−0.3 −0.9 −0.08 −0.2 −0.5 −0.03 −0.8 −0.07 −1.6 −0.08 −2.0 −0.14
8 10.6+0.3 2.3+0.8 0.16+0.08 21.8+0.3 2.9+0.8 0.30+0.03 34.9+1.0 0.25+0.06 – – – –
−0.3 −0.7 −0.05 −0.3 −0.7 −0.04 −0.8 −0.05
9 10.5+0.3 2.1+0.9 0.12+0.07 21.6+0.3 2.7+1.2 0.22+0.04 33.2+1.2 0.25+0.06 43.7+2.0 0.28+0.08 – –
−0.3 −0.7 −0.04 −0.4 −0.9 −0.03 −1.0 −0.06 −1.7 −0.07
10 10.3+0.4 2.0+1.2 0.09+0.10 21.4+0.4 3.9+1.4 0.25+0.06 34.0+1.3 0.31+0.07 44.0+2.3 0.28+0.09 – –
−0.5 −0.9 −0.04 −0.5 −1.0 −0.04 −1.0 −0.07 −1.9 −0.08
11 10.3+0.4 2.1+1.0 0.10+0.10 21.3+0.4 2.9+1.6 0.21+0.05 33.0+2.4 0.22+0.08 – – – –
−0.4 −1.0 −0.05 −0.4 −1.0 −0.03 −1.5 −0.07
12 10.1+0.3 2.3+1.0 0.14+0.13 20.8+0.3 2.3+1.0 0.21+0.05 34.6+1.8 0.20+0.10 – – – –
−0.3 −0.9 −0.06 −0.4 −0.8 −0.04 −1.9 −0.08
13 10.2+0.4 1.7+1.7 0.08+0.13 20.7+1.4 2.6+5.0 0.08+0.13 – – – – – –
−0.5 −1.0 −0.03 −2.2 −2.1 −0.05
14 10.7+0.5 0.5+4.0 0.06+0.04 20.3+1.6 5.9+2.8 0.38+0.19 – – – – – –
−0.8 −0.0 −0.03 −1.4 −3.2 −0.19
Notes.FitswereperformedwiththeCutoffPLmodelandavaryingnumberofCRSFs.Uncertaintiesandupperlimitsareatthe90%confidence
levelforoneinterestingparameter.(a)Epochfordatagrouping(seetextfordetails).(b)Absorbedluminosityinunitsof1037ergs−1.L coversthe
X
energyband3–50keVandwascalculatedusingadistanceof7kpc(Negueruela&Okazaki2001).(c)Inunitsofphotonss−1cm−2.
can be used to describe the broad band spectra of accreting differentbehaviorof the fundamentalcyclotronline, especially
neutron stars. Due to the early successes of fitting spectra of thecentroidenergy.
4U0115+634withtheNPEXandPLCUTmodels,thesecontinua
Given that the fits with NPEX and CutoffPLgive similarly
havebeenextensivelyusedwhenmodelingtheoutburstbehav- good results, what is the fundamental difference between both
iorofthispulsar.Unfortunately,however,inmostpreviousanal-
modeling approaches? NPEX models generally result in very
yses – including some of our own – no attempt was made at
broad cyclotron lines, with W and W often exceeding 5 keV
0 1
modelingthedatawithanyoftheotheravailablemodels.
(e.g., Nakajima et al. 2006, runs using the PLCUT model give
Asanexample,modelingthe2008outburstof4U0115+634 similar results, see, e.g., Li et al. 2012). In contrast, using the
withtheNPEXmodel(followingNakajimaetal.2006,fixingΓ2 CutoffPLcontinuumwiththe10keVfeatureresultsinwidths
to2.0)resultsinfitsthatareofonlyslightworsequalityasthose oftypicallylessthan3keV,muchmoreconsistentwiththenar-
usingtheCutoffPLmodelmodifiedbythe10keVfeature,and row shapesof the residualsshown in Fig. 2. When lines are as
we can recoverthe strong variationof the cyclotronline found broad as in the NPEX fits, they can influence the continuumfit.
in earlier analyses (Fig. 6, cyan data points)4. Depending on Toillustratehowstronglythesefeaturesactuallydistortthecon-
thechoiceofthecontinuum,wethereforefindafundamentally tinuum,Fig.8showsthecontinuumshapeinferredwhensetting
τ , τ , τ , andτ to zero.Theresidualsforthe CutoffPLcon-
0 1 2 3
4 ModelingthespectrawiththePLCUTmodelresultsinanunphysical tinuumwiththe10keVfeature(darkblue)showsharpabsorp-
breakaroundtheenergyofthefundamentalline,wethereforeconsider tion dips at the cyclotron line energies of the fundamentaland
thiscontinuumtobeunsuitableforstudyingthecyclotronlinebehavior. first harmonic line energies. For the NPEX model (green line),
A6,page6of10
S.Mülleretal.:NoanticorrelationbetweencyclotronlineenergyandX-rayfluxin4U0115+634
10
15
V
ke 5
0
5
−
3X 2
L 14
1
0.6
Γ 0.4 13 12
0.2 V]
e 11
12 k
V] [ 12
e 0
k 10 E 10
[
old 8 50 60 70
Ef 11 time(MJD-54500)
6
AFe 4 10
2
10
V] 1 2 5 10
e
[k 8 LX3−50keV [1037ergs−1]
G
E
Fig.6. Luminosity and time dependence (inset) of the energy of the
4 fundamental CRSF using the CutoffPL continuum model. The color
V] 3.5 codingisthesameasinFig.5.Cyandatapoints(diamonds)showthe
ke resultswhenusingtheNPEXcontinuum.
[ 3
G
σ 2.5
Using the results from the CutoffPL plus 10 keV feature
0.4 fitsforthelineenergyalsosolvesanotherproblemfoundinear-
G lierspectralmodeling.Here,usingNPEXfits,theratiosbetween
A
0.2 thefundamentalandhighercyclotronlineswereoftenfoundto
deviate significantly from integers. While slightly non-integer
2 ratios are expected when taking relativistic quantum mechan-
icsintoaccount(seethediscussionbyPottschmidtetal.2005),
d 1.5
2re thelargedeviationsfromintegermultiplesseenpreviously(e.g.,
χ
1 Tsygankovetal.2007;Nakajimaetal.2006;Heindletal.1999;
0.5 Santangelo et al. 1999) require rather complex model assump-
50 60 70 1 2 5 10 tionswhichintroducesecondordereffectssuchasstrongvertical
time (MJD-54500) LX3−50keV B-fieldgradients,crustalfieldstructuresonsmallscales,orther-
momagnetic effects for their explanation (see Schönherr et al.
Fig.5.Lefttemporalevolutionandrightluminositydependenceofthe
2007, and references therein for a discussion). Figure 9 shows
continuumparameters(seetextfordefinitions)and10keVfeaturefor
thatwecanrecoverthesenon-integerratiosinourNPEXfits,but
thefitswiththeCutoffPLcontinuum.Thecolorgradientfromreddish
whenusingtheCutoffPLplus10keVfeaturemodel,thesera-
toblueishdatapointsindicatesthetemporalevolutionoftheoutburst.
Thedata points show resultsfrom the initialfit run, where avariable tiosaremostlyinagreementwithintegervalues,oratleastonly
numberofcyclotronlineswasused.Greendatapointsintheleftcolumn slightlyhigher,asexpectedfromthemostsimplemodelsforcy-
correspondtothesecondfitrunwithafixednumberofthreecyclotron clotron line formation. We conclude that also from a physical
lines(onlyintheleftcolumn),whichwasbeenperformedasaconsis- pointofview,theCutoffPLcontinuumwiththe10keVfeature
tency check (see text for details). Where no green points are visible, givesamuchmoresatisfactorydescriptionofthedata.
bothfitsgavethesameresults.LXisinunitsof1037ergs−1.AGandAFe Finally, we check the dependence of the line position on
areinunitsofphotonss−1cm−2,whileA isinmultiplesof10−3.
Fe other free fit parameters when using the CutoffPL continuum
modifiedbythe10keVfeature.Sincetheoriginofthe10keV
feature remains still an open questtion, possible relations be-
on the otherhand,it is obviousthatrather than describingnar- tween the CRSF centroid energy and the parameters of the
rowcyclotronlines,themultiplicativebroadlinemodelstrongly 10keVfeaturehavetobeinvestigated,andothercontinuumpa-
influencesthe continuum.As discussed above,our spectral fits rameterssuchasE orΓcouldinfluencetheresultsforthecy-
fold
show that during the brightest phases of the outburst a signifi- lotronline parameters.Figure10showsthe behaviorof Δχ2 in
canthardeningofthecontinuumemissionisobserved,together thevicinityofthebestfitvaluesforthreeepochsrepresentative
witha changeofthe exponentialcutoff.While thisisalso seen fortheearlyphase,peak,andlatephaseofthe2008outburstfor
inthevariationofthecontinuumparameters,itisverylikelythat selectedcombinationsofimportantfitparameters.Thesecontour
theluminositydependenceoftheCRSFintheNPEXfitsisdueto plotsshowthatthe line energyisnotaffectedbycrosscorrela-
the line partially modelingthis behaviorof the continuum,and tions with other parameters, i.e., possible variations in the line
notarealphysicaleffect. energyofthefundamentalcyclotronlinearenotduetovariations
A6,page7of10
A&A551,A6(2013)
V] 4 1] 100
e −
k V 10 NPEX
[ e
0 2 k CutoffPL
W −1 1
s
0.5 s
ct 0.1
0.2 [
e
τ0 00.0.15 rat 0.01 a
0.02
0.01 1.4 E E E E
0 1 2 3
1.2
22
]
V
e 21 1
k
E[1 2109 ratio 00..86
0.4
38
V] 0.2
e 36 b
k 0
[ 34 5 10 20 50 100
2
E energy [keV]
32
48 Fig.8.a)PCA,HEXTE,andISGRIspectrafromepoch9(red)together
]
V 46 withthebest fit modelsusing the NPEX (green) and CutoffPL (blue)
e
k 44 continuaaftersettingtheopticaldepthofthecyclotronlinestozero.b)
[
3 42 Ratiobetweenthedataandthemodel.
E
40
6
50 60 70 1 2 5 10
time (MJD-54500) LX3−50keV [1037ergs−1] 5.5
4
Fig.7.Time(left)andluminosity(right)dependenceoftheparameters 5 =
oftheCRSF.ThecolorcodingisthesameasinFig.5.
n
4.5
3
observedintheparametersofthe10keVfeatureorthecontin- 4 =
uumparameters. E0 n
In contrast to the stable behavior of the line when using / 3.5
CutoffPLwith a 10 keVfeature,Fig. 11illustrates thedepen- En 2
denceoftheCRSFcentroidenergyonthecontinuumparameters 3 =
oftheNPEXmodel.Inmanycases,strongcorrelationsbetween n
E andΓ aswellasbetweenE andE arepresent.Thesecor- 2.5
0 1 0 fold
relationsareafurtherindicatorthattheLorentziancomponentis 1
usedto modelthe continuumratherthana cyclotronlinewhen 2 =
usingtheNPEXmodel. n
1.5
1
5. Summaryandconclusion
50 60 70 80
Inthispaperwehavepresentedastudyofthe2008outburstof time (MJD-54500)
4U 0115+634 based on RXTE and INTEGRAL data (Fig. 1).
Fig.9.Ratiosofthecyclotron lineenergies E /E withrespect tothe
We reproducedthe results from previouswork (e.g., Nakajima n 0
centroidenergyofthefundamental lineagainst time.Thedarkgreen,
et al. 2006;Tsygankovet al. 2007;Li et al. 2012, and others),
darkblue,redandpurpledatapoints(bars)correspondtoE /E ,E /E ,
that the spectra can be successfully modeledby,e.g., the NPEX E /E ,andE /E ,respectively,usingtheCutoffPLconti1nuu0mw2ith0a
model.We showedthatforthese fits the verybroadabsorption 3 0 4 0
10keVfeature.Thelightgreenandcyandatapoints(filledcircles)show
features,whicharethoughttodescribetheCRSFs,rathermodel theresultsforE /E andE /E usingNPEX.Thedashedlinesindicate
1 0 2 0
thebroadbandcontinuum.Ourresultthatthecontinuumparam- integervaluesfortheseratios.
eters are strongly variable over the outburst (Fig. 5), could be
responsibleforthechangein thecyclotronline energyin these
fits. energyofthefundamentalcyclotronline,thePLCUTmodeldoes
Wehaveshownthatthespectralcontinuumcanbealsowell not give a physical description of the continuum. Using the
describedwithamodelintroducedbyKlochkovetal.(2007)for CutoffPLtogetherwiththe10keVfeature,wehaveshownthat
EXO 2030+375 and Ferrigno et al. (2009) for 4U 0115+634, themodelingoflinesreflectsthetheoreticalexpectationinboth
i.e., the CutoffPL,modified by strong Gaussian emission fea- thelineshapeandtheratiosoftheharmonics.Inthismodel,the
ture around 10 keV and several (up to five) cyclotron lines luminositydependencyofthecentroidenergyisnotpresent.As
(Table 2 and Fig. 4). Due to an unphysical break close to the shownalsobyFerrignoetal.(2011),thelinecentroidenergyis
A6,page8of10
S.Mülleretal.:NoanticorrelationbetweencyclotronlineenergyandX-rayfluxin4U0115+634
5 15
4 ]
V
W0eV] 3 ke 10
k [
[ 0
2 W 5
1
3
0.6
2
Γ 0.4
1
Γ
1
0.2
0
12 8
]
dV] 10 V
Efol[ke 8 [ked 6
ol
] Ef 4
2
−
m
c 10 12 14 10 12 14 10 12 14
G−1 0.4 E0 [keV] E0 [keV] E0 [keV]
A s
s
n 0.2 Fig.11. Confidence contours between the fundamental cyclotron line
o
t energy and continuum parameters when using the NPEX model. The
o
h 0 displayedepochs,colorsandlinesarethesameasinFig.10.
p
[
GV] 10 inferred from the NPEX and PLCUT models was almost a jump
E ke ata3–50keV luminosityofaround3×1037ergs−1 andstrong
[ 8 hysteresiseffectswerepresent(Fig.6).Suchabehaviorhasnot
been seen in any of the other cyclotron line sources. The sec-
ondstrongestluminosity-dependentcyclotronlinevariabilityis
observedin V0332+53(Mowlaviet al. 2006;Tsygankovet al.
3.5
V] 2006;Nakajimaetal.2010),inwhichthecyclotronlinesareso
σGe 3 strongthatthechangecanbeseenbyeyeevenintherawdetec-
[k torspectra.Whiletheboundariesbetweenthedifferentaccretion
2.5
regimesestimatedbyBeckeretal.(2012)mightthereforehave
to be adjusted, the overall principal behavior of the accretion
10 12 14 10 12 14 10 12 14 columndiscussedbytheseauthorsstillholds.
E0 [keV] E0 [keV] E0 [keV] With the recently successful launch of NuSTAR (Harrison
etal.2010),alowbackground,focusingtelescopewithslightly
Fig.10. Confidence contours between the fundamental CRSF energy
higher resolution than the RXTE-HEXTE or the Suzaku-HXD
and other fit parameters. The columns show the results for epochs 2,
6, and 14, respectively. Contour lines correspond to the 68.3%, 90%, isnowavailable.Withitshigherresolutionandthemuchhigher
and99%confidencelevel.ColorindicatesΔχ2 withrespecttothebest expected signal to noise ratio in the spectra, we will be able,
fit value, withthe color scalerunning from orange (low Δχ2) to dark for the first time, to resolve cyclotron lines and study their
blue(largeΔχ2). luminosity-dependentbehaviorwithoutthedangerofasystem-
aticinfluenceofthechoiceofacontinuum.
remarkablystablethroughoutthebrightestphaseoftheoutburst. Acknowledgements. We thank the referee for his/her insightful comments.
Herewe showthatsuchstability ispresent,albeitatlowersig- We also thank the schedulers of RXTE and INTEGRAL for their role in
nificance,alsoduringtheearlyandlatephases.Inthismodel,the making this campaign possible, and the International Space Science Institute
in Bern, Switzerland, for their hospitality. We acknowledge funding by the
continuumvariationsareexplainedbyaslightanticorrelationof
Bundesministerium fürWirtschaft undTechnologie under Deutsches Zentrum
thecentroidenergyoftheGaussianfeaturewithluminosityand fürLuft-undRaumfahrtgrants50OR0808,50OR0905,and50OR1113,andby
asignificantvariationofΓ. theDeutscherAkademischerAustauschdienst. M.T.W.issupportedbytheUS
The strong systematic influence of the chosen continuum OfficeofNavalResearch.ICacknowledges financialsupportfromtheFrench
SpaceAgencyCNESthroughCNRS.S.M.N.andJ.M.T.acknowledgesupport
on the cyclotron line behavior illustrates a problem that is
from the Spanish Ministerio de Ciencia, Tecnología e Innovación (MCINN)
potentiallyinherenttoallcyclotronlinemeasurements.Weem- throughgrantAYA2010-15431andtheuseofthecomputerfacilitiesmadeavail-
phasize, however,that this does not mean that the general pic- able through the grant AIB2010DE-00057. This research is in part based on
ture of the different regimes of magnetized neutron star accre- observations with INTEGRAL, an ESA project with instruments and science
datacentrefundedbyESAmemberstates(especiallythePIcountries:Denmark,
tion and general luminosity dependence of the cyclotron line
France,Germany,Italy,Switzerland, Spain),CzechRepublic,andPoland,and
as outlined by Becker et al. (2012, and references therein) is
withtheparticipationofRussiaandUSA.WethankJohnE.Davisforthede-
incorrect. In the sample of sources discussed by Becker et al. velopment oftheSLxfigmodule,whichwasusedtocreate allfiguresinthe
(2012),4U0115+634wasanoutlier.Thechangeoflineenergy paper.
A6,page9of10