NASA Technical Reports Server (NTRS) 20150008984: NuSTAR AND SWIFT Observations of the Fast Rotating Magnetized White Dwarf AE Aquarii

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TheAstrophysicalJournal,782:3(7pp),2014February10 doi:10.1088/0004-637X/782/1/3
(cid:2)C2014.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedintheU.S.A.
NuSTARANDSWIFTOBSERVATIONSOFTHEFASTROTATINGMAGNETIZEDWHITEDWARFAEAQUARII
TakaoKitaguchi1,HongjunAn2,AndreiM.Beloborodov3,EricV.Gotthelf3,TakayukiHayashi4,
VictoriaM.Kaspi2,VikramR.Rana5,StevenE.Boggs6,FinnE.Christensen7,WilliamW.Craig6,8,
CharlesJ.Hailey3,FionaA.Harrison5,DanielStern9,andWillW.Zhang10
1RIKENNishinaCenter,2-1Hirosawa,Wako,Saitama351-0198,Japan
2DepartmentofPhysics,McGillUniversity,RutherfordPhysicsBuilding,3600UniversityStreet,Montreal,QuebecH3A2T8,Canada
3PhysicsDepartmentandColumbiaAstrophysicsLaboratory,ColumbiaUniversity,NewYork,NY10027,USA
4TheInstituteofSpaceandAstronauticalScience/JAXA,3-1-1Yoshinodai,Chuo-ku,Sagamihara252-5210,Japan
5CahillCenterforAstronomyandAstrophysics,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA
6SpaceSciencesLaboratory,UniversityofCalifornia,Berkeley,CA94720,USA
7DTUSpace-NationalSpaceInstitute,TechnicalUniversityofDenmark,Elektrovej327,DK-2800Lyngby,Denmark
8LawrenceLivermoreNationalLaboratory,Livermore,CA94550,USA
9JetPropulsionLaboratory,CaliforniaInstituteofTechnology,Pasadena,CA91109,USA
10NASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA
Received2013October31;accepted2013December17;published2014January17
ABSTRACT
AE Aquarii is a cataclysmic variable with the fastest known rotating magnetized white dwarf (P = 33.08 s).
spin
Compared to many intermediate polars, AE Aquarii shows a soft X-ray spectrum with a very low luminosity
(L ∼ 1031 erg s−1). We have analyzed overlapping observations of this system with the NuSTAR and the Swift
X
X-ray observatories in 2012 September. We find the 0.5–30 keV spectra to be well fitted by either an optically
thinthermalplasmamodelwiththreetemperaturesof0.75+0.18,2.29+0.96,and9.33+6.07 keV,oranopticallythin
−0.45 −0.82 −2.18
thermal plasma model with two temperatures of 1.00+0.34 and 4.64+1.58 keV plus a power-law component with
−0.23 −0.84
photonindexof2.50+0.17.Thepulseprofileinthe3–20keVbandisbroadandapproximatelysinusoidal,witha
−0.23
pulsedfractionof16.6%±2.3%.Wedonotfindanyevidenceforapreviouslyreportedsharpfeatureinthepulse
profile.
Key words: accretion, accretion disks – novae, cataclysmic variables – stars: individual (AE Aquarii) –
whitedwarfs–X-rays:stars
Online-onlymaterial:colorfigures
oftypicalIPs.ItsX-rayspectrumhasbeenmodeledasemission
1.INTRODUCTION
fromanopticallythinthermalplasmawithseveraltemperature
AE Aquarii (hereafter AE Aqr) is a cataclysmic variable components, similar to those seen from other IPs. However,
binary system classified as a member of the DQ Herculis or the highest temperature found in such models was 4.6 keV
intermediate polar (IP) class (Patterson 1994). It is a non- (Itohetal.2006),whichissignificantlylowerthantheaverage
eclipsingclosebinarysystematadistanceof102+42 pc(Fried- kT ≈22keVfoundinthe22IPsdetectedbyINTEGRAL(Landi
−23
jung1997),consistingofamagneticwhitedwarf(primary)anda etal.2009).Forthesereasons,themechanismandlocationof
K4–5Vstar(secondary)withanorbitalperiod,P =9.88hr. theX-rayemissionarestilluncertain(e.g.,Choietal.1999;Itoh
orbit
The 33.08 s period makes AE Aqr the fastest-spinning mag- etal.2006;Mauche2009).
netic white dwarf. The pulsations were originally discovered Another intriguing feature reported by a Suzaku observa-
in the optical (Patterson 1979), then confirmed in soft X-rays tion in 2005 is that AE Aqr may emit non-thermal hard
(Pattersonetal.1980)andtheultraviolet(Eracleousetal.1994). X-rays with a very narrow pulse profile at the spin period
IntheDQHerculisclass,thewhitedwarfisgenerallythought (Terada et al. 2008), suggesting that AE Aqr may acceler-
to possess a magnetic field (B ∼ 105–7 G) strong enough to ate charged particles in a fashion similar to rotation-powered
channel the accretion flow from the secondary to the poles of pulsars (e.g., Kaspi et al. 2006 for a review). However, the
thewhitedwarf.Accordingly,hardX-raysareproducedbythe Suzaku observation in 2006 did not reproduce the earlier re-
shock-heatedgas,whichreachestemperaturesofafewtensof sult,leavingthedetectionofnon-thermalX-raysfromAEAqr
keVnearthesurface.Thewhitedwarfphotosphere,heatedby uncertain.
thehardX-rays,emitsultravioletlight.Theseemissionsexhibit In this paper, we present broad-band X-ray observations of
spin modulation caused by the varying aspect of the accret- AE Aqr obtained with NuSTAR and Swift. Section 2 details
ing poles as the white dwarf rotates (see Patterson 1994 for a the observations, data reduction, and background modeling.
review). These more sensitive observations in the hard X-ray band can
AEAqrstandsoutasanunusualmemberoftheDQHerculis helpmeasurethemaximumtemperatureofthethermalplasma
class. It displays strong flares of broad-band emission, from in AE Aqr and test the presence of any beamed non-thermal
radio to X-rays. In addition, possible TeV γ-ray flares have component.WedescribethespectralmodelinginSection3and
been reported (Meintjes et al. 1994), although they have not the timing analysis in Section 4. The results and a possible
yetbeenconfirmedwithmorerecentTeVCerenkovtelescopes interpretationarediscussedinSection5.Throughoutthispaper,
(Mauche et al. 2012). The persistent X-ray luminosity of AE allerrorsaregivenatthe90%confidencelevelunlessotherwise
Aqr(∼1031ergs−1)istwoordersofmagnitudelowerthanthat stated.
1
TheAstrophysicalJournal,782:3(7pp),2014February10 Kitaguchietal.
10-2
10-3
1
-V
e
k
1
-Counts s10-4 -1-1 keVs10-3
s 10-4
1.4 nt
u
o
o C
ati 1
R
10-5
0.6
3 4 5 7 10 20 30 40 50 70
Energy (keV)
3 4 5 7 10 20 30 40
Figure1.Comparisonof(red)thebackgroundmodelproducedbynuskybgd
Energy (keV)
with (black) the actual blank-sky spectrum in the AE Aqr observation. The
bottompanelshowsthespectralratiooftheactualdatatothemodel. Figure2.X-rayspectraofAEAqrobservedwiththetwoNuSTARtelescopes,
(Acolorversionofthisfigureisavailableintheonlinejournal.) (black)FPMAand(red)FPMB.Thecrosspointsandsolidhistogramsarethe
background-subtractedspectraandbackgroundmodels,respectively.
(Acolorversionofthisfigureisavailableintheonlinejournal.)
2.OBSERVATIONSANDDATAREDUCTIONS
regions of the same observation, and then generates a back-
2.1.NuSTAR
groundspectrumwithinanyregionswiththebest-fitparameters
2.1.1.ObservationLogandDataScreening (Wiketal.inpreparationforamoredetailed).
Three blank-sky spectra for each telescope were extracted
WeobservedAEAqrwithNuSTAR(Harrisonetal.2013),the from annular regions of radius 120(cid:5)(cid:5)–270(cid:5)(cid:5), 270(cid:5)(cid:5)–370(cid:5)(cid:5), and
first focusing hard X-ray (3–79 keV) telescopes in orbit, from 370(cid:5)(cid:5)–740(cid:5)(cid:5)centeredonAEAqrtomodelthebackgroundspectra
2012September4,19:20UTtoSeptember7,18:50UT.NuSTAR
of the two NuSTAR telescopes. The blank-sky spectra were
was operated in its default mode throughout the observation. well fitted by the model with χ2/dof = 1217.6/1124. The
The acquired data with the observation ID of 30001120 were
background count spectrum in the source region was modeled
processedandscreenedinthestandardwayusingtheNuSTAR
withanexposure20timeslongerthantheactualonetoreduce
pipelinesoftware,NuSTARDASversion1.1.1,withtheNuSTAR
statistical errors. In order to verify the background model, the
calibration database (CALDB) version 20130509. The data backgroundspectruminablank-skyregion3.(cid:5)5fromthesource
werefilteredforintervalsofhighbackground,includingEarth
in a northeasterly direction was simulated and compared to
occultationsandSouthAtlanticAnomaly(SAA)passages.This
the actual spectrum extracted from the same region. The ratio
resulted in a total of 125 ks of dead-time corrected exposure
oftheactualspectrumtothemodelinFigure1wasfittedbya
time. constantfactorof0.98±0.04withχ2/dof =15.7/27,showing
All photon arrival times were corrected to the solar sys-
the background model is consistent with the actual spectrum
tem barycenter using the JPL DE200 ephemeris and the
withinstatisticalerrors.
Chandra -derived coordinates (20:40:09.185, −00:52:15.08;
Figure 2 shows the AE Aqr spectra obtained with the two
J2000)whichhavesub-arcseconduncertainties.Thesourcepho-
NuSTAR hard X-ray telescopes (FPMA and FPMB) and the
tonswereextractedfromacircularregionofradius1.(cid:5)0centered
background models. The highly ionized iron line can be seen
onAEAqr.Thesourcespectraweregroupedwithaminimumof
around 6.7 keV. The total count rate in the 3–30 keV energy
50countsperbin.Thebackgroundspectraweregeneratedwith
bandafterthebackgroundsubtractionanddead-timecorrection
the background modeling tool, nuskybgd (D. R. Wik et al. in is(9.2±0.1)×10−2 countss−1.
preparation). A detailed description of the background model-
ingisgiveninSection2.1.2.Thetelescoperesponsefiles,ARF
andRMFwerealsogeneratedbyNuSTARDAS.
2.2.Swift
2.1.2.BackgroundSpectralModeling
Swift(Gehrelsetal.2004)observedAEAqrsimultaneously
Since AE Aqr is a faint source in hard X-rays, the NuSTAR with NuSTAR from 2012 September 6, 06:07 to 07:53 UT.
background spectrum must be carefully subtracted from the The Swift X-ray telescope (XRT; Burrows et al. 2005) was
sourcespectrum.Blank-skyobservationsshowthebackground operated in the photon counting mode during the observation.
rate varies in an energy dependent way with detector position TheXRTdata(observationID00030295037)wereprocessedin
by a factor of ∼2 (Harrison et al. 2013). In addition, the thestandardmannerwithxrtpipeline(ver.0.12.6)inHEASoft
internal background rate varies in time due to changes in the (ver. 6.13). The total exposure time after the data screening
cosmicradiationintensityassociatedwiththegeomagneticcut- is 1.54 ks. The source photons were extracted from a circular
offrigidity,aswellastheelapsedtimesinceSAApassage. regionofradius1.(cid:5)2centeredonAEAqr.Thesourcespectrum
In order to perform accurate background subtraction, the wasrebinnedtohaveatleast20countsperbin.Thebackground
background spectrum was empirically modeled using the spectrumwasextractedfromanannularregionofradius1.(cid:5)7–7.(cid:5)0
NuSTAR background fitting and modeling tool, nuskybgd centered on the source and was scaled by an area ratio of the
(revision52).nuskybgdfitsblank-skyspectrainuser-selected sourceregiontothebackgroundregion.
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Table1
Best-fitParametersoftheThree-temperatureModel
0.1
Parameter Unit Value V1− 0.01
e
ZNkTH1 (10((2Zk0e(cid:6)cVma))−2) 00..<77658−−++20000...2..11147835 k s stnuo1− 1100−−34
F1b (10−12ergs−1cm−2) 1.97−+00..9845 C 10−5
kFT22b (10−12e(rkgeVs−)1cm−2) 32..5239−−++0000....95866524 10−46 (a) APEC x 3
kT3 (keV) 9.33−+62..0178 2
F3b (10−12ergs−1cm−2) 2.26−+00..9996 χ 0
CFPMB/CFPMAc 1.00±0.03 −2
CXRT/CFPMAd 0.94−+00..1292 −4
χ2/dof 247.1/201 0.5 1 2 5 10 20
Energy (keV)
Notes.
aSolarabundancefromWilmsetal.(2000),inwhichthesolarironabundance
relativetohydrogenis2.69×10−5.NotethatthesolarabundancefromAnders& 0.1
Grevesse(1989),inwhichthesolarironabundanceis4.68×10−5,isemployed V−1 0.01
e
inotherAEAqrarticles(e.g.,Itohetal.2006;Teradaetal.2008;Oruru& k
Meintjes2012). s −1 10−3
bUnabsorbedfluxin0.5–10keV. nts 10−4
cCross-normalizationfactorbetweenNuSTAR/FPMBandNuSTAR/FPMA. u
o
dCross-normalizationfactorbetweenSwift/XRTandNuSTAR/FPMA. C 10−5
(b) APEC x 2 + Power law
10−6
4
3.SPECTRALANALYSIS
2
3.1.SpectralFittingwithMulti-temperatureModels χ 0
−2
The X-ray spectra of AE Aqr observed with ASCA (Choi
−4
et al. 1999), XMM-Newton (Itoh et al. 2006), and Suzaku 0.5 1 2 5 10 20
(Teradaetal.2008)havebeenmodeledusinganopticallythin Energy (keV)
thermalplasmaemissionmodelwithafewdifferenttemperature Figure3.X-rayspectraof(green)Swift/XRT,(black)NuSTAR/FPMA,and
components in the same way as for other IPs. Therefore in (red) NuSTAR/FPMB overlaid with (a) the best-fit three-temperature APEC
the joint fitting of Swift and NuSTAR spectra, we adopted an modeland(b)two-temperaturemodelwithapower-lowemission.Themodel
emission model from a collisionally ionized diffuse plasma, componentsaredrawnwithdottedlines.Thebottompanelsshowfitresiduals
intermsofσ witherrorbarsofsizeone.
APEC(Smithetal.2001).EachAPECmodelwasconstrainedto
(Acolorversionofthisfigureisavailableintheonlinejournal.)
have common metal abundances relative to solar from Wilms
etal.(2000),butwasallowedtohavedifferenttemperaturesand
normalizations. We use the tbabs model (Wilms et al. 2000) Although other IP spectra commonly contain a strong neu-
to account for interstellar and self-absorption. In addition, the tral (or low-ionized) iron line around 6.4 keV with a compa-
cross-normalizationfactorsofXRT/FPMAandFPMB/FPMA rable equivalent width to highly ionized iron emission around
wereallowedtovary. 6.7keV(e.g.,Ezuka&Ishida1999;Yuasaetal.2010),wedo
InordertodeterminethenumberofAPECcomponentswith notdetectthisfeatureclearlyintheAEAqrspectrum.Infact,
differenttemperatures,weaddednewAPECcomponentsoneby XMM-Newtonspectraconstrainedtheupperlimittothe6.4keV
one until the fit was not significantly improved, as determined equivalentwidthtobe<61eV(Itohetal.2006).Wetriedtoadd
by the F-test. Improvement of the fit is significant with the a narrow Gaussian emission model with a center energy fixed
chanceprobabilityof(cid:7)1%untilthenumberofcomponentsis at6.4keVtothethree-temperatureAPECmodel,andsimultane-
increasedtothree,whereaslittleimprovementwasfoundbythe ouslyfitthespectraofSwiftandNuSTARwithit.Theintensity
addition of a fourth component with the chance probability of oftheneutralironlineis1.33+1.26 ×10−14ergs−1cm−2,which
−1.33
8.4%derivedfromtheχ2 reductionfrom247.1(dof=201)to correspondstoanequivalentwidthof37.9+41.7 eV.Ontheother
241.0(dof=199).The0.5–30keVspectraofSwiftandNuSTAR hand,theintensityofthehighlyionizediro−n3l7i.n9earound6.7keV
with the best-fit three-temperature APEC models are shown in was determined to be 1.11+0.11 ×10−13 erg s−1 cm−2, or an
−0.09
Figure 3(a), the parameters of which are listed in Table 1. equivalentwidthof495+365 eV,withthelinecentralenergyof
w9.i3t+h6.p1rekveiVo,usisrecsounlstsidfeorarbrleyfehriegnhceer. tThhaenhpirgehveiostustleymopbesraetruvreed, 6.732+−00..006134keV.Therefo−r5e3,theneutralironlineismuchweaker
−2.2 thanthehighlyionizedironline.
forthissource(4.6keVfromItohetal.2006),andapproaches
the average temperature of 22 keV for any other IPs (Landi
3.2.SpectralFittingwithNon-thermalEmission
et al. 2009). The metal abundance, 0.76+−00..1173Z(cid:6), is consistent
with that determined by XMM-Newton and Suzaku. The total In order to search for possible non-thermal hard X-rays as
luminosityin0.5–10keVis9.3+1.0×1030ergs−1foranassumed suggested by Terada et al. (2008), we simultaneously fitted
−2.2
distanceof100pc. the spectra of Swift and NuSTAR with a multi-temperature
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Table2
Best-fitParametersoftheTwo-temperatureModelwithPower-lawEmission (a) 3-10 keV
Parameter Unit Value 100
NH (1020cm−2) <82.2
Z (Z(cid:6)) 1.14−+00..6323
kT1 (keV) 1.00−+00..3243 2Z1
F1a (10−12ergs−1cm−2) 2.01−+11..2053 50
kT2 (keV) 4.64−+10..5884
F2a (10−12ergs−1cm−2) 1.86−+00..9861
Γ 2.50−+00..1273
FPLb (10−12ergs−1cm−2) 4.07−+11..6560 0
CFPMB/CFPMA 1.00±0.03 (b) 10-20 keV
CXRT/CFPMA 0.81−+00..2129 10
χ2/dof 244.5/201
Notes.
aUnabsorbedfluxofeachthermalemissioncomponentin0.5–10keV. 21
bUnabsorbedfluxofthepower-lawmodelin0.5–10keV. Z 5
plus power-law emission model. In the same way as the
determination of the number of APEC components described
above,wefirstfitthespectrausingasingleAPECwithapower-
0
lawmodel,andthenaddednewAPECmodelsonebyoneuntil 33.04 33.06 33.08 33.1
the fit was not significantly improved as determined by the Period (s)
F-test. We find two temperatures with power-law emission is
Figure4.Z2periodogramsacquiredwithNuSTARintheenergybandsof(a)
thestatisticallyfavoredmodel. 1
3–10and(b)10–20keV.
The Swift and NuSTAR spectra with the best-fit model are
shown in Figure 3(b), the parameters of which are listed in
Table3
Table2.Comparedwiththethree-temperatureAPECmodel,the PulseProfileParametersaforAEAqr
fit with the two-APEC model with the power-law emission is
EnergyRange PeakPhaseb PulseFraction χ2/dof
slightly but not significantly preferred. We cannot distinguish
(keV) (%)
whether the hard X-ray component detected with NuSTAR is
thermalornon-thermalemission. 3–20 0.30±0.02 16.6±2.3 17.2/16
3–6 0.31±0.04 14.8±3.1 11.7/10
6–10 0.31±0.03 19.9±3.7 8.4/10
4.TIMINGANALYSIS 10–20 0.22±0.10 14.2±8.0 2.5/4
4.1.SpinPeriodDetermination Notes.
aBest-fitparametersforafittotheNuSTARpulseprofilespresented
We examined the hard X-ray pulse profile for evidence of in Figure 5 using a sinusoid model with a constant offset, C:
the narrow pulsation reported by Suzaku (Terada et al. 2008). C(1.0+fPcos(2π(φ−φ0))), where φ0 is the peak phase and fP
Previous measurements have shown that soft X-rays below isthepulsefraction.
10 keV are sinusoidally modulated with a single peak in the bPhase0correspondstothatofFigure5.
spinphaseofthewhitedwarf(e.g.,Mauche2006).Inthiscase,
theZ12-statistic,orRayleightest(Buccherietal.1983)ismore 4.2.PulseProfile
sensitive to determine the spin period than the epoch folding
techniquewiththeχ2test(Leahyetal.1983). Figure5presentsthepulseprofilesinseveralenergybands,
Figure 4(a) shows the Z2 periodogram using NuSTAR folded on the best determined period, with the background
1
3–10 keV data with barycenter-corrected time. We found the subtracted using the model of Section 2.1.2. The profiles are
peakat33.0769±0.0004swithZ2 =124.1.Withintheuncer- wellrepresentedbysinusoidalfunctions(seeTable3).Wefind
1
tainty,theperiodisconsistentwithpreviouslymeasuredvalues no evidence for significant variation in the phase or relative
intheopticalband(Patterson1979;deJageretal.1994),andat modulationamplitude(orpulsefraction)withenergy.Themean
X-rayenergies(Choietal.1999;Choi&Dotani2006;Mauche pulse fraction, 16.6%±2.3%, is consistent with that found in
2006;Teradaetal.2008;Oruru&Meintjes2012). thequiescentphasefromXMM-Newtonobservations(Itohetal.
Wealsosearchedforasignalinthe10–20keVenergyband. 2006;Choi&Dotani2006).
The hard X-rays above 20 keV were ignored to improve the Wesearchedforanarrowpeakinthepulseprofileasreported
signal-to-noise ratio since the source rate from AE Aqr falls byTeradaetal.(2008)basedontheSuzakuobservationof2005.
below that of the background in that energy range (Figure 2). Weinvestigatedthepulseprofileusingdifferentnumbersofbins
Evidence for a pulsation was found also in this band at the percyclerangingfrom5to50.Furthermore,weexaminedthe
expected pulse period, 33.0765±0.0016 s, with Z2 = 11.1, profile using 29 bins per cycle as per the Suzaku analysis and
1
corresponding to a null hypothesis probability of 0.38%, or reproduced 1000 pulse profiles, each having a different start
2.9σ detectionsignificance(Figure4(b)). timeatphasezero.Thenwelookedforanysignificantoutliers
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fluxin2005(Teradaetal.2008).Thederivedpower-lawindex,
2.5±0.2,isalsoinconsistentwiththeSuzakuvalue,1.1±0.6,
0.1 and is steeper than those found for rotation-powered pulsars,
which range from 0.6 to 2.1 (Gotthelf 2003; Kaspi et al.
1 2006).HerewenotethatthepreviouslyreportedSuzakufitting
-s s 0.09 errorsdidnotincludethesystematicuncertaintyoftheSuzaku/
nt HXD-PIN background model because the authors described
u
o
C 0.08 that the hard X-ray flux was consistent with the narrow pulse
flux in the spin profile. However, the systematic uncertainty,
3% (Fukazawa et al. 2009), is comparable to the background-
0.07
subtractedcountrate,andthereforeshouldnotbeignored(see
(a) 3-20 keV
Figures6and7ofTeradaetal.2008forcomparisonoftherates).
Itispossiblethatthediscrepancyofthenon-thermalemission
parameters is due to the neglected systematic error in Terada
etal.(2008).
Furthermore,wefindnoevidenceofthenarrowpulseprofile
1 0.05
-s withapulsefractionofnearly100%andadutycycleof∼0.1
s
nt reported by Terada et al. (2008). In contrast with the Suzaku
u
o result, we marginally detect the sinusoidal modulation in the
C
10–20keVenergybandatasignificancelevelof2.9σ.Inorder
0.04 todeterminewhetherornotNuSTARissensitivetothenarrow
pulse profile suggested by Suzaku, we estimated its count rate
(b) 3-6 keV considering the NuSTAR telescope response and assuming the
Suzakuresult.Theexpected10–20keVcountratewithinaduty
0.04 cycle of 0.1 is 0.086±0.003 count s−1, 10 times higher than
thetime-averagedratemeasuredwithNuSTAR,indicatingthat
NuSTARwouldhavebeencapableofsignificantlydetectingthe
-1s 0.035 narrowpulsation.
nts ApossibleexplanationofthedifferencebetweentheSuzaku
u and NuSTAR results is that AE Aqr may have varied and
Co 0.03 its hard X-ray flux decreased. However, the soft X-ray flux
determined with NuSTAR in 2012 is consistent with previous
observationsmadewithSuzakuin2005(seeTable1),withinthe
0.025
cross-normalizationfactorofNuSTARtoSuzaku/XISof∼15%
(c) 6-10 keV
(basedonsimultaneousobservationsofcalibrationtargets).Itis
difficulttoexplainamechanismonlyinhardX-raysthatwould
0.012
makethefluxvarywithoutcorrelationofthermalsoftX-rays,a
partofwhichalsomodulateswiththespinperiod.
-1s 5.2.PossibleInterpretationoftheObservedEmission
s 0.01
unt Two energy sources could, inprinciple, power the observed
Co X-rayluminosityLX ∼1031ergs−1:liberationofgravitational
energyofaccretingmatterandtherotationalenergyofthewhite
0.008 dwarf.
The white dwarf was reported to spin down with a rate
(d) 10-20 keV P˙ =5.64×10−14ss−1(deJageretal.1994),whichcorresponds
0 0.5 1 1.5 2 tospindownpowerLsd ∼1034ergs−1 (cid:8)LX.Thissuggeststhe
Phase
possibilitythattheX-rayemissionofthewhitedwarfisfedby
Figure 5. AE Aqr pulse profiles folded on the best period of 33.0769 s in rotation(Ikhsanov&Beskrovnaya2012).Therotation-induced
(a) 3–20, (b) 3–6, (c) 6–10, and (d) 10–20 keV energy bands. The dashed electric field could accelerate particles to high Lorentz factors
curvesshowthebest-fitsinusoidalmodels.Theepochofphase0corresponds
ifthemagnetosphereisplasma-starvedandtheelectricfieldis
to56174.5inMJD.
notscreened.Synchrotronemissionwassuggestedasapossible
radiative mechanism in such a model (Terada et al. 2008). It
from the sinusoidal model with a 95% confidence level. We
is, however, unclear how the observed luminosity, spectrum,
concludethatthereisnoevidenceforanadditionalsharppulse
and pulse profile observed by NuSTAR would be produced by
componentinourdata.
relativisticparticles.Theefficiencyofsynchrotronemissionis
expected to be small, since the particles are accelerated along
5.DISCUSSION
themagneticfieldlines.Emissionfromacceleratedparticlesis
5.1.ComparisonofNon-thermalHardX-RayEmission alsoexpectedtobestronglybeamedalongthefieldlines,which
withPreviousSuzakuResults isinconsistentwiththeobservedbroadpulseprofileinFigure5.
Emission powered by accretion is a plausible mechanism.
The 10–30 keV flux determined with NuSTAR is 5.0+−11..34 × The standard model of accreting IPs involves an accretion
10−13ergs−1cm−2,abouthalfofthebest-fitSuzaku/HXD-PIN column heated by the accretion shock and cooled by thermal
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bremsstrahlung(Aizu1973).ItexplainswelltheX-rayspectra effect (Wynn et al. 1997), whereby most of the mass lost by
ofmanyIPs(seeCropperetal.1999forGingadata,Suleimanov thesecondaryisbeingflungoutofthebinarybythemagnetic
et al. 2005 for RXTE, Landi et al. 2009 for Swift/XRT and field of the rapidly rotating white dwarf. Norton et al. (2004,
INTEGRAL/IBIS, Brunschweiger et al. 2009 for Swift/BAT, 2008) performed numerical simulations of accretion flows in
and Yuasa et al. 2010 for Suzaku). The main parameters of magnetic cataclysmic variables with wide ranges of magnetic
the model are the free-fall velocity v = (2GM /R )1/2 moment,μ = 1032–36 Gcm3,andP /P = 10−3–1.They
and the accretion rate per unit area, aff= M˙/4πRW2Df,WwDhere demonstrated that IPs with a very smspainll raotrbioit of P /P <
WD spin orbit
f is the fraction of the white dwarf surface occupied by the 10−2 like AE Aqr (P /P = 9.3×10−4) show magnetic
spin orbit
accretion column. The standard model assumes that the shock propellereffects.Therefore,thetallaccretioncolumncouldbe
iTshreadshiaotcivkeaaltnidtudpeinhne(cid:2)d tvotbtrh,ewwhheriteevdw=avrfffs/u4rf∼ac1e,08hc(cid:7)msR−W1Dis. wfoormuleddailnsoIPbsewexitphecPtespdin/toPohrbaivte<lo1w0−X2-,rtahyoulugmhisnuocshitiseys,sttehmuss
thepostshockvelocityoftheaccretinggasandtbr ∼0.3/a sis making them more challenging to identify. AE Aqr is the
thebremsstrahlungcoolingtimescale(withaexpressedinunits only currently known example. It is also possible that nearly
ofgs−1cm−2).ThenetaccretionrateM˙ isdeterminedfromthe synchronoussystemssimilartoEXHydraecouldpossessatall
X-TrahyislusmceinnoasriiotywLoXul=dGimMplWyDMM˙˙/R∼W2D.× 1014 g s−1 for AE accocnrseitsitoenntcwoliuthmtnh.eTphicetulorewoafcrcinregt-iloinkeraatcecrientiosuncdhisscyusstesemdsbiys
Aqr, and the condition a (cid:8) 10−2 g s−1 cm−2 is satisfied if Nortonetal.(2008).
f (cid:7)10−3.Suchasmallfmaybepossibleif,e.g.,theaccretion
flow in AE Aqr is concentrated in a thin wall of the accretion 6.SUMMARY
column (as Basko & Sunyaev 1976 suggested for accreting
neutronstars).However,ontheassumptionofMWD ∼0.7M(cid:6) dwWaref AhaEveAqarnaolybzseerdveXd-wraiythdNatuaSToAfRthfeorm1a2g5nektsizaenddwSwhiitfet
determined with the optical measurements (e.g., Watson et al.
for 1.5 ks. The 0.5–30 keV spectra are well characterized
2006;Echevarr´ıaetal.2008),ourcalculationsofthespectrum
by either an optically thin thermal plasma model with three
predictedbythestandardaccretionmodeldonotprovideagood
temperatureswiththehighesttemperatureof9.33+6.07 keV,or
fitfortheNuSTARdata—theobservedspectrumwiththehighest −2.18
temperatureof9.3+6.1 keVissofterthanthepredictedpostshock anopticallythinthermalplasmamodelwithtwotemperatures
−2.2 plusapower-lawcomponentwithphotonindexof2.50+0.17.The
temperatureof29keV. −0.23
3–20 keV pulse profile shows a sinusoidal modulation, with a
Twomodificationsmightmaketheaccretionmodelconsistent
pulsed fraction of 16.6%±2.3%. We find no evidence for the
withNuSTARobservations.(1)TheshockaltitudehinAEAqr
previouslyreportedsharpfeatureinthepulseprofile.
couldbesignificant,perhapsevencomparabletothewhitedwarf
TheobservedsoftX-rayspectrumwiththebroadpulseprofile
radius.Suchtallaccretioncolumnswithlowaccretionrateswere
is hard to explain as a result of rotation-powered emission.
recently studied by Hayashi & Ishida (2014). The shallower
Instead accretion-powered emission is more likely, although
gravitationalpotentialatthehighaltituderesultsinareduction
the observed spectrum is softer than predicted by the standard
ofthepostshocktemperaturebelow20keVtypicalofotherIPs.
accretion column model. We suggest two modifications of the
ThetallaccretioncolumninAEAqrwaspreviouslyproposed
standard model to explain the AE Aqr spectrum: the shock
asthesourceofthelargehotspotsonthewhitedwarfsurface
altitude could be high, comparable to the white dwarf radius,
inferredfromobservationsbytheHubbletelescope(Eracleous
andcyclotronemissionwithB >106Gcouldadditionallycool
et al. 1994) (2) The X-ray spectrum emitted by the accretion
downtheaccretionplasma.Detailedcalculationsofsuchmodels
column could be affected by additional radiative losses due to
willhopefullyreproducethespectrumandpulseprofileofAE
cyclotronemissionintheinfraredband.Cyclotronemissionis
importantwhenB > 106 G,andmaycarryawayasignificant Aqrwiththewhitedwarfmassconsistentwiththatdetermined
withtheopticalmeasurements.
partoftheaccretioncolumnenergy,especiallyatlargealtitudes
wheretheplasmahasthehighesttemperature.Thiseffectcould
This work was supported under NASA Contract No.
explaintherelativelysoftextendedspectrumofAEAqrabove
NNG08FD60C,andmadeuseofdatafromtheNuSTARmission,
3 keV. Detailed calculations of such models will be presented
aprojectledbytheCaliforniaInstituteofTechnology,managed
elsewhere.
by the Jet Propulsion Laboratory, and funded by the National
In addition to the low X-ray luminosity and soft spectrum,
AeronauticsandSpaceAdministration.WethanktheNuSTAR
AE Aqr has another special feature—weak absorption of soft
X-rays,whichcorrespondstoN <8×1021cm−2,lowerthan Operations, Software and Calibration teams and the Swift Op-
H
theN >1022cm−2observedinmanyIPs(e.g.,Ezuka&Ishida erations team for support with the execution and analysis of
H
theseobservations.ThisresearchhasmadeuseoftheNuSTAR
1999;Yuasaetal.2010).Thisfeaturecouldbeexplainedbythe
Data Analysis Software (NuSTARDAS) jointly developed by
lowerdensityoftheaccretioncolumn,andneedstobefurther
theASIScienceDataCenter(ASDC,Italy)andtheCalifornia
investigatedwithdetailedmodels.AlsonotethatAEAqrdoes
InstituteofTechnology(Caltech,USA)andtheXRTDataAnal-
not show evidence for a neutral (or weakly ionized) iron line.
ysisSoftware(XRTDAS)developedundertheresponsibilityof
AmongotherIPspreviouslystudiedinthehardX-rayband,EX
HydraehasthelowestluminosityLX ∼ 1032 ergs−1 andalso ASDC. T.K. was supported by Japan Society for the Promo-
the lowest column with N (cid:2) 1022 cm−2 (Suleimanov et al. tion of Science (JSPS) Grant-in-Aid for Young Scientists (B)
H
(No.24740185).
2005).Theshapeofits3–30keVspectrumisnotfarfromthat
Facilities:NuSTAR,Swift
ofAEAqr,althoughEXHydraeisanearlysynchronoussystem
with an orbital period of 98.3 min and a long spin period of
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