XMM-Newton Spectroscopy of the Accretion-Driven Millisecond X-ray Pulsar XTE J1751-305 in Outburst

5 Pages·0.17 MB·English

by  J. M. Miller

DRAFTVERSIONFEBRUARY2,2008
PreprinttypesetusingLATEXstyleemulateapj
XMM-NEWTON SPECTROSCOPYOFTHEACCRETION-DRIVENMILLISECONDX-RAYPULSAR
XTEJ1751-305INOUTBURST
J.M.MILLER1,2,3,R.WIJNANDS1,4,5,M.MÉNDEZ6,E.KENDZIORRA7A.TIENGO8,M.VANDERKLIS8,D.CHAKRABARTY1,
B.M.GAENSLER2,ANDW.H.G.LEWIN1
Subject headings: binaries:close – pulsars: general – pulsars: individual: XTE J1751- 305 – stars: neutron –
x-rays:binaries–whitedwarfs
DraftversionFebruary2,2008
3 ABSTRACT
0
We presentan analysisof the first high-resolutionspectra measuredfroman accretion-drivenmillisecond X-
0
ray pulsar in outburst. We observed XTE J1751–305 with XMM-Newton on 2002 April 7 for approximately
2
35 ks. Using a simple absorbed blackbody plus power-law model, we measure an unabsorbed flux of 6.6±
n 0.1×10- 10 erg cm- 2 s- 1 (0.5–10.0 keV). A hard power-law component (Γ=1.44±0.01) contributes 83% of
a the unabsorbedfluxinthe0.5–10.0keV band,butablackbodycomponent(kT =1.05±0.01 keV)isrequired.
J
Wefindnoclearevidencefornarroworbroademissionorabsorptionlinesinthetime-averagedspectra,andthe
7
sensitivityofthisobservationhasallowedustosetconstrainingupper-limitsonthestrengthofimportantfeatures.
1
The lack of line features is at odds with spectra measured from some other X-ray binaries which share some

2 similaritieswithXTEJ1751–305. We discusstheimplicationsofthesefindingsontheaccretionflowgeometry
v inXTEJ1751–305.
6
6 1. INTRODUCTION verylow(∼0.01M⊙).
1 After the discovery of XTE J1751–305, we submitted a
Millisecond radio pulsars are thought to be created in neu-
8 target-of-opportunityrequestto XMM-Newton for the purpose
0 tronstarlow-massX-raybinaries(LMXBs). InthoseLMXBs,
ofstudyingthissourcewithhigh-resolutionspectroscopy. The
2 accretingmattermayspin-upthe neutronstar (see, e.g.,Bhat-
X-ray spectrum of the first system — SAX J1808.4–3658—
0 tacharya & van den Heuvel 1991). Therefore, it is expected
couldonlybestudiedduringoutburstusingRXTEandwiththe
/ that millisecond pulsars should also be found in LMXBs dur-
h WideFieldCamerasaboardBeppoSAX,whichhaveonlymod-
ingtheaccretionphaseofthebinaryasX-raypulsars.Although
p erate spectral resolution. In this Letter, we present an analy-
evidenceforrapidlyspinningneutronstarsinLMXBswasin-
- sisofthetime-averagedEPIC-pnandReflectionGratingSpec-
o ferredfromtheburstoscillationswhichwereseenduringtype-
trometer (RGS) data. The spectra obtained represent the first
r I X-ray bursts in several systems (see Strohmayer 2001 for a
t CCD- and gratings-resolution measurements from a millisec-
s review), the detection of millisecond pulsations in persistent
a ondX-raypulsarinoutburst.
emission remained elusive for many years. In 1998, the first
:
v suchsystemwasdiscovered(SAXJ1808.4–3658,whichhasa
Xi spin frequency of 401 Hz; Wijnands & van der Klis 1998a). 2. OBSERVATIONANDDATAREDUCTION
This source was extensivelystudied due to its obviousimpor-
r XMM-NewtonobservedXTEJ1751- 305beginningon2002
tanceforbinaryevolutionscenariosandaccretionflowgeome-
a
April 07.5 (UT). The EPIC-pn camera was operated in “tim-
tries and dynamics(see, e.g., Wijnands & van der Klis 1998b
ing”modetopreventphotonpile-upeffectsandtoallowpulse-
andreferencestherein).
phase-resolved spectroscopy . The RGS was operated in the
Inthespringof2002,Markwardt&Swank(2002a)reported
standard“spectroscopy”mode. The“thin”opticalblockingfil-
thediscoveryofthesecondaccretion-drivenmillisecondpulsar
ter was selected for the EPIC cameras. The EPIC-MOS cam-
XTE J1751–305(435Hz) in the Rossi X-rayTiming Explorer
eraswerenotoperatedinmodesoptimizedforasourceofthis
(RXTE)bulgescanobservationprogram. Aboutonemonthaf-
intensity,andwedonotconsidertheMOSspectrahere.
ter the discoveryof this system, XTE J0929–314was discov-
TheEPIC-pnandRGSdatawerereducedusingSASversion
ered (185 Hz; Remillard, Swank, & Strohmayer 2002.). The
5.3. We appliedthe standardreductionprocedure“epproc”to
neutron stars in both systems are in orbit around a compan-
ion star, with anorbitalperiodof∼42minutes(Markwardt& produceacalibratedpneventlist. In“timing”mode,thespatial
information is compressed into one dimension. We extracted
Swank2002b;Gallowayetal. 2002). Thesesystemsarevery
sourcedatainarectangle(withawidthof37”)alongthelength
tightbinariesandtheinferredmassoftheircompanionstarsis
oftheCCD, andbackgrounddata fromadjacentregions; only
1CenterforSpaceResearchandDepartmentofPhysics,MassachusettsInstituteofTechnology,Cambridge,MA02139–4307
2Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,[email protected]
3NationalScienceFoundationAstronomyandAstrophysicsFellow
4SchoolofPhysicsandAstronomy,UniversityofSt.Andrews,StAndrewsKY169SS,UK
5ChandraFellowwhileatMIT
6SRON,NationalInstituteforSpaceResearch,Sorbonnelaan2,NL-3584CA,Utrecht,NL
7InstitutfürAstronomieundAstrophysik,Abt.Astronomie,UniversitätTübingen,Sand1,D-72076,Tübingen,DE
8AstronomicalInstitute“AntonPannekoek”,UniversityofAmsterdam,Kruislaan403,NL-1098SJAmsterdam,NL
1
2 XTEJ1751- 305
“single” and “double” events were selected. The net pn ex-
posure was 33.7 ks. Using LHEASOFT version 5.1, the tool
“grppha”wasusedtorebinthespectrumtorequireaminimum
of20countsperbin. Weusedthetrial“timing”moderedistri-
butionandancillaryresponsefunctionsdevelopedbytheEPIC-
pncalibrationteam. We appliedthestandardreductionproce-
dure“rgsproc”toproducecalibratedRGSeventlists,first-order
spectra, and response functions. Periods of high instrumen-
tal backgroundwere excluded, giving a net RGS exposure of
34.2ks. The RGS spectra were groupedto requireatleast 20
countsperbin.
3. ANALYSISANDRESULTS
Preliminary spectroscopic results from this XMM-Newton
observationwerereportedinMilleretal.2002aandMilleretal.
2002b. TheaccuratepositionofXTEJ1751–305wasreported FIG. 1.— The EPIC-pn spectrum of XTE J1751- 305, fit with a
byEhleetal. (2002),alsobasedonanalysisofthisobservation
model consisting of blackbody and power-law components modified
(R.A.=17h51m13.5s,Decl.=- 30◦37′22′′,equinox2000.0). byphotoelectricabsorption(seeSection3fordetails). Thespectrum
TheinternalcalibrationaccuracyoftheEPICcamerasisbet- anddata/modelratioshownabovearerebinnedforvisualclarity.
terthan5%foron-axissources(Kirsch2002). Wefinddevia-
in the 0.5–10.0 keV band (an extrapolation from the 0.6–
tionsupto0.6keVwhicharenotremovedbyspectralmodel-
10.0keVbandasthe0.5–10.0keVrangeismoreconventional).
ing,andthereforerestrictouranalysisofthepnspectrumtothe
0.6–10.0keVband.Theaveragecountrate(79.36countss- 1)is Motivated by evidence for a Comptonizing volume in
SAXJ1808.4–3658(Gierlinski,Done,&Barret2002),wealso
wellbelowthatatwhichphotonpile-upoccursintimingmode
(1500countss- 1). Testsoftheeffectiveareacalibrationofthe fit the EPIC-pn spectrumwith the “compTT”Comptonization
model (Titarchuk 1994), which attempts to account for the
RGS reveal accuracies of 5% in the 7–36Å (0.35–1.77 keV)
Compton-upscattering of cool photons in a hot corona self-
rangeand10%atwavelengthsshorterthan7Å (energiesabove
consistently. Thismodeldoesnotprovideanacceptablefitby
1.77 keV; den Herder 2002). Although our best-fit model for
itself(χ2/ν=2- 4,ν=1884).Wefindadegeneracybetweena
the EPIC-pn spectrum (see below) providesan adequatefit to
the RGS spectrum(χ2/ν =1.314,ν=1463), broad deviations coolcorona(kT ∼3keV)withhighopticaldepth(τ ∼8),and
a hotcorona(kT ∼40 keV)with loweropticaldepth(τ ∼1),
are apparent at energies above 1.8 keV. We therefore rely on
eachup-scatteringaseedphotondistributionpeakingbetween
theEPIC-pnspectrumtocharacterizethecontinuumemission,
kT =0.5–0.6keV.Finally,wetriedfittingablackbodycompo-
and search the RGS bandin contiguous3Å slices for narrow
nentinadditiontocompTT.Thismodelmightcorrespondtoa
features.
scenario in which the blackbody emission region is only par-
TheEPIC-pnandRGSspectrawereanalyzedusingXSPEC
tiallycoveredbythecorona. Thismodelalsoyieldedapoorfit
version11.1.Allerrorsquotedinthispaperare90%confidence
(χ2/ν∼2).
errors. Notethatsystematicerrorsarenotaddedtothespectra
The strength, ionization state, and profile of Fe Kα emis-
toaccountforfluxcalibrationuncertainties.
sion lines — if produced through irradiation of the accretion
disk—canrevealagreatdealabouttheaccretiongeometryof
3.1. TheEPIC-pnSpectrum low-massX-raybinaries. Asaietal. (2000)reportthepossible
detectionofanFeKαlinein4U1820–30(P =11.4minutes;
We first considered a model consisting of blackbody and orb
Stella, White, &Priedhorsky1987). Thelinewasfoundtobe
power-law components, modified by neutral absorption in the ionized(E=6.6±0.1keV),broad(FWHM=0.7+0.2keV),and
ISM (using the “phabs” model in XSPEC). We measure a - 0.5
best-fit equivalent hydrogen column density of N = (9.8± weak(WKα=33+- 1121eV).AssumingalineofequivalentFWHM
0.1)×1021 atoms cm- 2. The blackbody tempeHrature ob- inthe 6.40–6.97keVrange(FeI —Fe XXVI),we measurea
tained is moderate: kT =1.05±0.01keV. For spherical sym- 95%confidenceupper-limitofjust6 eVin theEPIC-pnspec-
metry, this translates into a blackbody radius of R = f2× trum of XTE J1751–305. Using a Gaussian with zero width
3.01+0.09 (d/10 kpc) km, where the spectral hardening fac- tomodelalinenarrowerthantheEPIC-pnresolution,the95%
- 0.06 confidenceupperlimitontheequivalentwidthofanysuchnar-
tor f is the ratio of the color temperature and the effective
rowFeKαlinesislessthan4eV(6.40–6.97keV).
temperature, and d is the distance to the source. The mea-
sured power-law index is Γ= 1.44±0.01; the normalization
is (6.4±0.1)×10- 2 (photonskeV- 1 cm- 2 s- 1 at 1 keV). This 3.2. Pulse-Phase-ResolvedSpectroscopy:FirstResults
model produces an adequate fit to the data: χ2/ν = 1.107 We applied the SAS task “barycen”, and using the binary
(whereνisthenumberofdegreesoffreedom;ν=1884forthis systemparametersreportedbyMarkwardtetal.(2002)wepro-
model;seeFigure1). We notethatasimplepower-lawmodel ducedspectrafromthe“high”and“low”partsofthepulse. We
returnsaverypoorfit(χ2/ν>4,ν=1886);thesoftblackbody findthatthemodelappliedtothetime-averagedspectrumisan
componentisstronglyrequiredbythedata. acceptable descriptionof these spectra (no convincingnarrow
With this simple but standard model, we obtain an unab- or broad emission or absorptionfeatures are found). Interest-
sorbed 0.5–10.0 keV flux of (6.6±0.1)×10- 10 erg cm2 s- 1, ingly,theslopeofthepower-lawcomponentisconsistentinthe
or(0.170±0.004)photonscm- 2s- 1. Thehardpower-lawcom- two spectra butthe normalizationchanges, and the blackbody
ponentcontributes83%oftheunabsorbedflux componentchangesintemperaturebutnotinnormalization.
J.M.Milleretal. 3
FIG. 2.— TheRGS spectra of XTE J1751- 305 (RGS-1inblack, thFeING.e3X.—Ly-TαhelinReGaSt-122s.1p3ecÅtr,ufimtwoifthXaTEsimJ1p7l5e1p-o3w0e5r-ilnawtheplruesgiGoanuosf-
RGS-2 in red; both are rebinned for clarity and narrow features are
sianmodel.Ourbestfitmodeldoesnotrequireanemissionlinewithin
instrumental)fitlocallyintheregionoftheNeKedge(14.25Å)with
12.13±0.1Å,assumingaFWHMequivalent totheprominent NeX
asimplepower-lawplusedgemodel. Theedgedepthwasfixedatthe
Ly-α emission line seen in the 42-minute X-ray pulsar 4U 1626–67
solarvaluebutthepower-lawindexandnormalizationwereallowedto
(Schulzetal.2001).Narrowfeaturesareinstrumental.
vary;atrendinthedata/modelratioisclearlyvisible. Fitsmadewith
avariableedgedepthsuggestNeislikelyunder-abundant(seetext).
the 95%confidenceupper-limitisthattheabundanceofNe
isonly77%ofthesolarvalue. AsolarNeabundancedoesnot
Similar behavior was discovered in pulse-phase-resolved
spectra of SAX J1808.4- 3658 (Gierlinski, Done, & Barret allowforanoptimalcharacterizationofthelocalspectrum(see
Figure2).
2002).
ThebinaryX-raypulsar4U1626–67alsohasa42-minuteor-
Wenotethatapossiblefeatureatapproximately2.9keVap-
bitalperiod.Moreover,thedistancetothissystemmaybeclose
pears to vary in width, intensity, and centroid energy in the
to 8 kpc (Chakrabarty 1998); we anticipate a Galactic center
“high”and“low”spectra;however,theerrorlimitsontheline
location for XTE J1751- 305and therefore a similar distance.
parameters overlap. This feature is also visible in the time-
A Chandra/HETGS spectrum of 4U 1626- 67 reveals broad-
averaged spectrum (see Figure 1). As there are no astrophys-
enedemissionlinesandDoppler-shiftedlinepairs(Schulzetal.
icallyabundantelementswithtransitionsnearthisenergy,itis
temptingtoassociatethisfeaturewithared-shiftedabsorption 2001).TheNeLy-αline(12.13Å)isparticularlystrong(aflux
linefromtheneutronstarsurface(giventhetemperatureofthe of 8.15±0.93×10- 5 photons cm- 2 s- 1 is measured; FWHM
blackbody,perhapsFeXXV orFeXXVI).AlthoughanF-test =2860±330 km s- 1). We report the 95% confidence upper-
finds that the addition of a Gaussian to model this feature is limits on emission lines from helium-like and hydrogenicNe,
significant at the 7σ level of confidence in the “low” part of Mg,andSiinTable1(upperlimitsonOemissionlinesarenot
the pulse (3σ in the “high” part), a similar featureis apparent constrainingduetopoorstatisticsatlowenergies).
when the EPIC-pn spectrum of the Galactic black hole candi-
dateXTEJ1650- 500(Milleretal. 2002c)isfitusingthesame 4. DISCUSSION
responsematrix. Itislikelythatthisfeatureis duetoa defect We have analyzed the first CCD- and grating-resolutionX-
inthetiming-moderesponsematrix. rayspectraof a millisecondX-raypulsarin outburst. We find
noconvincingevidenceforbroadornarrowemissionorabsorp-
3.3. TheRGSSpectra tionfeaturesinthetheEPIC-pnandRGSspectra(seeFigure1
andTable1).Withoursimpleblackbodypluspower-lawmodel
We analyzed the RGS spectra in contiguous 3Å slices for
fortheEPIC-pnspectrum,we findthatthepower-lawcompo-
narrowemissionorabsorptionlines.Ineach3Å slice,asimple
nentcomprises83%oftheunabsorbedfluxinthe0.5–10.0keV
power-lawplusISMedge(s)modelwasfittothedata. Wefind band. The power-law component is hard: Γ = 1.44±0.01,
no convincingevidence for narrow absorption features in this butwithinarangetypicalforX-raypulsars(e.g.,Γ=1.0–1.5;
band (0.3–2.5 keV, or 5–40Å). At energies below ∼0.6 keV
White,Swank,&Holt1983). TheEPIC-pnspectrumrequires
(wavelengths longer than ∼21Å) the sensitivity is low due to ablackbodywithkT =1.05±0.01keVandanimpliedemitting
therelativelyhighcolumndensityalongthelineofsighttoXTE radiusofR= f2×3.01+0.09(d/10kpc)km. Itisfairtosaythat
- 0.06
J1751–305. ourknowledgeofthespectralhardeningfactor f isratherpoor
Recently, evidence for enhanced Ne abundances has been (foradiscussion,seeLewin,vanParadijs,&Taam1993). If f
foundincompactbinaries(Schulzetal. 2001;Juett,Psaltis,& isless than1.3, itwouldindicatethatonlypartofthe neutron
Chakrabarty2001).WefittheRGSspectraaroundtheposition starsurface(a“hotspot”)isinvolved.
of the Ne K edge(14.25Å) with a power-lawmodelbetween Markwardt et al. (2002) fit a simple absorbed power-law
13.25Å and 15.25Å(seeFigure 2). The power-lawindex and model to the RXTE spectrum of XTE J1751–305, and mea-
normalizationwere bothallowed to varyin this fit. Using the sure a softer power-law index (1.7 < Γ < 1.9). The effec-
cross-sectionsofVerneretal. (1993)andsolarabundancesrel- tivelowersensitivityboundoftheRXTEProportionalCounter
ativetoHasperMorrison&McCammon(1983),wefindthat Array (PCA) is 3 keV; fitting the 3–10 keV EPIC-pn spec-
Nemaybeunder-abundantinXTEJ1751–305: trumwithonlyapower-law,wefindΓ=1.72±0.01forN =
H
4 XTEJ1751- 305
(9.8±0.1)×1021atomscm- 2andΓ=1.90±0.02allowingN spectrum of XTE J1751–305 is also consistent with a highly
H
tofloat(inthiscase,N =[2.5±0.1]×1022atomscm- 2).Thus, ionized disk in this source. Secondarily, the absence of such
H
thediscrepantindicesareeasilyexplainedintermsofdifferent lines may be due to differences in the accretion flow geome-
instrumentalranges. try due to the neutron star magnetic field. Whereas the mag-
InmodelingsimultaneousChandra/LETGSandRXTEspec- netic field in XTE J1751- 305is likely to be similar to that in
traofXTEJ0929–314,Juett,Galloway,&Chakrabarty(2002) SAX J1808.6- 3658(B=(2- 6)×108 G; Wijnands& vander
measuresimilarlydiscrepantpower-lawindices. Theysuggest Klis 1998a),the magneticfield in 4U 1626- 67is likely much
anastrophysicaloriginforthediscrepancyinXTEJ0929–314 higher (B=3×1012 G, Orlandini et al. 1998). The stronger
and XTE J1751–305in the form of a power-law with a break magneticfieldin4U1626–67maydisrupttheinnerdisk,allow-
or a roll-over in the 1.4–4.4 keV range. A model consist- ingcoolmaterialintheouterdisktobeirradiatedbyaweaker
ing of blackbodyand brokenpower-law components(E = corona.Emissionlineshavealsobeenseeninneutronstarsys-
break
3.7±0.1 keVx, Γ = 1.34±0.03, Γ = 1.60±0.04) temswhichareviewededge-on(so-calledaccretiondiskcoro-
E<3.7 E>3.7
provides an improved fit to the spectrum of XTE J1751–305 nae or “ADC” sources; see, e.g., Kallman et al. 2002); the
(χ2/ν=1.090,ν=1882).Althoughtheseresultssuggestthatan absenceofemissionlinesinXTEJ1751–305mayalsobepar-
astrophysicaloriginfor the apparentspectralevolutionis pos- tially due to a low system inclination (Markwardtet al. 2002
sible, anumberofconcernsremain. Giventhat: (1)the effec- reportnoevidenceofdipsoreclipses).
tivearea(flux)calibrationoftheEPIC-pnhas5%uncertainties Studies of some ultra-compact systems suggest a Ne-rich
(Kirsch 2002), (2) the LETGS (plus ACIS-S array) flux cali- companion(forobservationalresults, see: Schulzetal. 2001;
brationhas10%uncertaintiesabove1keV (H.Marshall, priv. Juett, Psaltis, & Chakrabarty2001; for theoreticaldiscussions
comm.), (3) that RXTE PCA and High EnergyX-Ray Timing see Yungelson, Nelemans, & van den Heuvel 2002; Bildsten
Experiment(HEXTE)observationsofthe“Crab”nebula(taken 2002). FitstotheNephotoelectricabsorptionedgeintheRGS
tobea purepower-lawforcalibration)differinpower-lawin- spectra of XTE J1751–305 constrain the abundance of Ne to
dex by δ(Γ)≃0.1 (see, e.g., Wilms et al. 1999), and (4) that beatmost77%ofthesolarvaluealongthislineofsight(95%
Markwardtetal. (2002)didnotfita blackbodycomponentto confidenceupper-limit;seeSection3.3).Thismaysuggestthat
the RXTE spectra of XTE J1751–305, we do not consider an aNe-richcompanionisunlikelyinthecaseofXTEJ1751–305.
astrophysicaloriginforthediscrepantindicestoberequiredby
thepresentdata.
5. ACKNOWLEDGMENTS
The upperlimits on the strength of narrow (consistentwith
instrumentresolution;FWHM∼0.1keV)andbroad(FHWM= We are grateful to the XMM-Newton Project Scientist Fred
0.7keV)FeKαemissionlinesare4eVand6eV,respectively Jansen for granting time to observe this source. We thank
(95% confidence). This stands in contrast the detection of a Michael Nowak for useful discussions. J. M. M. is grateful
weakFeKαlineinoutburstspectraofthe401Hzmillisecond for support from the NSF. R. W. was supported by NASA
X-raypulsarSAX J1808.4–3658obtainedwithRXTE(Heindl through Chandra fellowship grant PF9-10010, which is oper-
&Smith1998;Gierlinski,Done,&Barret2002). Itispossible atedbytheSmithsonianAstrophysicalObservatoryforNASA
thatastrong,hotcorona(orthemagnetosphere,ifthecoranae undercontractNAS8-39073. W. H. G. L. gratefullyacknowl-
in these LMXBs is small or disrupted) which producesto the edgessupportfromNASA.Thisworkwassupportedinpartby
power-lawcomponentmayionizethediskinXTEJ1751- 305 the Netherlands Organization for Scientific Research (NWO).
toadegreewhichpreventsFeKαemission. Thisresearchhasmadeuseofthedataandresourcesobtained
TheabsenceofH-likeandHe-likeresonanceemissionlines through the HEASARC on-line service, provided by NASA-
like thoseobservedin 4U1626–67(Schulzetal. 2001)in the GSFC.
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TABLE 1
EmissionLineUpper-Limits
Helium-like Hydrogenic
Broada Narrowb Broada Narrowb
(10- 5phcm- 2s- 1) (10- 5phcm- 2s- 1) (10- 5phcm- 2s- 1) (10- 5phcm- 2s- 1)
Ne 1.6 1.4 0.9 0.5
Mg 1.4 1.3 11 8.8
Si 29 26 3.7 3.5
NOTE.—The95%confidenceupperlimitsonhighlyionizedresonanceemissionlineswithin0.1Å oftheirexpectedrest-framewavelengthsintheRGSspectra
ofXTEJ1751–305. aAssumingFWHM=3000km/s,similartotheDoppler-shiftedpairsin4U1626–67(Schulzetal. 2001). bTheupperlimitswerecalculated
usingGaussiansofzerowidthtomodelfeaturesnarrowerthantheRGSresolution.LimitsonthestrengthofOlinesarenotconstrainingduetopoorstatisticsatlow
energies. Theseupperlimitsaresimilartoorbelowthefluxesmeasuredfromthelinesdetectedin4U1626–67,indicatingaconstrainingsensitivitywasachievedin
thisXMM-Newtonobservation.