BeppoSAX observations of the X-ray pulsar MAXI J1409-619 in low state: discovery of cyclotron resonance features

1.1 MB·English

by  Mauro Orlandini

THEASTROPHYSICALJOURNAL,747:,2012MARCH1
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
BeppoSAXOBSERVATIONSOFTHEX–RAYPULSARMAXIJ1409- 619INLOWSTATE:DISCOVERYOF
CYCLOTRONRESONANCEFEATURES
MAUROORLANDINI1,FILIPPOFRONTERA1,2,NICOLAMASETTI1,VITOSGUERA1,LARASIDOLI3
TheAstrophysicalJournal,747:,2012March1
2
1 ABSTRACT
0 Thetransient500sX–raypulsarMAXIJ1409- 619wasdiscoveredbytheslitcamerasaboardMAXIonOc-
2 tober17,2010,andsoonafteraccuratelylocalizedbySwift. Wefoundthatthesourcepositionwasserendip-
itously observed in 2000 during BeppoSAX observations of the Galactic plane. Two sources are clearly de-
n
a tected in the MECS: one is consistent with the position of IGR J14043- 6148and the other one with that of
J MAXI J1409- 619. We reporton the analysis of this archival BeppoSAX/MECSobservationintegrated with
newlyanalyzedobservationfromASCAandasetofhigh-energyobservationsobtainedfromtheoffsetfields
3
2 oftheBeppoSAX/PDSinstrument. FortheON-sourceobservation,the1.8–100keVspectrumisfitbyanab-
sorbedpowerlawwithaphotonindexΓ=0.87-+00..1299,correspondingto2–10and15–100keVunabsorbedfluxes
] of 2.7×10- 12 and 4×10- 11 erg cm- 2 s- 1, respectively, and a 2–10 keV luminosity of 7×1034 erg s- 1 for a
E
15kpcdistance.ForaPDSoffsetfieldobservation,performedaboutoneyearlaterandshowinga15–100keV
H flux of 7×10- 11 ergcm- 2 s- 1, we clearly pinpointthree spectralabsorptionfeaturesat 44, 73, and128keV,
. resolvedbothinthespectralfitandinthe Crabratio. We interpretthesenotharmonicallyspacedfeaturesas
h
duetocyclotronresonances.Thefundamentalenergyof44±3keVcorrespondstoamagneticfieldstrengthat
p
- theneutronstarsurfaceof3.8×1012(1+z)G,wherezisthegravitationalredshift.Wediscussthenatureofthe
o sourceinthelightofitspossiblecounterpart.
tr Subject headings: X–rays: binaries — Stars: individual: MAXI J1409- 619 — Stars: individual:
s IGRJ14043- 6148
a
[

2 1. INTRODUCTION position(Kenneaetal.2010b).
The source was also observed on October 22 by the Pro-
v On October 17, 2010 Yamaokaetal. (2010b) reported on
8 anoutburstfromanewsource,namedMAXIJ1409- 619,re- portional Counter Array (PCA; Jahodaetal. 1996) aboard
1 vealed by the Gas Slit Camera (GSC; Miharaetal. 2002) of RXTE(Bradtetal.1993).TheX–rayspectrumshowsastrong
2 theMonitorofAll-skyX-rayImage(MAXI;Matsuokaetal. 6.5 keV Iron line (E.W. 200 eV), and a continuum modeled
1 2009)experimentaboardtheInternationalSpaceStation.The byreflectionorpartiallycoveringabsorption(Yamaokaetal.
2. 4–10keVfluxdecreasedfromitsinitialvalueof∼40mCrab 2010a). The 2–10 keV flux was about 10- 10 erg cm- 2 s- 1,
1 onOctober17to∼30mCrabonthefollowingday. and was strongly variable on time-scales of hundreds of
0 On October 20 Swift (Gehrelsetal. 2004) began a target seconds. An observation performed on December 4 found
1 ofopportunityobservationoftheMAXIJ1409- 619errorcir- MAXI J1409- 619 at a flux level 6–7 times higher than the
: cle(0.◦2radius). Kenneaetal.(2010b)usedtheX–RayTele- October observation (Yamamotoetal. 2010), and confirmed
v scope(XRT;Burrowsetal.2005) to pinpointthepositionof the500spulseperiodobservedbySwift.
Xi the new source with an estimated 90% confidence level un- FromobservationsperformedonDecember2and3bythe
certaintyof1.′′9andcoordinatesRA(J2000): 14h08m 02.s56, GLAST Burst Monitor (GBM; Meeganetal. 2007) aboard
r
a Dec(J2000): - 61◦ 59′ 00.′′3. Its spectrum is fit by an ab- theFermisatellite,thepresenceofa500sdoublepeakedpul-
sorbedpowerlaw,withphotonindexΓ=- 0.5+0.1. Afollow- satingsignalwasconfirmed(Camero-Arranzetal.2010).
- 0.6 Followingthe MAXIJ1409- 619localizationbySwift, we
up XRT observation on November 30 found the source ∼7
searched for BeppoSAX (Boellaetal. 1997a) archival obser-
times brighter than on October 20, with an average 0.3–10
keVfluxof7×10- 10ergcm- 2s- 1(Kenneaetal.2010a).Only vations with the transient position within the Narrow Field
Instruments (NFIs) Field of View (FoV). We found that an
inthislatterobservationa∼500spulsationwasdetected,with
observationperformedin the framework of a Galactic plane
a 42% sinusoidal rms modulation. The source was detected
surveycontainedtheMAXIJ1409- 619position(Sgueraetal.
since October 18 also in the 15–50 keV energy band by the
2010).
Swift Burst Alert Telescope (BAT; Barthelmyetal. 2005) at
InthisPaperwereportonthespectralanalysisofthisBep-
thelevelof∼30mCrab(Kenneaetal.2011).
poSAXobservation,integratedwithhigh-energydataobtained
Neither catalogued radio nor X–ray sources were present
fromtheoffsetfieldsofthePDS(Fronteraetal.1997)instru-
in the Swift error circle, while a 2MASS IR star, the likely
ment. We further present the analysis of a newly analyzed
IRcounterpartoftheX–raytransient,lays2.′′1fromtheXRT
ASCAobservationperformedduringaGalacticplanesurvey.
1INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF) 2. OBSERVATIONS
Bologna,viaGobetti101,40129Bologna,Italy 2.1. BeppoSAX/PDSObservations
2DipartimentodiFisica,UniversitàdegliStudidiFerrara,viaSaragat
1,44122Ferrara,Italy We searched our BeppoSAX archive for observations cov-
3INAF/IstitutodiAstrofisicaSpazialeeFisicaCosmica(IASF)Milano, eringtheMAXIJ1409- 619position. Fromourlocalarchive
viaBassini15,20133Milano,Italy weareabletoextractnotonlyproductsfromtheNFInominal
2 Orlandinietal.
GAL PLANE SUR 02+16 2−10 keV
SAX MECS 2000 Feb 7 Exposure: 47747 s
14h10m00s 08m00s 06m00s 04m00s 02m00s
0
0
5
−61o30’00"
0
0
4
MAXI #1 45’00"
0
0
els3
x
Pi #2
Y −62o00’00"
0
0
2
Agile
00 PDS OFF 15’00"
1
100 200 300 400 500
Figure1. RegioncenteredontheMAXIJ1409- 619position(thesmallblack
X Pixels
circle)withthePDSpointings:ONpointings(red),POFFpointings(green),
andMOFFpointings(cyan). ThePDSFoVis1.◦3(FWHM).Notethetwo
0 4 8 13 34
PDSobservations(oneONandoneMOFF)fullycoveringthesourceposi-
tion,andthree(oneONandtwoPOFF)partiallycoveringtheposition(with Figure2. 2–10keVMECSimageofamosaicofthetwo,partiallyoverlap-
theterm“partially”weintendthatthesourceisbeyondthecollimatorFWHM ping,ON-sourceBeppoSAXobservationsOP08317andOP08386,smoothed
butbeforeitsFWZI).ThebolddottedlinecorrespondstotheGalacticplane. withaGaussianfilterwithaσof24′′.Twosourcesareclearlydetected.The
pointings(alsoavailablefromASDC–theASIScientificdata onepresentintheMAXIJ1409- 619errorboxislistedas#2inTable2.The
two“cuts-out”areduetotheremovalofthecalibrationsourceevents.
Center4),butalsonetspectrafromthePDSoffsetfields.
Indeed,becauseoftherockingtechniqueusedtoderivethe LECScanbeoperatedonlywhenintheEarthshadow. Good
PDSbackground,twopositionsoffsetby3.◦5withrespectto data were selected when the instrument configurations were
theNFIdirectionwerealternativelyobservedevery96s. For nominal, and with an elevation angle above the Earth limb
fieldsfreeofsources,weexpectthatdifferencesbetweenthe >4◦.
countspectra in the “plus OFF” (thereafter POFF) and “mi- A mosaic of the two, partially overlapping, ON-source
nusOFF”(thereafterMOFF)positionswillbeconsistentwith OP08317 and OP08386 MECS images is shown in Fig-
zero. If,ontheotherhand,acontaminatingsourceispresent ure2, wheretworelativelybrightsourcesareclearlypresent
inoneofthetwooffsetfields,thenthespectraldifferencecor- (see Table 2). Source #2 position is consistent with
respondstoanet,backgroundsubtractedPDSspectrumofthe MAXIJ1409- 619,whilethesource#1positionisconsistent
offsetfield. with the X–ray source IGR J14043- 6148(Birdetal. 2010).
We found three sets of observations containing the Thethirddetection(source#3)correspondstoextendedlow-
MAXI J1409- 619 position in their FoVs. In Figure 1 we energy emission observed in the LECS image, and may be
showafindingchartcenteredontheMAXIJ1409- 619posi- associatedwitharadiosourceatapproximatelythesamepo-
tionandshowingboththeON-sourceandOFF-sourcepoint- sition(Cohen&Green2001). Theuncertaintyinthe MECS
ings(thePDS FoV is 1.◦3 FWHM). InTable 1 we detailthe positionsis30′′(Perri&Capalbi2002).
wholesetofBeppoSAXobservationsusedinouranalysis.All We extracted LECS and MECS data from a 4′ (30 pixel)
errorsare givenat the 90%confidencelevel for a single pa- radiuscircularregioncenteredonthe MAXIJ1409- 619po-
rameter. sition.ConcerningLECSdata,therearenotenoughcountsto
allow a spectral reconstruction. On the other hand, we are
2.1.1. ON-sourceobservations(OP08317andOP08386)
able to extract a 1.8–10 keV MECS spectrum, that we re-
The first ON-source BeppoSAX pointing with binned to a minimum of 30 counts per bin to allow the use
MAXI J1409- 619 in the NFI FoV was performed on of χ2 statistics. We used a response matrix appropriate for
January 29, 2000 within a program aimed at performing a the off-axis position of the source. Because the position of
systematic study of part of the Galactic plane (OP08317 MAXIJ1409- 619isclosetotheGalacticplane,wechecked
– Galactic Plane Survey Field 02). Another ON-source forpossiblecontaminationfromtheGalacticridgeemission.
observation (OP08386 – Galactic Plane Survey Field 16), Tothisend,fromtheMECSimageweextractedbackground
was performed a week later. This observation partially spectrafromtwosource-freeregions,offsetbyaboutthesame
overlapstheOP08317field(seeFig.1). angleasthesource(totakeintoaccountvignetting),andwith
The Galactic plane Field 02 net exposures of the two the same extraction radii. These backgroundsare consistent
imaging instruments Low-Energy Concentrator Spectrome- withthemeanbackgroundusedroutinelyforMECSobserva-
ter (LECS; 0.1–10 keV; 37′ FoV; Parmaretal. 1997) and tions. For this reason the mean backgroundwas used in the
Medium-EnergyConcentratorSpectrometer (MECS; 1.8–10 spectralanalysis.
keV;56′ FoV;Boellaetal.1997b)are6812and22060s, re- Given that the PDS is not an imaging instrument, before
spectively. The difference is due to the constraint that the performing joint MECS/PDS spectral fits it is necessary to
addresstheproblemofdeterminingwhetherthehighenergy
4http://www.asdc.asi.it/bepposax emission observed by the PDS is due to MAXI J1409- 619
BeppoSAXobservationsofMAXIJ1409- 619:discoveryofCRFs 3
Table1
ListofPDSpointingscoveringtheMAXIJ1409- 619errorbox
Seq OPn Date Duration SourceName Dist RA DEC Type PDSrate S/N
(ks) (′) (J2000) (J2000) (15–100keVc/s)
1 08317 29/01/2000 42 GalPlaneSur2 19.6 211.3474 - 61.88437 ON 0.247±0.069 3.6
2 10480 07/01/2001 220 CircinusGalaxy 27.3 212.8952 - 61.80250 MOFF 0.450±0.032 14.1
3 08386 07/02/2000 52 GalPlaneSur16 49.5 210.2841 - 61.83828 ON 0.086±0.049 1.7
4 01652 23/02/1997 90 AlphaCen 53.6 213.5273 - 62.53019 POFF 0.086±0.052 1.6
5 08484 19/02/2000 47 GalPlaneSur23 70.0 213.5936 - 61.09281 POFF 0.144±0.072 2.0
Note. —The“SourceName”refers,incaseofoffsetfields,tothenominalONpointing. Thedistancelistedincolumn6refersto
theangulardistanceofthePDSpointingfromtheMAXIJ1409- 619position.
Table2
SourcesdetectedintheMECSfieldforOP08317
# Ratea pixel R.A. Dec S/N Counterpart
(2–10keV) x y (2000)
1 45.5±3.6 373.2 317.1 140431.3 - 614710.8 12.6 IGRJ14043- 6148
2 10.6±1.3 188.2 233.2 140800.4 - 615824.1 8.15 MAXIJ1409- 619
3 3.77±0.79 293.9 250.1 140600.5 - 615609.6 4.77
aInunitsof10- 3Counts/s
Dickey&Lockman 1990) and unabsorbed 2–10 and 15–
100 keV fluxes of 2.7×10- 12 and 4×10- 11 erg cm- 2 s- 1,
V−1 respectively.
Counts sec ke−1 10−3 From F2.i1g.u2r.eM1OwFFe ocfafnsetsefieeldthoabtswerevahtiaovne(OthPre1e04s8e0t)s of PDS
Normalized 10−4 Ffiofocfirseetnwttoosbtoasfteitsrhvtieacmtsiot(onOspPec0ro1fvo6er5rmi2naagnutdhseOefuPMl0Aa8n4Xa8lI4yJ)s1iws4e(0s9de-oe6nth1oe9thopabovsseeitrsivouenfd-.
countratesinTable1). Ontheotherhand,fortheMOFFoff-
) 1
sResiduals ( −10 sheatFvefiireseltnd,ouowgfehthscethaCeticiskrtciecidnsuffsoorGraattlhhaeoxryopuoregbshseensrpcveeactitoorfanl,aaOnnyPal1yc0sa4its8a.0lo,gduaetda
2 5 10 20 50 100 X–ray sources in the MOFF observation FoV. The only
Energy (keV) source present is the EGRET (Hartmanetal. 1999) source
Figure3. 1.8–100keVjointMECS/PDScountratespectrum(plussigns) 3EGJ1410- 6147,thatwasrecentlyassociatedwiththe50ms
and the power law best fitmodel (histogram) ofMAXIJ1409- 619 in the radiopulsarPSRJ1410- 6132(O’Brienetal.2008). Nosig-
OP08317field.Thefitresidualsareshowninthebottompanel.
nificantX–rayemissionhasbeendetectedaroundtheEGRET
or to the INTEGRAL source. Bear in mind that, in comput- source(Dohertyetal.2003);thereforeweassociatethePDS
ing fluxes, it is necessary to take into account the triangu- detectionwithMAXIJ1409- 619.
lar angular response of the PDS collimators (see Fig. 2 in Westartedbyextractinga15–100keVcoarserbinnedPDS
Fronteraetal.2007). spectrumoftheMOFFfield(assumingasitsbackgroundthe
First, we analyzedthe ON-sourceOP08386MECS obser- corresponding POFF spectrum). The fit with a power law
vation, that contains IGR J14043- 6148 (detected at >15σ) gives a photon index of 2.2±0.3, with a normalized χ2 of
ν
butnotMAXIJ1409- 619.Forthisobservationwehaveonly 1.31for11dof. Fromtheshapeoftheresidualsandfromthe
a marginal PDS detection (see Table 1), therefore it seems quite different photon index with respect to the ON-source
plausible to associate the high-energy emission observed in observation,we suspected the presenceof a changeof slope
the OP08317 observation with MAXI J1409- 619. To fur- inthespectrumbetween20and30keV.Thisbreakisnotde-
ther confirmthis association, we verifiedthat the PDS spec- tectedintheON-sourceobservationbecauseofitspoorstatis-
trum extrapolated at lower energies is consistent with the tics. We triedbotha brokenanda cutoffpowerlaw. By fix-
MAXIJ1409- 619MECSspectrum. ing the cutoff energy at 25 keV (we were only able to con-
In Figure 3 we show the joint MECS/PDS 1.8–100 keV strainthebreak/cutoffenergybetween20and40keV),acut-
MAXI J1409- 619 count rate spectrum, together with its off power law fit gives a power law index Γ=0.9+0.4, with
- 0.3
power law best fit (normalized χ2ν of 0.18 for 15 de- a normalized χ2ν of 1.06 for 11 dof. While from a statisti-
grees of freedom – dof). It is characterized by a hard calpointofviewthetwofitsareequivalent(anF-testgivesa
spectrum, with power law photon index 0.87+- 00..2199, NH = 36%probabilityofchanceimprovement(PCI)oftheχ2–see
2.8+3.4 ×1022 cm- 2 (consistent with galactic absorption, AppendixA for the discussion on the correctnessof the use
- 2.2
4 Orlandinietal.
∼44keVlineisstatisticallysignificant(PCIis6×10- 3).Sec-
V−1 0.01 (otnhde,ltihneedwifidfitchulhtiaedstionbdeetfierxmedin,iwnghitlheethoethleirnelinneorpmaraalimzaettieorns
Counts sec ke−1 10−3 iasstraeatifssfeteieccntsetdaosbc“ylheaolarllreygse”reeicnroronthrssetr)ucacortnetthidneuuleuinmteo:ptrhwoeefiflahec.atvTethhenatolittnheeenwoliiundgethhs
Normalized 10−4 ilesivnitedhedeneretpfftohorrewtheceocmlainnpeuisntefaedtrhfoirgnohlmyerathneneuep“rhgpoieerlsel)”i.mwitid(tthhi,swwhiilllebefomr othree
Following our detection strategy, we performed a second
1 fit on the 15–80 keV energy range. The fit with only one
)
sResiduals ( −2−10 20 En5e0rgy (keV) 100 da2τGo×as=fui.g19snsB01ii+-afiy∞7nc)aa.,idnnwdTtalihhynbiegslbeosearettnphtceoteoirtonhtnfihdeterw,liGlawnisnaeieunthpsowsatχiriaaad2νncmtchoieenfwpte0taara.bssb4sl8afioerrx:efpeoχtdEri2νoc3aynwt0caw7=dseok73feo3.6Vb+-(P45.tfaoCNkirnIeo3eVti2des,
- 42
Figure4. 15–200keVMOFFPDScountratespectrum(plussigns)andthe howthetwolineenergiesarenotinanharmonicratio.
bestfitpowerlawplus3Gaussiansinabsorptionmodel(histogram). Thefit
Then we extended the energy range to the whole 15–
residualsareshowninthebottompanel. TheGaussianenergiesareleftfree
4an,danadre74k4eV±f3o,r7t3h+-e45t,harnedel1i2n8es+-.58keV.Thelinewidthswereconstrainedto4, 2(χ02ν0koefV4.b5afnodr.4A6gdaionf)t,hesofitwweiathddtwedoalinnoetshwerasabnsootrapctcioenptlaibnlee,
the width of which was fixed at 7 keV. The fit significantly
oftheF-test), thecutofffitnicelymatchesthespectralindex improved(χ2ν of0.58for44dof,PCIof10- 10),withthebest
observedat lower energies. The corresponding15–100keV fitE =128+5keV.τ wasnotconstrained(thiswasnotasur-
cyc - 8
flux,correctedforthetriangularresponseofthecollimator,is prise because, as discussed above, we have no signal in the
7.0×10- 11ergcm- 2s- 1. “hole”,andthereforewearenotabletoputalimittotheline
Intriguedby the shape of the residuals, we tried a smaller normalization).
rebinningfactorandawider(15–200keV)energyrange. We Finally, we constrained the line energies to be in an har-
were surprised to find the spectrum shown in Figure 4. Ab- monic ratio. In this case we obtain a χ2 of 1.22 for 46
ν
sorption features at ∼40, ∼70, and ∼120 keV are quite ev- dof. Thebestfitparametersofthefundamental5wereE =
cyc
ident. We also noted that the points that trail these features 41±1keV,τ =11+9. Allthelinewidthswerefixed,asinthe
at ∼50, ∼90, and ∼150keV seems to be in agreementwith - 7
previouscasewiththelineenergiesleftfree,butwewerenot
a hard continuumup to 200 keV. To test this hypothesis,we
abletoconstrainthe(1:2)and(1:3)linenormalizations.
performedafittothedatabelow30keVtogetherwiththese
In order to verify that the observed features were not of
trailingpoints.Nocutoff(thebreakenergyisnotconstrained)
instrumental origin, and at the same time to better charac-
orbrokenpowerlaw(thetwophotonindicesareequal)model
terizethecyclotronresonancefeatures(thereafterCRFs),we
areabletofitthedata, whileapowerlaw withphotonindex
performedanormalizedCrabratioanalysis(Orlandini2004)
1.0±0.2does(χ2 =0.56for20dof).Thereforethechangeof
ν on the MOFF spectrum, as we successfully employed for
slope in 20–40keV derivedfrom the analysis of the coarser
the detection of CRFs in numerous X–ray pulsars (see, e.g.
binned energy spectrum is really due to the presence of the
Orlandinietal. 1998). As itis evidentfromthe top panelof
absorption features. The first absorption feature drives the Figure 5, the ratio between the MAXI J1409- 619 and the
positionofthebreak,whilethehigherenergyonesarespread
Crab count rate spectra shows a “hole” in the 38–44 keV
bytherebinning,resultinginlow-count,highenergybinsthat
range, due to the fundamentalCRF. To enhance the feature,
arefitbyachangeofslope. we multipliedthe MAXI J1409- 619/Crabratio by the func-
Tofurthersupportthisinterpretation,weperformedajoint
tionalformoftheCrabspectrum,i.e.asimplepowerlawwith
fit with the MECS spectrum (thisanalysis is detailed in Ap-
photonindexequalto2.1. Theresultisshowninthesecond
pendixB).Wefindthatacutofforabrokenpowerlawmod-
panelofFigure5. Finally,inthelowerpanel,weshowthera-
elsarenotabletofitthisbroadbandspectrumwiththecon- tiobetweenthepreviousfunctionandtheMAXIJ1409- 619
straintΓ∼0.9below10keV. Thisimpliesthattheoriginof
bestfitcontinuumpowerlawmodel,togetherwithaGaussian
thechangeofslopeisnotduetothepresenceofacutoffbutto
fittothefundamentalCRF. TheGaussianwidthwasfixedat
somethingelse,likelyabsorptionfeatures.Forthisreasonsas 4keV,whiletheGaussiancentroidenergyis43+2keV.Please
thecontinuumwewilluseapowerlawwithΓfixedat0.87, - 1
notethatthis value is a lower limit, because it doesnottake
asderivedbytheanalysisonthebroadbandspectrumofthe
intoaccounttheintrinsicenergyresolutionofthePDSinstru-
firstobservation.
ment.
As throughlydiscussed in AppendixA, the assessment of
InordertogiveaquantitativeevaluationoftheCRFinthe
thestatisticalsignificanceoftheseabsorptionfeatureshasto
normalizedCrabRatio weperformedaruntest(seedetailed
be done with great care. Our strategy was to evaluate the
discussioninAppendixA).Upto∼34keVtheresidualsare
significance of each feature one at a time. We started with
a power law fit in the energy range 15–55 keV, and we ob- consistentwithrandomfluctuations(N+=12;N- =9;Nr=13;
consistentwithrandomfluctuationatthe84%). Ontheother
tained an unacceptable χ2 of 1.4 for 25 dof. The inclusion
ν
of a Gaussian in absorption(gabsin XSPEC) with centroid
energyE =44±3keV,τ =16+14,andwidthfixedat4keV 5Thereafterwewillindicatethefundamentalresonanceas(1:1),meaning
cyc - 7 withthistheordinalnumberoftheresonancewithrespecttothefundamental.
significantlyimprovedthefit(χ2ν equalto0.48for23dof). Withthisnotationthefirstharmonicwillbeindicatedas(1:2),thethird(1:3),
Twocommentsonthismeasurementareinorder: first,the andsoon.
BeppoSAXobservationsofMAXIJ1409- 619:discoveryofCRFs 5
of this source are consistent with those of a late O/early
B-typestar(Wegner1994)withareddeningofA ≈20mag.
V
This,usingtheformulaofPredehl&Schmitt(1995),implies
acolumndensityofN ∼3.6×1022cm- 2,whichisconsistent
H
with that inferred from the Swift and BeppoSAX spectral
analysisresults. Thislargeextinctionisalsosupportedbythe
non-detection of the optical counterpart of the source. All
this points to a likely HMXB nature of this source, hosting
a heavily absorbed early-type star, similar to several cases
of INTEGRAL hard X–ray sources identified as HMXBs
at optical and/or NIR wavelengths (see, e.g., Masettietal.
2010).
Assuming then A ≈20 mag along the MAXI J1409- 619
V
line of sight and a B0 spectral type for the companion star
in this system, we can infer its distance, depending on the
luminosity class (main sequence, giant or supergiant) of the
star. Using the tabulated absolute magnitudes for this type
ofstar(Lang1992)wefind, forthesethreecases, respective
distancesof∼4.6,∼7.9and∼14.5kpc. Giventhatthelarge
absorptionfoundalongthelineofsightofthissourceatboth
NIR and X–ray wavelengths is consistent with the Galactic
one, we consider most likely that the correct distance is the
largest one. Thus, MAXI J1409- 619 is quite likely located
Figure5. Top:RatiobetweenMAXIJ1409- 619MOFFandCrabcountrate in the far side of the Galaxy, i.e. in the farthest parts of the
spectra. Middle: theMAXIJ1409- 619/Crab ratiomultipliedbyE- 2.1,the Sagittarius-Carina arm, and its mass donor star is likely an
functionalformoftheCrabspectrum.Thedottedlinecorrespondstothebest
early-typesupergiant.
fitpowerlawcontinuum. Bottom: ratiobetweentheformerexpressionand
thebestfitpowerlawcontinuum,togetherwithaGaussianfittothe∼43keV
4. DISCUSSION
CRF.ThestatisticalassessmentoftheCRFhasbeenevaluatedwitharuntest,
findingthatwecanrejectthelinebeingrandomatthe99%confidencelevel We found five sets of observations containing the posi-
(seetextandAppendixAfordetails). tionofMAXIJ1409- 619inourBeppoSAX/PDSarchive,per-
hand, in the 34–50 keV band we see a clear structure in the formedin1997,2000,and2001,andoneintheASCAarchive
residuals. In this case from a run test with N+ =2, N- =14, (March 1998). In all our observations the source was in a
N =2wewecanrejectthenullhypothesisofrandomnessat lowstate,with15–100keVfluxesintherange∼2–8mCrab,
r
the99%confidencelevel. andnospectralvariabilityduringtheobservations. Forcom-
parison, an integrated exposure (over 5 years) of 2.4 Ms by
2.2. ASCAobservation INTEGRAL/IBIS provides 2σ upper limits on the persistent
The region around MAXI J1409- 619 was observed by quiescent emission of 0.2 and 0.4 mCrab in the 20–40 and
40–100keV energybands, respectively(Sgueraetal. 2010).
ASCA on March 2, 1998 for a net exposure time of about
Whenassumingthesourcefluxesinoutburstasmeasuredby
18ks. Thesourceisclearlydetected,anditsspectrumiscon-
Swift(Kenneaetal.2010b,2011)andRXTE(Yamamotoetal.
firmedto be quite hard, with a best fit powerlaw photonin-
dexΓ=0.1+0.8andanunabsorbed1–10keVfluxof3×10- 12 2010), from our low state measurement we can infer a dy-
- 0.6 namicrangeof400in15–50keV,andof300in2–10keV.
ergcm- 2 s- 1. Theabsorptioninthedirectionofthesourceis
The discovery of CRFs in the low state spectrum of
notwellconstrainedbythefit,withanupperlimitof3×1022 MAXI J1409- 619, together with its ∼500 s pulsations, un-
cm- 2,consistentwiththeSwiftandBeppoSAX/MECSresults. ambiguously identify the source as an accreting X–ray bi-
nary pulsar. Only few sources show multiple CRFs, and
3. AVERYRED(DENED)COUNTERPARTFORMAXIJ1409- 619 only two show resonances above the (1:2). Three CRFs
AsstressedbyKenneaetal.(2010b),aNIRsourcebelong- were observed during the 2004–2005 outburst of the X–ray
ing to the 2MASS catalogue(Skrutskieetal. 2006) is found pulsar V0332+53 (Coburnetal. 2005; Kreykenbohmetal.
within 2.′′1 of the MAXI J1409- 619position as determined 2005; Tsygankovetal. 2006), and five (possible six) CRFs
bytheXRT.Accordingtothatcatalogue,thissource(labeled were discovered in the spectrum of 4U 0115+63 during
as 2MASS J14080271- 6159020) has NIR magnitudes J = its 1999 giant outburst (Santangeloetal. 1999; Heindletal.
15.874±0.086, H = 13.620±0.022 and K = 12.560±0.021. 1999; Ferrignoetal. 2009). In both cases, deviations from
Nocataloguedopticalcounterpartispresentatthepositionof a pure harmonic ratio among the CRFs were observed, and
this NIR source, and inspection of the DSS-II-Red digitized were explainedin terms of departuresfroma classical dipo-
archival plates6 does not show any evident optical object at lar structure of the magnetic field in the line-formingregion
thatlocation.FollowingMonetetal.(2003)wecanthusplace (Nishimura2005,2008).
aconservativeupperlimit,R>20,totheR-bandmagnitudeof The magnetic field strength at the neutron star surface
theopticalcounterpartofthissource. Theabovemagnitudes corresponding to E = 44 keV is 3.8×1012(1+z) G
cyc
thusindicatethatthisisanextremelyredobject. (Canuto&Ventura1977),wherez,thegravitationalredshift,
Indeed, assuming the Milky Way extinction law foratypicalneutronstarofmass1.4M andradius10Km,
⊙
(Cardellietal. 1989), we find that the NIR color indices is about 0.3. When left as free parameters in the fit, we
found that the best fit CRF line energies do not follow the
6http://archive.eso.org/dss/dss harmonic relation E = nE . A slightly non-harmonicity is
n 1
6 Orlandinietal.
Figure7. ErrorboxofAGLJ1410- 6143(bigredcircle)superimposedon
the20–100keVINTEGRAL/IBISdeepmosaicimage(∼2.3Msexposure).
TheBeppoSAX/MECSpositionofMAXIJ1409- 619ismarkedbythegreen
square. The two small yellow ellipses represent the Fermi γ–ray sources
2FGLJ1409.9- 6129and2FGLJ1413.4- 6204,respectively. Whitecontours
(from50%to99%)refertotheEGRETsource3EGJ1410- 6147.
Second, at variance with the V0332+53 and 4U 0115+63
observations,bothperformedduringgiantoutbursts,ourBep-
poSAXobservationswereperformedwhileMAXIJ1409- 619
was in a low state. This did not allow the study of the de-
pendence of the CRF parameters as a function of luminos-
ity, an importanttool for study of the physicalconditionsin
Figure6. Harmonicity ofthe MAXIJ1409- 619 CRF line energies. The the line-forming region (Miharaetal. 2004; Nakajimaetal.
dashedblacklinecorrespondstoalinearfit(thatis,En=nE1)tothedata, 2006;Klochkovetal.2011).
whilethecoloredstripstakeintoaccountrelativistic effects, asdetailedby
Thelikelyearly-typeopticalcounterpartandthe500spul-
Eq.(1).Eachstripcorrespondstoafixedvalueofthemagneticfieldstrength
B12=B×1012 G,andtoarange0.0001–1forsin2θ,whereθistheangle sation makes MAXI J1409- 619 a HMXB pulsar. Accord-
betweenthephotondirectionandthatofthemagneticfield. WhileE1 and ingtothenatureofthesecondarystarwehavetwopossibil-
E3areconsistentwithB12∼3.8,E2isconsistentwithamagneticfieldabout ities: the source is a supergiantfast X–ray transient (SFXT;
20%lower. Sgueraetal. 2005; Negueruelaetal. 2006) or a Be/HMXB.
In favor of the former interpretation is the highly reddened
expectedwhenrelativisticeffectsaretakenintoaccount(see,
supergiant as possible counterpart, the typical outburst X–
e.g.,Meszaros1992)
ray luminosity of 2×1037 erg s- 1 (assuming a distance of
14.5 kpc), and the dynamic range of more than two orders
E =m c2q1+2n(B/Bcrit)sin2θ- 1 1 (1) ofmagnitude(∼300)thatistypicaloftheso-called“interme-
n e sin2θ 1+z diate”SFXT (Sgueraetal. 2007; Clarketal. 2010). Against
wehavethatthesourceactivephase,abouttwomonthslong
whereme istheelectronrestmass,cthespeedoflight,θ the (Uenoetal.2010;Kenneaetal.2011),issignificantlylonger
angle between the photon and the magnetic field direction, thatthattypicalofSFXT(see,e.g.,Sidoli2009). Ifthiswere
andBcrit=4.414×1013Gisthecriticalmagneticfieldstrength thecase,thenourmagneticfieldmeasurementwouldruleout
wherethecyclotronenergyequalstheelectronrestmass. themagnetarnatureforSFXTs(Bozzoetal.2008). Theob-
Theobserved(1:2)and(1:3)ratiosofthelineenergieswith servedpropertiesofMAXIJ1409- 619arealsoinagreement
respectto the fundamental, 1.7±0.2and 2.9±0.3, cannotbe withthoseobservedinotherBe/HMXBs,like1A1118- 615,
explainedin termsof Eq. (1), as it is evidentfromFigure 6. a 400 s X–ray pulsar with a hard X–ray spectrum (Γ∼1), a
Afittotheharmonicrelation,shownasthedashedline,gives CRF at ∼55 keV, and long (tens of years) periods of quies-
E1=41±3keV (in agreementwith the best fit value Ecyc = cence interrupted by giant (Type-II) outbursts lasting weeks
41±1 keV found when imposing the harmonic relation in tomonths,inwhichtheX–rayluminosityincreasesbyafac-
the spectral fit), but with poor significance (χ2ν =2.27 for 2 tor∼200(see,e.g.,Rutledgeetal.2007). Thiswouldputthe
dof). In the same figurewe also showthe harmonicrelation sourceafactor∼2–3closer.
fromEq.(1)fordifferentvaluesofthemagneticfieldandthe Finally, we note that MAXI J1409- 619 is located within
angleθ. Takingintoaccountrelativisticeffects,wefoundthat the 0.◦5 error box of the unidentified transient MeV source
themagneticfieldresponsibleforthe(1:1)and(1:3)CRFsis AGL J1410- 6143 (see Fig. 7) discovered on February 21,
about20%higherthanthatresponsibleforthe(1:2)CRF. 2008bythe γ–raysatellite AGILE (Tavanietal. 2009a) dur-
Two pointsareworth noticing: it is alwaysthe (1:2)reso- ingabrightMeVflarelastingonlyaboutoneday(Pittorietal.
nance that shows the larger disagreementwith the harmonic 2008; Orlandinietal. 2008). The AGILE large position un-
relation, and this could be due to the fact that this CRF is certainty makes very difficult the identification of its lower
duetopureabsorption(Nishimura2003),whilefortheother energy counterpartresponsible for the γ–ray emission. De-
resonancesother effects, like multiple scattering and photon spite this drawback, it is intriguing to note that the flaring
spawning,enterintoplay(see,e.g.,Schönherretal.2007,and source MAXI J1409- 619is the only cataloguedhardX–ray
referencestherein).Unfortunately,becausewearenotableto source above 20 keV (20–100 keV) to be located inside the
reconstruct the CRF profiles, we cannot extract more infor- AGILEerrorbox,accordingtoalltheavailablecatalogsinthe
mation,liketheelectrontemperatureandthegeometryofthe HEASARC database. Thisspatialcorrelationis furthersup-
emittingregion. portedbyasimilartransientnatureforbothMAXIJ1409- 619
BeppoSAXobservationsofMAXIJ1409- 619:discoveryofCRFs 7
and AGL J1410- 6143. We also point out that this is not a Ferrigno,C.,Becker,P.A.,Segreto,A.,Mineo,T.,&Santangelo,A.2009,
unique case. To date, a few HMXBs have been unambigu- A&A,498,825
ously detected as flaring MeV sources lasting only a few Frontera,F.,Costa,E.,dalFiume,D.,etal.1997,A&AS,122,357
days (Sabatinietal. 2010; Tavanietal. 2009b; Abdoetal. Frontera,F.,Orlandini,M.,Landi,R.,etal.2007,ApJ,666,86
Gehrels,N.,Chincarini,G.,Giommi,P.,etal.2004,ApJ,611,1005
2009). Inaddition,thereareseveralotherHMXBsproposed
Hartman,R.C.,Bertsch,D.L.,Bloom,S.D.,etal.1999,ApJS,123,79
as best candidatecounterpartsof unidentifiedtransientMeV Heindl,W.A.,Coburn,W.,Gruber,D.E.,etal.1999,ApJ,521,L49
sources located on the Galactic plane (Sgueraetal. 2009, Jahoda,K.,Swank,J.H.,Giles,A.B.,etal.1996,inSPIEConf.Proc.,Vol.
2011; Sguera 2009). For the sake of completeness, we note 2808,X-RayandGamma-RayInstrumentationforAstronomyVII,59
that within the largeAGILE errorbox there are a numberof Kennea,J.A.,Curran,P.,Krimm,H.,etal.2010a,ATel,3060,1
highenergyMeVsources(seeFig.7):i)2FGLJ1409.9- 6129 Kennea,J.A.,Krimm,H.,Romano,P.,etal.2010b,ATel,2962,1
Kennea,J.A.,Romano,P.,Mangano,V.,etal.2011,inProceedingsof4th
and 2FGL J1413.4- 6204 have been reported in the second InternationalMAXIWorkshop,toappear(arXiv:1101.6055)
Fermisource catalog (Abdoet al. 2011)as firmly identified Klochkov,D.,Staubert,R.,Santangelo,A.,Rothschild,R.E.,&Ferrigno,
γ–raypulsarsandthisunambiguouslyexcludestheirassocia- C.2011,A&A,532,A126
tionwiththetransientAGLJ1410- 6143,ii)3EGJ1410- 6147 Kreykenbohm,I.,Mowlavi,N.,Produit,N.,etal.2005,A&A,433,L45
Lampton,M.,Margon,B.,&Bowyer,S.1976,ApJ,208,177
isstillunidentifiedalthoughithasbeenlikelyassociatedwith
Lang,K.R.1992,AstrophysicalData:PlanetsandStars(NewYork:
the γ–ray pulsar 2FGL J1409.9- 6129(O’Brienetal. 2008). Springer-Verlag)
However,Wallaceetal.(2000)reportedapossibleMeVflare Masetti,N.,Parisi,P.,Palazzi,E.,etal.2010,A&A,519,A96
from3EGJ1410- 6147lastingafewdaysonNovember1991. Matsuoka,M.,Kawasaki,K.,Ueno,S.,etal.2009,PASJ,61,999
This behavior is at variance with the proposed association Meegan,C.,Bhat,N.,Connaughton,V.,etal.2007,inAmericanInstituteof
with the γ–ray pulsar 2FGL J1409.9- 6129while it is more PhysicsConferenceSeries,Vol.921,TheFirstGLASTSymposium,ed.
S.Ritz,P.Michelson,&C.A.Meegan,13
compatiblewiththeflaringnatureofAGLJ1410- 6143. Fur- Meszaros,P.1992,HighEnergyRadiationfromMagnetizedNeutronStars
thermulti-wavelengthstudies(radio,NIR,X–rayandγ–ray) (Chicago:UnivertyofChicagoPress)
oftheskyregionarestronglyneededandencouragedtoshed Mihara,T.,Kawai,N.,Yoshida,A.,etal.2002,inSPIE,Vol.4497,X-Ray
andGamma-RayInstrumentationforAstronomyXII,173
morelightonthenatureofsuchhighenergyemitters.
Mihara,T.,Makishima,K.,&Nagase,F.2004,ApJ,610,390
Monet,D.G.,Levine,S.E.,Canzian,B.,etal.2003,AJ,125,984
Nakajima,M.,Mihara,T.,Makishima,K.,&Niko,H.2006,ApJ,646,1125
ACKNOWLEDGEMENTS
Negueruela,I.,Smith,D.M.,Reig,P.,Chaty,S.,&Torrejón,J.M.2006,in
Wethankananonymousrefereeforhelpfulcommentsand ESASpecialPublication,Vol.604,TheX-rayUniverse2005,165
suggestionsthatgreatlyimprovedthepaper. TheBeppoSAX Nishimura,O.2003,PASJ,55,849
satellite was a joint Italian-Dutch programme. Part of this Nishimura,O.2005,PASJ,57,769
Nishimura,O.2008,ApJ,672,1127
work is based on archival data, software or online services
O’Brien,J.T.,Johnston,S.,Kramer,M.,etal.2008,MNRAS,388,L1
providedbythe ASI Scientific Data center (ASDC), and the Orlandini,M.2004,inX–rayandGamma–rayAstrophysicsofGalactic
High Energy Astrophysics Science Archive Research Cen- Sources,4thAGILEScienceWorkshop,119,(astro-ph/0402628)
ter(HEASARC),providedbyNASA’sGoddardSpaceFlight Orlandini,M.,dalFiume,D.,Frontera,F.,etal.1998,ApJ,500,L163
Center. This publication makes use of data products from Orlandini,M.,Frontera,F.,Bassani,L.,Landi,R.,&Sguera,V.2008,ATel,
1419,1
the2MASSarchive. Theauthorsacknowledgesupportfrom
Parmar,A.N.,Martin,D.D.E.,Bavdaz,M.,etal.1997,A&AS,122,309
ASI/INAF grants I/088/06/0, I/009/10/0 and I/033/10/0 and Perri,M.&Capalbi,M.2002,A&A,396,753
grantPRININAF/2009. Pittori,C.,Verrecchia,F.,Santolamazza,P.,etal.2008,ATel,1394,1
Predehl,P.&Schmitt,J.H.M.M.1995,A&A,293,889
Press,W.H.,Teukolsky,S.A.,Vetterling,W.T.,&Flannery,B.P.2007,
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8 Orlandinietal.
APPENDIX
ONTHESTATISTICALSIGNIFICANCEOFABSORPTIONFEATURES
Theanalysisofthesignificanceofspectralabsorptionfea-
tures, and in particular CRFs, has historically been fraught
withseriousmisunderstanding,thatweattributetotheappli-
cationoftechniquesmadeforemissionfeaturestoabsorption
ones. Thishasaveryprofoundimpactonthestatisticalanal-
ysis,asdetailedinthefollowing.
First, when searching for spectral features, and especially
absorption features, the binning of data is crucial: indeed a
narrowfeaturecanbelostifthedatabinningistoohigh(see,
e.g., Eadieetal. 1971, page 259), as we showed in the data
analysis detailed in Section 2.1.2. But before searching for
thebestbinningfactor,ithasbecomecustomarytorebindata
inordertoachieveaminimumnumberofcountsineachbin.
Thereasonofthisprocedureisthatwemustavoidbinswith
few counts if we want to use the χ2 statistics to test for the Figure8. Exampleonhowthegoodnessoffitestimationperformedbythe
goodnessof ourfit (in other words, errorsmustbe normally χ2statisticsnotalwaysisabletofindthebestfitmodeltothedata. Inthis
distributed). Aminimum20countsperbinfilterissufficient casethelinearfittothe12datapointsyieldsanormalizedχ2 of1,butthe
toachievethisgoal(Cash1979). deviationwithrespecttothestraightlineisevident. Aruntestshowsthat
thesedeviationshaveonly1.3%probabilitytobeduetorandomfluctuations
The application of this criterion to backgroundsubtracted
(seetextfordetails).
spectra is correct for instruments that are not background
dominated(that is, they have an intrinsically low numberof this routine is not incorporated in XSPEC). This same test
counts per bin), like a detector on the focal plane of X–ray mustbeusedtoassessiftwodifferentmodelsarestatistically
optics. On the other hand, when the net source spectrum is equivalent or not (and indeed we used it in Section 2.1.2 to
obtained by subtracting two high counts spectra this correc- compare the power law and the cutoff power law fits). It is
tionmustnotbeapplied(instrumentslikeBeppoSAX/PDSor worthnotingthattheapplicationoftheF-testtothestatistical
RXTE/HEXTE are backgrounddominated, with a very high assessmentofCRFsisnotaffectedbytheconcernsraisedby
number of counts per energy channel). Although the net itsusewithemissionlines(Protassovetal.2002).
source counts can be very low, they are still Gaussian dis- Third, CRFs are very shallow features, with their count
tributed, because resulting from the difference of two nor- ratesoftenattheinstrumentsensitivitylevel.Wethereforeex-
mallydistributedcounts. pecterrorstobedominant,andconsequentlytheχ2couldnot
Second,iffeaturesarepresentinthesourcespectrum,they bethebestestimatoroftheCRFsignificance. Weneedatest
(clearly)showupin thefit residuals, andthestandardgood- thattakesintoaccountthestructureoftheline(aninformation
ness of fit estimator, the minimization of the χ2 statistics whichislostintheχ2,becausewecomputethesquareofthe
(Lamptonetal.1976), should(clearly)indicatethatthefitis differencebetweenthe data andthe model). In otherwords,
not“good”andanadditionalcomponentisnecessary.Thein- we need to compute what is the probabilitythat a particular
clusionofanewcomponenttothecontinuumshouldresultin structurevisibleinthefitresidualsoccursbychance.
areductionofthereducedχ2. Inordertoassesswhetherthe The run test (also known as Wald-Wolfowitz test; Barlow
improvementof the χ2 is due to chance or it is because the 1989;Eadieetal.1971)worksonthesignsofthedeviations,
newcomponentissignificantitiscustomarytousetheF-test thatisontheformoftheresiduals. Tobetterclarifyhowthe
(see,e.g.,Barlow1989,page160). runtest worksgivea look atFigure 8, adaptedfromBarlow
A very important point has to be raised here, because we (1989): when fitting the 12 data points with a straight line
have two completely different approaches whether we are thenormalizedχ2isexactly1(likelyduetoerroroverestima-
dealing with emission or absorption features. In the former tion). Butitisevidentbyeyethatthefitisnotgood(indeed
casewearedealing,usingaXSPECterminology,withanad- thedatacomefromaparabolicmodel). Thereasonisthatif
ditivecomponent,andwetestthenullhypothesisthattheco- thefitweregoodweshouldexpectthatthenumberofpoints
efficientofthenewtermiszero(Bevington1969,page200). “above”thefittinglineshouldnotgrouptogether,butshould
ThisistheF-testroutinethatisincorporatedintherecentver- be intermixed with points “below” the fitting line (and this
sionsofXSPEC. shouldbemoretrueasthenumberofdatapointsincreases).
Ontheotherhand,whendealingwithanabsorptionfeature, If, on the other hand, we observe only small groups of data
thecomponentisnotadditivebutmultiplicative(seeroutines with the same “sign” (called runs), this means that our data
gabs or cyclabs). It is therefore obviousthat we cannot arenotrandomlydistributedwithrespecttothefittingmodel,
usethe F-testforan additivecomponent,becausewe cannot butthereisanunderlyingtrend.
varyto zerothe new componentcoefficientwithoutmodify- Aswe cansee fromFigure8, we have 12points, 6 points
ing the parameters of the original model. The two compo- “above”thefittingline(letuscallthemN+)and6points“be-
nents (the multiplicative model and the original continuum) low” the fitting line (N- ). The number of runs Nr is only 3,
arestronglycoupled,andvariationsinonewillaffectalsothe suspiciouslysmall.IndeedtheprobabilityofobtainingN ≤3
r
other. Thereforewemustuse adifferentF-test, asdescribed is1.3%,tellingusthatthestructureobservedintheresiduals
inPressetal.(2007,page730),thatteststhenullhypothesis isnotduetorandomfluctuationsbuttoawrongmodelization
thattheobservedvariancescomesfromthesamesample(and (thelinearfit).
BeppoSAXobservationsofMAXIJ1409- 619:discoveryofCRFs 9
From this example, and from the real-case analysis per- losetheabsorptionfeature;
formedontheresidualsshowninFigures5and9,shouldbe
clearthatthe goodnessoffitin thecase ofdatastructuresin 2. In order to assess the significance of a CRF the F-test
theresidualsshouldnotbeaddressedonlywiththeχ2estima- is perfectly suitable, but we must use the correct rou-
tor,butithastobesupportedbytheruntest. tine:theF-testroutineincludedinXSPECisapplicable
Inconclusion,thisisasortofcookbookforacorrecteval- only to an additivecomponent(in particular, an emis-
uationofthestatisticalsignificanceofabsorption(CRFs)fea- sion line), while both gabs and cyclabs are multi-
turesinthespectraofX–raysources: plicative;
1. Be careful with the binning: although CRFs are usu- 3. The evaluationof the statistical significance of a CRF
allybroadfeatures,atoosmallbinningcanhidestruc- mustbesupportedbyothertests,especiallyinthepres-
tures. When dealing with backgrounddominateddata enceofstructuresinthefitresiduals:theruntestisable
(like, for example, BeppoSAX/PDS or RXTE/HEXTE to discriminatewhetherthestructureis duetorandom
net spectra) never rebin data in order to have a mini- fluctuationsornot.
mum number of counts per bin, otherwise we risk to
THECONTINUUMSPECTRALMODELOFMAXIJ1409- 619
Theanalysisperformedonthebroadbandspectrumofthe thelowenergypart,below10keV.Thesystematicdeviations
pointedMAXIJ1409- 619observationyieldsasbestfitcon- fromthemodel(informationlostbytheχ2 test,asdiscussed
tinuumapowerlawwithindex∼0.9(seesection2.1.1). On inAppendixA)canbequantifiedbymeansoftherunteston
the other hand, the analysis performed on a coarser binned theMECSresiduals(N+=8,N- =6,Nr =3). Theprobability
spectrum of the offset observation indicates the presence of ofobtainingN ≤3is0.5%,thereforethestructureobserved
r
achangeofslopearound20–40keV(seesection2.1.2). Be- in the residuals is not due to random fluctuations but to the
causeafinerrebinningofthissecondobservationrevealedthe wrongmodelizationofthecontinuum.Thesameresultisob-
presenceoffeaturesthatcanbeexplainedasCRSFs,itisim- tainedwiththebrokenpowerlawfit(seepanelc.inFigure9):
portanttounderstandwhetherthischangeofslopeisduetoa thereisnomatchtotheMECSdata.
cutoffinthecontinuumorisacombinedeffectofthebinning
andthepresenceoftheabsorptionfeatures. Table3
From the pointed observation we know that data below BestfitparametersforthejointMECS/PDSspectralfits
10keV mustbe describedbya powerlaw with Γ∼0.9. To
PowerLaw Cutoff BrokenPL PowerLaw
takeintoaccountthisconstraintweperformedajointfitofthe
+3gabs
highenergyPDSspectrumfromthesecondobservationwith
thelowenergyMECSspectrumfromthefirstobservation.
Xbe–AsriadltyehsoputuhglashattrahstealtrowewoknesnopewercngtrytaodcnuooemtteoshfrrooewpmraodnciyeffsessprienengcttrooafblsXvea–rrvriaaaytbisoilnibtsyy, EcΓΓut2off 0.87··±····0.07 - 01.4·7±·5·+- 400..35 02..209763+-+-+-740010....2296 1.0····-+··00..23
χ2ν(dof) 2.48(66) 0.91(65) 0.78(64) 0.54(59)
thecircumstellarmaterial(see,forexample,thecaseofVela
X–1orGX301–2).Becauseoursourcedoesnotshowanyin-
trinsic absorption or emission lines, signatures of reprocess-
ing,weareconfidentthatourMECSspectrumdidnotchange These results demonstrate that the change of slope is not
betweenthetwoobservations. in the continuumbut is due to somethingelse: we madethe
Firstwetriedafitwithapowerlaw,modifiedatlowenergy hypothesis that its origin are absorption features. Therefore
by photoelectric absorption. All the parameters are left free thetruecontinuummodelisapowerlawtogetherwithCRSF
buttheN (fixedatthe2.8×1022cm- 2 valueobtainedfrom features(seepaneld.inFigure9). Becausewearenotableto
H
theON-sourceobservation),andtheresultisshowninpanel disentanglethecontinuumandtheCRSFs, duetothebroad-
a.ofFigure9. ThefitparametersarelistedinTable3. nessofthefeaturesandtheirlowstatistics,weneedtofixthe
Thefitisnotstatisticallyacceptable,andtheslopeisdriven powerlawindextothevalueobtainedbythebroadbandfit.
by the low energypart of the spectrum. To fit also the PDS Justasatest,weperformedalltheanalysisontheCRSFsby
data we need something that is able to change the slope at fixingthepowerlawindexat1.0,1.1,and1.2,corresponding
about40keV. to the 1.0±0.2 value obtained from the fit to the PDS data
Nowletustestwhetherwecanexplainthischangeofslope below30keVandthepointstrailingtheCRSFs. TheCRSF
in termsofa cutoffin thecontinuum. We first tried a cutoff parametersare all consistent with each other, demonstrating
powerlaw(seepanelb.inFigure9): thefitisstatisticallyac- thatthey do notdependon the particularvalue of the power
ceptablebutthepowerlawindexdoesnotmatchwiththatof lawindex.
10 Orlandinietal.
0.01
0.01
V−1 V−1 10−3
ke ke
nts sec −1 10−3 nts sec −1 10−4
ou ou 10−5
C C
d d
malize 10−4 a) malize 10−6 b)
Nor Nor 10−7
2 1
) )
suals ( 0 suals ( 0
Resid −2 Resid −1
−2
10 100 10 100
Energy (keV) Energy (keV)
0.01 0.01
V−1 V−1
ke ke
nts sec −1 10−3 nts sec −1 10−3
u u
o o
C C
malized 10−4 c) malized 10−4 d)
or or
N N
1 1
) )
suals ( 0 suals ( 0
Resid −1 Resid −1
−2 −2
10 100 10 100
Energy (keV) Energy (keV)
Figure9. 1.8–200keVjointMECS(fromthepointedobservation)/PDS(fromtheoffsetobservation)spectrum.a.Fitwithapowerlawcontinuum.b.Fitwith
acutoffcontinuum. c.Fitwithabrokenpowerlawcontinuum. d.FitwithapowerlawcontinuumandthreeCRSFs.Theonlymodelabletofitsimultaneously
thelow(below10keV)andhighenergydataisthepowerlawplusthreeCRSFs.ThebestfitparametersarelistedinTable3.