X-Ray Properties of the Point Source Population in the Spiral Galaxy NGC 5055 (M63) with Chandra

0.47 MB·English

by  Bing Luo

ChineseJournalofAstronomyandAstrophysicsmanuscriptno.
(LATEX: N5055.tex; printedonFebruary5,2008; 15:17)
X-Ray Properties of the Point Source Population in
the Spiral Galaxy NGC 5055 (M63) with Chandra
7
0
0
1⋆ 1 2 2
2 BingLuo ,JiyaoChen ,ZhongliZhang ,YuWang ,Jingying
Wang2 andHaiguangXu2
n
a
J

5
1 DepartmentofPhysics,FudanUniversity,220HandanRoad,Shanghai

1 200433,PRC
v 2 DepartmentofPhysics,ShanghaiJiaoTongUniversity,800Dongchuan
5 Road,Shanghai200240,PRC
2
1
1
0
7 Abstract By analyzing the Chandra ACIS S3 data we studied
0 theX-raypropertiesofthelow-massandhigh-massX-raybinary
/ populationsin the nearbyspiralgalaxyNGC5055.A totalof43
h
p X-raypointsourcesweredetectedwithinthe2effectiveradii,with
- 31sourceslocatedonthediskandtherest12sourcesinthebulge.
o
The resolved pointsources dominatethe total X-ray emission of
r
t thegalaxybyaccountingforabout80%ofthetotalcountsin0.3–
s
10keV.Bycarryingoutthespectralfittingswecalculatedthe0.3–
a
: 10.0keVluminositiesofallthedetectedX-raypointsourcesand
v
foundthattheyspanawiderangefromafew1037ergs−1toover
i
X 1039 erg s−1. After compensating for the incompleteness at the
r low luminosity end, we find that the correctedXLF of the bulge
a
population is well fitted with a broken power-law model with a
break at 1.57+0.21 × 1038 erg s−1, while the profile of the disk
−0.20
population’sXLFagreeswithasinglepower-lawdistributionwith
aslopeof0.93+0.07.Thediskpopulationissignificantlyricherat
−0.06
>2 × 1038 erg s−1 than the bulge population, inferring that the
∼
disk may have undergonerelatively recent, strong starbursts that
significantly increased the HMXB population, although ongoing
starbursts are also observed in the nuclear region. Similar XLF
profiles of the bulge and disk populations were found in M81.
However, in most other spiral galaxies different patterns of the
spatial variation of the XLF profiles from the bulge to the disk
havebeenobserved,indicatingthatthestarformationandevolu-
tionhistorymaybemorecomplexthanwehaveexpected.
Keywords: galaxies:individual(NGC5055)—X-ray:binaries—
stars:luminosityfunction—stars:formation
2 Luoetal.
1 INTRODUCTION
ThestudyoftheX-raypropertiesofthebrightpointsourcesin spiralgalaxies,
mostofwhicharelow-massX-raybinaries(LMXBs)andhigh-massX-raybi-
naries(HMXBs),mayprovideuswithvaluableobservationalconstraintsonthe
starformationandevolutionhistoryinthediskandbulgeofthehostgalaxy.With
the superb high spatial resolution of the Chandra X-Ray Observatory, a large
populationofX-raypointsourceshavebeenresolvedforthefirsttimeinsome
nearbyspiralgalaxies.Inafewspecificcases,distinctX-raycharacteristicshave
been revealedforthe sourceslocated in the bulgeand those in the disk, which
can beinterpretedasthe evidenceforthe spatialvariationof stellar population
compositionthatreflectsthedifferencesinstarformationhistory(e.g.,Tennant
et al. 2001;Kong et al. 2002;Soria & Kong 2002). In this paper we present a
Chandra study of the point sources in the nearby starburst galaxy NGC 5055
(M63;SA(rs)bc),whichisoneoftheprototypeArmClass3flocculentgalaxies
(Elmegreen& Elmegreen1987)thatshowsregular,two-armspiralstructureto
aradiusof4.0kpcinthenear-infraredband.Theopticalcenterofthegalaxyis
identifiedatRA=13h15m49.25sDec=+42d01m49.3s(J2000;Maozetal.1996).
TheLINERnucleusisUVbrightandissurroundedbyluminousyoungstarclus-
ters, showingclearstellar absorptionsignatures(Maozetal. 1998;Leithereret
al. 2002). The inclination and position angles of the galaxy are deduced to be
58◦ and103◦,respectively(Garcia-Gomez&Athanassoula1991).Weorganize
the paper as follows. In §2, we describe the observationand data reduction.In
§3, we presentthe imaginganalysis. In §4, we investigatethe X-rayproperties
of the detected point sources, which includes the temporal and spectral analy-
sisaswellasthecalculationsofhardnessratiosandX-rayluminosityfunctions
(XLFs).Finally,wediscussandsummarizetheresultsin§5and§6,respectively.
Throughoutthepaper,wequoteerrorsatthe90%confidencelevelunlessmen-
tioned otherwise. We adopt a distance of 7.2 Mpc to NGC 5055 (Michele &
Serra 1997), which is consistent with the distance calculated from its redshift
(z =0.001681)ifcosmologicalparametersH0 =70kms−1Mpc−1,Ωm =0.3
andΩΛ =0.7areadopted.Atthisdistance1′correspondstoabout2.1kpc.
2 OBSERVATIONANDDATAREDUCTION
NGC 5055 was observed with Chandra with the CCD 0, 1, 2, 3, 6 and 7 of
theChandraAdvancedCCDImagingSpectrometer(ACIS)inoperationontwo
separateoccasions,whichstartedonAugust27,2001foratotalexposureof28.4
ks,andonApril15,2001forashortdurationof2.4ks,respectively.Theevents
weretelemeteredinfaintmodeandthedatawerecollectedwithaframetimeof
3.2s.TheCCD temperaturewassetat−120oC. IntheAugust27observation
fromwhichthedataofthisworkwasdrawn,thecenterofgalaxywaspositioned
ontheACISS3chip(CCD7)withanoffsetof31′′fromthenominalpointingfor
theS3chip,sonearlyalltheemissionofthegalaxywascoveredbytheS3chip.
In the analysisthatfollows,we used the CIAO softwareversion2.3to process
thedataextractedfromtheS3chiponly.Inordertoapplythelatestcalibration,
westartedwiththeLevel-1data.WekepteventswithASCAgrades0,2,3,4and
6,andexcludedbadpixels,badcolumns,andcolumnsadjacenttobadcolumns
andnodeboundaries.Inordertoidentifyoccasionalperiodsofhighbackground,
weextractedthelightcurveofthesource-freeregionsontheS3chipin2.5–7.0
keVwherethebackgroundflaresareexpectedtobemostvisible.Wefoundthat
uptoabout5%ofthetotalexposuretimewasaffectedbyhighbackgroundflares.
⋆E-mail:[email protected]
X-rayPointSourcesinNGC5055(M63) 3
Byexcludingthecontaminatedintervalsweobtainedacleanexposureof27.0ks
fortheanalysis.
3 X-RAYIMAGE
In Figure 1a we plot the Chandra S3 image of NGC 5055 in 0.3–10.0keV in
logarithmic scale. The image has been corrected for both exposure and back-
ground, and has been smoothed by using a minimum signal-to-noise ratio of
3 and a maximum signal-to-noise ratio of 5 per beam. We find that the X-ray
emission from the galaxy is dominated by a large population of X-ray point
sources, which contribute about 80% of the total counts of the galaxy in 0.3–
10.0keV.ThediffuseX-rayemissionisnearlysymmetricwithinabout20′′,and
isslightlyelongatedineast-westdirectioninouterregions.NodiffuseX-raysis
significantlydetectedbeyond≃33′′,whichisapproximatethesizeofthebulge
(≃ 35′′;Baggettetal.1998).WefindthatthediffuseX-rayemissionispeaked
atRA=13h15m49.3sDEC=+42d01m45.5s(J2000)whereabrightnuclearX-ray
pointsourceisdetected.ThepositionoftheX-raypeakalsocoincideswiththe
opticalandinfraredcentersofthegalaxytowithin0.5′′.InFigure1bweshow
the opticalimage drawnfromthe Digital Sky Survey(DSS) in linear scale, on
whichthelocationsofthe43detectedX-raysourcesaremarkedwithcirclesfor
comparison(§4.1).
4 X-RAYPOINTSOURCES
4.1 Detections
We detected X-Ray point sources on the ACIS S3 image using the CIAO tool
celldetectwithasignal-to-noisethresholdof3.Wefirstrestrictedthedetections
in0.3–10.0keV,andthencrosscheckedtheresultsin0.5–7.0keV.Wealsohave
crosscheckedtheresultsbyusingthewavelet-basedCIAOtoolwavdetectandby
visional-examination.OnthewholeS3CCD 46sourcesaredetected,ofwhich
43 sources are located within the 2 effective radii (1 effective radius = 1Re =
1.6′;Thornley1996).Inthisworkwefocusourstudyonthese43sourcesonly.
The spatial distribution of the 43 sources show a clear concentration towards
thegalaxycenter,inferringthatmostofthemarephysicallyassociatedwiththe
galaxy.BasedontheresultsofthedeepChandraobservationsoftheblankfields
(Mushotzkyetal.2000),weperformedMonte-Carlosimulationsandfoundthat
only few (<5) of the 43 point sources may be unrelated background sources.
∼
By overlaying the positions of these sources on the DSS image (Fig. 1b), we
findthat31ofthe43sourcesaredetectedonthediskwithatendencytoreside
on the spiral arms. The rest 12 sources are detected in the bulge, of which up
toabout1sourcemaybea diskpopulationsourcethatismisclassifiedintothe
bulge population due to the project effect. At the X-ray peak a nuclear source
isdetected,howeveritdoesnothavethehighestcountrateamongthedetected
sources.Welistthepropertiesofthe43sourcesinTable1,wherewesortthem
intheorderofincreasingprojecteddistancefromthecenterofthegalaxy.
4.2 TemporalVariabilities
Afterremovingtheintervalsofstrongbackgroundflares,weextractedthe0.3–
10.0keVlightcurvesofthedetectedsourcesthateachhasmorethan60counts
in total. We calculated the Kolmogoroff-Smirov(K-S) statistic for each of the
lightcurvesagainstthe null hypothesisthatthe countrate of the source is tem-
porally invariant over the effective exposure time; if the source is temporally
4 Luoetal.
invariant,thecumulativefractionofthecountisadiagonalfrom0to1.Wefind
that Src 22 (243 counts), Src 35 (600 counts) and Src 38 (102 counts) show
significant evidence for temporal variations on the 90% confidence level (Fig.
3). This is not likely to be caused by the variations of the local background,
since the K-S test gives negative results in the backgroundvariability. For the
centralsource(Src1),thetemporalvariabilityislesssignificantintermsofthe
K-Stest,possiblybecauseK-Stestsaremostsensitivearoundthemedianvalue
of the independent variable. However, by calculating the variability parameter
S =(fmax−fmin)/ σf2max +σf2min =1.1,wherefmaxandfminarethemaxi-
q
mumandminimumcountrates,respectively,andσ andσ arethecorre-
fmax fmin
spondingerrors,wefindthatSrc1ismarginallyvariableontimescalesof1∼5
hrduringwhichitscountratechangedbyabout50%.
4.3 HardnessRatios
Sincethebackground-correctedcountratesofmostoftheresolvedX-raypoint
sources are low, which makes it impossible to carry out spectral analysis for
each source, we turned to study the hardness ratios of all the resolved sources
that are defined as H21 = (M −S)/(M +S) and H31 = (H −S)/(H +
S), where S, M and H are the background-correctedcounts extracted in 0.3–
1.0 keV (S), 1.0–2.0 keV (M) and 2.0–10.0 keV (H), respectively. The same
approachhasbeenadoptedinearlierworks(e.g.,Sarazinetal.2000).Welistthe
calculatedhardnessratiosandthe1σerrorsincolumn7and8ofTable1,andplot
H31againstH21inFigure4.Inthefigureweshowthepredictedhardnessratio
distributionsforanabsorbedpower-lawmodelbyadoptingcolumndensitiesof
1.31×1020 cm−2 (the Galactic value; Dickey & Lockman 1990) and 3.93×
1020 cm−2, and photon indices of Γ = 0.0, 1.0, 2.0, 3.0 and 4.0, as well as
the hardness ratios for an absorbed blackbodymodel by adopting the Galactic
absorptionandtemperaturesofkT =0.5, 0.4, 0.3, 0.2and0.1keV.
WefindthatthedistributionofthecolorsissimilartothatofM31(Konget
al.2002)andthenearlyface-on,gas-richspiralgalaxyM83(Soria&Wu2003).
Mostofthesourceslieinabroaddiagonalbandextendingfrom(H21,H31)=
(−1,−1) to (1,1). One source (Src 19) that is located at a moderate distance
(1.1′) to the galaxy center has hardness ratios of about (−1,−1) and is thus
identifiedasasupersoftsource(SSS).IftheselectioncriterionsofSupperetal.
(1997)andKahabka(1999)areapplied,5othersources(Src6,7,9,13and14)
canbeidentifiedascandidatesofSSSsaswellsincetheysatisfyH31+σH31 <
−1andH21 < 0,orH21+σH21 < −0.8.Ifweslackentheselectionlimitto
H21 < 0.5 and H31 < 0.5 (e.g. Swartz et al. 2002), Src 12 and 15 can also
be classified as SSS candidates.As is shown in Figure4 the hardnessratiosof
thesesupersoftsourcescanapproximatelybedescribedwithanabsorbedblack-
body model with the temperatures ranging from about 0.2 to 0.3 keV. Src 31
has a hardness ratio of (1,1), which appears to have been heavily absorbed by
a column density significantly larger than the Galactic value. Since Src 31 is
locatedfarawayfromthecenterofthegalaxy(d>110′′)anditshardnessratios
agreewiththoseexpectedbyanabsorbedpower-lawmodelwithanabsorption
of≥ 0.4×1022cm−2 andaphotonindexofΓ ∼ 0,itisprobablyanunrelated
backgroundAGN.
For comparison,within the 2Re region the measured mean hardness ratios
forthetotalX-rayemissionofthegalaxyare(H21,H31)=(−0.28,0.18).For
thesameregion,themeanhardnessratiosare(H21,H31)= (−0.14,0.13)for
X-rayPointSourcesinNGC5055(M63) 5
alltheresolvedX-raypointsourcesand(−0.46,0.23)fortheunresolveddiffuse
emission.
4.4 SpectralAnalysis
We first extracted and studied the cumulativespectrum of all the 42 off-center
pointsourcesresolvedwithin2Re.Thebackgroundswereextractedfromacare-
fully selected annulus around each source. By applying the latest CALDB we
have corrected for the charge transfer inefficiency (CTI) and the continuous
degradationintheACISquantumefficiency,whichisespeciallysevereatlower
energies.To avoid the effectsof calibrationuncertaintiesat lower energiesand
instrumentalbackgroundathigherenergieswerestrictedthespectralanalysisto
the0.7–7.0keVenergyband.Wefoundthatthecumulativespectrumcannotbe
fitted with a single absorbedpower-lawmodel, if the absorptionis fixed at the
Galacticvalue.Whentheabsorptionwasallowedtovary,however,theabsorbed
power-lawmodelcangiveanacceptablefit(χ2/dof = 152.7/116).Theresult-
ingabsorptionandphotonindexare0.14±0.01×1022cm−2and2.05±0.06,
respectively.Withtheseparametersthetotalfluxofallthe42off-centersources
in 0.3–10.0keV is calculatedto be 7.79×10−13 erg s−1 cm−2. We also have
divided the resolved off-centersourcesinto the disk and the bulge populations
andstudiedtheircumulativespectrain0.7–7.0keV.Thecumulativespectrumof
thediskpopulationcanbemarginallyfittedwithanabsorbedpower-lawmodel
withanabsorptionof0.27±0.03×1022cm−2andaphotonindexof2.45±0.08
(χ2/dof = 138.9/112).The spectrumof bulgepopulation,on the otherhand,
can be well fitted with a model that consists of a power-law componentand a
blackbodycomponent,bothsubjectedtoacommonabsorptionthatisconsistent
with the Galactic value (χ2/dof = 60.1/55). The obtained photon index and
temperatureareΓ=1.36±0.13andkT =0.12±0.01keV,respectively.
By excluding all the detected point sources we examined the spectrum of
the diffuse emissions extracted in < 2Re that includes the contributions from
boththeunresolvedpointsourcesandtheinter-stellarmedium(ISM).Theback-
ground was extracted in a source-free boundary region on the S3 chip as far
awayaspossiblefromthegalaxy.Wefirstattemptedtofitthespectrumwithan
absorbed power-lawor an absorbedMEKAL model, but neither of them gives
anacceptablefitto thedata,unlesstheabsorptionisallowedto increaseto un-
reasonable,physicallymeaninglessvalues.Thusweattemptedtoapplyamodel
thatconsistsofaMEKALcomponenttorepresenttheemissionofthehotISM,
andapower-lawcomponenttorepresentthecontributionofX-raybinaries,with
both components subjected to a common absorption. Since the abundance of
theMEKALcomponentispoorlyconstrained,wetentativelyfixeditto0.1Z⊙,
which is obtained by Tyler et al. (2004) for the central region of NGC 5055.
WefoundthatwhentheabsorptionisfixedtotheGalacticvalue,themodelpro-
videsuswithanacceptablefit(χ2/dof = 63.4/51),withagastemperatureof
0.30±0.02keVandaphotonindexof2.47±0.26.Thetotalfluxoftheemis-
sionin0.3–10.0keVis7.24×10−13ergs−1cm−2,ofwhichabout54%canbe
ascribedtothepower-lawcomponent.
Of the 43 X-ray sources resolved within 2Re, 10 sources (Src 1, 5, 8, 9,
16,18,22,25,27and35)haveatotalcountslargerthan100.Weextractedthe
individual spectra of these sources and the corresponding background spectra
inannularregionsadjacenttowherethesourcespectrawereextracted.Wefitted
eachspectrumwithanabsorbedpower-lawmodeland/oranabsorbedmulticolor
diskblackbodymodel(DBB),andlisttheresultsinTable2.Thespectrumofthe
centersource(Src1)iswellfittedbyanabsorbedpower-lawmodelwithaphoton
6 Luoetal.
indexof1.61±0.15,whichistypicalforthoseofsupermassiveblackholesinac-
tivegalaxies.The0.3–10.0keVluminosityofSrc1correctedfortheabsorption
is calculated to be 3.16×1038 erg s−1, which is lower than that of most low-
luminosity AGNs. The spectrum of the brightest source Src 35, which showed
significanttemporalvariationsduringtheobservation(§4.2),canbefittedwithan
absorbedDBBmodelwithaninnerdisktemperatureofkTin =0.49±0.04keV.
Thissourceisacandidateultra-luminousX-raysource(ULX)sinceits0.3–10.0
keVluminosityis1.25×1039ergs−1.
AccordingtothepreviousstudiesoftheSSSsinspiralgalaxies(e.g.,M101,
Pence et al. 2001; M31, Kong et al. 2002; M83, Soria & Wu 2003), SSSs are
probably white dwarfs fueled by accretion from their low-mass companions.
Indeedwe find thatthe spectrumof the SSS candidateSrc 9 can be fitted with
anabsorbedDBBmodelwiththeinnertemperatureof0.15±0.01keV,which
isconsistentwiththatofanaccretingwhitedwarf.
4.5 X-RayLuminosityFunctionsoftheResolvedOff-CenterPointSources
AllOff-CenterSourcesDetectedwithin2Re
Assuming that all the off-center X-ray point sources resolved within 2Re
are located at the distance of NGC 5055, we calculated their X-ray luminosi-
ties in 0.3–10.0keV using the best-fit spectral parametersfor their cumulative
spectrum. The conversion factor of the counts is 2.13 × 1036 erg cts−1, and
the resultingluminositiesrange from2.1×1037 to 1.25×1039 erg s−1. With
theseweconstructtheXLFandillustrateitinFigure5a.Sincethedetectionof
thepointsourcesisnotcompleteatthefaintendoftheluminosityfunction,by
adoptingamethodsimilartothatutilizedin,e.g.,KimandFabbiano(2004)and
Xuetal.(2005)weranMonte-Carlosimulationstocreatefakepointsourceson
theS3imageofNGC5055in0.3–10.0keV.Inthesimulationswecreatedfake
sourcesby using the MARX package(Wise et al. 1997),and assumed that the
radial distribution of the fake sources at any given luminosity follows the r1/4
law(deVaucouleurs1948).Atagivenluminosity,wedeterminedhowmanyof
thefakesourcescanbedetectedwiththesametechniqueusedin§4.1.Insucha
waywecorrectedboththeobservedXLFandthebackgroundfortheunresolved
sources.ThecorrectedXLFarealsoshowninFigure5b.
WefittedboththeuncorrectedandcorrectedcumulativeXLFswiththesoft-
ware Sherpa by using either a single power-law profile or a brokenpower-law
profile
N(>L)=N0(cid:26)((LLb/)Lα3h8−)α−lα(lL/L38)−αh LL><LLbb ,
whereL/L38isthe0.3–10.0keVluminosityinunitsof1038ergs−1,andαl and
α aretheslopeindicesforthelowerandhigherluminosityends,respectively.
h
We find that for the uncorrected XLF, the fittings with the single power-law
modelcanbeimmediatelyrejected(χ2/dof = 150.2/21).Itoverestimatesthe
dataatthehighluminositiesandunderestimatesthedataatthelowluminosities.
The use of the broken power-law model, on the other hand, can significantly
improvethefittingsandprovideuswithanacceptablefit(χ2/dof = 15.8/19).
Thebest-fitparametersatthe90%confidencelevelareL = 2.53+0.56×1038
b −0.45
erg s−1, α = 0.61+0.02 and α = 1.96+0.58. For the XLF corrected for the
l −0.03 h −0.18
effectofincompletenessatthefaintendoftheXLF,thesinglepower-lawmodel
also cannot give an acceptable fit (χ2/dof = 125.8/22), while the broken
power-law model can improve the fittings significantly (χ2/dof = 23.1/20)
withthebest-fitparametersL = 3.11+0.37×1038 ergs−1,α = 0.71±0.05
b −0.48 l
X-rayPointSourcesinNGC5055(M63) 7
andα =2.36+0.54.
h −0.43
BulgeandDiskPopulations
We studied the XLFs of the bulge and disk population sources (Fig. 5)
and found that the single power-law model is inadequate to describe both the
uncorrected XLFs of the bulge population (χ2/dof = 68.9/9) and the disk
population(χ2/dof = 30.3/19).Thebrokenpower-lawmodel,however,gives
a good fit to the data with L = 1.68+0.12 ×1038 erg s−1, α = 0.31±0.12
b −0.22 l
and α = 3.95+1.44 for the bulge population (χ2/dof = 7.4/7), and
h −1.19
L = 3.11+0.37 ×1038 erg s−1, α = 0.71± 0.05 and α = 2.36+0.54 for
b −0.48 l h −0.43
the disk population(χ2/dof = 23.1/20).We then correctedthe XLFs for the
incompletenessat the low energiesusing the method as is described above for
alltheresolvedoff-centersources.Forthebulgepopulation,abrokenpower-law
is still needed to describe the XLF with L = 1.57+0.21 × 1038 erg s−1,
b −0.20
α = 0.38+0.20 and α = 2.28+1.30 (χ2/dof = 7.4/7). The corrected XLF
l −0.23 h −0.65
of the disk populationis foundto be nicely consistentwith a single power-law
modelwithaslopeofα=0.93+0.07(χ2/dof =16.3/19).
−0.06
WehavecrosscheckedourresultsontheXLFprofilesofalltheresolvedoff-
centersourcesanddiskpopulationbyexcludingSrc35,thebrightestsourcethat
maybiasthefittings.Wefoundthatwithintheerrorsthebest-fitparametersare
consistentwiththoseobtainedwithSrc35included.
5 DISCUSSION
Wecalculatedthe0.3–10.0keVluminositiesof43X-raypointsourcesdetected
within 2Re of NGC 5055 and found that they span a wide range from about
2.1×1037 ergs−1 to 1.25×1039 ergs−1, whichis typicalforspiralgalaxies
(M81;Tennantetal.2001,NGC1637;Immleretal.2003)andearly-typegalax-
ies(Xuetal.2005andreferencestherein)atsimilardistances.Aftercompensat-
ingfortheincompletenessatthelowluminosityend,wefindthatthecorrected
XLFofthebulgepopulationiswellfittedwithabrokenpower-lawmodel,while
theprofileofthediskpopulation’sXLFsatisfiesasinglepower-lawdistribution.
Thediskpopulationissignificantlyricheratthehigh-luminosityend(>2×1038
∼
erg s−1) than the bulge population, inferring that the star formation history of
the bulge is distinct from that of the disk. In other words, the disk may have
undergonerecent,strongstarburststhatsignificantlyincreasedtheHMXBpop-
ulation, although ongoing starbursts are also observed in the nuclear region of
NGC5055.QuitesimilarphenomenonhasbeenfoundinM81(alsoidentifiedas
a LINERor Sy1.8)byTennantetal. (2001),who reportedthatthe XLFof the
bulgepopulationexhibitsabreakat∼4×1037ergs−1,andtheXLFofthedisk
populationcanbefittedwithasinglepower-law.
DifferentpatternsofthespatialvariationoftheXLFprofilesfromthebulge
tothediskareobservedinotherspiralgalaxies.InM83,aSAB(s)cgalaxythat
hostsastarburstnucleus(Soira&Wu2003),theprofileofthebulgeXLFiscon-
sistentwithasinglepower-lawwithaslopeof0.8,whileabreakappearsonthe
XLFofthediskpopulationatabout8×1037ergs−1.Thismightsuggestthat,un-
likeinNGC5055andM81,thereisnostrongstarburstsinM83onthediskinthe
recentpast,andtheHMXBsdominatethesourcepopulationonlyinthenuclear
starburstregions,ratherthanovertheentiredisk.InM31Priminietal.(1993),
Shireyetal.(2001),Kaaret(2002)andKongetal.(2002)foundthatthecumula-
tiveXLFofallthedetectedpointsourceshasadistinctbreakat2−3×1037erg
s−1,aslopeof1.36±0.33forthehighluminosityendandaslopeof0.47±0.09
forthelowluminosityend.Kongetal.(2002)alsostudiedthecumulativeXLFs
8 Luoetal.
ofthepointsourcesintheinnerbulge,outerbulgeanddiskregions.Theyfound
thatthereisaspatialvariationintheXLFprofiles;boththebreakluminosityand
theslopeofthecumulativeXLFincreaseoutwardsmonotonouslyfromtheinner
bulge,wheretheLMXBs dominatethe X-rayemissionofpointsources,to the
disk.InafewotherspiralgalaxieswheretheXLFshavebeenstudiedindetail,
however,the reportofa breakon theXLF isnotavailablein literatures.These
includesM101(Penceetal.2001),IC342(Baueretal.2003;Kong2003),NGC
891(Templeetal.2005),NGC1637(Immleretal.2003)andM51(Terashima
&Wilson2004),forwhichtheslopesoftheXLFsrangefromabout0.5toabout
1.
ThediversityoftheXLFprofilesoftheX-raysourcesinspiralgalaxiesin-
dicatesthatthestarformationandevolutionhistorymaybemorecomplexthan
wehaveexpected,sothatfurthercarefulmulti-bandinvestigationsareneeded.In
addition,wesuggestthat,sincetypicallyonly50-150X-raypointsourcesarede-
tectedpergalaxy,theuncertaintyintroducedbythesmallnumberstatisticsmay
biasourconclusionsontheXLFprofiletoacertainextent.InXuetal.(2005),
byperformingMonte-Carlosimulationsweshowedthatevenifthereisanuni-
versalbreakonthe XLFs of early-typegalaxies,the statistical errorsdefinitely
precludeus from measuringit correctly.The same result can be applied to the
measurementsoftheXLFprofilesofspiralgalaxies.
6 SUMMARY
Wedetectedatotalof43X-raypointsources(12inthebulgeand31onthedisk)
withinthe2effectiveradiiofNGC5055,whose0.3–10.0keVluminositiesrange
fromafew1037ergs−1toover1039ergs−1.Aftercompensatingfortheincom-
pletenessatthelowluminosityend,thecorrectedXLFofthe bulgepopulation
iswellfittedwithabrokenpower-lawmodel(L = 1.57+0.21×1038 ergs−1),
b −0.20
while the XLF of the disk population satisfies a single power-law distribution
(α=0.93+0.07).Thediskpopulationissignificantlyricherat>2×1038ergs−1
−0.06 ∼
thanthebulgepopulation,inferringthatthediskmayhaveundergonerelatively
recent, strong starbursts that significantly increased the HMXB population,al-
thoughongoingstarburstsarealsoobservedinthenuclearregion.Thisissimilar
towhathasbeenfoundinM81.Inmostotherspiralgalaxies,however,different
patternsofthespatialvariationoftheXLFprofilesfromthebulgetothediskare
observed,indicatingthatthe star formationandevolutionhistory maybe more
complexthanwehaveexpected.
Acknowledgements This work was supported by the National Science
Foundation of China (Grant No. 10273009 and 10233040), Shanghai Key
ProjectsinBasicResearchNo.04JC14079.
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10 Luoetal.
30.0
(a)
03:00.0
30.0 Buldge 17
20 16
11
J2000) 02:00.021 13 98 3 2 14 12 19 22
Dec. ( 30.0 14 6 5 710 15
18
01:00.0
28 24
26 23
30.0
58.0 54.0 13:15:50.0 46.0 42.0
R.A. (J2000)
Fig.1 (a):X-rayimageofthe central3.6′×3.6′ regionof NGC 5055
in 0.3–10 keV in logarithmic scale, which has been smoothed with a
minimum significance of 3 and a maximum significance of 5, and has
beencorrectedforbothexposureandbackground.(b):DSSblueimage
with the field of view of (a) and the bulge marked with a box and a
circle,respectively.OnbothimagesthedetectedX-raypointsourcesare
markedwithsmallcircles.
Fig.2 Background-correctedlightcurvesofSrc22,Src35andSrc38.