X-ray spectral evolution of SAX J1747.0-2853 during outburst activity and confirmation of its transient nature

0.26 MB·English

by  N. Werner

Astronomy & Astrophysics manuscript no. wernerfinal February 5, 2008
(DOI: will be inserted by hand later)
X-ray spectral evolution of SAX J1747.0-2853 during outburst
activity and confirmation of its transient nature
N. Werner1,2, J.J.M. in ’t Zand2,3, L. Natalucci4, C.B. Markwardt5,6, R. Cornelisse2,3,7, A. Bazzano4,
M. Cocchi4, J. Heise2,3, and P. Ubertini4
4 1 Faculty of Sciences, P.J.Safarik University,Moyzesova 16, 040 01 Kosice, Slovak Republic
0
2 SRONNational Institutefor Space Research, Sorbonnelaan 2, NL - 3584 CA Utrecht,theNetherlands
0
3 Astronomical Institute, UtrechtUniversity,P.O. Box 80000, NL - 3508 TA Utrecht,theNetherlands
2
4 Istituto di Astrofisica Spaziale (CNR), Area Ricerca Roma Tor Vergata, Via del Fosso del Cavaliere, I - 00133

n Roma, Italy
a 5 NASAGoddard Space Flight Center, Code 662, Greenbelt, MD 20771, U.S.A.
J
6 Dept.of Astronomy,University of Maryland, College Park, MD 20742, U.S.A.
3 7 Dept.of Physics and Astronomy,University of Southampton, HampshireSO17 1BJ, U.K.
1

2 Received, accepted
v
7 Abstract.SAXJ1747.0-2853 isanX-raytransientwhichexhibitedX-rayoutburstsyearlybetween1998and2001,
7 andmostprobablyalsoin1976.Theoutburstof2000wasthelongestandbrightest.WehaveanalyzedX-raydata
3 sets that focus on the 2000 outburst and were obtained with BeppoSAX, XMM-Newton and RXTE. The data
2 cover unabsorbed 2–10 keV fluxes between 0.1 and 5.3×10−9 erg s−1cm−2. The equivalent luminosity range is
1 6×1035to2×1037ergs−1.The0.3–10keVspectrumiswelldescribedbyacombinationofamulti-temperaturedisk
3
blackbody,ahotComptonizationcomponentandanarrowFe-Kemissionlineat6.5to6.8keVwithanequivalent
0
width of up to 285 eV. The hydrogen column density in the line of sight is (8.8±0.5)×1022 cm−2. The most
/
h conspicuousspectralchangesinthismodelarerepresentedbyvariationsofthetemperatureandradiusoftheinner
p edge of the accretion disk, and a jump of the equivalent width of theFe-K line in one observation. Furthermore,
- 45 type-I X-ray bursts were unambiguously detected between 1998 and 2001 which all occurred during or close
o
r to outbursts. We derive a distance of 7.5±1.3 kpc which is consistent with previous determinations. Our failure
t to detect bursts for prolonged periods outside outbursts provides indirect evidence that the source returns to
s
a quiescence between outburstsand is a true transient.
:
v
Key words.Accretion disks – X-rays:binaries – X-rays: bursts– X-rays:individual: SAXJ1747.0-2853
i
X
r
a
1. Introduction Eddingtonlimitexpectedforaneutronstarwasmeasured
by Sidoli et al. (1998) and Natalucci et al. (2000) to be
ThetransientX-raysourceSAXJ1747.0-2853wasdiscov-
between 8 and 10 kpc. Given that the currently best esti-
ered during an observationof the Galactic bulge with the mateforthedistancetotheGalacticcenteris8.0±0.4kpc
BeppoSAX Wide-Field Cameras (WFCs) in March 1998
(Eisenhauer et al. 2003) and that the angular distance of
(in ’t Zandet al.1998).The positionof the source is con- ′
19 translatestoalineartangentialdistanceofonly44pc,
sistent with that of the transient source GX+0.2-0.2 de-
it is quite possible that the source is located within only
tectedbyArielVin1976(Proctoretal.1976).Thesource ≈ 100 pc from the Galactic center. According to Sidoli
wentintooutburstagaininMarch2000(Markwardtetal.
et al. (1998) the severe interstellar absorption in this re-
2000)andSeptember 2001(Wijnands etal.2002).Type I
gionhampersthesearchfortheopticalcounterpartofthe
X-ray bursts were detected at the discovery proving that
source.TheapparentJ magnitudeispredictedtobelarger
SAX J1747.0-2853 is a weakly magnetized neutron star
than 30 for the low mass companion.
(type I X-ray bursts are thought to be due to thermonu-
The source was serendipitously observed with
clear flashes on the surface of a neutron star; for reviews
Chandra/ACISin2001July(Wijnandsetal.2002),when
see Lewin et al. 1993 and Strohmayer & Bildsten 2003).
it was not in a bright outburst. The 0.5–10 keV un-
Thedistanceasdeterminedfromequalizingthepeakbolo-
absorbed X-ray luminosity for a distance of 9 kpc was
metricfluxofphotosphericradiusexpansionburststothe
3×1035ergs−1.Thespectrumwasconsistentwithapower
Send offprint requests to: N. Werner,email [email protected] law witha photonindex ofabout2 while showingnosign
2 N. Werner,J.J.M. in ’t Zand et al.: SAX J1747.0-2853
Fig.1. The PCA lightcurve of the SAX J1747.0-2853ob- Fig.2. The PCA lightcurve of the SAX J1747.0-2853ob-
tained between 1999 February 6 and 2003 March 30. The tainedbetween2000February10andOctober30.Thefive
times of the five BeppoSAX NFI observations are indi- BeppoSAXNFI observationsandthe laterXMM-Newton
cated by the horizontal line and the observations with observation are indicated.
XMM-Newton and Chandra are indicated by the vertical
dashed lines.
2. Long-term lightcurve and observations
of an Fe-K emission line. Since the luminosity is at least Since February 1999, the PCA instrument on board the
anorderofmagnitudelargerthanexpectedforquiescence, RXTE satellite is programmedto scanthe Galactic bulge
it appears that the classification of SAX J1747.0-2853 as twice a week with a sensitivity close to 1 mCrab (Swank
transient was premature. The possibility of the implied & Markwardt 2001). Observations occur throughout the
lowaccretionlevelsmakesitaparticularlyinterestingob- year, except for three months centered on December and
ject for further study. one month in June, when solar constraints on spacecraft
In this light we have analyzed X-ray data of SAX pointing prevent them. Figure 1 shows the lightcurve of
J1747.0-2853 that were obtained in 2000 when the SAXJ1747.0-2853resultingfromtheseobservations,from
source was fairly active. The data were collected with thebeginningofthebulgescanprogramupto2003March
the Narrow-Field Instruments (NFIs) and Wide Field 30. In source confused regions like this, it is very difficult
Cameras (WFCs) on BeppoSAX, the European Photon toperformanaprioribackgroundsubtraction.Ratherwe
ImagingCamera(EPIC)PNandMOScamerasonXMM- evaluated a background level a posteriori per each of two
Newton and the Proportional Counter Array (PCA) on scan directions and subtracted that level from the data
RXTE. This represents the densest X-ray coverageof the in the plots presented here. This level was verified from
source, although it must be said that the spectral mea- independent measurements with BeppoSAX and XMM.
surements are significantly hampered by the high inter- The light curve is characterized by six peaks in March
stellar absorption (at energies below 2 keV) and source 1999,March 2000,April 2000, August 2000,August 2001
confusion(above10keV).TheGasImagingSpectrometer and September 2001. The 1999 outburst is reported for
(GIS) onboard ASCA also performed a long observation the first time. We furthermore know that there was also
of SAX J1747.0-2853 in March 2000. Since this obser- an outburst in March 1998 (Sidoli et al. 1998, Natalucci
vation duplicates the coverage of the other instruments, et al. 2000).
both in time and instrumental capabilities,we choose not The X-ray activity of SAX J1747.0-2853 appears
to analyze these data in great detail but merely checked strongest in 2000, with three peaks within 5 months of
the standard data products made available by NASA’s which the first one had the highest peak flux measured
High-EnergyAstronomyScienceArchiveResearchCenter. for the source thus far. SAX J1747.0-2853 was found to
Using all X-ray data, we studied the spectrum as a func- be in outburst on 2000 March 2 with an average inten-
tion of luminosity and tried to constrain accretion flow sity of 42 mCrabin the 2–10keV band (Markwardtet al.
geometriesat different states of the source.We startwith 2000).Thepeakof140mCraboccurred6dayslater.After
a discussion of the long-term light curve in Sect. 2, con- that the brightness steadily decreased over the course of
tinue with a treatment ofthe 0.3–10keVBeppoSAX-NFI a month and reached a minimum at about 2000 April 7.
spectrum during various stages of activity in Sect. 3, dis- Then the brightness increased again to a second peak of
cusstheASCAdatainSect.4andtreattheXMM-Newton 80 mCrabon2000April 30.Subsequently,the sourcehad
datainSect.5.InSect.6wereviewtheX-rayburstactiv- another peak four months later in mid August at about
ity. We end with an interpretation of all data in Sect. 7. 20 mCrab. Some outbursts may have been missed during
N. Werner,J.J.M. in ’t Zand et al.: SAX J1747.0-2853 3
observation interruptions. In conclusion, it appears SAX showsanincreaseof50%(notethatthisobservationlasted
J1747.0-2853exhibitedoutburstactivitywitharecurrence 1.6 d).
time of approximately one year between 1998 and 2001, The time intervals corresponding to two X-ray bursts
with a peak flux of approximately one tenth that of the were excluded from the MECS data (see Sect. 6). The
Crabandadurationbetweenoneweekandafewmonths. LECS was not observing during the bursts. We found
Figure 2 focuses on part of the lightcurve (in the year that the standard LECS and MECS background spectra
2000) where the X-ray activity was the highest and the obtained from blank field observations underestimate the
X-raycoveragethedensest.FiveBeppoSAXNFIobserva- background present in empty regions close to the source
tionswereperformedandoneserendipitousXMM-Newton by a factor of ∼5. We therefore used backgroundspectra
observation. The first four NFI observations are covering for the LECSandMECSinstruments obtainedfroma re-
the decreasing part of the strongest peak, ending with an gion near to the source and void of point sources. Since
observation at the minimum between the two peaks (on the source was very bright during the first two observa-
April 7).The April12 observationcoversthe risingphase tions, the tails of the point-spread function became too
of the 2nd peak. The XMM-Newton observation covers bright and the local background determination became
the tail of the third 20 mCrab peaked outburst. Exact questionable. Therefore, the background subtraction for
observation and exposure times are provided in Table 1. the MECS and LECS instrument was performed using
The long ASCA observation was performed between backgrounds from the longest observation on April 12.
March 11.1 and 22.0, 2000, with a net exposure time for For the other three observations the background spectra
the GIS of 331.0 ks. SAX J1747.0-2853 was outside the and source spectra were extracted from the same image.
fieldofviewoftheSolid-stateImagingSpectrometer.The Due to calibration uncertainties the bandpasses for the
observationcoversthefirsthalfofthedecayingpartofthe spectral analysis were limited to 0.3-4 keV for the LECS,
brightestpeakandoverlapsthefirsttwoNFIobservations. 1.8–10 keV for the MECS and 15-200 keV for the PDS.
It is worthwhile to note that the Chandra observation In order to accommodate cross calibration uncertainties,
occurreda few weeks before the 2001outburst. It is quite the normalisations of the LECS and PDS data sets with
possible that this observation caught the source in the respect to the MECS data were left as free parameters.
onset of that outburst, given that the sensitivity of the Furthermore, a 1% systematic error per channel due to
PCA observations for SAX J1747.0-2853 is a moderate 3 calibration uncertainties of the instruments is included.
mCrab which is roughly equivalent to 5×1035 erg s−1. We fittedourspectrausingavarietyofmodelsinXSPEC
(version 11.2.0; Arnaud 1996).
3. BeppoSAX NFI observations and data analysis By fitting the spectra with various models we found
that the normalisation constant for the PDS data is un-
3.1. Data extraction acceptably high (larger than 1.5). Since the PDS has a
relatively large field of view and SAX J1747.0-2853 is in
The NFIs on BeppoSAX (Boella et al. 1997a) consist
thecentralregionofourGalaxywherethedensityofX-ray
of two imaging devices: the Low-Energy Concentrator
sourcesishigh(c.f.,Sidolietal.1999),thehighnormalisa-
Spectrometer (LECS; Parmar et al. 1997) operating
tionconstantindicatesthatthePDSdataaresignificantly
in the 0.1–10 keV bandpass and the Medium-Energy
contaminated by other sources. This is further illustrated
Concentrator Spectrometer (MECS; Boella et al. 1997b)
bythesmallvariabilityofthePDSspectrumascompared
operating at 1.6–10 keV. The source was also ob-
by the (uncontaminated) MECS spectrum (Fig. 3). Also,
served by the non-imaging Phoswich Detector System
the contaminating sources (most notably 1E 1740.7-2942
(PDS; Frontera et al. 1997) operating in the 15-220
′
which is at a 64 off-axis angle) are known to be strongly
keV bandpass, while the High-Pressure Gas Scintillation
variable which precludes the usefulness of other PDS ob-
ProportionalCounterwasturnedoff.Thecircularfieldsof
servationswhen SAX J1747.0-2853wasoffas background
viewoftheLECSandMECSare40arcminand30arcmin
measurements.We decidedto exclude the PDS datafrom
indiameter.ThePDShasahexagonalfieldofviewwitha
◦ ◦ the analysis.
diameter ranging between 1.3 and 1.5 (full width at half
maximum FWHM). The energy resolution at 6 keV for
the MECS is 8% (FWHM). 3.2. Spectral modeling
To study the source spectrum we extracted the LECS
and MECS data from circular regions centered at the We tested different spectral models to describe the con-
sourceusingradiiof4.6arcminand4arcminandrebinned tinuum. To describe the soft component we tested the
them in order to have at least 20 counts per photon en- blackbody and the multicolor disk blackbody model, for
ergy bin, so that we could apply the χ2 statistic in fitting the hard component we tested a powerlaw, an exponen-
models to the data. tially cut off powerlaw and a Comptonization model. A
The photon flux as measured by the MECS remains more elaborate discussion is presented in Natalucci et
constant within 10% for all observations except for the al. 2003. The best fitting model of the continuum con-
2nd observation (on March 16) during which it steadily sists of a soft multicolor disk blackbody component and
decreases by 40% and for the April 12 observation which a hard Comptonized component, all with interstellar ab-
4 N. Werner,J.J.M. in ’t Zand et al.: SAX J1747.0-2853
Fig.4. Spectrum of the SAX J1747.0-2853obtained dur-
ing the observation on 2000 April 7, when the source was
Fig.3. MECS (1.8–10 keV) and PDS (15-200 keV) spec-
in the minimum between the two peaks. The spectrum is
traofSAXJ1747.0-2853from2000March13(1stfromthe
fitted withComptonization,diskblackbody andanarrow
top),March16(2nd fromthetop),March27(3rd fromthe
top)April7(1st fromthebottom)andApril12(2nd from line, allwith absorption.The equivalentwidth ofthe iron
line at this observation is the highest of all observations.
the bottom). The MECS spectra are illustrating well the
changesofthe brightnessofthe sourceandthe changesof
the shape of the spectra. The small changes in the PDS
spectra are indicative of contamination by other sources.
Only the PDS data from the first observation are some-
whatabovethe noiselevelcreatedbyother sourcesinthe
field of view of the PDS.
sorption. This is in agreement with results obtained by
Natalucci et al. (2000) for NFI data on the 1998 out-
burstofSAXJ1747.0-2853,andwiththoseonmanyother
LMXBs at similar flux levels (e.g., Sidoli et al. 2001).
However, we note that the combination of a disk black
body and a power law fits just as well due to the absence
of PDS data (in contrast to the BeppoSAX study of the
1998 outburst by Natalucci et al. 2000). Fig.5. Spectrum of the SAX J1747.0-2853 obtained by
The above-mentioned continuum model describes the the XMM-Newton EPIC cameras during the observation
data reasonably. However, the reduced χ2 of the fit still on2000September 21,withthebestfitand1σ residuals.
exceeds 2 and by investigating the residuals we noticed Thespectrumisfittedasthesumofadiskblackbodyand
that there is a narrow-band spectral component between aComptonizedcomponentallwithabsorption.Theupper
5.5 and 7.5 keV, which is not represented by these mod- curve showsthe data obtainedby the EPIC-pn,the lower
els. The peak of the narrow-bandemissionfeature is near curves show the MOS1 and MOS2 data.
the energy expected for the Fe-K line complex. To de-
scribe this feature we included a gaussian in our model.
We found that a gaussian with the width fixed to zero (a obtained in the last three observations when the bright-
narrow line) describes the emission feature well. The 1σ ness of the source was lower. Since it is most likely that
widthofthe lineis with90%confidencesmallerthan0.28 NH is constant in the direction of the source (circumstel-
keV. We note that the background in the vicinity of the lar NH variations usually are accompanied with dipping
sourcedoesnotshowanysignofanFe-Kline.Thisiscon- activity inthe lightcurvewhichis notdetected),we fixed
sistentwiththeexpectedfluxfromdiffuseFe-Klineinthis NH to8.8×1022cm−2.Wedecidedtochoosethisvaluebe-
region of at most 3×10−5 ph s−1cm−2 for a 4′ extraction causeit wasdeterminedfromdata withthe highestcount
radius(e.g.,Koyamaetal.1986andWangetal.2002).In rate and highest accuracy.
Table1wepresenttheparametervaluesforeachobserva- It is evident from Fig. 3 and Table 1 that the spec-
tiontogetherwithsingle-parameter1σerrors,foramodel trum is changing over time. The contribution of the disk
consisting of a soft component (disk blackbody), a hard black body flux to the total observed photon flux in 2–
Comptonized component and a Gaussian, all with inter- 10 keV is decreasing with the overall flux. The spectrum
stellarabsorption.We getlowervalues ofNH for the data is also slightly hardening with time. If we replace the
N. Werner,J.J.M. in ’t Zand et al.: SAX J1747.0-2853 5
Table 1.ResultsofmodelingtheBeppoSAX-NFIandXMM-Newtonspectra(lastentry)withacombinationofadisk
black body model (Mitsuda et al. 1984 and Makishima et al. 1986), Comptonization of soft photons in a hot plasma
(Titarchuk 1991), and a narrow emission line.
Observation date in Exposure‡ Model Parameters∗ 2–10 and 0.3–10 χ2ν (dof)
2000 keV unabs. flux†
Mar. 13.19-13.85 21.4 ksec NH=8.8+−00..63 3.9×10−9 (79) 0.96 (70)
kTin=1.9±0.1, Ri2ncosθ=16.5+−21..50 5.9×10−9
kT0=0.48+−00..0230, kTe=unconstrained,τ <3
Eline=6.82±0.06, EW=35 eV, f=(9.2±2.1)
Mar. 16.71-17.48 34.4 ksec NH=9.3+−00..73 3.0×10−9 (82) 1.09 (73)
kTin=1.83±0.02, Ri2ncosθ=14.4+−10..89 4.6×10−9
kT0=0.4±0.1, kTe=unconstrained,τ <0.2
Eline=6.78+−00..0075, EW=41 eV, f=8.6+−02..61
Mar. 27.63-28.45 36.6 ksec NH=8.8¶ 1.7×10−9 (80) 1.04 (70)
kTin=1.87+−00..0027, Ri2ncosθ=7.3+−50..56 2.6×10−9
kT0=0.41+−00..0045, kTe=unconstrained,τ=unconstrained
Eline=6.53+−00..0087, EW=33.2 eV, f=4.1+−11..10 ‡
Apr.7.09-7.82 27.9 ksec NH=8.8¶ 9.3×10−11 (17) 1.00 (55)
kTin=0.43±0.04, Ri2ncosθ=340+−218600 4.3×10−10
kT0=0.16+−00..0021, kTe=unconstrained,τ=unconstrained
Eline=6.75+−00..0036, EW=285 eV,f=1.31+−00..1197
Apr.12.82-14.40 57.2 ksec NH=8.8¶ 4.0×10−10 (58) 1.02 (64)
kTin=2.9±0.3, Ri2ncosθ=0.20±0.04 7.1×10−10
kT0 <1.3, kTe=unconstrained, τ=unconstrained
Eline=6.56+−00..0093, EW=58.5 eV, f=2.3+−00..43
Sept.21.39-21.63 MOS NH=8.8¶ 8.7×10−11 (1) 1.38 (139)
16.7 ksec, kTin=0.3±0.1, Ri2ncosθ=7.0+−509.6×102 6.6×10−10
EPIC-pn kT0=0.6±0.1, kTe=39+−1728, τ=0.15+−00..4120
7.6 ksec
Net exposure time as it applies to theMECS data;
∗ Parameters and units: NH: hydrogen column density in units of 1022 cm−2; kTin: temperature at inner disk radius in keV;
Ri2ncosθ:θ isthediskinclinationangle,Rin isthediskinnerradiusinkilometersforasourceat10kpc;kT0:temperatureofthe
Comptonized soft seed photons in keV; kTe: temperature of the electron plasma in keV; τ: optical depth of the plasma; Eline:
centroid energy of Gaussian line profilein keV;EW: equivalent width of theline; f: line flux in 10−4 phot s−1 cm−2
† unabsorbed flux in units of ergs cm−2 s−1. The typical uncertainty in this number is 10% for 2–10 keV and a factor of 2 for
0.3–10 keV.Thenumberbetweenparenthesesreferstothefraction inpercentsofdetectedphotonscomingfrom theblackbody
component.
¶ thevalue was frozen
Comptonization model by a power law to quantify the eter values ofthe Comptonized component,but note that
changes in a straightforwardmanner, the power law pho- the data are not particularly constraining due to the lack
ton index decreases from 2.64 on March 13 to 2.30 on of data above 10 keV. The spectrum obtained during the
April 12. The most significant decrease of the 0.3–10 keV observation on April 7 is shown in Fig. 4.
unabsorbedfluxhappenedbetweenMarch28andApril7.
In this time interval there were significant changes in the
disk blackbody component andin the equivalent width of 4. ASCA observation
theironline.BetweenMarch28andApril7theinnerdisk
temperature decreasedby a factor of∼5 andthe disk ra- SAX J1747.0-2853decayed during the ASCA observation
dius increased. At the same time the equivalent width of by 40%. Furthermore, the light curve shows two X-ray
the iron line increased by a factor of ∼ 7. Between April bursts. The overall spectrum as measured with ASCA-
7 and April 12 the inner disk temperature jumped from GIS2 shows the same components as the NFI spectrum
∼0.4 keV to ∼3.1 keV, the disk radius decreased and the (χ2r =1.22for669dof)withanFe-Klineat6.70+−00..0042 keV
equivalent width of the iron line decreased by a factor of with a flux of (1.1±0.2)×10−3 phot s−1cm−2 and an
∼5. We do not find any significant changes in the param- equivalentwidth of 45eV. These results confirmthe find-
ings of the first two NFI observations.
6 N. Werner,J.J.M. in ’t Zand et al.: SAX J1747.0-2853
5. XMM-Newton observation and data analysis Table 2. Summary of the twelve Galactic center ob-
servation campaigns of the WFCs. Shown are the time
TherearethreeidenticaltelescopesonboardoftheXMM-
frames of the campaigns, the number of observations, the
Newtonsatellite.For twoofthese,50%ofthe radiationis
net exposure time and the number of bursts seen from
divertedtoreflectiongratingsandtheother50%ofthera-
SAX J1747.0-2853.
diationiscollectedbytheEPICmetaloxidesemoconduc-
tor MOS1 and MOS2 CCD arrays.100% of the radiation Campaign # obs. texp(ks) # bursts
fromthethirdtelescopeiscollectedbytheEPIC-pnCCD 1996 Aug.15–Oct.29 67 1017 0
array.The 3 EPICcameras have∼6′′ resolutionin a 30′- 1997 Mar.02–Apr.26 21 654 0
diameter field of view. The energy resolution intrinsic to 1997 Sep.06–Oct.12 13 302 0
all CCDs is E/δE=20 to 50 in a 0.1 to 15 keV bandpass. 1998 Feb.11–Apr.11 17 551 14
1998 Aug.22–Oct.23 10 410 0
During the observationon SAX J1747.0-2853all cam-
1999 Feb.14–Apr.11 14 470 3
eraswereoperatedinfull-window mode withthe medium
1999 Aug.24–Oct.17 24 801 0
filter. The data analysis was performed with the Science
2000 Feb.18–Apr.07 21 633 8
AnalysisSystem(SAS,version5.4.1).Severalbackground
2000 Aug.22–Oct.16 29 767 8
flaresoccurredduring the observation,relevantdata were 2001 Feb.14–Apr.23 5 215 0
excludedfrom the analyzis.We didnot use the data from 2001 Sep.04–Sep.30 7 284 9
timeintervalsduringwhichthecountrateofphotonswith 2002 Mar.05–Apr.15 5 91 0
energyhigherthen10keVexceeds0.5countss−1forMOS
and2countss−1fortheEPIC-pn.Theresultingnetexpo-
sure times are 7.6 ksec for the pn camera and 16.7 ks for and lasted approximately 20 s. Its time profile has a fast
each of the MOS cameras. The source spectrum was ex- riseandanexponentialdecay.Wefittedthedecayingpart
tractedfromacircularregioncenteredatthesourceusing with an exponential and find an e-folding decay time of
a diameter of 1.5 arcmin. The background spectra were 6.6 ± 0.4 s. The second burst started on 2000 April 12
extracted from a circle with the same radius, from a re- at 23:14:23 UT and lasted 50 s. It took about 15 s for
gioncloseto SAX J1747.0-2853fromthe same CCD. The the decay to set in for this burst while for the previous
extractedspectrawererebinnedintobinswithaminimum burstthe decaysetinafterabout2s.Thee-foldingdecay
of 20 counts per bin. The response matrices and ancillary time is 8.0±0.3 s. The neutron star photospheric radius
response files were generated using SAS. expansionaswell asa plateauin the peak flux during the
The bandpasses for the spectral analyses are, due to burstssuggeststhattheburstsareEddington-limitedtype
calibration uncertainties, limited for all three EPIC cam- I X-ray bursts. When fitted with a blackbody spectrum,
erasto0.3–10.0keV.Wefittedtheobtainedspectrausing the burst peak intensity corresponds to an unabsorbed
XSPEC. In order to accommodate the cross calibration bolometric flux of (4.47±1.4)×10−8 ergs s−2. Assuming
uncertaintiesthenormalisationsforthe MOS1andMOS2 isotropic burst emission, Eddington limited type I X-ray
datasetswithrespecttotheEPIC-pndatawereleftasfree burstscanbeusedtoconstrainthedistanceofthesource.
parameters. We started with the same continuum model Usingastandardcandleluminosityof(3.0±0.6)×1038ergs
asweusedfortheBeppoSAXobservations.Thespectrum s−1 (Lewin et al. 1993; see also Kuulkers et al. 2003), we
with the best fit and residuals is shown in Fig. 5. There calculated a distance of 7.48±1.34 kpc for SAX J1747.0-
was no need to include an iron line feature in the model. 2853.
Our data constrain the highest possible value for the iron We investigatedthe bursthistoryofthe SAXJ1747.0-
line flux to 1.85×10−5 phot s−1cm−2 with a 90% con- 2853 by employing the WFCs (for an instrument descrip-
fidence level. This value is an order of magnitude lower
tion, see Jager et al. 1997). The PCA lightcurve is over-
thenthelowestvaluedetectedbytheBeppoSAXobserva- plotted in Fig. 6 with the time intervals of the WFC ob-
tionsaftertheoutburst.Theupperlimitoftheequivalent servationsandthetimesofdetectedbursts.Therewere30
width of the iron line is 24.7 eV. Taken literally, the tem- burstsdetectedduringorclosetooutburstsinthisperiod.
perature at the inner disk radius was small and the disk It is interesting to note that the BeppoSAX WFCs had
was far away from the neutron star compared to the pre- SAX J1747.0-2853 in the field of view during the whole
vious observations.The unabsorbed2–10keV flux during first peak of the outburst, but bursts were detected only
this observation was 8.7×10−11 erg cm−2s−1. This value at or after peak flux. The chance probability of detecting
is comparableto the April 7 BeppoSAX observation.The
zero bursts prior to the peak, assuming that the average
data obtained by the EPIC-pn show some excess with re- burst rate is identical to that after the peak and that the
spect to the model at low energies,but the data obtained bursts arrive randomly, is 4.6%.
by the MOS cameras do not confirm the excess.
The WFCs did not detect any bursts during its fall
1999(MJD51410–51470)campaignontheGalacticcenter
when there was no strong persistent X-ray activity, while
6. X-ray bursts
the net exposure time of this campaign was 801 ks. At
Two type-I X-ray bursts were detected with the MECS. other times when the PCA lightcurve also does not show
The first burst started on 2000 March 17 at 0:14:30 UT strong persistent activity, there are considerable amounts
N. Werner,J.J.M. in ’t Zand et al.: SAX J1747.0-2853 7
”This suggestion was recently reaffirmed by the report of
anX-rayburstfromSAXJ1747.0-2853in1991(Grebenev
et al. 2002)which is sevenyears before the source discov-
ery. However, we find indirect evidence that it does go to
quiescencebetweenoutbursts.Thelackofburstdetections
in 3 WFC campaigns, each with a coverage in excess of
600 ks, is that evidence. If there were any significant ac-
cretion, there would have been bursts as shown by Sidoli
et al. (1999).
From the long-term PCA lightcurve we estimate the
average 0.3–10 keV luminosity. The average count rate is
87±2c s−1 per 5 PCUs. This is within 5%fromthe value
measured within a day from the XMM-Newton observa-
tion.Applying theresultsofthatobservationweestimate
an average flux of (6±3)×10−10 erg cm−2s−1 or lumi-
nosityof(4±2)×1036ergs−1.Thebolometricluminosity
Fig.6. The PCA lightcurve of SAX J1747.0-2853 with is estimated to be at most 50% more. Thus, the system
the times (as indicated by horizontal lines) when the appears to be close to the critical luminosity below which
BeppoSAX WFC had the source in its field of view with the accretion onto the compact object changes from per-
sufficient sensitivity, and with the times of the detected sistent to transient in nature (Van Paradijs 1996; White
X-ray bursts (as indicated by vertical dotted lines; this et al. 1984).
includes the four NFI and ASCA detected bursts). The The multiple-peak outburst of 2000 is an interesting
number of the detected X-ray bursts in an observation feature, as is the otherwise somewhat irregular outburst
window is indicated over the horizontal lines. behavior.Thesuggestionpresentsitselfthattheaccretion
disk is not fully emptied during mosts outbursts due to
heatingfrontsinthediskthatdonotmakeittoeitheredge
of bursts (for instance in the period around MJD 51800). of the disk (see Lasota 2001 and references therein). The
This strongly suggests that the source was truly in qui- fact that during the rise to the first peak no X-ray bursts
escence during the fall of 1999. The presence of truly were detected while they were during the decay suggests
quiescent periods is furthermore supported if we con- that this outburst was caused by an outside-in heating
sider the full WFC database of 42 burst detections from frontintheaccretiondisk.Ifitwouldhavebeenaninside-
SAXJ1747.0-2853,seeTable2.Therearemanyprolonged out front, one would have expected immediate accretion
periods when there were no bursts detected, the cover- onto the neutron star surface and, therefore, ignition of
age was particularly extensive in the fall of 1996 and the X-ray bursts fairly soon after the onset of the outburst.
spring of 1997 with a combined exposure time of 1.7 Ms. Our series of five NFI observations nicely probes the
Finally,wenotethattheChandraobservationasdiscussed outburst evolution, with coverage of a decaying state of
by Wijnands et al. (2002) was close to the 2001 outburst the firstpeakandthe risingstateofthe secondpeak.The
during which WFC also detected 9 bursts and the above- most conspicuous changes in the spectrum during the de-
quiescent flux very likely reflects the onset of that out- cay are the softening of the soft component (as diagnosed
burst. by the disk inner edge temperature) and the jump of the
equivalentwidthoftheFe-Kline.Thischangeismostob-
vious in the last NFI observation of the decay with the
7. Discussion
lowest flux (see Fig. 3) for which the MECS spectrum
Whenever its location was covered by 2–10 keV instru- shows an upturn at low energies (when kTin decreases
mentationwith∼10−12 ergcm−2s−1 sensitivityorbetter from roughly 2 to 0.4 keV) and a larger Fe-K line (the
(we exclude soft X-rays here because the low-energy ab- equivalent width increases from about 40 eV to about
sorptionis very high), SAX J1747.0-2853was detected to 300 eV). Also, the temperature of the seed photons for
be above quiescence. The lowest reported flux is 1.9 × the Comptonization drops threefold in this observation.
10−11 erg cm−2s−1 (2–10 keV) during a Chandra obser- The temperature measurements are qualitatively consis-
vationin July 2001(Wijnands et al.2002).The next low- tent with a picture of a changing inner radius of the op-
est flux value was measured by Sidoli et al. (1998) at tically thick part of the accretion disk (e.g., review by
5 × 10−11 erg cm−2s−1 in April 1998 in the tail of the Lasota 2001). When the flux becomes lower during the
1998 outburst. For a distance of 7.5 kpc this translates firstpeak, the optically thick partofthe disk movesaway
to 2 and 4×1035 erg s−1. An X-ray burst was detected from the neutron star and the disk spectrum cools. The
during the observation that Sidoli et al. discuss, indepen- rise to the second outburst peak was covered by the last
dently testifying to low-level accretion activity. Based on NFI observation. The spectrum is characterized by a rel-
the failure to find SAX J1747.0-2853 in true quiescence, atively hot disk temperature, hotter than any previous
it appears that SAX J1747.0-2853is not a true transient. measurement. Perhaps the optically thick part of the ac-
8 N. Werner,J.J.M. in ’t Zand et al.: SAX J1747.0-2853
cretiondisk here is closer to the neutron star than during Jager R., Mels, W.A., Brinkman, A.C., et al., 1997, A&AS,
thedecayofthepreviouspeak.Perhapsthisreflectsadisk 125, 557
instability that occurred in the inner parts of the disk in- Koyama, K., Makishima, K., & Tanaka, Y. 1986, PASJ, 38,
stead and the outburst is the result of an inside-out disk 121
Kuulkers, E., Hartog, P.R., In ’t Zand, J.J.M., Verbunt, F.,
instability. Unfortunately, we were not able to verify this
Harris, W.E., & Cocchi, M. 2003, A&A,399, 663
with WFC monitoring observations of X-ray bursts.
Lasota, J.-P. 2001, NewAR,45, 449
There is one striking measurement regarding the Fe-
Lewin, W.H.G., van Paradijs, J., & Taam, R.E. 1993, Space
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