The puzzling symbiotic X-ray system 4U1700+24

1.8 MB·

by  A. A. Nucita

Astronomy & Astrophysics manuscript no. 4u1700+24 (cid:13)c ESO 2014
January 10, 2014
The puzzling symbiotic X-ray system 4U1700+24
A.A. Nucita1,2, S. Stefanelli1, F. De Paolis1,2, N. Masetti3, G. Ingrosso1,2, M. Del Santo4, and L. Manni1,2
1 Departmentof Mathematics and Physics“E. De Giorgi”,Universityof Salento, ViaperArnesano, CP193, I-73100,
Lecce, Italy
4 2 INFN,Sez. diLecce, via perArnesano, CP 193, I-73100, Lecce, Italy
1 3 INAF– Istitutodi Astrofisica Spaziale e Fisica Cosmica diBologna, via Gobetti 101, I-40129 Bologna, Italy
0 4 INAF/IAPS,via del Fosso del Cavaliere, 100, I-00133, Roma, Italy
2
Submitted:XXX;Accepted: XXX
n
a
ABSTRACT
J

9 Context. Symbiotic X-ray binaries form a subclass of low-mass X-ray binary systems consisting of a neutron star

accreting material from a red giant donor starvia stellar wind orRochelobe overflow. Onlya few confirmed members
]
arecurrentlyknown;4U1700+24 isagood candidateasitisarelatively brightX-rayobject, possibly associated with
E
thelate-typestar V934 Her.
H Aims. We analysed the archive XMM-Newton and Swift/XRT observations of 4U 1700+24 in order to have a uniform
. high-energy (0.3−10 keV) view of the source. Apart from the 2003, 2010, and 2012 data, publicly available but still
h
unpublished,we also took theopportunity tore-analyze a set of XMM-Newton data acquired in 2002.
p Methods. After reducing the XMM-Newton and Swift/XRT data with standard methods, we performed a detailed
-
o spectral and timing analysis.
r Results. We confirmed the existence of a red-shifted O VIII Ly-α transition (already observed in the 2002 XMM-
t Newton data) in the high-resolution spectra collected via the RGS instruments. The red-shift of the line is found in
s
a all the analysed observations and, on average, it was estimated to be ≃ 0.009. We also observed a modulation of the
[ centroid energy of thelineon short timescales (afew days) and discusstheobservations in theframework of different
scenarios. If the modulation is due to the gravitational red-shift of the neutron star, it might arise from a sudden
1 re-organization of the emitting X-ray matter on the scale of a few hundredsof km. Alternatively, we are witnessing a
v uni-polarjetofmatter(withtypicalvelocityof1000−4000 kms−1)possibly emittedbytheneutronstarinanalmost
3 face-on system. The second possibility seems to be required by the apparent lack of any modulation in the observed
5 X-raylightcurve.Wealsonotealsothatthelow-resolutionspectra(bothXMM-NewtonandSwift/XRTinthe0.3−10
0 keVband)showtheexistenceofablackbodyradiation emittedbyaregion (possibly associated with theneutronstar
2 polar cap) with typical size from a few tens to hundreds of meters. The size of this spot-like region reduces as the
1. overall luminosity of 4U 1700+24 decreases.
0
Key words.(X-rays)binaries – (Stars:) binaries: symbiotic – (Stars) neutron – (Stars) individual: 4U 1700+24
4
1
:
v 1. Introduction ever,X-raystudies of these sources arestill quite sporadic,
i
X withonly ahandful ofobjectshavingbeenexploredinthis
Symbiotic X-ray binaries (SyXBs) form a tiny subclass of
spectral window (Masetti et al. 2002, 2007a,b, Rea et al.
r Galactic low-mass X-ray binaries (LMXBs) characterized
a 2005, Paul et al. 2005, Tiengo et al. 2005, Mattana et al.
by a red giant star (generally of spectral type M) which
2006, Patel et al. 2007, Corbet et al. 2008, Marcu et al.
losesmattertoacompactobject,mostlikelyaneutronstar 2011, Gonz´alez-Gal´lan et al. 2012).
(NS), via stellar wind, or (less frequently, as in the case of
GX1+4;Chakrabarty&Roche1997)Rochelobeoverflow.
Only seven confirmed members are currently known, while One of these sources is 4U 1700+24. It was discovered
for other candidates like 1RXS J180431.1-273932 (Nucita (Cookeetal.1978,Formanetal.1978)asarelativelybright
et al. 2007) follow-up observations allowed us to exclude X-rayobject,withvariabilityonbothlong-termtimescales
the SyXB nature (see Masetti et al. 2012 and references (months to years; Masetti et al. 2002, Corbet et al. 2008)
therein).However,accordingtostellarpopulationsynthesis and short-term timescales (tens to thousands of seconds:
studiesperformedbyLu¨etal.(2012),between100and1000 Garcia et al. 1983, Dal Fiume et al. 1990); This character-
oftheseobjectsareexpectedtobeintheGalaxy(although istic suggested that the source might be an accreting sys-
one should note that half of the SyXBs considered by the tem; Garcia et al. (1983)proposed the brightlate-type star
authors were found to be either spurious or unconfirmed V934 Her as the optical counterpart of 4U 1700+24 (see
cases). also Gaudenzi & Polcaro 1999 and Masetti et al. 2002) on
The SyXBsubclassonlystartedgainingsomeattention the basis of its position and the detection of emission lines
fromthe scientificcommunityinthelastdecadeonly;how- in its ultraviolet spectrum. This association was later con-
firmedbyMasettietal.(2006)withaChandraX-raysatel-
Send offprint requests to: A.A. Nucita, e-mail: lite observation that provided a localization of the source
[email protected] with subarcsecond precision.
2 Nucita et al.: XMM-Newton and Swift observation of 4U1700+24
Periodicity studies of the object’s light curve were per- MOS and pn, respectively. For each observation, the ex-
formedinX-rays(Masettietal.2002,Gallowayetal.2002, posure time resulting from this procedure is reported (in
Corbetetal.2008)andoptical(Hinkleetal.2006),without ks) in the last column of Table 1 for the MOS 1, 2 and
findinganyconcludingevidenceofeitherorbitaloraccretor pn,respectively.The eventscollectedduring the goodtime
spin modulation. intervals were only used in the spectral analysis, being the
X-rayspectroscopyofthe source,obtainedoverthe last timinganalysisperformedwithoutapplyinganytimefilters
decades(Garciaetal.1983,DalFiume etal.1990,Masetti to avoid the introduction of artifacts.
etal.2002),showsacontinuumtypicalofaccretingLMXBs, The X-rayemissionfromthe source was firstextracted
with a thermal component probably originating on or near from a circular region centred on the nominal position of
the accretor and a Comptonized emission detected up to 4U 1700+24 as determined by Masetti et al. (2002) when
100 keV. In particular, Masetti et al. (2002) examined the analyzingtheChandra/HRCdata(α=17h06m34.517s,δ=
X-rayspectroscopicpropertiesofthesourceusingdatacol- +23058′ 18.66′′,witherrorsonbothcoordinatesof≃0.6′′)
lected with several satellites over 13 years, from 1985 to and with a radius chosento containat least 80%of the to-
1998.After this study,Tiengo etal.(2005)published a pa- tal source energy. The backgroundsignal was accumulated
per on the X-ray behaviour of 4U 1700+24: the authors from circular regions on the same chip.
analysed an observation collected with the XMM-Newton We noted that the observations labeled as 0155960601,
satellite in 2002 and found an emission feature at ≈ 0.5 0151240301,and0151240201wereaffectedbypile-up.This
keVandanemissionlineat≃0.64keVwhichwaspossibly effect consists in two or more photons hitting nearby CCD
identified as the red-shifted O VIII Ly-α transition. pixels during an exposure, thus producing an event which
NofurtherinvestigationsontheX-rayspectroscopicbe- mimics a single, larger energy photon. If not severe, the
haviour of 4U 1700+24 have been performed since then; pile-upeffectcanbemitigatedbyfollowingthemethodde-
however,three more XMM-Newton pointings performedin scribedintheXRPSUser’smanual(2008).Inparticular,we
2003 and seven Swift/XRT observations made in 2010 and extractedthe sourcesignalby using anannularregioncen-
2012 are publicly available but still unpublished. In this tredonthesourcenominalcoordinateswithinnerandouter
paper we present an analysis of these data, together with radii of ≃ 10′′ and ≃ 40′′, respectively. With this choice,
an independent examination of the 2002 observation first and in accordance with the signal found by Tiengo et al.
reportedbyTiengoetal.(2005)inordertohaveauniform (2005) when analyzing the 2002 observation, we were able
analysis of the whole XMM-Newton and Swift/XRT data tocorrectthepile-upfortheEPICpndata,butnotforthe
setsconcerning4U1700+24.Wefoundthatthefeatureob- MOS 1 and 2 cameras for which the correctionwould have
servedat≃0.5keVis possiblyanartifactdue primarilyto greatly reduced the number of good counts. Consequently,
the instrumentaloxygenedge(see e.g.Armas-Padillaetal. we avoided using these data to prevent spurious effects in
2013),whileweconfirmtheexistenceofthe≃0.6keVline. the spectral and timing analysis.
Moreover,we found that the line evolves in time. Finally,sourceandbackgroundX-rayspectra,together
Thepresentworkisstructuredasfollows:Sect.2reports with the associated ancillary and response matrix files,
the observations and the data analysis; in Sect. 3, the re- were extracted and imported within XSPEC (Arnaud et
sultsfromthesefourpointingsarepresented;whereasSect. al. 2007) for a simultaneous fitting procedure.
4 provides a discussion of our results.
3. Swift/XRT observation and data reduction
2. XMM-Newton observation and data reduction Swift/XRT observed 4U 1700+24 in 2010 with two dedi-
catedpointingsandin2012withfiveobservations,because
Thesource4U1700+24hasbeenobservedseveraltimes(see
the source was in the same field of view as the gamma ray
Table 1 for details) by all the X-ray instruments (RGS 1
burst GRB121202A (see Table 2 for details on the archive
and 2, EPIC-MOS 1 and 2, EPIC-pn; Jansen et al. 2001,
data sets).
Stru¨der et al. 2001, and Turner et al. 2001) on board the
TheSwiftdatawereanalysedusingstandardprocedures
XMM-Newton satellite. Here, we report the observation
(Burrows et al., 2005)and the latest calibration files avail-
identificationnumber,thenominaltargetcoordinates(right
able at http : //heasarc.nasa.gov/docs7swift/analysis/. In
ascensionanddeclination),the positionangle,theobserva-
particular, we processed the XRT products with the xrt-
tiondatetogetherwiththestartandendtime,thenominal
pipeline (v.0.12.6) tasks, applied standard screening crite-
duration, and the esposure time after removing the high-
ria by using ftools (Heasoft v.6.13.0), and extracted with
energy flares.
xselect the source spectra and light curves (in the 0.3-10
The observation raw data files (ODFs) were processed keV band) from a circular region (with radius of ≃ 40′′)
using the XMM-Science Analysis System (SAS version
centred on the nominal coordinates of the target. When
13.0.0) and with the most updated calibration constituent
possible,thebackgroundspectraandlightcurveswerealso
files.Toobtainthecalibratedlow-andhigh-resolutionspec-
extracted from circular regions. We noted that the 2012
tra, we ran the emchain and epchain tools for the EPIC
Swift/XRT observations were affected by pile-up because
cameras products, while the rgsproc pipeline was executed
the corresponding source count rates were above ≃ 0.5
for the RGS 1 and RGS 2 instruments. count s−1. Thus, we followed the recipe presented in the
We followed the standard analysis recipes described in Swift on-line threads1 and discarded the central part (up
theXRPSUser’smanual(2008).Inparticular,weextracted to ≃ 15′′) of the source extraction region until the source
thelightcurvesabove10keVforthefullMOSandpncam- count rate was below the threshold value. We then used
eras. We then identified and discarded parts of the obser- the xrtmkarf task to create the ancillary response files and
vations affected by high levels of background activity, by
using a threshold of 0.35 and 0.40 counts s−1 for the two 1 Seehttp://www.swift.ac.uk/analysis/xrt/pileup.php
Nucita et al.: XMM-Newton and Swift observation of 4U1700+24 3
Table 1. Log of thearchive XMM-Newton observations analysed in thispaper.
OBS.ID REV NOM.RA NOM.DEC POS.ANGLE DATE START END NOM.DUR. EXP.TIME
(deg) (deg) (deg) (yr-m-d) (h:m:s) (h:m:s) (ks) (ks)
0155960601* 489 256.64370 23.97183 295.63034 2002-08-11 15:55:49.0 19:32:02.0 13 −,−,3.2
0151240301* 593 256.64385 23.97183 79.403876 2003-03-07 01:08:08.0 04:32:20.0 12 −,−,1.3
0151240201* 594 256.64385 23.97183 79.399512 2003-03-09 01:03:49.0 04:27:57.0 12 −,−,5.5
0151240401 673 256.64385 23.97183 294.30723 2003-08-13 15:23:12.0 19:40:38.0 15 9.4,9.2,7.8
Note:Theobservationslabelledwithanasteriskpresentedpile-upintheEPICcameras.Wewereabletocorrectforthepile-uponlytheEPICpndata.
Hence,inthesecases,weavoidedusingtheMOS1andMOS2eventsinordertopreventspuriouseffects(seetextfordetails);asaconsequence,asymbol
−appearsinthelastcolumn.
Table 2. Log of the archive Swift/XRT observations analysed transitions,therelativedistanceamongthecentralenergies
in thispaper. was frozen to the value predicted by the atomic physics.
We used the C-statistics (Cash 1979) as the estimator
OBS.ID DATE START END EXP.TIME
(yr-m-d) (h:m:s) (h:m:s) (ks) of the goodness of the performed fit and, for any line ini-
0009080001 2010-05-30 20:29:06 23:39:56 1.4 tiallyrecognizedassuchbyeye,the featurewasconsidered
0009080002 2010-06-04 12:55:00 00:16:58 9.7 detected if, repeating the fit without any Gaussian profile,
0054025500* 2012-12-02 05:07:33 13:12:24 12.8
0054025501* 2012-12-02 13:13:56 23:14:27 6.4 the newly obtained value of the statistics differed by at
0054025502* 2012-12-03 14:56:26 23:03:44 5.3 least2.3 fromthe previous one. This choice correspondsto
0054025503* 2012-12-04 00:47:17 10:28:36 5.8
68% confidence level (or, equivalently, 1 σ for one interest-
0054025504* 2012-12-05 13:21:41 18:11:36 5.9
ing parameter) for the detected emission line (Arnaud et
Note:Theobservationslabeledwithanasteriskpresentedpile-up.
al.,2007).Finally,ourestimateoftheerrorassociatedwith
the centroid energy was evaluated as the quadratic sum of
the error in output from XSPEC and the calibration un-
certainty quoted above.
took into account the corrections for the different extrac-
tionareasofthesourceandbackgroundandforvignetting.
Finally, the light curves were background corrected.
data and folded model
4. Results 5
4.1. XMM-Newton RGS spectral analysis of 4U1700+24
Our study of the X-ray properties of 4U1700+24 started eV−1 4
wXiMthMt-hNeeawntaolnysgisraotfintghse.fiTrhsteosrpdeecrt-rsaplercetsroaluotbitoaninoefdRbGySthine nts s k−1 3
u
the first-order spectrum is FWHM=72 m˚A and the cali- d co
binrgattioonFiWnwHaMve≃len6g2t0hkismacsc−u1raatnedup∆vto≃8m69˚Ak,mcosr−re1spaotn3d5- ormalize 2
˚A (XMM-Newton Users Handbook 2009).In the following, n
we use the unbinned RGS 1 and RGS 2 spectra for the
1
quantitative analysis and searched for emission lines.
We then imported the spectra (and the associated re-
sponsematrices)withinXSPECandsimultaneouslyfitthe 0
0.62 0.64 0.66 0.68
data. In this respect, the phenomenological spectral anal- Energy (keV)
ysis followed a local fit method2, i.e. the unbinned spec- Fig.1.AzoomoftheRGS1and2spectraof4U1700+24around
tra were first divided in intervals of ≃ 100 channels wide the O VIII Ly-α emission line (at about ≃ 19 ˚A ) for all the
and then Gaussian profiles were added to account for all observationsquotedinTable1.Thespectrawerebinnedinorder
identified emission lines. In this procedure,the line energy, to have a signal-to-noise ratio of 5 in each bin, and we give
as well as its width and normalization were considered as the horizontal axis in energy instead of wavelength. The data
free parameters of the fit. In addition, the local continuum in yellow/orange (corresponding to the 2002 observation, i.e.
0155960601) clearly show a large continuum component with
was modelled as a power law with a fixed photon index
respect to the other data sets, thus reflecting the high activity
Γ = 1 and free normalization. In the fit procedure, con-
of thesource at that time. The solid lines represent thebest-fit
sistently with Tiengo et al. (2005), we fixed the column
model as described in thetext.
density of Galactic neutral hydrogen to the average value3
observed along the line of sight towards 4U1700+24, i.e.
4.4×1020 cm−2 (Dickey & Lockman 1990). For line dou- In particular, this procedure resulted in the identifica-
blets andtriplets andfor emissionlines close to free-bound tion of an emission feature at ≃ 0.645 keV (≃ 19 ˚A ),
i.e. consistent with that already observed by Tiengo et al.
2 Although in a different context, the local fit method is de- (2005) in the RGS 1 and 2 spectra of the 2002 observa-
scribed in Guainazzi & Bianchi (2007). tion.Wedostillidentifythesamelineinallthesubsequent
3 We used the on-line NH calculator (available at XMM-Newtonobservationsconsideredinthiswork.InFig.
http://heasarc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl) to 1, we give a zoom of the RGS 1 and 2 spectra around the
get theaverage column density. identifiedemissionline(binnedinordertohave5σ perbin,
4 Nucita et al.: XMM-Newton and Swift observation of 4U1700+24
and with the horizontal axis in energy) of 4U1700+24, for
all the observations quoted in Table 1. We note that the
data in yellow/orange correspond to the 2002 observation
(Obs. ID 0155960601)and clearly show a continuum com-
ponent largerthan that presentin the other data sets; this
reflectsthehighactivityofthesourceatthattime(seealso
next paragraph).
Some emission lines within a few σ of the observed
feature are found in the CHIANTI database (Dere et al.,
2001), with the Ly-ǫ line of the NVII having a centroid
energypositioncompatiblewith thatobservedin ourspec-
tra.However,as firstdiscussedbyTiengo etal.(2005),the
probability of the occurrence of this transition is very low,
thus pushing toward the interpretation of this line as the
red-shifted Ly-α transition of the H-like oxygen, i.e. the
OVIII feature with a rest-frame wavelengthof 18.9671˚A.
In Table 3, we give the best-fit parameters (C =
Stat
2234.33 for 2044 degrees of freedom) obtained via the
method described above4 for the emission line observed at
≃19˚A foreachoftheXMM-Newtonobservationsanalysed
in this work. Here, we report the mean time of the obser-
vation(inModifiedJulianDate,MJD),the linenormaliza-
tion, the best-fit central energy (E ) and the line width
obs Fig.2.Here,weshowthecentroidpositioninkeV(upperpanel)
(σobs).Assumingthattheobservedemissionlineisthered- as determined by the Gaussian line fit, the line width (middle
shiftedLy-αtransitionoftheH-likeoxygen(OVIII),inthe panel),andthered-shiftfactoroftheOVIIILy-αlineidentified
last two columns we give the red-shift factor and associ- inthehigh-resolutionspectraof4U1700+24asafunctionofthe
ated equivalent velocity (defined as ∆λ/λ = v/c, being c observationtime(definedastheaveragetimeofeachexposure)
the speed of light), respectively. in modified Julian date. In each panel, thedot-dashed line rep-
On average, the emission line has an energy of ≃0.648 resents afit with aconstant, while thedashed line accountsfor
keV, corresponding to a wavelengthof ≃19.136˚A, and an any linear trend possibly associated with the data (see text for
details).
average red-shift of ≃ 0.009 (see also below). In Figure 2,
we show the position (upper panel) in keV, the line width
(middle panel), and the red-shift factor (bottom panel) of state shell as the resonanceline, r: 1s2 1S -1s2p1P , the
the O VIII Ly-α line as a function of the observation time two inter-combination lines (often blended0), i: 1s121S -
(heredefinedastheaveragetimeofeachexposure)inMJD. 1s2p3P ,andthe forbiddenfeature,f : 1s21S -1s2s3S0.
We note that the line energy for observation 0155960601 2,1 0 1
These transitions are particularly important since, as
is consistent with that already derived by Tiengo et al.
demonstrated by Porquet & Dubau (2000), their relative
(2005). We first fitted the central energy of the emission
emission strengths are good indicators of the physical con-
feature, its width, and the red-shift factor (assuming the
ditions of the gas density and temperature.
rest-frame energy to be that of the O VIII Ly-α line) with
Because of the poor statistics of the RGS data, the fit
constantvalues(seethedot-dashedlinesinthethreepanels
theOVIIcomplexwithamodelconstitutedbyapowerlaw
in Fig. 2) finding 0.648±0.001 keV (χ2 = 2.8 for 3 dof),
and three Gaussians (with all the parameters free, except
0.0067±0.001keV(χ2 =0.45for 3dof), and0.009±0.001
the relative distances among the lines, as well as the con-
(χ2 = 2.1 for 3 dof), respectively. Then, we searched for a
tinuum power law index) did not converge. We then used
globaltrendinthedatabyusingalinearmodelasafitting
a different approach and, in particular, we fixed the cen-
function. Assuming that the overall behaviour of the data
troid energy of the interested lines to that expected by the
varies linearly with time, we obtained the rate of changes
atomic physics, after correcting for the average red-shift
of the interested parameters to be (5.9±5.4)×10−6 keV
(∆λ/λ ≃ 0.009) previously found. We also set all the line
day−1 (χ2 = 3.6 for 2 dof), (2.8±5.4)×10−6 keV day−1
widths to zero. Leaving as free parameters the Gaussian
(χ2 =0.5for2dof),and(−1.1±0.8)×10−5day−1(χ2 =2.1
line and power law normalizations allowed us to get a rea-
for2dof),respectively.AlthoughacloseinspectionofFig.2
sonable fit (see Fig. 3) characterized5 by C =664.3 for
might allow us to conclude that the emission line position Stat
520 degrees of freedom. However, this procedure only re-
changes on time scales of ≃ 2 days (see e.g. the bottom
sulted in upper limits to all the line normalizations, thus
panelwherethiseffectisamplifiedbythefactthatthered-
making impossible to infer the physical condition of the
shift factor is given and also the log of the observations in
X-ray emitting gas. In this case, our best-fit model works
Table 1),aconstantmodelseemsto be preferredwhenone
only as a guide for the eye when searching for the O VII
considers the long-term trend.
complex lines. In this respect, we note that the RGS data
We alsotriedtheidentificationofthemostintenselines
show the existence (although with a signal-to-noise ratio
of He-like ions of oxygenin the range 5-35 ˚A, i.e. the tran-
sitions between the n = 2 shell and the n = 1 ground 5 Performing the fit procedure with a normalized power law
only resulted in a best-fit with CStat=673.0 for 532 degrees of
4 WhenrepeatingthefitwithouttheGaussianlineprofile,the freedom. Intheframework of thelocal method discussed in the
newvalueofthestatisticsisCStat=3270.75for2056degreesof text,thecomparisonofthisbest-fitwiththepreviousoneallows
freedom, thusimplying thenecessity of a Gaussian component. us tobe confident with theexistence of theO VII complex.
Nucita et al.: XMM-Newton and Swift observation of 4U1700+24 5
Table3.ForeachoftheXMM-Newtonobservations,wegivethemainbest-fitparameters(normalization,centroidpositionenergy
Eobs,line width σobs, and wavelength λobs) of theemission feature observed at ≃19 ˚A (see text for details).
OBS.ID MJD N2 Eobs σobs λobs ∆λ/λ v
(day) (cm−2s−1) (keV) (keV) (˚A) kms−1
0155960601 52498.21 (9.0±1.0)×10−4 0.646+−00..000011 0.006+−00..000011 19.195+−00..003300 0.012+−00..000012 3600+−360000
0151240301 52705.58 (1.1±0.2)×10−3 0.651+−00..000011 0.008+−00..000011 19.048+−00..003300 0.004+−00..000022 1200+−660000
0151240201 52707.99 (8.4±1.0)×10−4 0.646+−00..000011 0.006+−00..000011 19.195+−00..003300 0.012+−00..000022 3600+−660000
0151240401 52865.22 (9.9±1.0)×10−4 0.648+−00..000011 0.007+−00..000011 19.136+−00..003300 0.009+−00..000012 2400+−360000
less than ≃ 1) of emission lines in the positions where the ing that the use of the modelEDGE in XSPEC to account
O VII complex lines are expected; this makes us confident for its shape did not dramatically improve our best-fit.
thatthelineidentifiedat≃0.19˚AisreallytheOVIIILy-α The existence of something missing in the model at
transition red-shifted by an average red-shift of ≃0.009. ≃ 0.6 keV is particularly clear in the residuals associated
to the 2003 EPIC spectra; we interpret this feature as the
data and folded model fingerprint of the O VIII Ly-α observed in the RGS data.
Thus, our best-fit model consisted of an absorbed black
body plus a Comptonization to which two emission lines
3 (the instrumental feature at ≃0.5 keV and a real emission
line at ≃ 0.6 keV) were added. For convergence purposes,
we left the centroid energy (and the associated width) of
V−1 thebroadfeatureobservedat≃0.5keVasfreeparameters,
e
nts s k−1 2 btoutthfiexevdalutheeopbotasiitnieodnwanhdenwaindathlyzoifntghReGOSV1IIaInLdy2-αsplience-
u
d co tra (see Table 3). With reference to Fig. 4 (left panel), the
alize black, green, blue, and red data points correspond to the
m EPICpndataoftheobservations0155960601,0151240301,
or
n 1 0151240201, and 0151240401, respectively. The pile-up af-
fected most of the data sets, so that the MOS 1 and MOS
2 data (purple and cyan points in the same figure) were
only available for the last observation. The EPIC best-fit
parameters (χ2 = 1.3 for 1379 degree of freedom - d.o.f)
0
0.555 0.56 0.565 0.57 0.575 0.58
arereportedinthefirstfourrowsofTable4.Here,kT is
Energy (keV) BB
the temperature of the black-body component, kT and τ
c
the temperature and optical depth of the Comptonization,
Fig.3. A zoom around theO VII triplet (data points rebinned
E and σ the broad feature (at ≃ 0.5 keV) position and
in order to have a signal-to-noise ratio of 5 in each bin) and 1 1
thebest-fitmodelsuperimposedafterred-shiftingtherest-frame linewidth,respectively.Allthenormalizationsofthemodel
energy of the complex by ∆λ/λ ≃ 0.009, i.e. the energy found components are free to vary. In particular, NBB, NC, N1
when analyzing theO VIIILy-α. andN2 arethe blackbody, the Comptonization,the broad
feature and the O VIII Ly-α line normalizations, respec-
tively. In the BBODYRAD normalization, R and D
km 10
arethesourceradius(inunitsofkm)anddistance(inunits
of 10 kpc), respectively. For the COMPST normalization,
4.2. XMM-Newton EPIC spectral analysis of 4U1700+24
N represents the total number of photons from the source
Following Masetti et al. (2002) and Tiengo et al. (2005), and f a factor depending on the injected photon energy
we simultaneously fitted all the low-resolution spec- and spectral index.
tra of 4U1700+24 with an absorbed black body plus In Table 5, we also give the flux in the same band for
Comptonization (COMPST in XSPEC). As in the case eachoftheXMM-Newtonobservations(firstfourrows)and
of the RGS analysis, we fixed the Galactic neutral hydro- theestimatedfluxesintheenergybands0.3−2keV,2−10
gen column density to the average value observed towards keV, and 0.3−10 keV , respectively. We note that the ab-
the target, i.e. 4.4×1020 cm−2 (Dickey & Lockman 1990). sorbed 0.3-10keV band flux results in 2.35×10−10 erg cm
However, we noted the existence of large residuals at low −2 s −1 when averaged over the four XMM-Newton obser-
energies (close to ≃ 0.5 keV and ≃ 0.6 keV) particularly vations.As canbe seeninthe table,the sourcewasin high
similartothetypicalshapeofemissionlines.Theexistence state during the 2002 observation (see also Tiengo et al.
ofabroademission(σ ≃0.1keV)line at≃0.5keVwasal- 2005),becausethefluxwasafactorof2largerthantheav-
readynotedbyTiengoetal.(2005)andexplainedasdueto eragevalue. We alsonote that archivalEXOSAT, ROSAT,
the blending of several lines possibly observed in the RGS ASCA, RXTE, andBeppoSAX observations(spanning the
data, but no other emission line feature was reported. We years1985-1998)haveshownthatthe X-rayemissionfrom
note that the feature observed at ≃ 0.5 keV may be due the source appears to become harder as its luminosity in-
to the instrumental absorption oxygen edge at 23 ˚A as re- creases (see e.g. Table 2 in Masetti et al. 2002). However,
centlyfoundbyArmas-Padillaetal.(2013).Forsimplicity, we did not find this behaviour in the XMM-Newton data
we model this feature as a large Gaussian line after verify- analysedin the present paper.We also note that, based on
6 Nucita et al.: XMM-Newton and Swift observation of 4U1700+24
Table 4. The best fit model describing the EPIC data (first four rows) and Swift/XRT data (last seven rows) consists in an
absorbed black body plus a Comptonization to which two emission lines (at ≃ 0.5 keV and ≃ 0.6 keV) were added (see text for
details).
ObsID kTBB NBB kTc τ NC E1 σ1 N1 N2
(keV) R2 /D2 (keV) Nf/4πD2 (keV) (keV) (cm−2s−1) (cm−2s−1)
km 10
0155960601 0.90+−00..0096 9.00+−12..1363 2.11+−00..0086 34.56−+02..9230 0.0183+−00..00000074 0.464+−00..002181 0.131+−00..000294 0.017+−00..000013 ≤0.0004
0151240301 0.82+−00..0073 6.94+−11..7369 2.12+−00..2009 27.93−+22..6688 0.0072+−00..00000056 0.485+−00..002208 0.093+−00..003254 0.005+−00..000021 0.0007+−00..00000054
0151240201 1.13+−00..1100 2.66+−00..2246 2.45+−00..4252 24.00−+13..8680 0.0071+−00..00000032 0.490+−00..001121 0.043+−00..001142 0.0019+−00..00000044 0.0010+−00..00000011
0151240401 0.71+−00..0055 6.98+−21..2451 1.96+−00..0055 33.07−+11..8287 0.0075+−00..00000024 0.487+−00..000282 0.108+−00..001180 0.007+−00..000011 0.0007+−00..00000013
0009080001 1.02+−00..1018 0.50+−00..1100 (2.16) 16.29−+32..4849 0.0008+−00..00000011 – – – 0.0003+−00..00000011
0009080002
0054025500 0.89+−00..1193 5.02+−31..2906 (2.16) 28.96−+23..8180 0.0070+−00..00000089 – – – 0.0012+−00..00000055
0054025501 1.05+−00..2283 2.3+−21..00 (2.16) 27.34−+59..3740 0.0040+−00..00000085 – – – 0.0009+−00..00000051
0054025502 0.77+−00..1170 5.6+−42..16 (2.16) 32.58−+74..9900 0.0031+−00..00000085 – – – 0.0011+−00..00000055
0054025503 0.98+−00..3106 4.7+−31..59 (2.16) 31.94−+93..3900 0.004+−00..000011 – – – 0.0013+−00..00000066
0054025504 0.88+−00..2197 1.4+−10..47 (2.16) 28.90−+44..2880 0.0020+−00..00000033 – – – 0.0011+−00..00000033
a set of Chandra/HRC observations, the source again ap- radiusthatisatleastonefourthofthatestimatedwiththe
pearedinahighstateinApril2005(witha2-10keVfluxof 2003 XMM-Newton data.
≃2×10−10 erg cm −2 s −1 corresponding to a luminosity
of ≃4×1033 erg cm s −1, Masetti et al. 2006).
4.4. XMM-Newton and Swift/XRT temporal analysis results
We also note that the black-body component required
by the best-fit model allowed us to estimate the radius of As discussed before, when extracting the light curve of the
theX-rayemittingregiontobe≃30−130m,i.e.consistent target,wedidnotfilteroutanyperiodoflargebackground
with the expected size of a polar cap emission in a NS. activityasthisproceduremightintroducespuriousfeatures
inthetiminganalysis.Hence,weusedtheoriginaleventlist
(corrected for the solar system barycenter when needed)
4.3. Swift/XRT spectral analysis results files and extracted the light curves in the 0.3 − 10 keV
energy band (and bin size of 10 seconds) for the source
The 2010 and 2012 Swift/XRT source spectra (as well as and background. In this procedure, we avoided using the
the correspondingbackground,ancillary,andresponsema- data affected by severe pile-up while, for the observation
trixfiles)wereimportedwithinXSPEC,groupingtogether 0151240401,all the EPIC cameras were used to get a final
the spectra acquired in 2010. We note that when a row (averaged) light curve. The obtained (synchronized) light
is empty, each parameter remains unchanged with respect curves were given in input to the SAS task epiclccorr to
to the previous value. Each spectrum was rebinned with accountforthebackgroundsubtractionandfortheabsolute
a minimum of 25 counts per energy bin. We first tried to and relative corrections. The EPIC light curves associated
use the model providing the best-fit for the XMM-Newton withthe fourobservationsanalysedinthis work(see Table
EPIC data (see the previous section), but soonwe realized 1) are shown in the four first panels of Fig. 5. During the
that the Gaussian line component at ≃ 0.5 keV is not re- fourXMM-Newtonobservations,thetargethadanaverage
quired. On the contrary, residuals appeared at ≃ 0.6 keV, count rate of 105±7 count s−1, 35±4 count s−1, 28±3
thus forcing us to maintain a Gaussian component to ac- count s−1, and 18±2 count s−1, respectively: clearly, as
count for this line feature: in particular, we fixed (as be- alsodiscussedbefore,thetargetwascharacterizedbyahigh
fore) the line centroid energy to the average value ≃0.646 state during the 2002 observation.
keV. In order to avoid bad convergences, we fixed the the For 2010 and 2012 Swift/XRT observations, we ex-
temperatureparameterkTc oftheCOMPSTcomponentto tractedthe sourceandbackgroundlightcurves(in the 0.3-
the average value (≃ 2.16 keV) estimated with the XMM- 10 keV energy band and bin-size of 10 sec) by using the
Newton EPIC data only, while leaving the optical depth xselect, accounting for the pile up when necessary. Since
τ and the normalizations of the model components free to these light curves were also synchronized, we subtracted
vary. thebackgroundandscaledfortheextractionareasbyusing
The best-fit procedure(χ2 =1.2 for 621d.o.f.) resulted the lcmath tool. The Swift/XRT light curves are reported
in the parameter values reported in the last seven rows of inthe lasttwopanelsofFig.5with,inparticular,the2010
Table 4 (from which one can note that the spectral prop- time series on the left and the 2012 on the right part of
erties of 4U1700+24 did not change substantially) and in the figure, respectively. We note that 4U 1700+24 had an
the estimates ofthe 0.3-2.0keV,2.0-10.0keV,and0.3-10.0 average count rate of 0.27±0.23 count s−1 in 2010, and
keV band fluxes (see Table 5), respectively. The best-fit 0.47±0.43 count s−1 in 2012.
modelis superimposedonthe Swift/XRT datain the right The source is clearly variable and we gave an estimate
panel in Fig. 4. As one can note, in accordance with the of this temporal variability by using the normalized excess
XMM-Newton data, a decrease in the X-ray luminosity is variance(σ2 ;seee.g.Nandraetal.1997andEdelsonet
NXS
always accompanied by a decrease in the emitting region al. 2002) to which we associated an uncertainty according
size. In particular, the 2010 observations clearly show that toeq.(11)inVaughanetal.(2003).IneachpanelinFig.5,
the black-body component may come from an area with a we give the excess variance for the EPIC 0.3-10 keV light
Nucita et al.: XMM-Newton and Swift observation of 4U1700+24 7
data and folded model data and folded model
0.1
V−1 1 V−1
normalized counts s ke−1 00.0.11 normalized counts s ke−1 0.01
10−3
10−3
2
2
0
0
χ χ
−2
−2
−4
−4
0.5 1 2 5 10 0.5 1 2 5
Energy (keV) Energy (keV)
Fig.4.Leftpanel:theEPICspectraof4U1700+24 duringthefourobservationsanalysedinthiswork.Black,green,blue,andred
datapointscorrespondtotheEPICpndataoftheobservations0155960601,0151240301,0151240201,and0151240401,respectively.
Since the pile-up affected most of the data sets, MOS 1 and MOS 2 data (purple and cyan data points) were only available for
the last observation (i.e. 0151240401). Here, the solid lines correspond to the best-fit model described in the text. Right panel:
theSwift/XRTspectraof4U1700+24 duringthe2010and2012 observations(datapoints)togetherwiththebest-fitmodel(solid
lines).Here,theblackandreddatacorrespondtotheSwiftobservations0009080001 and0009080002, respectivelywhiletheother
datasetsshown(purple,blue,yellow,greenandcyan)correspondtotheobservationIDsfrom0054025500 to0054025504 ofTable
4.
Table 5.The0.3−2keV,2−10keVand0.3−10keVbandfluxestogetherwiththeestimatedluminosity(fullband)forasource
distance of ≃420 pc.
ObsID MJD F0.3−2.0kev F2−10.0kev F0.3−10.0kev L0.3−10.0kev
(day) (10−10 ergcm−2 s−1) (10−10 ergcm−2 s−1) (10−10 ergcm−2 s−1) (1034 ergs−1)
0155960601 52498.22 0.669+−00..000261 4.35+−00..0122 5.02+−00..0137 1.060+−00..001400
0151240301 52705.58 0.290+−00..000251 1.23+−00..0129 1.52+−00..0136 0.320+−00..001400
0151240201 52707.99 0.245+−00..000054 1.25+−00..0019 1.49+−00..0018 0.310+−00..000220
0151240401 52865.22 0.284+−00..000024 1.41+−00..0013 1.69+−00..0014 0.355+−00..000120
0009080001 55346.92 0.031+−00..001048 0.080+−00..001057 0.110+−00..002100 0.023+−00..001036
0009080002 55351.78
0054025500 56263.38 0.246+−00..000270 1.30+−00..0160 1.54+−00..0163 0.324+−00..001237
0054025501 56263.76 0.145+−00..000166 0.773+−00..019436 0.920+−00..118600 0.193+−00..003384
0054025502 56264.79 0.143+−00..000156 0.739+−00..014452 0.880+−00..111600 0.185+−00..002334
0054025503 56265.23 0.186+−00..000377 1.09+−00..0282 1.27+−00..1207 0.267+−00..002517
0054025504 56266.66 0.077+−00..000132 0.373+−00..002896 0.450+−00..003903 0.095+−00..000260
curve with a bin-size of 10 seconds. Keeping in mind that odogramandcalculatedthe global probability as explained
negative values of σ2 indicate the absence of or very in Benlloch et al. (2001). In particular, the global signifi-
NXS
small variability in the time series, we conclude that the cance ofa peak atagivenfrequency andwithgivenampli-
4U1700+24 light curve shows a certain degree of intrinsic tudeisevaluatedbycountingthenumberofpeakswiththe
variabilitywhichseemstobeconstantintime.Consistently same height (or larger) in the full range of tested frequen-
with the results of Masetti et al. (2002), a variability on cies. As a result, we did not detect any clear periodicity
time-scalesoftenstothousandsofsecondscanbeidentified with significance larger than ≃1σ.
in the high-energy light curve.
We searchedforperiodicmodulationsinthe time range
5. Discussion and conclusions
from 20 s to a few hours by using the the Lomb-Scargle
technique (Scargle 1982). We tested the significance of The large collecting area of the XMM-Newton telescope
eachpeak observedin the periodogramby simulating 5000 allowed us to study in detail the accreting binary 4U
simulated red-noise light curves each of which with the 1700+24.By using the high spectralresolutionofthe RGS
same statisticalproperties(mean, variance,time gaps,and instruments, we identified an emission line at ≃ 0.6 keV
red-noise index) as the observed time series following the (firstobservedbyTiengoetal.2005in2002),alsopresentin
method described in Timmer & Koenig (1995). For each the pn spectrum of the source.We associatedthe observed
simulatedlight curve,we evaluated the Lomb-Scargleperi- emissionfeaturewiththeOVIIILy-αtransitionred-shifted
8 Nucita et al.: XMM-Newton and Swift observation of 4U1700+24
Fig.5. The XMM-Newton and Swift/XRT light curves associated to all the observations analysed in this work (see Table 1)
are shown in separated panels. In each panel, we also give the estimate of the excess variance of the X-ray signal during each
observation. Time is in daysfrom thebeginning of the 2002 XMM-Newton observation.
by the quantity ∆λ/λ=0.009±0.001 as obtained averag- tational red-shift of the photons emitted by a plasma blob
ing the red-shift factors estimated in each single pointing at distance R from an object with mass M, i.e. ∆λ/λ =
(see Fig. 2). 1/(g )0.5−1withg =1−2GM/Rc2.Ascanbeseen,the
00 00
We also searched in the range 5−35 ˚A for the most possibility that 4U 1700+24hosts a white dwarfcaneasily
be ruled out because, for the typical values of white dwarf
intense lines of He-like ions of oxygen: the transitions be-
mass and radius (M ≃ 1 M and R ≃ 2×104 km), the
tween the n = 2 shell and the n = 1 ground state shell as ⊙
the resonance line, r: 1s2 1S −1s2p 1P , the two inter- expected gravitational red-shift is a factor of 10 (or more)
0 1
combinationlines(oftenblended),i: 1s21S −1s2p3P , lowerthantheobservedvalue.InagreementwithGarciaet
0 2,1
andtheforbiddenfeature,f : 1s21S −1s2s3S .However, al.(1983),thissupportstheideathat4U1700+24isaneu-
0 1
tron star that accretes matter from a red giant. Assuming
because of the poor statistics of the RGS data, a blind fit
a neutron star mass of ≃ 1.4 M in the 4U 1700+24 bi-
procedure to the RS data around the O VII complex with ⊙
nary system, the detected red-shift range corresponds to
a model constituted by a power law and three Gaussians
thegravitationalred-shiftofaphotonemittedatadistance
(with all the parameters free, except the relative distances
of 160− 1000 km from the central object, i.e. consistent
among the lines and the continuum power law index) did
with the value found by Tiengo et al. (2005) when ana-
not converge. Hence, we fixed the centroid energy of the
lyzing the 2002XMM-Newton observation.Furthermore,a
interested lines to that expected by the atomic physics af-
close inspection of Fig. 2 allows us to conclude that the
ter correcting for the average red-shift previously found.
red-shift of the O VII Ly-α line is variable on a time scale
Interestingly, the RGS data show the existence (although
of few days (see the log of the observations in Table 1). In
with a small signal-to-noise ratio) of emission lines in the
particular,the red-shifts estimated for the centralobserva-
positions where the O VII complex lines are expected: this
makesusconfidentthatthelineidentifiedat≃0.19˚Aisthe tions0151240301and0151240201are≃0.004and≃0.012,
respectively. Since these estimates differ from the average
OVIIILy-αtransitionred-shifted(onaverage)by≃0.009.
red-shift value by more than 3−5σ, we are confident that
A red-shift of the O VIII Ly-α line in the range
the effect is real. Excluding Doppler contributions due to
0.002−0.013 (see the estimated values given in Table 3)
the orbital motion of any blob of plasma around the neu-
can be explained (see also Tiengo et al. 2005)as the gravi-
Nucita et al.: XMM-Newton and Swift observation of 4U1700+24 9
tron star (as the associated signatures would be different is the 0.5 − 8 keV luminosity and E˙ is the spin-down
to the observations presented here), we conclude that we energy rate (related to the NS period P and first pe-
are witnessing the re-organization of matter at a distance riod derivative P˙ by E˙ ≃ 1046P˙/P3 in cgs units), and
of a few hundred kms around the accreting object. An al- assuming an average efficiency η = 10−4, we obtained
ternative picture would be a jet of matter (with typical E˙ ≃ 1037−1038 erg s−1. Since the measured P˙ values are
velocity of 1000−4000 km s−1) possibly emitted away by in the range ∼ 10−13−10−20 s s−1 (see the on-line cata-
the neutron star in an almostface-on system. The alterna- logue http : //www.astro.ufl.edu/ anuviswanathan/cgi−
tive condition seems to be required by the apparent lack bin/psrcat.htm and also Becker 2001), we get P in the
of any periodicity and/or modulation (as we have verified range 2× 10−4 − 10−1 s, which is clearly not detectable
via a Lomb-Scargle analysis) in the observed X-ray light inthe XMM-NewtonandSwift dataanalysedin this work.
curve. However, as also observed by Tiengo et al. (2005), Interestingly enough, assuming a polar cap model for the
thepuzzlinglackofanyblue-shiftedcomponentimpliesthe wind accreting NS (see e.g. Becker et al. 2012), it is possi-
necessityofanad-hocgeometrytoexplaintheobservations ble to estimate the NS surface magnetic field which turns
or one could invoke a uni-polar jet emitted by the neutron out to be <2×1010 G, i.e. in agreement with the typical
∼
star.
magnetic field values of the X-ray emitting pulsars.
Based on these facts, we prefer a scenario in which the
Obviously, a long XMM-Newton observation or a
mass coming from the M-type companion stellar wind (see
plannedexposurewiththepncameraintimingmodecould
Postnov et al. 2011 and references therein for details on
allow us to infer the NS period.
thewindaccretioninsymbioticX-raybinaries)iscaptured
directly onto a small zone of the NS surface. The X-ray Acknowledgements. This paper is based on observations by XMM-
photons emitted are reprocessed by a blob of matter at a Newton, anESAsciencemissionwithinstruments andcontributions
few hundred kms from the NS surface so that the output directlyfundedbyESAMemberStatesandNASA.Partofthiswork
isbasedonarchivaldata,softwareoron-lineservicesprovidedbythe
emission features are gravitationally red-shifted.
ASIScienceDataCenter(ASDC),Italy.AANisgratefultoSaraA.A.
Weestimatedthe0.3-10keVbandfluxtobe2.35×10−10 Nucitafortheinterestingdiscussionswhilepreparingthismanuscript.
erg cm −2 s −1 when averagedoverthe four XMM-Newton MDS thanks the Department of Mathematics and Physics E. De
observations. In Table 5 we also give the flux in the same Giorgi at the University of Salento and the astrophysics group for
thehospitality.
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