Investigating the nature of absorption lines in the Chandra X-ray spectra of the neutron star binary 4U 1820-30

0.3 MB·English

by  E. M. Cackett

DRAFTVERSIONFEBRUARY4,2008
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
INVESTIGATINGTHENATUREOFABSORPTIONLINESINTHECHANDRAX-RAYSPECTRAOFTHENEUTRON
STARBINARY4U1820- 30
E.M.CACKETT1,2,J.M.MILLER1,J.RAYMOND3,J.HOMAN4,M.VANDERKLIS5,
M.MÉNDEZ5,6,7,D.STEEGHS3,R.WIJNANDS5
DraftversionFebruary4,2008
ABSTRACT
We use four Chandra gratings spectra of the neutron star low-mass X-ray binary 4U 1820- 30 to better
understandthe nature of certain X-ray absorption lines in X-ray binaries, including the NeII, NeIII, NeIX,
OVII,andOVIIIlines. Theequivalentwidthsofthelinesaregenerallyconsistentbetweentheobservations,
8 asexpectediftheselinesoriginateinthehotinterstellarmedium. Noevidencewasfoundthatthelineswere
0 blueshifted,againsupportingtheinterstellarmediumorigin,thoughthismaybeduetopoorstatistics. There
0 is apparentvariabilityin the OVIII Lyα line equivalentwidth providingsomeevidencethatat least some of
2 theOVIIIabsorptionariseswithinthesystem. However,thesignificanceismarginal(2.4-σ),andthelackof

n variationinthe otherlinescasts somedoubtonthe realityofthevariability. Fromcalculatingtheequivalent
a hydrogencolumndensitiesforarangeofDopplerparameters,wefindtheyareconsistentwiththeinterstellar
J originofthelines. Additionallywefitthespectrawithphotoionizationmodelsforlocallyabsorbingmaterial,

9 and find that they can reproduce the spectrum well, but only when there is an extremely low filling factor.
WeconcludethatboththeISMandlocalabsorptionremainpossiblefortheoriginofthelines,butthatmore

2 sensitiveobservationsareneededtosearchforlow-levelvariability.
v Subjectheadings:accretion,accretiondisks—stars: neutron—X-rays: individual(4U1820- 30)—X-rays:
1
binaries—X-rays:ISM
7
3
3
1. INTRODUCTION blackholesystemsalsoshowevidenceforhighly-ionizedout-
0 flows such as H 1743- 322, GX 339- 4 and GRS 1915+105
7 High-resolution X-ray spectroscopy from Chandra and
(Milleretal. 2006b, 2004; Leeetal. 2002). Disk winds are
0 XMM-Newton allows for the study of absorption and emis-
also observed from the accretion disks around supermassive
/ sion lines in Galactic X-ray binaries. While emission lines
h black holes in Active Galactic Nuclei (e.g. Crenshawetal.
are clearly associated with the sources themselves (e.g.,
p 1999;Kaastraetal.2000;Kaspietal.2000).
Cottametal. 2001a,b; Schulzetal. 2001), that is notalways
- Several neutron star low-mass X-ray binaries also display
o the case for absorption lines. In some systems, the absorp-
clear evidence for absorption local to the source. For ex-
r tionlinesarealso clearlyassociatedwiththe source,asthey
t ample, GX 13+1 shows blueshiftedabsorptionlines indicat-
as a2r0e0o2b;Psearrvmeadrteotbael.v2a0r0ia2b;lUeeadnad/eotrabl.lu2e0s0h4if;tBedoi(rei.ng.e,tLael.e2e0t0a5l;. ing an outflow with a velocity of ∼400 km s- 1 (Uedaetal.
: 2004).Moreover,intheX-raydippingsources,strongabsorp-
v DíazTrigoetal. 2006; Milleretal. 2006a,b). However, in
tion lines are observed as these objects are inclined close to
i other sources the lines are interpreted as being associated
X the orbital plane (e.g., Sidolietal. 2001; Parmaretal. 2002;
withabsorptionbythehotionizedinterstellarmedium,which
Boirinetal.2005;Churchetal.2005;DíazTrigoetal.2006).
r is thought to be present in the disk and halo of the galaxy,
a with temperatures around 106 K (e.g., Futamotoetal. 2004; On the other hand, the nature of the absorption lines ob-
servedinmanyotherneutronstarX-raybinariesislessclear
Yao&Wang2005;Juettetal.2006,andreferencestherein).
- the lines are weaker and there is no evidence of signifi-
From studies of the absorption lines in the black hole X-
ray binary GRO J1655- 40, the plethora of highly-ionized cant blueshifts, supporting the interpretation that these lines
are due to the interstellar medium (e.g., Yao&Wang 2005;
lines that are detected are all seen to be blueshifted – ev-
Juettetal.2006).
idence for a mass outflow into our line of sight, presum-
AspartoftheChandraHETGSZ/AtollSpectroscopicSur-
ably from the accretion disk. In fact, detailed modeling of
vey(CHAZSS)wehaveobtainedsensitiveobservationsof6
the lines in this system suggest that magnetic fields may
neutronstarlow-massX-raybinariesat2separateepochsus-
be the force driving the wind (Milleretal. 2006a). Other
ingtheChandrahighenergytransmissiongratingspectrom-
Electronicaddress:[email protected] eter (HETGS). The multiple epochs allow for a search for
1DepartmentofAstronomy,UniversityofMichigan,500ChurchSt,Ann variability in the absorption lines present in these systems;
Arbor,MI48109-1042,USA this is key for learning about the nature of the absorption
2DeanMcLaughlinPostdoctoralFellow
lines. In this paper, we present the observations of the sys-
3Harvard-Smithsonian CenterforAstrophysics,60GardenStreet,Cam-
bridge,MA02138,USA tem4U1820- 30fromthissurvey. Tightlimitscanbeseton
4MITKavliInstituteforAstrophysicsandSpaceResearch,MIT,70Vas- the size of any absorbing region associated with a binary if
sarStreet,Cambridge,MA02139-4307 the system is ultra-compact in nature. Therefore, the ultra-
5Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, compactnatureof4U1820- 30(Stella,Priedhorsky,&White
Kruislaan403,1098SJ,Amsterdam,theNetherlands
1987) makes it a particularly interesting source to study ab-
6SRON-NetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584
CAUtrecht,theNetherlands sorptionlines.
7AstronomicalInstitute,UniversityofUtrecht,POBox80000,3508TA
Utrecht,TheNetherlands
2 Cackettetal.
2. PREVIOUSCHANDRAOBSERVATIONSOF fluxacrosstheline). Currently,thereisnounambiguousway
4U1820- 30 to model pileup in Chandra gratings spectra. To minimize
4U 1820- 30 is an accreting neutron star low mass X- pileup we operated Chandra with the High Energy Trans-
mission Grating Spectrometer in continuous clocking mode
ray binary in the globular cluster NGC 6624, which has a
welldetermineddistance(7.6±0.4kpcKuulkersetal.2003) tosignificantlyincreasethetemporalresolution. Continuous
and reddening (E(B-V) = 0.32±0.03 Kuulkersetal. 2003; clocking mode allows milli-second timing at the expense of
onedimensionofspatialinformation.
Bohlinetal.1978). Thisreddeningisconsistentwiththeto-
AspartofCHAZSS,4U1820- 30wasobservedtwicewith
tal HI column density of our Galaxy in this direction deter-
minedfrom21cmradioobservations,N =1.5×1021 cm- 2 Chandrafor approximately25 ksec each time, first on 2006
H Aug 12 (ObsId 6633), and later on 2006 Oct 20 (ObsId
(Dickey&Lockman1990). Weadopttheabovecolumnden-
6634). The HETGS-dispersed spectrum was read-out with
sitythroughoutthispaper.
theACIS-Sarrayoperatingincontinuous-clockingmode.We
Absorption lines and edges in the X-ray spectrum of
reduced these data using CIAO version 3.3.0.1 and follow-
this source have been studied previously by Futamotoetal.
ing the standard analysis threads8. Using the tgextract tool
(2004), Yao&Wang (2005) and Yao&Wang (2006) using
spectrathe+1and-1orderwereextractedatthenominalin-
an LETG/HRC Chandra observation (ObsId 98), and by
strument resolution for both the high-energy grating (HEG)
Juettetal.(2004),Yao&Wang(2005),Juettetal.(2006)and
and the medium-energy grating (MEG). Using the standard
Yao&Wang (2006) using HETG/ACIS Chandra observa-
redistributionmatrixfiles(RMFs) fromtheCALDB, the an-
tions (ObsIds 1021 and 1022). A summary of the Chandra
cillaryresponsefiles(ARFs)weregeneratedusingthefullgarf
observations of this source is given in Table 1. We briefly
script. We carefullyinspected the MEG +1 and -1 ordersto
summarizethemainresultsofthesepreviousstudiesbelow.
confirm there was no wavelength shift between the two or-
Futamotoetal.(2004)clearlydetecttheOVIIHeα,OVIII
ders. The spectra and ARFs were then added together us-
Lyα,andNeIXHeαabsorptionlinesintheLETGspectrum,
ingtheadd_grating_spectra,givingafirst-orderspectrumfor
andputupperlimitsontheOVIIHeβ line. Fromtheircurve
theMEG.Similarly,wecreatedafirst-orderspectrumforthe
ofgrowthanalysisandphotoionizationmodeling,theydeduce
HEG.
thatalloxygenwillbefullyphotoionizediftheabsorbingcol-
In addition to analyzing these two new Chandra observa-
umn is located close to the binary system, and therefore at-
tions, we also re-analyzethe two previousChandraHETGS
tribute these lines to hot gas in the ISM. However, we note
observationsof 4U 1820- 30(ObsId 1021 & 1022) to allow
that they make two tacit assumptions - that the filling factor
forasearchforvariabilityintheabsorptionlines. Thesetwo
ofthegasis1,andthatalltheabsorptionarisesfromthesame
datasetswerenotoperatedincontinuous-clockingmode,and
gas.
suffer from pile-up. We reduce the data following the stan-
ThestudiesbyJuettetal.(2004)andJuettetal.(2006)are
dardanalysisthreads, extractingthe spectraandARFs using
concernedwiththeabsorptionedgesfromoxygen,neonand
the same methoddescribedabove. For consistency,we only
iron,comparingthespectraofanumberofX-raybinaries. In
analysis the HETGS observations here and not the LETGS
Juettetal. (2004) the structure around the oxygen edge in 7
observationwhichhasalowerspectralresolution.
X-ray binaries is studied, looking at the OI, OII, and OIII
absorptionlines in thatregion. The edgeand these low ion-
izationlinesareattributabletoISMabsorption. InJuettetal. 4. ANALYSISANDRESULTS
(2006)theneonandironLedgesarestudied,andthenature 4.1. Absorptionlines
of the neon absorption lines discussed. These authorscome
Whensearchingforabsorptionlines,weuseonlytheMEG
to the conclusion that the neon lines in the low-mass X-ray
spectra, as the MEG has a higher effective area over the re-
binaries studied are consistent with predictionsfor the ISM,
gionofinterestcomparedtotheHEG.Weinspectedthespec-
exceptfortheNeIXlinedetectedintheblackholeX-raybi-
nary GX 339- 4 Milleretal. (2004) which shows a column traover2Åsegmentsfrom2Åto24Å usingtheISISspectral
density significantlyhigherthan predictedfor the ISM, con- fitting package. In each segment, we fitted a simple power-
firmingthatthelargestcontributiontothisline(inthissource, law(modified,whereappropriate,byanabsorptionedgedue
atleast)isfromlocalabsorption. to neutral elements in the ISM). Within each segment, we
Fromastudyof10low-massX-raybinaries,Yao&Wang searchedforanysignificantabsorptionlinespresent. We fit-
(2005)findthatthedetectedNeIX,OVIIandOVIIIabsorp- tedaGaussiantothedetectedabsorptionlineswhosecentral
tionlinesareconsistentwiththehotISMorigin.Yao&Wang wavelength,width,andnormalizationwereallallowedtobe
(2006) focuses specifically on 4U 1820- 30, discussing the freeparameters. The equivalentwidth of the line was deter-
implicationsoftheNeandOabundanceintheISMfromthe mined from the fitted Gaussian. For line identifications we
absorptionlines/edgesintheChandraobservations. use Verner,Verner,&Ferland (1996) as a reference. How-
ever,forNeIIandNeIIIweusethewavelengthsdetermined
by Juettetal. (2006). They find the weighted mean of the
3. OBSERVATIONSANDDATAREDUCTION
central wavelengths of these lines measured in multiple X-
All the previousobservationsdiscussed above suffer from ray binaries. The wavelengths for these lines from different
pileup,whereahighcountrateleadstomorethanonephoton sourcesareseentoconsistentwitheachother,andareallsys-
arrivingata pixelperreadout. Thisaffectstheshapeofthe tematicallyshiftedbyasmallamount(∼20mÅ)fromthethe-
spectrum, typically makingit harderas the sum of the ener- oreticalvaluedeterminedbyBehar&Netzer(2002), though
gies of the multiple photonsthat arrivedbetween each read- within their stated errors. The Juettetal. (2006) values are
outisdetectedasonecount. Itisunclearhowpileupaffects knownmostaccurately,thusweusethemthroughouttherest
the measurement of absorption lines. It likely modifies the ofthepaper.
continuumleveland shape, which wouldin turn changeany
measuredequivalentwidths(whichdependsonthecontinuum 8http://http://cxc.harvard.edu/ciao/threads
X-rayabsorptionlinesin4U1820- 30 3
In the two new Chandra observations, we detect several would be expected from the oscillator strengths. We do not
X-ray absorption lines. We define a detected line as one determine upper limits for these lines from the ObsId 1021
for which fits with a Gaussian give an equivalentwidth that and 1022 datasets, as they are substantially less sensitive,
excludes zero at more than the 2σ level of confidence (note andthereforethelinescouldnotbedetectedatthepredicted
thatwearebeingconservativebyusingthesignificancewith equivalentwidths. Whiletheupperlimitsdonotruleoutthat
whichwecanmeasuretheequivalentwidthasthesignificance thelineissaturated,wecannotbeconclusive.
of the detection). In increasing wavelength (theoretical/best Similarly,wetrytodeterminewhethertheOVIIIlinesare
observationalwavelengthsaregiveninbrackets),thedetected saturated by determining limits on the O VIII Lyβ line at
linesare: NeIX(13.447Å),NeIII(14.508Å),NeII(14.608 16.006Å. Fitting a Gaussian, with central wavelength fixed
Å),OVII(18.629Å),OVIII(18.967Å).Alltheseabsorption at 16.006Å to ObsId 6633 and 6634 we measure equivalent
lines are detected in both observations, however, some were widthsof3.0±1.5mÅ,and3.4±2.8mÅrespectively. This
notdetectedinthepreviousChandraobservations,likelydue givesthe ratio of the equivalentwidths of the OVIII Lyβ to
tothelowersensitivityofthoseshorterobservations. Whilst Lyαas0.47±0.32and0.18±0.16forObsId6633and6634
the OI absorptionline at ∼23.5Å was detected in all obser- respectively.Forcomparison,ifthelinesarenotsaturatedwe
vations,wehavenotattemptedtofitthislineduetothecom- wouldexpectthisratiotobe0.14giventheoscillatorstrengths
plicated structure around this regiondue to multiple oxygen and wavelengths. The measured value from ObsId 6634 is
lines. The structure in this region has been studied in detail clearlyconsistentwiththisratio,andinObsId6633theratio
previously by Juettetal. (2004). The interstellar absorption isnotsignificantlydifferent.ThereforetheOVIIIlinesarenot
atthiswavelengthalsosignificantlyincreasesthenoiseinthe highlysaturated.
data, therefore the LETG observation of 4U 1820- 30previ- Another check on saturation can be performed using the
ouslystudiedpresentthebestavailabledatasetforanalysisof measured equivalentwidth of the OVII Lyα line (measured
thisregion.WealsonotethattheOVIIlineat∼21.6Åwasnot in previous Chandra LETG observations by Futamotoetal.
detectedinanyoftheseobservations,duetothelowsignal-to- 2004) and comparing it with the weighted average of the
noise in that region. Again, the previousLETG observation O VII Lyβ equivalent widths measured here. We find that
of4U1820- 30givesthebestdetectionofthisline. the ratio of O VII Lyα equivalent width to Lyβ equivalent
Incaseswhereanabsorptionlinehadbeendetectedinone width is 5.5±2.2. Fromthe line wavelengthsand oscillator
oftheotherobservations,butnotintheobservationwewere strengths(takenfromVerneretal.1996),theexpectedratioin
studying,a95%confidencelevelupperlimitforthelineflux thenon-saturatedregimeis6.4, consistenttheratiofromthe
andequivalentwidthwasdetermined. Thiswasachievedby observedequivalentwidths,suggestingthattheOVIIlinesare
fixingthe line wavelengthat the theoreticalvalue, fixing the nothighlysaturated.
width at a value smaller than the instrument resolution (as
would be the case if the line was unresolved), and then in- 4.2. ColumnDensities
creasing the line flux until the χ2 value for the fit increased
From the measured equivalentwidths of the lines, we de-
by 2.7. We justify fixing the width smaller than the instru-
termine the column densities. Firstly we assume that the
mentresolutionasfortheMEG,theresolutioncorrespondsto
lines are not saturated and on the linear part of the curve
a FWHM of ∼350km s- 1 at 18.967Å,and higherat shorter
of growth. Under this assumption, the column density and
wavelengths. This is significantly higher than the expected equivalentwidthareassociatedviaWλ=(πe2/Mec2)Njλ2fij=
tthieesrmdeatlebrmroiandeednfinrogminththeesIpSeMct.raTlafibtltein2gg.ivInesFtihge.li1newpersohpoewr- 8.85×10- 13Njλ2fij, where Wλ is the equivalent width (in
cm),N isthecolumndensityofagivenspecies,λistheline
theabsorptionlinesdetectedin thetwo newChandraobser- j
wavelength(in cm) and f is the oscillator strength (Spitzer
vations. ij
1978).
AsNeIXisdetectedat13.447Å oneshouldalsoexpectto
However,aswecannotconclusivelydeterminewhetherthe
find additional NeIX lines at 11.547Å and 11.000Å. How- linesaresaturated,wealsocalculatethecolumndensityfora
ever,wedonotclearlydetecttheselinesinanyoftheobser-
rangeofDopplerparameters,b. Foralowerlimitfortheline
vations. If the line is not saturated, the ratio of the equiva-
broadening,we estimate the FWHM ofthe Galactic rotation
lentwidthsshouldfollowtheratiooftheoscillatorstrengths
in the directionof the sourcefrom a Besancon modelof the
timestheratioofthewavelengthssquared.Themeanvalueof Galaxytobe50kms- 1.Wealsoevaluatethecolumndensities
the equivalentwidthof the NeIX (13.447Å)line is approxi- assumingb=100kms- 1andb=200kms- 1. Thelatterbeing
mately5mÅ.Fromtheoscillatorstrengths,thiswouldputthe anupperlimitonthe widthofOVI emission linesfromUV
expectedequivalentwidthsfortheNeIXlinesat11.547Å and observationswithFUSE(Otte&Dixon2006). Giventheos-
11.000Å to be 0.8mÅ and 0.3mÅ, respectively. In Ob- cillator strength, line wavelength, observedequivalentwidth
sId 6633, we note that there may be a weak absorption line and assumed Doppler parameter, we determine the required
present at λ=11.550±0.007 Å , with an equivalent width column density for each line in ObsIds 6633 and 6634, us-
of EW = 1.8±0.9 mÅ, though this is only a 2σ detection. ing the curve of growth equations in Spitzer (1978).For the
blendedOVIIIfinestructurecomponentswecomputethecol-
Thismeasuredequivalentwidthisconsistentwithwhatispre-
umndensities numerically. Table 3 givesthe calculated col-
dicted from the oscillator strengths. The line at 11.000Å is
umndensities.
notdetected,andwedeterminethe95%confidencelevelup-
Inorderto testwhetherthesecolumndensitiesareconsis-
per limit to be 1.7 mÅ - the line would be too weak to de-
tent with the interstellar medium, we converted the column
tect given the noise in the data. In ObsId 6634 these NeIX densityforeachspeciesintoanequivalenthydrogencolumn
lines are not detected, and we find the upper limits on the
density. Todothiswerequiretheionicfractionforeachion,
equivalent width for the lines at 11.547Å and 11.000Å to whichforcollisionalionization(themechanismpresentinthe
be 1.6mÅ and 1.3mÅ respectively, consistent with what ISM) depends on the temperature. The low ionization lines
4 Cackettetal.
(NeIIandNeIII)willbeinalowertemperaturegasthanthe f =1.9×10- 7for6634. Since f scaleswithR,smallerradii
higher ionization lines (NeIX, OVII, OVIII), thus we esti- withinthesystemmaketheproblemworse.
mated the temperaturesseparately. Comparingthe observed It seems unlikely that the gas would be in a shell, as the
ratio of NeII/NeIII with the ratio of the ionic fractions for fillingfactorwouldmeanthatthethicknessoftheshellwould
thoseionsatarangeoftemperatures(ascalculatedforcolli- be very small (on the order of only tens of metres for gas
sionalionizationbyMazzottaetal.1998),wedeterminedthe within the system). Additionally it is also unlikely to be a
best-fitting temperature of the gas. Given that temperature, shell covering the entire 4π or else one would also observe
theionicfractionsatthattemperatureandtheISMabundances emission lines. One possibility is that thermal instabilities
fromWilmsetal.(2000)weconvertedtheioniccolumnden- produce small, dense structures, or alternatively there could
sitiesintoequivalenthydrogencolumndensities(seeTable3). bepartialcoveringbylargerblobs,whichcouldbedrivenby
We foundthetemperaturesforthe NeII andNeIIIgasto be a wind. Therefore, while it seems possible that locally pho-
intherange(4.8- 5.2)×104K. toionizedmaterialcouldaccountfortheabsorption,itdoesre-
Similarly for NeIX, OVII, O VIII we determined a tem- quirefairlyextremeparameters. WenotethatFutamotoetal.
peraturefromtheratiosofOVII/OVIIIandOVII/NeIX(as- (2004) use CLOUDY to run a numericalsimulation for pre-
sumingNe/OfromWilmsetal.2000) comparedtotheionic viousobservationsof4U1820- 30. Theseauthorsconcluded
fractionsfromMazzottaetal.(1998),andthenusedtheesti- thatphotoionizedgaswithin the binarysystem couldnotre-
matedionicfractionsandabundancestoconverttoequivalent producetheobservedlinecolumndensities. Insummary,the
hydrogencolumn densities (see Table 3). The temperatures ISMmaybethemorelikelysourceoftheabsorptionlines,but
wedetermineforthisgasisinthe range(1.3- 2.0)×106K, ourmodelsdemonstratethatphotoionizationcannotberuled
consistentwiththehotphaseoftheISM. out,especiallygiventhepresenceofvariability.
Theequivalenthydrogencolumndensitiesthatweestimate
4.4. Sourceflux
are all consistent with having a lower column density than
the column to the source as seen in H I (1.5×1021 cm- 1, In order to compare absorption line properties with the
Dickey&Lockman1990). sourceproperties,wedeterminethesourcefluxfromthebest
fittingcontinuummodeltotheHEGspectrum.Forthisbroad-
band spectral fitting we use XSPEC (Arnaud 1996). We do
4.3. Photoionizationmodeling
notfit the MEG and HEG simultaneouslydue to the signifi-
In order to assess the possibility of the observed absorp- cant pileup in the first two observations, where the observa-
tion lines originating from local absorption in the X-ray bi- tionswerenotoperatedincontinuousclockingmode. Asthe
nary system (rather than the ISM) we used the photoioniza- MEG has a larger effective area than the HEG at low ener-
tion code XSTAR v2.1(Kallman&Bautista 2001) to model gies(byabouta factorof 2) it is moresubstantially affected
theobservedspectruminObsID 6633and6634. Forthe in- by pileup. This is clearly apparentwhen trying to fit a con-
putionizing spectrumwe use the unabsorbedsource spectra tinuummodeltojusttheMEGspectraintheseobservations.
foreachobservation,scaledtotheobservedluminosities(see There is an excess of counts at high energies and deficit at
section4.4fordetailsondeterminingthesourceflux,weas- low energies, as expectedby pileup. The HEG is muchless
sumed=7.6kpcgiving0.5-10keVluminositiesof5.7×1037 affected,andthuswe choosetofit tojustthe HEG 1storder
and8.1×1037ergs- 1forObsID6633and6634,respectively). spectra. Forconsistency,weusethismethodforfittingtothe
Foreachobservationwethencreatedagridofabsorptionline newChandraobservationsalso,eventhoughtheydonotsuf-
models allowing the column density and ionization parame- ferfrompileup.
ter,ξ=L/nR2,tovary. Inourmodelsweassumesolarabun- We choose to fit an absorbed blackbody plus power-law
dances,acoveringfractionof0.5,andturbulentvelocitiesof modeltotheHEGspectraovertheenergyrange1.2- 8keV
200 km s- 1. From these grids of models we then fit the ob- withthecolumndensityisfixedatthegalacticvaluetowards
served spectra (again using ISIS) from 13Å to 20Å with a this globular cluster for all observations. Models to neu-
power-law(withGalacticabsorptionfixedat1.5×1021cm- 1) tron star X-ray binary spectra can be degenerate (Linetal.
convolved with the absorption line models calculated from 2007), especiallyovera shortenergyrange,such as covered
XSTAR. This allows us to determine under what conditions by the HEG. The model we adopt is fiducial and reflects a
theobservedspectrumcanbereproducedbyphotoionization physically-motivatedscenarioconsistingofthermalandnon-
oflocalabsorbinggas. thermalemission. Itallowsustocharacterizethesourceflux
In Fig. 2 we show the best-fitting models from the pho- well because it provides a good fit to the spectrum. Impor-
toionizationmodeling.Wefindthatforbothobservations,the tantly,itisasimplecontinuummodelandsoiseasilyrepro-
observed NeIX, OVII, OVIII lines can be reproduced well ducible, but, due to modeling degeneracies the specific pa-
with χ2ν = 1.13 for ObsID 6633, and χ2ν = 1.15 for ObsID rametervaluesderivedshouldberegardedwithcaution. The
6634. Note that the NeII and NeIII lines are not included fittedspectralparametersaregiveninTable4.
inthecode,andthusarenotreproduced. We determine the 0.5 - 10 keV source flux, by extending
ForObsID6633wefindthebestfitwithNH =(6.1±1.5)× a dummy response to lower and higher energies. We deter-
1019 cm- 2 and logξ =1.36±0.05 (where ξ = L/nR2). For minetheuncertaintyinthefluxbypropagatingtheerrorson
ObsID6634wefindthebestfitwithN =(7.6±1.3)×1019 theindividualparameters. Thecalculatedfluxesaregivenin
H
cm- 2andlogξ=1.34±0.03.Giventhatthebinaryseparation Table 4. We note that the uncertaintiesin the fluxesmay be
for 4U 1820- 30 is ∼1010 cm (Stellaetal. 1987), with the dominatedbycalibrationuncertainties,andcouldbecloserto
ionizationparameterswe find the gas density would have to around6% (the differencein flux we find between fitting to
ben=2.5×1016cm- 3forObsID6633andn=4×1016cm- 3 the MEG and HEG) than those we determine from spectral
forObsID6634forthelargestplausibleradiusforabsorbing fitting.
gas within the system. The implies that the filling factor, f, Wecheckwhetherpileupaltersthedeterminedfluxessub-
is extremelylow with f =N /nR=2.4×10- 7 for 6633and stantiallybycomparingthecalculatedsourcefluxeswiththe
H
X-rayabsorptionlinesin4U1820- 30 5
1-day average RXTE All-Sky Monitor (ASM) countrate for suggestingtheyareinterstellarinorigin.However,ifthelines
the dayof eachChandraobservation(see Tab. 4). Compar- aresaturated,thenevenifthecolumndensitychangessignif-
ingthesourcefluxagainsttheASMcountrate,theyareseen icantly(forinstanceinresponsetochangesinthesource)the
tobecorrelated,withnolargeoffsets(seeFig. 3). equivalentwidth would remainmoreor less constant. Simi-
Tolookforvariabilityoftheabsorptionlinesbetweeneach larly,inthesaturationregimeeveniftheionizingfluxchanges
epoch, in Fig. 4 we plot the measured equivalent width (or byafactorof2(asobservedbetweenObsId6633or6634)the
upperlimit,ifthelineisnotdetected)foreachobservationvs equivalentwidthandionrelativeabundancefractionsmaynot
the0.5-10keVunabsorbedsourceflux(seeTable4). Theer- changebyasimilaramount.However,eventhoughwedonot
rorbarsindicate1σuncertaintiesintheequivalentwidth. The observetheα- β- γ sequenceforeachline, upperlimitson
dashed lines indicates the weighted mean (weighted by the β/αindicatethatthelinesarenothighlysaturated.
uncertainties) of the equivalent widths of the detected lines. We find thattheremaybe a low levelofvariability(2.4-σ
Generally, the equivalent widths of the lines are consistent significance)intheOVIIILyαline,whichmayindicateitis
between the observations. However, we note that the OVIII associated with the source. If the absorption is local to the
linedoesseemto displayvariability,althoughthevariability source,thisvariabilitycouldbe,forexample,duetochanges
isnothighlysignificantanddoesnotappeartobecorrelated inmassaccretionrateleadingtochangesinmassoutflow. If
withthesourceflux(seeFig.4). Theobservedwavelengthof onenaivelyassumesthatthecontinuumfluxtracesthe mass
theOVIIIlineisalsofoundtobeconsistentwiththetheoret- accretion rate one would therefore see correlated variability
icalwavelengthandnotblueshifted. betweenthesourcefluxandthelineequivalentwidth. Alter-
Wecalculatetheχ2 valueforafitoftheequivalentwidths natively,ifthepropertiesoftheabsorbingcolumnareapprox-
for the OVIII line to their weighted mean to assess the sig- imatelyconstantandtheionizingfluxchanges,thefractionof
nificance of this variability. For the non-detection in ObsId oxygen in OVIII should change, which could lead to corre-
1021, we use the equivalentwidth determinedfromfitting a lated variability. However, there appears to be no such cor-
Gaussian fixed at the wavelength of the line in the calcula- relation observed (see Fig. 4) though more observations are
tion. Thisequivalentwidthisfoundto2.9±2.9mÅ.Weget requiredtobeconclusive.
χ2=14.1(3degreesoffreedom)whenfittingtheequivalent Additionally, if the gas in the system respondsto changes
widths to their weighted mean. For the hypothesis that the intheionizingfluxmoreslowlythantheinherentchangesin
equivalent widths are constant, this corresponds to a proba- theionizingflux(i.e. iftherecombinationtimeislong),this
bilityforachievingahigherχ2 valueof0.003. However,we wouldhavetheeffectofgreatlydampeningtheamplitudeof
note that we have searched 5 lines to find variability of this variationsseenintheabsorptionrelativetotheincidentflux.
level, and thus this increases the probability to 0.015. This For OVIII originating within the system, the size of the ab-
is equivalent to the line being variable at the 2.4σ level of sorbingregionis<1011cm. Thecolumndensityweobserve
confidence. Thus, while it is not a highly significant result, tobeabout1020cm- 2,thusthedensityisne∼109cm- 3. The
low-levelvariabilitycannotberuledoutforthisline. recombination rate coefficient for OVIII is α=1.5×10- 12
In comparing equivalentwidth measurements from multi- cm3 s- 1 at 106 K, so the recombinationtimescale = 1/(neα)
pleepochs,itisimportanttoconsiderwhetheranysystematic ∼700 s. This is significantly shorter than the timescale be-
effectscancausevariabilityinthelines. Importantly,neither tweentheobservationsandthusshouldnotbeafactor.
thefirsttwoobservations(ObsId1021and1022)wereoper- If the absorption lines were associated with a disk wind,
atedincontinuous-clockingmode,andthusbothareaffected then blueshifts would be expected. We find that the wave-
by pileup. It is not clear how pileup alters equivalentwidth length of the absorption lines are consistent with their rest
measurements, though we note that as the OVII and OVIII wavelengths (after accounting for the absolute instrumental
linesareclosetogetherinwavelengththeyshouldbeaffected calibrationuncertaintyof0.05%),asexpectedforinterstellar
similarly.Anyeffectmustbefairlysmall,astheOVIIIlineis lines. However, we note that in the dipping X-ray binaries,
seentovarywhereastheOVIIisnot. absorptionlinesthatarelocaltothesourcearenotseentobe
blueshifted and are thought to be associated with the accre-
5. DISCUSSION tiondiskcoronaratherthananaccretiondiskwind.Similarly,
We have analyzed four Chandra HETGS observations of herethelackofblueshiftscouldjustbeexplainedbyabsorp-
theneutronstarlow-massX-raybinary4U1820- 30andde- tionfromanaccretiondiskcorona(DíazTrigoetal.2006).
tectavarietyofabsorptionlinesinthespectra. Thefirsttwo Totestbothpossibleoriginsforthelines,wefirstcalculated
archivalobservationssufferfrom pile-up, and thus measure- theequivalenthydrogencolumndensities(impliedbytheob-
ments of lines from these observations may not be robust. servedequivalentwidths)foravarietyofDopplerparameters,
However,thetwonewobservationspresentedheredonotsuf- andfindthattheyareconsistentwiththeinterstellaroriginfor
ferfromthisproblemandhaveanincreasedsensitivitycom- the lines. Additionally we perform photoionization model-
paredtothepreviousobservations,allowingforasignificantly ing of absorption by material local to the system. We find
improvedstudyoftheabsorptionlinesinthissystem. thatphotoionizationcanreproducetheobservedspectrawell.
Toinvestigatethenatureoftheselines,wecomparetheline However,todothisrequiresaverylowfillingfactor,butthis
equivalentwidthsbetweentheobservations. Ifthelineisun- couldpossiblybeexplainedbyabsorptionfromdenseblobs.
saturated and on the linear part of the curveof growth, then WeconcludethatwhiletheNeIIandNeIIIlinesareproduced
theequivalentwidthisadirectmeasureoftheabsorbingcol- intheISM,theoriginoftheNeIX,OVIIandOVIIIlinesre-
umn. Iftheequivalentwidthdoesnotvarybetweenobserva- mainsunclear,withboththeinterstellarandlocalabsorption
tions,thiswouldthereforeindicatethattheabsorbingcolumn remainingpossible.Thekeytodeterminingtheoriginofthese
hasremainedunchangedbetweentheobservations,aswould linesmaylieinvariability,thusadditionalsensitiveobserva-
beexpectedforabsorptionlinesassociatedwiththeISM.The tionsof4U1820- 30areneededtoconfirmvariabilityinthe
majority of the absorption lines present in 4U 1820- 30 are OVIIIline,andsearchforlow-levelvariabilityinotherlines.
consistentwithremainingconstantbetweentheobservations,
6 Cackettetal.
Wethankananonymousrefereeforhelpfulsuggestionsthat teamforthequick-lookresultsusedinthispaper.JMMgrate-
improvedthepaper. WeacknowledgeB.Otteforusefuldis- fullyacknowledgessupportfromChandra.
cussionson observationsof OVI. We thankthe RXTE/ASM
REFERENCES
Arnaud, K. A. 1996, in ASP Conf. Ser. 101: Astronomical Data Analysis Lin,D.,Remillard,R.A.,&Homan,J.2007,ArXivAstrophysicse-prints,
SoftwareandSystemsV,17 astro-ph/0702089
Behar,E.&Netzer,H.2002,ApJ,570,165 Mazzotta,P.,Mazzitelli,G.,Colafrancesco,S.,&Vittorio,N.1998,A&AS,
Bohlin,R.C.,Savage,B.D.,&Drake,J.F.1978,ApJ,224,132 133,403
Boirin,L.,Méndez,M.,DíazTrigo,M.,Parmar,A.N.,&Kaastra,J.S.2005, Miller,J.M.,Raymond,J.,Fabian,A.,Steeghs,D.,Homan,J.,Reynolds,C.,
A&A,436,195 vanderKlis,M.,&Wijnands,R.2006a,Nature,441,953
Church,M.J.,Reed,D.,Dotani,T.,Bałucin´ska-Church,M.,&Smale,A.P. Miller, J. M., Raymond, J., Fabian, A. C., Homan, J., Nowak, M. A.,
2005,MNRAS,359,1336 Wijnands,R.,vanderKlis,M.,Belloni,T.,Tomsick,J.A.,Smith,D.M.,
Cottam,J.,Kahn,S.M.,Brinkman,A.C.,denHerder,J.W.,&Erd,C.2001a, Charles,P.A.,&Lewin,W.H.G.2004,ApJ,601,450
A&A,365,L277 Miller,J.M.,Raymond,J.,Homan,J.,Fabian,A.C.,Steeghs,D.,Wijnands,
Cottam,J.,Sako,M.,Kahn,S.M.,Paerels,F.,&Liedahl,D.A.2001b,ApJ, R.,Rupen,M.,Charles,P.,vanderKlis,M.,&Lewin,W.H.G.2006b,
557,L101 ApJ,646,394
Crenshaw, D. M.,Kraemer, S.B., Boggess, A.,Maran, S. P.,Mushotzky, Otte,B.&Dixon,W.V.D.2006,ApJ,647,312
R.F.,&Wu,C.-C.1999,ApJ,516,750 Parmar,A.N.,Oosterbroek,T.,Boirin,L.,&Lumb,D.2002,A&A,386,910
DíazTrigo,M.,Parmar,A.N.,Boirin,L.,Méndez,M.,&Kaastra,J.S.2006, Schulz,N.S.,Chakrabarty,D.,Marshall,H.L.,Canizares,C.R.,Lee,J.C.,
A&A,445,179 &Houck,J.2001,ApJ,563,941
Dickey,J.M.&Lockman,F.J.1990,ARA&A,28,215 Sidoli,L.,Oosterbroek,T.,Parmar,A.N.,Lumb,D.,&Erd,C.2001,A&A,
Futamoto,K.,Mitsuda,K.,Takei,Y.,Fujimoto,R.,&Yamasaki,N.Y.2004, 379,540
ApJ,605,793 Spitzer, L.1978,Physicalprocesses intheinterstellar medium(New York
Juett,A.M.,Schulz,N.S.,&Chakrabarty,D.2004,ApJ,612,308 Wiley-Interscience,1978.333p.)
Juett,A.M.,Schulz,N.S.,Chakrabarty,D.,&Gorczyca,T.W.2006,ApJ, Stella,L.,Priedhorsky,W.,&White,N.E.1987,ApJ,312,L17
648,1066 Ueda,Y.,Murakami,H.,Yamaoka,K.,Dotani,T.,&Ebisawa,K.2004,ApJ,
Kaastra,J.S.,Mewe,R.,Liedahl,D.A.,Komossa,S.,&Brinkman,A.C. 609,325
2000,A&A,354,L83 Verner,D.A.,Verner,E.M.,&Ferland,G.J.1996,AtomicDataandNuclear
Kallman,T.&Bautista,M.2001,ApJS,133,221 DataTables,64,1
Kaspi, S.,Brandt, W.N.,Netzer, H.,Sambruna, R.,Chartas, G.,Garmire, Wilms,J.,Allen,A.,&McCray,R.2000,ApJ,542,914
G.P.,&Nousek,J.A.2000,ApJ,535,L17 Yao,Y.&Wang,Q.D.2005,ApJ,624,751
Kuulkers,E.,denHartog,P.R.,in’tZand,J.J.M.,Verbunt,F.W.M.,Harris, —.2006,ApJ,641,930
W.E.,&Cocchi,M.2003,A&A,399,663
Lee,J.C.,Reynolds,C.S.,Remillard,R.,Schulz,N.S.,Blackman,E.G.,&
Fabian,A.C.2002,ApJ,567,1102
X-rayabsorptionlinesin4U1820- 30 7
TABLE1
ChandraOBSERVATIONSOF4U1820- 30
ObsID ObservationDate Grating/Detector Exposure(ks)
98 2000Mar10 LETG/HRC 15.12
1021 2001Jul21 HETG/ACIS 9.70
1022 2001Sep12 HETG/ACIS 10.89
6633 2006Aug12 HETG/ACIS 25.18
6634 2006Oct20 HETG/ACIS 25.13
NOTE.—ObsID6633and6634arethenewobservationspresentedinthiswork.
TABLE2
ABSORPTIONLINEPROPERTIESIN4U1820- 30
ObsID Ion Transition Theoretical Observed Equivalentwidth FWHM Flux
(Å) (Å) (mÅ) (mÅ) (10- 4photonscm- 2s- 1)
1021 NNeeIIXII 1s12s--12sp2p 1134..454078 13.455±0.003 7.<3±3.17.8 3.8+- 73..35 4.<3±2.11.0
NeII 1s- 2p 14.608 <8.1 <4.7
OVII 1s2- 1s3p 18.629 <10.2 <4.8
OVIII 1s- 2p 18.967 <10.8 <4.8
1022 NeIX 1s2- 1s2p 13.447 13.448±0.005 6.4±2.2 19.3±10.9 5.2±1.8
NeIII 1s- 2p 14.508 <3.5 <2.7
NeII 1s- 2p 14.608 <3.6 <2.8
OVII 1s2- 1s3p 18.629 <6.6 <6.0
OVIII 1s- 2p 18.967 18.966±0.007 22.7±6.3 33.4±14.1 1.4±0.4
6633 NNeeIIXII 1s12s--12sp2p 1134..454078 1134..455005±±00..000026 43..05±±01..93 181.6.4±+- 911..743.5 22..95±±00..79
NeII 1s- 2p 14.608 14.611±0.002 5.1±1.2 0.07+- 10.10.67 3.5±0.8
OVII 1s2- 1s3p 18.629 18.634±0.006 9.8±3.0 24.0±11.0 6.3±2.0
OVIII 1s- 2p 18.967 18.972±0.006 6.4±2.9 7.1+- 27.11.2 4.1±1.8
6634 NNNeeeIIIXIII 1s112ss---122spp2p 111344...456400788 111344...456400582±±±000...000000235 535...014±±±011...812 2731.8..24±+-+-1381.31..2.6420.2 435...901±±±011...802
OOVVIIIII 1s12s--12sp3p 1188..692697 181.89.66525±+- 000..00.0007106 178..04±±24..55 450..902±+- 101.402.42.6 166..10±±24..30
NOTE. —Seetextfordetailsonspectralfittingprocedure. TheoreticallinewavelengthsforNeIX,OVII,andOVIIIarefromVerner,Verner,&Ferland
(1996). ThelinewavelengthsforNeIIandNeIIIarethebestobservationally determinedvaluesfromJuettetal.(2006). Uncertainties andupperlimitsare
1σconfidencelimits. Wherethelinewasnotdetectedandwedetermineupperlimits,thewavelengthwasfixedatthetheoreticalvalue,andthewidthofthe
Gaussianfixedbelowtheinstrumentresolution.
TABLE3
LINECOLUMNDENSITIESFORVARIOUSDOPPLERPARAMETERS
ObsID Line Linearcurveofgrowth b=50kms- 1 b=100kms- 1 b=200kms- 1
Nion NH Nion NH Nion NH Nion NH
(1016cm- 2) (1020cm- 2) (1016cm- 2) (1020cm- 2) (1016cm- 2) (1020cm- 2) (1016cm- 2) (1020cm- 2)
6633 NeIX(13.447Å) 0.3 0.4 0.6 0.7 0.4 0.5 0.4 0.4
NeIII(14.508Å) 1.8 7.1 2.6 12.5 2.1 8.9 1.9 7.8
NeII(14.608Å) 4.4 7.1 8.3 12.6 5.6 8.9 4.9 7.8
OVII(18.629Å) 2.2 0.5 9.7 2.1 3.3 0.8 2.6 0.6
OVIII(18.967Å) 0.5 0.6 0.7 2.1 0.6 0.9 0.5 0.7
6634 NeIX(13.447Å) 0.4 0.5 0.9 1.1 0.6 0.7 0.5 0.6
NeIII(14.508Å) 1.6 7.1 2.2 13.4 1.8 9.0 1.7 7.9
NeII(14.608Å) 4.6 7.1 9.4 13.4 6.0 9.1 5.2 8.0
OVII(18.629Å) 1.6 0.6 3.2 1.3 2.1 0.8 1.8 0.7
OVIII(18.967Å) 1.4 0.8 6.4 3.2 2.9 1.5 1.9 1.0
NOTE. —Seetextfordetailsondeterminingcolumndensities. WeonlydeterminecolumndensitiesforObsIDs6633and6634. Theequivalenthydrogen
columndensity(NH)isdeterminedusingtheISMabundancesfromWilmsetal.(2000).
8 Cackettetal.
TABLE4
SPECTRALFITSTO4U1820- 30
ObsID Blackbody Power-law χ2ν 0.5- 10keVunabsorbedflux RXTE/ASMcountrate
kT(keV) Normalization Spectralindex Normalization (10- 9ergcm- 2s- 1) (countss- 1)
1021 0.72±0.07 (3.5±0.6)×10- 3 1.67±0.02 0.85±0.02 0.87 5.8±0.2 14.0±0.3
61603232 1.01.696±+- 000..11.0512 ((24..51±±10..01))××1100-- 23 11..7466±±00..0032 11..1123±±00..0012 00..9848 89..37±±00..23 2214..33±±00..63
6634 1.20±0.02 (3.2±0.2)×10- 2 1.74±0.02 1.50±0.01 1.24 11.7±0.3 27.7±1.0
NOTE.—InallfitsthecolumndensitywasfixedattheclustervalueofNH=1.5×1021cm- 2.WeusedtheXSPECblackbodymodelnamedbbody,wherethe
normalizationisdefinedasL39/(D10)2,whereL39istheluminosityinunitsof1039ergs- 1andD10thedistancetothesourceinunitsof10kpc.Thepower-law
normalizationisdefinedat1keV,withunitsofphotonskeV- 1cm- 2s- 1.ThespectralfitsaretotheHEG1storderspectrum(seetextfordetails).Forcomparison
withthesourceflux,theRXTE/ASMcountratefor4U1820- 30isalsogiven.
FIG. 1.—13- 15Åand18- 20Åsegmentsofthecombinedfirst-orderMEGspectraof4U1820- 30. Forillustrativepurposesthedatahavebeenrebinnedby
afactorof2.ThetoppanelsshowtheseregionsinObsId6633,andthebottompanelsshowtheseregionsinObsId6634. Thedataisinblack,andthe1σerror
barsareinblue.Thebest-fittingmodelisinred.Thecontinuumisfitbyapower-lawmodifiedbyneutralISMabsorptionedges,whereappropriate. Absorption
linesaremodeledbyaGaussian.Table2givesthelineproperties.
X-rayabsorptionlinesin4U1820- 30 9
ObsId: 6633
ObsId: 6634
FIG.2.—Best-fittingXSTARphotionizationmodelsforObsID6633(top)and6634(bottom).TheNeIX,OVIIandOVIIIlinesarereproducedwellbythe
models.
FIG.3.—0.5-10keVunabsorbedsourcefluxdeterminedfromfittingtotheHEGspectraisshownversusthe1-dayaverageRXTEAll-SkyMonitorcountrate
onthedayofeachobservation.
10 Cackettetal.
FIG. 4.—Equivalentwidths(mÅ)ofthevariousabsorptionlinesin4U1820- 30versus0.5-10keVunabsorbedsourcefluxinunitsof10- 8 ergcm- 2 s- 1
. Dashed lines indicate the weighted mean of the measured equivalent widths. Arrows indicate 95% confidence upper limits and errorbars indicate 1- σ
uncertainties.