Absorption and scattering by interstellar dust XMM-Newton observation of Cyg X-2
a r X i v :a s t r o -p h /0508129v 1 4 A u g 2005
Astronomy &Astrophysics manuscript no.(will be inserted by hand later)
Absorption and scattering by interstellar dust:
XMM-Newton observation of Cyg X-2
E.Costantini 1,2,M.J.Freyberg 3&P.Predehl 3
1SRON National Institute for Space Research,Sorbonnelaan,2,3584CA,Utrecht,The Netherlands 2Astronomical Institute,Utrecht University,P.O.Box 80000,3508TA Utrecht,The Netherlands 3
Max-Planck-Institut f¨u r extraterrestrische Physik,Giessenbachstr.1,D-85748Garching bei M¨u nchen,Germany
Received /Accepted
Abstract.We present results of the XMM-Newton observation on the bright X-ray binary Cyg X-2.In our analysis we focus upon the absorption and scattering of the X-ray emission by interstellar dust distributed along the line of sight.The scattering halo around Cyg X-2,observed with the CCD detector EPIC-pn,is well detected up to ～7arcmin and contributes ～5-7%to the total source emission at 1keV,depending on the dust size distribution model considered.For the ?rst time spatially resolved spectroscopy of a scattering halo is performed.In the halo spectrum we clearly detect the signature of the interstellar dust elements:O,Mg,and Si.In the 0.4?2keV band,the spectral modeling of the halo shows a major contribution of silicates (olivine and pyroxene).The spatial analysis of the halo surface brightness pro?le shows that the dust is smoothly distributed toward Cyg X-2at least for ～60%of the path to the source.However,given the substantial pile-up,we could not investigate fainter or narrower components of the halo.Within this observation limits,the data do not show preference for a speci?c dust size distribution;namely the Mathis,Rumpl &Nordsieck (1977)or the Weingartner &Draine (2001)model.In this analysis we used the Mie theory to compute the di?erential scattering cross section.The RGS data were used to investigate the ISM absorption.In particular the absorption spectrum shows complexity around the oxygen edge at ～0.54keV,which cannot be explained in a unique way:absorption by molecular oxygen or ionized atomic oxygen,as proposed in other studies of Cyg https://www.wendangku.net/doc/556654778.html,bining the RGS results with the additional information on dust grains provided by the EPIC-pn spectrum of the scattered radiation we estimate a column density for dust absorption by oxygen,provided that it is locked in silicate grains.Key words.ISM –Dust scattering halos–Cyg X-2–Interstellar Dust
1.Introduction
The observed light from a source is obscured by the in-terstellar matter (ISM)through the combination of two processes:absorption and scattering.Absorption is due to both gas and dust,whereas scattering is attributed to dust alone.Di?erently from the IR to UV wavelengths range,in the X-ray regime the observation of absorption and scat-tering by interstellar dust (ID)are strongly coupled.Thus,in the X-ray regime,the two extinction mechanisms can be simultaneously observed and studied.If an X-ray emit-ter is located behind a layer of dust,its radiation will be absorbed and at the same time scattered into the direction of the observer.In the X-rays the scattering mechanism is no longer explained by the simple Rayleigh formula.In particular the scattering angle is in this case very small (θscatt ∝(λ/a )?1?),forward directed,dependent on the wavelength of the incident photon (λ)and the size (a )of the grain.The small scattering angle results in a halo of di?use emission around the source (Overbeck 1965).The
energy range in which absorption and scattering can be studied,is a strong function of the equivalent hydrogen column density of the medium (N H ).Indeed the X-ray ra-diation is obscured by absorption depending on the value of N H :I =I 0e ?N H σ,where I 0is the source radiation and σis the absorption cross section.Through the analysis of the absorbed spectrum,information on the chemistry,column density and abundances of the ID grains can be inferred.Simultaneously,the spectral and spatial properties of the X-ray halos can be analyzed.The halo intensity,angular extension,and spectral distribution are a function of the size distribution and composition of the scatterers (the dust grains),their distribution along the line of sight,and the spectral properties of the source illuminating them.Sources with faint halos have a hydrogen column density which is low enough to not completely absorb the soft X rays.High sensitivity instruments are needed to study the emission of the scattering halo,which is very weak com-pared to the brightness of a background source (up to 20%of the soft emission,Predehl &Klose 1996).Faint halos
2Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-2
are important to study ID chemistry.Indeed the energy range where scattering occurs(approximately0.3?2keV), includes possible features of ID components,primarily oxygen(0.54keV),magnesium(1.3keV),and silicon(1.84 keV)(Predehl&Klose1996;Draine2003,hereinafter D03).An energy resolution of～80?150eV over the en-ergy band of interest for the scattering process,makes it possible to spectroscopically investigate the features of a faint halo.Previous studies of scattering halo pro?les were carried out with Einstein(e.g.Mauche&Gorenstein 1986;Gallagher,Cash,&Green1995)and ROSAT (Smith&Dwek1998;Predehl&Schmitt1995,here-inafter PS95).According to those?ndings,the ID size distribution appeared to be consistent with the Mathis,Rumpl,&Nordsieck(1977)(hereinafter MRN) model.The MRN model includes a mixture of car-bonaceous and silicate materials,with size distribu-tion a?3.5,for0.001 Finally,it has been recognized that a simple analytical computation of the di?erential scattering cross section,the so called Rayleigh-Gans(hereinafter RG)approximation ,could be misleading if applied to halo energies<1keV and/or large grains a>0.25μm.The full Mie theory(Mie 1908),from which the RG approximation is derived,had to be used(Smith&Dwek1998;Draine&Tan2003). The models applied to X-ray scattering halos are the re-sult of a deeper knowledge of ID properties gathered at longer wavelengths.Due to the low resolution of early X-ray instruments,only integral properties of the dust could be studied,adding relatively little information on the na-ture of ID.On the contrary,with the X-ray observatories now?ying,we can address other issues like:(i)the chem-ical properties of dust particles that scatter X-rays,(ii) abundances and depletion in the ISM,and(iii)the actual distribution of dust along the line of sight. In this paper we present the RGS and EPIC-pn analysis of Cyg X-2,located at Galactic coordinates l=87.33?,b=?11.32?,behind a dust layer with equivalent N H column density of the order of～2×1021cm?2,which produces a relatively weak scattering halo.This makes Cyg X-2an ideal candidate to study both the spatial and spectral dis-tribution of the halo at energies softer than2keV.The fractional halo intensity of Cyg X-2,de?ned as the inten-sity of the halo extended emission over the total observed emission,was estimated from ROSAT-PSPC to be3.9%at 1.06keV(PS95).Now,with the high sensitivity of XMM-Newton,the halo can be resolved and analyzed down to 0.4keV with the EPIC-pn.Absorption by the ISM toward the line of sight of Cyg X-2was studied with the RGS. Recently Cyg X-2was studied by Takei et al.(2003),us-ing Chandra-LETG and by Juett,Schulz,&Chakrabarty (2004)using Chandra-HETG.In each analysis,the absorp-tion features in the spectral region of the oxygen edge were interpreted in di?erent ways.Takei et al.(2003)claimed to have detected absorption by molecular oxygen,while Juett,Schulz,&Chakrabarty(2004)interpret those fea-tures in terms of mildly ionized oxygen in the ISM. The paper is organized as follows:In Sect.2the princi-ples of the scattering halo theory are presented.In Sect.3 the analysis of RGS and EPIC-pn data of Cyg X-2is shown.Sect.4describes the careful extraction of the infor-mation on scattered radiation.Sect.5is then devoted to the spatial and the spectral modeling of the scattered halo. Finally,in Sect.6we discuss our results,and in Sect.7 the conclusions of this work are shown. 2.The Halo Theory The intensity of the light scattered by dust at a scattering angleθsca,assuming spherical grains and single scattering has the following general form(e.g.,Mathis&Lee1991): I(θsca)= E max E min F(E)dE a max a min n(a)da× × 10?f(x)d?dx.(1) F(E)is the spectral energy distribution of the source;a is the grain radius with number density n(a); x is the fractional distance of the total path,from the source(x=1)to the observer(x=0),at which the scattering occurs;and?f(x)is the normalized spatial distribution of the scattering sites.Since we consider dust that is evenly distributed,?f(x)=1.For the scattering angleθsca,it holds thatθsca=θobs/(1?x).The term dσ/d?is the di?erential scattering cross section,a function ofθ,a and E.There are two critical terms in Eq1.One is n(a),which depends on the physical and chemical state of the dust grains.The other crucial term in evaluating the scattered emission is the di?erential scattering cross section for which the exact solution is given by the Mie theory(Mie1908;van de Hulst 1957).This describes the scattering and absorption of an electromagnetic wave by spherical solid particles. Quantitatively,the refraction index m of a given material Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-23 can be written as(e.g.,Henke,Gullikson,&Davis1993): r eλ2 m=1? π ∞0f′′(ω′)ω′dω′ ,(4) 2πn q r e c whereμ(ω)is the absorption coe?cient at the incident energy E=ˉhω.Here the assumption is that for a given compound the dielectric function is the sum of the sin- gle atoms contributions.However,at the threshold energy, the absorption coe?cient strongly depends on the chem- ical compound.The interaction between a photoelectron wave and all the other waves backscattered by the neigh- boring atoms creates modulations in the absorption cross section.These are in general called X-ray Absorption Fine Structure(XAFS).The XAFS features,which are few tenth of angstroms wide,have been recently recognized in high energy resolution absorption spectra of some astro- nomical sources(e.g.,Lee et al.2002).Only instruments with energy resolution better than?λ～0.02?A can re- solve these features.The di?erential scattering cross sec- tion,calculated using the Mie approach,as a function of energy is displayed in Fig.1for various scattering angles, considering the case of Mg2SiO4and a grain size of0.1μm. The presence ofμ(ω)in the scattering cross section causes spikes at the K-edge of a given element,as seen in the?g- ure.We note also that at the softer energies we do not expect a dramatic change in the spectral shape of the dif- ferential cross section as a function of the scattering angle. In our observation,the Cyg X-2halo is visible down to 0.4keV,as the hydrogen column density toward the source is relatively low(N H～2.17×1021cm?2,as measured from the H i emission,Dickey&Lockman1990).Therefore we need to evaluate the dσ 4Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-2 spectrum(0.4-10keV)of the central source.In princi-ple the absolute?ux can be evaluated from the OOT events.In the case of extreme pile up this is no longer possible,due to the“pseudo-MIP”e?ect:If the charge within one pn-CCD pixel exceeds a threshold of about15 keV this event is regarded as being due to a minimum ionizing particle(MIP)and all events in this CCD col-umn and the neighboring columns for this read-out frame are rejected on board(pseudo-MIPs,Freyberg2003). However,in the case of very strong pile-up this threshold can be triggered by normal X-rays.In extreme cases,in almost all frames the columns at the center of the PSF are rejected.These spatial exposure variations are not fully re?ected in the event data?les and in the XMMSAS software.The“pseudo-MIPs”have no appreciable in?u-ence on the source spectral shape;however,these rejected columns a?ect the determination of the?ux measured from the OOT events.Indeed,the pseudo-MIP rejection occurs preferentially in the columns corresponding to the PSF core and therefore in the same columns as the bulk of the OOT events,which then get rejected.We modeled the spectrum extracted from the OOT events.Note that the source position,rather than the recorded position on the CCD,was used for the Charge Transfer Ine?ciency (CTI)correction using XMMSAS.The soft EPIC-pn spec-trum is well?tted with a multi-temperature black body for the accretion disk emission(Mitsuda et al.1984)with kT～0.36keV at the inner radius,plus a comptonized black body spectrum for the emission of the neutron star (Titarchuk1994).The soft(seed)photons have a temper-ature kT0～0.8keV before being Compton scattered to reach a temperature of kT～6.2keV in an electron cloud of thicknessτ(Tab.1).We found evidence of an emission line at energy E～6.7keV,consistent with?uorescent emission by ionized iron(e.g.,Di Salvo et al.2002).The soft spectrum is both absorbed by gas and dust and scat-tered by ID,i.e.light is deviated from our line of sight “subtracting”photons from the central source spectrum (PS95),We implemented in XSPEC a model for the scat-tering correction which is based on handy empirical rela-tions:τsca=0.05×N H?0.083and A V=0.56N H+0.23, whereτsca is the scattering optical depth and N H is in units of1021cm?2.This relation is based on the study of 25ROSAT sources(PS95)and it is not critically model dependent as long as we are dealing with a relatively small correction forτ.The optical extinction A V value is only 1.3for Cyg X-2(Bradt&McClintock1983),therefore the in?uence of scattering in the spectrum is practically neg-ligible in the?t for such a low intervening column density (PS95). 3.2.Cyg X-2RGS spectral analysis As the RGS covers only the0.35-2keV band,the hard component cannot be constrained,therefore the thermal comptonization parameters are?xed to the EPIC-pn best-?t values.The background spectrum contribution was Table 1.Best?t parameters for the EPIC-pn spec-trum of Cyg X-2in the energy band0.4-10keV, with a disk black body at temperature kT db(DISKBB in XSPEC,Mitsuda et al.1984)and a comptonized spectrum(COMPTT in XSPEC,Titarchuk1994),af-fected by extinction of gas and dust(TBABS in XSPEC, Wilms,Allen&McCray2000).See text for the de?nition of the parameters.Errors are given at90%con?dence level for one interesting parameter. N H(×1022cm?2)0.19±0.05 kT db(keV)0.36±0.05 kT0(keV)0.81±0.03 kT(keV)6.2±0.1 τ1.62±0.07 E Fe(keV)6.67±0.16 σFe(keV)0.2+0.3 ?0.1 EW Fe(eV)73±53 χ2/dof1115/1111 evaluated using the“blank?eld”observations speci?c for RGS.This guarantees the omission of any halo contam-ination in the background.In the case of Cyg X-2,the halo contribution to the source spectrum is almost negli-gible,since the?ux of the source is more than20times larger than the?ux of the di?use halo.We extracted the ?rst and second order from RGS1and RGS2for a total of four data sets.The data below7?A are a?ected by low and poorly calibrated sensitivity and are rejected. Absorption by oxygen in the ISM could be studied in de-tail in the spectral region around0.54keV.Takei et al. (2003)found a complex structure for the oxygen edge re-gion in a Chandra-LETG observation of Cyg X-2.In their analysis they interpreted the spectrum in terms of ab-sorption lines and edges from oxygen in both atomic and molecular form.Although the RGS energy resolution is approximately44%less than LETG,we?nd similar com-plexity in the oxygen region:in particular a single oxy-gen edge at energy0.543keV is an unsatisfactory?t to the data.We?rst considered the approach of Takei et al. (2003)(model1in Tab.2).We included an additional edge in the?t,which improves the?t by?χ2/?ν=39/1(cor-responding to a signi?cance higher than99.5%).The two edge energies are?xed:0.536(23.13)and0.543(22.83) keV(?A),corresponding to compound and atomic oxygen, respectively.On the other hand,a third edge,strongly re-quired by LETG data at0.549keV(22.58?A,atomic oxy-gen),improves our?t only by?χ2/?ν=5/1(signi?cance 97.5%).At an energy of0.524±0.003keV,consistent with the atomic oxygen1s?2p transition,an absorption line of equivalent width(EW)1.45+0.22 ?0.14 eV is clearly detected. Finally,at0.530±0.003keV some absorption line-like residuals,in addition to the known instrumental absorp-tion line(de Vries et al.2003),still remain.Including at this position a second absorption line in the?t yields an energy that is interpreted as the1s?2p transition of com-pound oxygen(Fig.2,upper panel).The measured EW is Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-25 1.41±0.26eV,consistent with the?ndings of Takei et al. (2003). However,large uncertainties still remain in the laboratory measurements of oxygen bound with other elements and the identi?cation of such features is not conclusive.Other laboratory measurements(e.g.,McLaughlin&Kirby 1998;Gorczyca&McLaughlin2000)of atomic oxygen around the K edge region would interpret the absorption structures as absorption lines from neutral and ionized oxygen.This is called model2in Tab.2.The only two features in common with the Takei et al.interpretation are:the absorption edge at0.543keV(22.83?A)and the 1s?2p transition line at0.524±0.003keV.The region between these two“standard”features is?t by an ab- sorption line at22.89?A,consistent with the1s-3p transi- tion of neutral atomic oxygen,and with another absorp- tion line which would be consistent with a blend of un- resolved lines of O iii(at23.05?A).Finally,the evident absorption line at23.35?A,also found by Takei et al.,is interpreted as ionized atomic oxygen(O ii),as predicted by Gorczyca&McLaughlin(2000)measurements(Fig.2, lower panel).This interpretation(model2)was also ap- plied to Chandra-HETG data of a sample of bright galac- tic sources(Juett,Schulz,&Chakrabarty2004).Such an ionized component would be interpreted as ionization of the ISM,localized in the vicinity of the source.In Tab.2 the results of the two models are shown;there is no sig- ni?cant di?erence in terms of goodness of?t. Table 2.RGS?tting results for the oxygen region in Cyg X-2.The parameters for the continuum emission are taken from the broad-band spectrum.The energies are measured in keV and the Equivalent Width(EW)in eV. Model1?ts the O vicinity with3edges(two of which are from atomic O)and2absorption lines(one from atomic neutral O and the other from molecular O).Model2in- terprets the spectrum in terms of absorption by atomic O, either neutral or mildly ionized.Errors are given at90% con?dence for one interesting parameter. model1model2 6Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-2 so derived and the N H found in our best-?t model (Tab.3).We note that iron shows an overabundance of ～20%compared to the Wilms,Allen&McCray(2000) ISM value. Table 3.Relevant absorption edges in the RGS spec- trum of Cyg X-2.The energy,the corresponding wave- length,and the optical depth(τ)were obtained from the data.The equivalent total hydrogen column density N H was derived from the ISM abundances of Wilms,Allen& McCray(2000).These are to be compared with the best- ?t N H=(2.20±0.02)×1021cm?2,measured by RGS. Errors are given at90%con?dence level. Element Energy WavelengthτN H keV?A1021cm?2 1http://simbad.u-strasbg.fr Costantini,Freyberg &Predehl:XMM-Newton observation of Cyg X-2 7 Fig.5.Ratio between single events and all events (up-per dash-dotted curve)and ratio between double and all events (lower solid curve),for EPIC-pn,around 0.75keV.The dotted lines represent the values expected in absence of pile-up. ent exposures of each quadrant of the detector selecting just the time intervals when all four quadrants were on at the same time.Any excluded region (serendipity sources,OOT events,regions outside the chip boundaries)was of course taken into account in the computation of the ex-traction area. Pile-up is close to 100%at the center of the source.This causes the characteristic “hole”in the spatial pro?le,but to a lesser degree also distorts the pro?le shape up to many arcsecs from the source.In order to evaluate the dependence of pile-up as a function of the distance from the source,we extracted the radial pro?le of the source at di?erent energies for single events,double events and the total events,selecting di?erent pattern from the data.Where the count rate is low (i.e.no pile-up)the ratio of the radial pro?les extracted with these di?erent patterns (single,double and all events), should be a constant value (Fig.5).We see that for single events pile-up a?ects the pro?le up to ～40′′;thus we studied the data only outside this radius.We divided the photon histogram by the ex-posure map,and the areas of the annuli,that was also corrected for the zones excluded in the photon extraction.Moreover,each photon is vignetting corrected by the ra-tio of the e?ective area at the aim point of the telescope and at the position where the photon itself is detected.The resulting radial intensity distribution is now in units of cts/s/arcsec 2.The next step is the subtraction of the PSF from the data.The model for the XMM-Newton PSF as a function of energy and o?-axis angle was derived by https://www.wendangku.net/doc/556654778.html,parison between the PSF-model (solid line),Mrk 421(asterisks)between 0.9and 1keV.For radii smaller than ～30′′,the ?atness of the pro?le is mainly due to the mask used in the observation.The counts in this region are non zero due to a slight misalignment of the mask with respect to the source coordinates in this particular observation. the analysis of 110point-like sources (Ghizzardi 2002).The instrumental PSF is described by a King pro?le: P SF ∝ 1 2 The o?-axis dependence of the PSF is neglected as the sources we study are located on-axis. 8Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-2 the Cyg X-2halo radial pro?le.We made the two pro?les overlap in the interval between～40′′-100′′,where Cyg X- 2does not show signi?cant scattered extended emission and therefore the pro?le is dominated by the instrumen- tal PSF.We have seen above that the PSF pro?le slowly changes with energy,hence overlapping the source pro?le with the PSF considering a too large energy band would introduce additional uncertainty.For the largest energy bin we considered(0.25keV,§5.1)the net uncertainty on the slopeαis±2%.As the halo analysis is con?ned to radii>40′′,the uncertainty introduced by r c(E)is negli- gible.We note that in principle,to evaluate the spectral ?ux we could use either the OOT events or the RGS high resolution spectrum.The uncertainties in the cross cali- bration between PN and RGS?ux may reach20%and make the RGS data inappropriate for this purpose.The OOT events were also unusable for the normalization due to the pseudo-MIP e?ect(§3.1). At the end of the procedure just described,the resulting SBP of Cyg X-2,which is the summed contribution of an extended emission and the instrumental pro?le,could be subtracted by the PSF and then modeled(§5.1). 4.2.Halo spectral analysis For the study of the spatial variation of the spectrum we ?rst selected radial distances from the source from1.3′to 7.3′divided in annuli of1′.The background was extracted from the same region as for the halo spatial analysis.A spatial selection3,was applied to ensure that the energy response stays constant across the detector.In order to have a model independent estimate of how the halo spec- tra may change as a function of the distance from the source,we normalized the spectra in the rings(in units of raw detector counts)to the source spectrum estimated from the OOT events(§3.1).In this way,?rst,all the fea- tures belonging to the X-ray emitter itself cancel out and, second,assuming that absorption by ID is rather uniform within the few arcmin across the halo,also the absorption component is eliminated from the halo spectrum.The re- sult of this procedure is shown in Fig.7.The spectra of the Cyg X-2halo are plotted in order to illustrate the be- havior of the halo with increasing angular distance from the source,as the PSF contribution becomes less impor- tant.The extraction regions of these spectra are marked in Fig.4.Here the inner radii range from3.3′to5.3′,where the halo is more relevant.Note that at this stage,the in- strumental mirror scattering spectral energy distribution is still to be subtracted.The vertical axis of Fig.7is in arbitrary units as the absolute normalization of the source spectrum cannot be recovered from the OOT events(see §3.1).Below2keV,a decline of the curve is evident and Costantini,Freyberg &Predehl:XMM-Newton observation of Cyg X-2 9 Fig.8.XMM EPIC-pn pure-scattering mirror spectrum at a radius 180′′from the source,extracted from the halo-free source Mrk 421.We note a smooth increase of the scattering above 2keV for larger radii. the Si feature and the continuum just below 1.84keV,as the scattered emission becomes weaker with respect to the PSF.In this region,a small di?erence in the chosen nor-malization may a?ect the results of the halo modeling.The PSF subtraction has the e?ect of making the Si fea-ture appear even deeper than what already observed from the raw data (Fig.7).If,for example,the normalization of the halo spectrum were ±10%of what we calculated,the error bars for the Si feature in the PSF-subtracted spectrum (Fig.11)would increase of an additional 6%,considering that the ratio between the PSF and the raw data at this position is ～30%at θobs =4.8′. 5.The halo modeling There are two critical terms in Eq.1.One is the di?eren-tial scattering cross section and the other is the assumed grain size distribution.For both the spatial and the spec-tral modeling of the scattering halo,we used the di?er-ential scattering cross section calculated using the Mie theory.For each value of the parameter X =2πa/λ,where a is the grain size and λis the wavelength of the incoming wave,we calculated the parameter dσ/d ?for 115scattering angles θsca from 0to 3?.The scat-tering cross section depends also on the di?raction in-dex m which is determined by the material and is en-ergy dependent.In this study we use the tabulated values of m for graphite,olivine,and pyroxene as determined by Henke,Gullikson,&Davis (1993)except in one case (§6.1),when we test the speci?c ID composition proposed in D03.In that case,the calculation for m takes into ac-count the XAFS near the edges. The code 4(Wiscombe 1980)used to generate the scat-Scatt/ Homogen Mie/ tering cross sections is reliable for X <2×104.For large values of a (say a >1.2μm for E =2keV),the anomalous di?raction theory should be used (van de Hulst 1957).We ignore very large grains in our calculation since,as shown in Draine &Tan (2003),radii >0.4μm contribute less than 1%at large scattering angles,and less than 20%at θsca <100′′.In bright sources,like Cyg X-2,pile-up hampers the possibility to investigate the halo at small scattering angles,where the e?ect of very large grains,or grains located very close to the source may be relevant (Predehl &Klose 1996).We also restricted our modeling to energies <2keV,above which the halo contribution drops dramatically in the case of Cyg X-2. Thus,the chosen grain size interval is 0.005-0.25μm or 0.00035-0.8μm when the MRN dust size distribution model or WD01model is adopted,respectively.The dust size intervals were divided in 200logarithmically spaced size bins.We allowed the power law index of the MRN distribution to vary by 20%around the typical value 3.5.WD01tested their grain size distribution for two dif-ferent values of the ratio of the total over selective op-tical extinction:R V =3.1and 5.3,and for di?erent car-bon abundances.Cyg X-2is located at galactic latitude b =?11.3?where the ISM is di?use (no CO detected,Dame,Hartmann,&Thaddeus 2001),thus we considered R V =3.1.We used the set of parameters for slopes and coe?cients of the dust distribution corresponding to a carbon abundance in PAH alone of 6×10?5(Tab.1of Weingartner &Draine 2001).For both models,the lower and upper limit of the integral on the dust distribution parameter x were left as free parameters. 5.1.Spatial Modeling of the Halo The SBP was extracted and subtracted from the PSF con-tribution as described in §4.1.At a ?xed energy,the model has three free parameters (Eq.1):dσ/d ?,x ,and n (a ).The best ?t was reached through χ2minimization.We con-sidered rays only scattered once before being observed.Double scattering occurs for optical depths τsca close to unity,indicating a very high dust column density (PS95,Costantini &Predehl 2005),which is not observed to-ward Cyg X-2.In Fig.9,the Cyg X-2SBP of the halo at 1keV is shown.We tested the MRN and the WD01for the dust size distribution.Both models provide an accept-able ?t in terms of χ2(χ2red =1.29and 1.33,respectively).The WD01distribution spans a wider range of grain sizes.In particular,scattering by grains with size a in the range 0.25?0.4μm have the e?ect of enhancing the halo at smaller radii (< ～200′′).The intensity of the halo is pa-rameterized by the scattering optical depth τsca ,de?ned as:I frac =I halo /I tot =1?e ?τsca ,where I halo is the ?ux of the scattered emission,and I tot is the total source emis-sion (PS95).At 1keV we measured τsca =0.054±0.018and 0.067±0.018for the MRN or the WD01model,re-spectively.The error quoted here is statistical and does not include any uncertainty in the background subtrac- 10Costantini,Freyberg &Predehl:XMM-Newton observation of Cyg X-2 tion.The best ?t shows a minimum and maximum value for the fractional path at which the scattering occurs,x ,of ～0.001and ～0.6,respectively.In this interval the halo pro?le shows a smooth distribution of dust along the line of sight up to a fractional distance x ～0.6,after which the halo is unaccessible due to pile-up.However,if the up-per limit of x is constrained to be close to 1(we ?xed it at 0.99)indicating that we are actually observing scatter-ing occurring at all distances,the ?t worsens for both the WD01and MRN models (?χ2=37for ?ν=1,corre-sponding to a signi?cance >99.5%).We then extended this analysis to the energy range at which the halo is ob-servable.In Fig.10we show the total scattering optical depth,derived from the SBP using energy intervals of 0.25keV,in the energy range 0.4-1.9keV.The large bin size smoothes out any features,leaving just the general shape the spectral energy distribution of the scattering optical depth.The solid line in Fig.10refers to the theoretical value of τsca as predicted by D03at the mean energy of the extraction bin.This was derived by multiplying the theoretical value of σsca (D03)with the hydrogen column density toward Cyg X-2that we measure. Fig.9.The data (halo+PSF)(asterisks)compared to the PSF (dotted line)around 1keV.The dashed and the solid thin lines are two di?erent halo models (MRN and W01),while the solid and dashed thick lines indicate the best-?t to the total data (model+PSF)relative to MRN and WD01,respectively. 5.2.Spectral Modeling of the Halo In Fig.11we show the pure scattered radiation energy dis-tribution (i.e.PSF subtracted),collected in a ring centered at 4.8′.The subtraction of the mirror scattering and of the central source contribution was described in §4.We avoid the data below 0.4keV as the e?ective area calibration may still su?er from uncertainties in this range.At soft en- Fig.10.The scattering optical depth as a function of energy,as measured from the SBP of Cyg X-2halo for the MRN model (?lled squares)and WD01(empty squares).The data were collected in energy bins 0.25keV wide.The solid line refers to the value of the total τat the center of the bin,as predicted in D03. ergies,the halo spectra collected in di?erent annuli,shown in Fig.7,do not vary dramatically (within statistical er-rors)among each other after the PSF subtraction.Above 2keV the di?erential scattering cross section does change dramatically as a function of the scattering angle (Fig.1);however the contribution of the PSF wings is dominant at this energy and this change is unobservable in the present data.In principle,after the PSF subtraction,the model-ing of the halo spectra,at each angular distance,would reveal the local properties of the dust grains.Indeed,in-homogeneity in the dust spatial distribution,size of the dust particles,and chemical composition would result in a spectral change.Unfortunately,these changes are tiny with respect to the instrumental uncertainties.The model-ing here refers just to the ring around 4.8′,where the halo is brighter and thus the statistics are highest.The spec-tral features are still very well observable after the PSF subtraction.If arti?cially ?tted with Gaussian pro?les,the signi?cance is 2.3σ,3.2σ,and 4σfor O,Mg,and Si,respec-tively.Although the dust features of oxygen,magnesium and silicon are clearly detected in the scattered emission,still the statistics do not allow a unique interpretation of the data.In the attempt to give a quantitative picture of the observational evidence,we chose a model with only the dominant well-known compounds of the di?use interstellar dust environment.We calculated the scattered intensity expected at the chosen angular distance,for the most com-mon constituents of interstellar grains,i.e.graphite (car-bon)and silicates.Silicates are found mostly in the form of Mg 2x Fe 2(1?x )SiO 4and MSiO 3,where M is either Mg or Fe,thus we included olivine in the three di?erent forms of MgFeSiO 4(ρ=3.8g cm ?3),Fe 2SiO 4,(ρ=4.39g cm ?3) Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-211 and Mg2SiO4(ρ=3.27g cm?3).Pyroxene are in the form of MgSiO3(ρ=3.2g cm?3)and FeSiO3(ρ=3.8g cm?3). For a given dust size distribution model(MRN or WD01), the relative contributions of the di?erent grains materials are left as free parameters.Moreover,we considered also a model where only MgFeSiO4is taken as representative compound for silicates(D03).For this model,the dust size is distributed according to WD01,and in the scattering cross sections XAFS are included.The goodness of the?t was evaluated by minimizingχ2. The data are ignored below0.4keV,therefore we do not expect to put any constraint on the carbon component, whose main feature is at0.28keV.However,the carbon component is included in the?t since it may in?uence the shape of the continuum at energies above the car- bon edge.Carbon contribution was allowed to vary be- tween20%and30%of the total amount of dust(Whittet 2003),for all the models we tested.The data are mod- eled by a mixture of silicate and pyroxene.In Fig.11we display the three dust models tested.A mixture of sili- cate compounds(labeled MRN and WD01)seems to ac- ceptably interpret the data.C,O,Fe,Mg and Si alone account for95%of the dust components.All elements ex- cept O account for15-30%of the total amount of dust, in various forms(Whittet2003).Here we assume that these elements describe100%of the observed scattering. The linear combination of the compounds contribution is shown in Fig.12for the MRN distribution.In the WD01 case,the combination of compounds is not signi?cantly di?erent from MRN(Fig.11).The relative contribution to unity is Mg2SiO4=0.42,C=0.26,FeMgSiO4=0.054,and FeSiO3=0.25.Fe2SiO4and MgSiO3contribute for a negli- gible fraction(<0.001).Counting the contributions of the single elements,we obtain that roughly42%of O,26%of C,11%and13%of Si and Mg,and5%of Fe are needed to?t the data. 6.Discussion 6.1.The chemistry of dust grains For the?rst time the signature of the elements locked in dust grains and responsible for the scattering of the X-rays is detected.The best?t indicates a major contri- bution by olivine and pyroxene.We cannot exclude the presence of other di?erent compounds.Being the best?t a linear combination of scattered intensities for a given compound,adding too many components would not be necessarily a true physical interpretation of the data.In principle,the depth of the spikes in the scattered spectrum tells us about the intrinsic properties of the dust grains. Magnesium and silicon are not detected but marginally in absorption(Fig.3),while they are prominent features in the scattered spectrum,even in the raw data at the angles where the statistics is maximal(Fig.7).Only a Fig.11.EPIC-pn data of the Cyg X-2halo,extracted at～4.3′from the source and PSF subtracted(asterisks), compared with the best-?t models:WD01(dashed dot- ted line),MRN(solid line),and D03+WD01model(dia- monds).O-K,Mg-K,and Si-K edge energies are indicated by dotted lines. Fig.12.Relative contributions to the best-?t model,us- ing the MRN distribution.The data are the same as in Fig.11. deeper observation will allow us to quantify this possible discrepancy by comparing the column densities for scat- tering and absorption derived for Mg and Si.As Fig.3 shows,for an absorbing equivalent hydrogen column den- sity of2.2×1021cm?2,the iron L-shell at0.706keV is clearly measured.In the scattered spectrum iron is hardly detectable.The best?t model(Fig.12)requires a certain amount of Fe,although not precisely quanti?ed,locked in silicates,with a Mg to Fe ratio of～5:(1.4-2.5).The deple- 12Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-2 tion of Mg,as well as Si,depends on the density of the dust environment.Di?erently,Fe is found to be highly de-pleted(80-100%)from the intercloud medium to the dens-est environments.Thus,we can suppose that Mg is more abundant in silicates than Fe.In particular,a mixture of olivine and pyroxene with a ratio of5:2for Mg:Fe,would totally account for the depletion of Mg and Si(Whittet 2003).In this frame,less than half of the available iron grains are locked in silicates and the rest in other forms. The best?t of the scattered halo at～4.8′,within errors,is in agreement with this simple prediction.If the lower limit is taken,there would be more room for iron to be locked in other forms.Weingartner&Draine(1999)suggested,for instance,that up to60%of Fe could be associated with graphite to form very small grains(a～10?15?A).A Mg:Fe ratio of1:1,as prescribed by the D03dust mixture, fails to interpret the spectral energy distribution of the dust toward Cyg X-2.This again reinforces the idea that more compounds containing Mg and Si should play a role in the scattering.As shown in Fig.12,the data requires a major contribution of Mg2SiO4rather than iron com-pounds like FeSiO3.This dust mixture leads to a stronger contribution of Mg with respect to the D03model. This interpretation is in?uenced by the PSF subtraction, which can arti?cially enhance the depth of the Si fea-ture(but not signi?cantly for Mg which lies at1.3keV, where the halo is well above the the PSF)and the con-tinuum around it(§4.2).Another unknown is the role of carbon.If the contribution of carbon between0.4and 2keV varies substantially from the20?30%of the total budget,then the contribution of silicate would need to be revisited.The quality of spectral data does not al-low to appreciate a substantial discrepancy between the WD01and MRN dust size distribution.As discussed be-low(§6.3),at the scattering angles we consider for the spectral analysis(around4.8′),the bulk of the scatter-ing is caused by grains with quite“standard”sizes and in this case the MRN and WD01do not di?er dramatically. However,with a deeper observation we would di?erenti-ate more between the two models at each scattering angle and this would in?uence the relative contribution to the scattering of the di?erent compounds.We have seen that the modeling of Si su?ers from the highest uncertainty in this analysis and we cannot draw any conclusion on the basis of this feature.However,the prominence of the Mg feature suggests a signi?cant contribution of magne-sium compounds.The oxygen region of the scattered spec-trum is well interpreted by D03.This is because of the dielectric functions,which include XAFS near the edge energy(D03).When convolved with the spectral resolu-tion of these data,such sub-structures are almost totally canceled:the di?erence in depth between the D03and the Henke,Gullikson,&Davis(1993)dielectric functions are indeed～1%for Mg and Si,and～20%for iron.Oxygen is the only element where the discrepancy is noticeable(～a factor two).Evaluating the contribution of the single ele-ments,assuming that C,O,Fe,Mg,and Si are causing the totality of the scattering,we?nd that oxygen can be com-pletely explained in terms of silicate.This supports the idea that oxygen is preferably locked in these materials, leaving little room to other O compounds(e.g.OH,H2O, Whittet et al.2001),at least in the di?use ISM.The fea-tures of the scattered halo,as the one shown in Fig.11, were predicted by the scattering theory(e.g.Hayakawa 1970;Predehl&Klose1996,D03).The dust physical pa-rameters extracted from our modeling seems to bolster this interpretation.Another possibility for producing a complex halo spectrum would be an uneven absorption at an angular scale of arcminutes around the source.If, for example,the absorption toward the central source is few percent less than at the outer parts,the halo/source ratio will show extra absorption at soft energies,similar in shape to what we observe(Fig.7).In this case,at the en-ergy of the absorption edges,we would expect also broad and asymmetric residuals.However,we found the spectral shape of the halo to have circular symmetry around the central source and this is in con?ict with a clumpy struc-ture of the medium,and the features we observe in the scattered spectrum seem not to have an asymmetric,edge-like,shape.Moreover,there should be regions where,on the contrary,N H is lower than toward the central source. In this case,the halo/source ratio should show an excess at soft energy and this was not observed.Although the assumption that in the di?use ISM dust is homogeneously distributed is surely simplistic,in the case of Cyg X-2we cannot prove that spatial variations of N H on arcminutes scales play a major role in the halo shape. 6.2.Oxygen in the ISM The most prominent absorption features in the RGS band arise from oxygen transitions.The O i resonant line cor-responding to the1s-2p transition is clearly detected. Whether we can observe O(Takei et al.2003)in molecu-lar form is not obvious.The uncertainty is enhanced con-sidering the lack of laboratory measurements for oxygen locked in compounds at these energies.The RGS resolu-tion does not allow us to give a unique interpretation of the spectral region at energies higher than the oxygen edge. In terms ofχ2,the addition of one or possibly two more absorption edges or,alternatively,modeling the spectrum with three absorption lines(O i1s-3p,O ii1s-2p,O iii,1s-2p),provides similar results.From the curve of growth analysis,Juett,Schulz,&Chakrabarty(2004)calculate that the contribution of the ionized oxygen should be10% of the neutral phase.The mildly ionized oxygen would then be produced by charge exchange with O i.However, in principle,absorption by oxygen in compounds is ex-pected.In the present XMM-Newton observation,in ad-dition to absorption,we could also study the scattering by ID and this allows us to state that oxygen locked in dust is actually present on the line of sight of Cyg X-2 (Fig.11).The modeling of the halo spectrum shows that, to?rst order approximation,the oxygen feature is well explained if all the oxygen available in dust is locked in Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-213 silicates.This translates into an oxygen to hydrogen ratio O dust/H=180ppm(Cardelli et al.1996)in the line of sight of Cyg X-2.If the oxygen abundance O/H is taken to be490ppm(Wilms,Allen&McCray2000),then the depletion value for O,de?ned as the gas to total ISM ratio, is0.63.With the total hydrogen column density derived from the RGS measurement(N H～2.2×1021cm?2),we can then predict an absorption column density for oxy-gen locked in(silicate)dust.We obtain N O abs for dust ～3.85×1017cm?2.In the Takei et al.(2003)model(col-umn1,Tab.2),the edge attributed to oxygen in dust has an absorption optical depth of0.27±0.06at0.536keV. Using the absorption cross sectionσO abs=1.78×10?19cm2, (D03,Li et al.1995),the column density for this edge is 1.5+0.3 ?0.4 ×1017cm?2,which is too low compared to the the-oretical value we obtained above.One possibility is that other absorption edges by oxygen in solid form are missing the detection.Indeed the energy and the structure of the edges slightly changes depending on which compound of oxygen and silicon is considered(e.g,Owens et al.2002; Wong et al.1995),and therefore some of them may be blurred and di?cult to detect with the present resolution. This calculation strongly depends on the adopted value of σO abs.Here we considered the peak absorption cross sec-tion for oxygen locked in Mg2SiO4(D03,Li et al.1995), as determined by D03,with the underlying assumption that the oxygen edge pro?le is the same as the Si pro-?le.However,small variations ofσO abs,lead to substan-tial variations of the value of N H.In fact,if the value σO abs is closer to what predicted for atomic oxygen(σ= 4.8×10?19,Verner&Yakovlev1995;σ=3.85×10?19, Henke,Gullikson&Davis1993),then the column density associated to the0.536keV edge reconciles to the value we obtained from the scattered radiation analysis.As a cautionary note,the e?ective area of EPIC-pn diminishes toward lower energy(60%less at0.54keV than at1keV) and the statistics at the oxygen feature may prevent us from a full description of the dust contribution to the to-tal extinction.Moreover,the value for the total O/H ratio is not yet solidly established(Jensen et al.2005,and ref-erences therein).Finally,the knowledge of the theoretical wavelengths of these dust absorption features is very frag-mentary,making their interpretation somehow aleatory. Taking the N O abs derived starting from the scattering anal-ysis(andτ=N O absσO abs=0.68,if D03value is taken forσ)as an upper limit,absorption by oxygen in dust should be visible in absorption(edges and lines)using the RGS,and instruments with even higher energy resolution (Takei et al.2003). 6.3.The dust size distribution In this XMM-Newton observation,the pile-up prevents us from studying the halo at angular radii smaller than ～40′′.This observational limit also hampers any detection of scattering either by large grains or standard grains lo-cated very close to the emitting source(Predehl&Klose 1996).Indeed these two conditions have the same e?ect of producing very narrow halo components in the pro?le. The data modeling of both the halo spatial pro?le and the halo spectral distribution show that both the MRN and WD01dust size distribution can be applied despite the di?erent amount of dust predicted for di?erent scat-tering angles(Fig.9).Grains with sizes>0.25μm play a more important role in the WD01model for radii<～200′′at1keV.In the size range0.001-0.25μm,the MRN and WD01model do not di?er dramatically for R V=3.1 (see Fig.2of Weingartner&Draine2001)and for the grain sizes roughly between0.1-0.2μm typically produce the bulk of the scattering halo at angular radii between 100′′and1000′′(e.g,Mathis&Lee1991;Draine&Tan 2003),which is the region that could be directly studied in the SBP of Cyg X-2.The contribution of small grains (a<～0.05)would be best investigated if we could access the region beyond1000′′,but unfortunately,in this ob-servation the halo begins to fade,making the modeling challenging beyond～650′′. The value of the scattering optical depth derived from the SBP at1keV(τsca=0.054±0.018)is larger,but consistent within the errors,with the value derived by the ROSAT halo(τsca～0.039,PS95)obtained at～1.06 keV,if the MRN distribution is used.The PS95model-ing indeed started also from the MRN model,but leaving free some parameters in it,among which the maximum grain size and the slope of n(a).Their best?t requires a maximum grain size of only0.15μm.This makes the halo model?atter and the derived value ofτsca lower than what we measure.The WD01distribution predicts instead almost the double(τsca=0.067±0.018)of the ROSAT result.This trend is visible at all energies(Fig.10).This is mostly due to the increased scattering“power”by larger grains at smaller radii predicted by the WD01(Fig.9). When the total scattering optical depth(for both MRN and WD01case)as a function of energy is compared with a theoretical model(D03,Fig.10),we see that it does not measure a substantial part of the scattered ra-diation.Indeed the model over-predicts the data at all energies apart from perhaps the last point.The distri-bution ofτsca?attens toward lower energies,while the model,is signi?cantly steeper.As noted in D03,the dis-crepancy was also found for other halo analysis(e.g., PS95,Smith,Edgar,&Shafer2002;Woo et al.1994). The Draine&Tan(2003)measurement instead has been found in agreement with the model(D03). The value of I frac(and thus ofτsca),extracted at each energy,is model dependent and,moreover,can be sub-stantially in?uenced by instrumental e?ects.For exam-ple,the scattering angle range accessible to this study (200′′?600′′)strongly privilege the observation of the scat-tering of1?2keV photons(e.g.,Mathis&Lee1991, D03),while the bulk of the emission for softer photons peaks at larger scattering angles.The estimated values for the total scattering optical depth,and in particular the ones related to the soft scattered photons are certainly lower limits.A signi?cant fraction of the halo,in the form 14Costantini,Freyberg&Predehl:XMM-Newton observation of Cyg X-2 of fainter or narrower components,may be masked by the PSF wings(at small radii)or unaccessible because of the faint scattered emission with respect to the background (at larger radii). 6.4.The dust distribution along the line of sight The distribution of dust along the line of sight could be studied through the SBP of the halo.The distribution of dust seems to be evenly distributed,at least for a frac-tional distance of the total path x<0.6,corresponding to a linear distance between4.3-6.7kpc,(depending on the distance estimates for Cyg X-2;Orosz&Kuulkers 1999;Smale1998).However,if the dust distribution is imposed to be uniform up to a fractional distance of0.99 the?t worsens signi?cantly suggesting that our line of sight passes through di?erent dust clumps.A contribution from scattering events closer to the source is indeed likely to be present,but fainter or narrower halo components, as detected in other sources(e.g.Smith,Edgar,&Shafer 2002;Draine&Tan2003;Costantini&Predehl2005), cannot be investigated using XMM-Newton for radii<12′′(and in any case not for Cyg X-2because of the pile-up). 7.Conclusions We have presented XMM-Newton results on the e?ect of scattering and absorption by ID along the line of sight to the bright X-ray binary Cyg X-2.This study led to the unprecedented detection of the elements in the ID respon-sible for the X-ray scattering:oxygen,magnesium,and silicon.To?rst order,the modeling of the pure scattered radiation suggests a major contribution of silicates in the form of olivine and pyroxene,in the energy range0.4-2 keV.The best?t of the scattered spectrum shows that the ratio of Mg to Fe,locked in dust grains,is5:(1.4:2.5). This is consistent with a picture in which Mg and Si are for the most part locked in silicates(Whittet2003). The contribution of carbon,a fundamental constituent of ID,could not be quanti?ed as its most prominent feature (0.28keV)lies below our selected EPIC-pn energy band. In the RGS spectrum,we studied the complexity around the oxygen edge,investigating the possibility of absorption by atomic and molecular oxygen,as suggested by Takei et al(2002),in comparison with absorption by atomic neu-tral and ionized oxygen(Juett,Schulz,&Chakrabarty 2004).The RGS resolution is not su?cient to prefer one interpretation to the other,but the complementary infor-mation from the EPIC-pn analysis of the scattered spec-trum allows us to detect oxygen locked in dust,preferably in the form of silicates.Starting from the scattered halo spectral modeling,we estimated that the absorption col-umn density we expect by oxygen locked in silicates is indeed measurable using the RGS.The value of this col-umn density seems too high compared to what measured for the absorption edge that Takei et al.(2003)interpret as arising from dust.However,instrumental/theoretical uncertainties makes this result not conclusive.The study of the spectral energy distribution of the scat-tered radiation,performed in the halo region where the signal-to-noise ratio was best(around～4.8′),stressed the need of using an accurate theoretical approach to the data.The full Mie theory had to be used to model the data satisfactorily,especially below2keV where the chem-istry of the halo can now be studied.This approach was already applied to ROSAT data(Smith&Dwek1998; Draine&Tan2003).With XMM-Newton we could ex-tend this analysis,performing spatially resolved spec-troscopy of the halo that could not be interpreted unless the Mie di?erential scattering cross section was used. The modeling of the SBP shows that the dust is uniformly distributed along the line of sight at least for a fractional distance of the total path x<0.6,corresponding to a lin-ear distance between4.3-6.7kpc,depending on the source distance estimates.However,a uniform dust distribution along the complete path toward Cyg X-2is not required by the data,hinting to a clumped structure of the dust for x>0.6.Within the instrumental uncertainties,the data are acceptably?t by both a MRN and a WD01dust size distribution.The inferred scattering optical depth is approximately0.054and0.067at1keV for the MRN and WD01distribution,respectively.We extended the mod-eling of the SBP of the halo to the0.4-2keV band.The derived values of the totalτsca as a function of energy are systematically lower than what predicted by the theory (D03),pointing out that some halo components may be easily missed due to instrumental e?ects. Acknowledgements.The authors thank the referee, Prof. B.T.Draine,for his valuable comments which im-proved the quality of the paper.This project was developed for the most part during the stay of E.C.at the Max-Planck-Institut f¨u r extraterrestrische Physik.E.C.wishes to thank also K.C.Steenbrugge and F.Verbunt for carefully reading this manuscript.The XMM-Newton project is supported by the Bundesministerium f¨u r Bildung und Forschung/ Deutsches Zentrum f¨u r Luft-und Raumfahrt(BMBF/DLR), the Max-Planck-Gesellschaft and the Heidenhain-Stiftung. References Andr′e,M.K.,et al.2003,ApJ,591,1000 Bradt,H.V.D.&McClintock,J.E.1983,ARA&A,21, 13 Cardelli,J.A.,Meyer,D.M.,Jura,M.,&Savage,B.D. 1996,ApJ,467,334 Cartledge,S.I.B.,Lauroesch,J.T.,Meyer,D.M.,& So?a,U.J.2004,ApJ,613,1037 Costantini,E.&Predehl,P.2005,in preparation. Dame,T.M.,Hartmann,D.,&Thaddeus,P.2001,ApJ, 547,792 den Herder,J.W.,et al.2001,A&A,365,L7 de Vries,C.P.,den Herder,J.W.,Kaastra,J.S.,Paerels, F.B.,den Boggende,A.J.,&Rasmussen,A.P.2003, A&A,404,959 Dickey,J.M.&Lockman,F.J.1990,ARA&A,28,215 Di Salvo,T.et al.2002,A&A,386,535 Costantini,Freyberg &Predehl:XMM-Newton observation of Cyg X-215 Draine,B.T.2003,ApJ,598,1026(D03)Draine,B.T.&Tan,J.C.2003,ApJ,594,347Freyberg,M.J.,The XMM-Newton EPIC-pn camera:Spectral and Temporal properties of the internal back-ground (XMM-Newton Calibration Report,June 2003)Gallagher,D.,Cash,W.,&Green,J.1995,ApJ,439,976Ghizzardi,S.2002,“In Flight Calibration of the PSF for the PN Camera”,XMM-Newton Calibration Report,EPIC-MCT-TN-012,http://xmm.vilspa.esa.es/docs/documents/CAL-TN-0029-1.0.ps.gz Gorczyca,T.W.&McLaughlin,B.M.2000,Journal of Physics B Atomic Molecular Physics,33,L859Hayakawa,S.1970,Prog.Theor.Phys.,43,1224Juett,A.M.,Schulz,N.S.,&Chakrabarty,D.2004,ApJ,612,308Henke,B.L.,Gullikson,E.M.,&Davis,J.C.1993,Atomic Data and Nuclear Data Tables,54,181Jensen,A.G.,Rachford,B.L.,&Snow,T.P.2005,ApJ,619,891 Li,D.,Bancroft,G.M.,Fleet,M.E.&Feng.X.H.,1995,Phys.Chem.Minerals,22,115Landgraf,M.,Baggaley,W.J.,Gr¨u n,E.,Kr¨u ger,H.,&Linkert,G.2000,J.Geophys.Res.,105,10343 Lee,J.C.,Reynolds,C.S.,Remillard,R.,Schulz,N.S.,Blackman,E.G.,&Fabian,A.C.2002,ApJ,567,1102Mathis,J.S.,Rumpl,W.,&Nordsieck,K.H.1977,ApJ,217,425(MRN) Mathis,J.S.&Whi?en,G.1989,ApJ,341,808Mathis,J.S.,&Lee,C.-W.1991,ApJ,376,490 Mathis,J.S.,Cohen,D.,Finley,J.P.,&Krautter,J.1995,ApJ,449,320 Mauche,C.W.&Gorenstein,P.1986,ApJ,302,371McLaughlin,B.M.&Kirby,K.P.1998,Journal of Physics B Atomic Molecular Physics,31,4991Mie,G.1908,Ann.Phys,Lpz 25,377Mitsuda,K.et al.1984,PASJ ,36,741Overbeck,J.W.1965,ApJ,141,864 Orosz,J.A.&Kuulkers,E.1999,MNRAS,305,132Owens, A.,Fraser,G.W.,&Gurman,S.J.2002,Radiation Physics and Chemistry,65,109Predehl,P.&Klose,S.1996,A&A,306,283 Predehl,P.&Schmitt,J.H.M.M.1995,A&A,293,889(PS95) Read,A.M.,&Ponman,T.J.2003,A&A,409,395Smale,A.P.1998,ApJ,498,L141 Smith,R.K.&Dwek,E.1998,ApJ,503,831 Smith,R.K.,Edgar,R.J.,&Shafer,R.A.2002,ApJ,581,562 Snow,T.P.,&Witt,A.N.1995,Science,270,1455Str¨u der,L.,et al.2001,A&A,365,L18 Takei,Y.,Fujimoto,R.,Mitsuda,K.,Onaka .T.2003,ApJ,581,307 Titarchuk,L.1994,ApJ,434,570 Turner,M.J.L.,et al.2001,A&A,365,L27 van de Hulst,H. C.1957,Light Scattering by Small Particles,New York:John Wiley &Sons Verner,D.A.&Yakovlev,D.G.1995,A&AS,109,125Weingartner,J.C.&Draine,B.T.1999,ApJ,517,292 Weingartner,J.C.&Draine,B.T.2001,ApJ,548,296(WD01) Whittet,D.C.B.,Pendleton,Y.J.,Gibb,E.L.,Boogert,A.C.A.,Chiar,J.E.,&Nummelin,A.2001,ApJ,550, 793 Whittet,D.C.B.2003,Dust in the galactic environ-ment,2nd ed.by D.C.B.Whittet.Bristol:Institute of Physics (IOP)Publishing,2003Series in Astronomy and Astrophysics,ISBN 0750306246.Wilms,J.,Allen,A.&McCray,R.2000ApJ,542,914Wiscombe,W.J.1980,Applied Optics,19,1505Witt,A.N.,Smith,R.K.,&Dwek,E.2001,ApJ,550,L201 Wong,J.,et al.1995,Physica B Condensed Matter,208,220 Woo,J.W.,Clark,G.W.,Day,C.S.R.,Nagase,F.,&Takeshima,T.1994,ApJ,436,L5