A&A 365, L225–L230 (2001)DOI: 10.1051/0004-6361:20000048c© ESO 2001

Astronomy&

Astrophysics

Cassiopeia A: On the origin of the hard X-ray continuum andthe implication of the observed O vııı Ly-α/Ly-β distribution

J. A. M. Bleeker1, R. Willingale2, K. van der Heyden1, K. Dennerl3, J. S. Kaastra1,B. Aschenbach3, and J. Vink4,5,6

1 SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands2 Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK3 Max-Planck-Institut fur extraterrestrische Physik, Giessenbachstraße, 85740 Garching, Germany4 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany5 Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA6 Chandra Fellow

Received 6 October 2000 / Accepted 24 October 2000

Abstract. We present the first results on the hard X-ray continuum image (up to 15 keV) of the supernovaremnant Cas A measured with the EPIC cameras onboard XMM-Newton. The data indicate that the hard X-raytail, observed previously, that extends to energies above 100 keV does not originate in localised regions, like thebright X-ray knots and filaments or the primary blast wave, but is spread over the whole remnant with a rather flathardness ratio of the 8–10 and 10–15 keV energy bands. This result does not support an interpretation of the hardX-radiation as synchrotron emission produced in the primary shock, in which case a limb brightened shell of hardX-ray emission close to the primary shock front is expected. In fact a weak rim of emission near the primary shockfront is discernable in the hardest X-ray image but it contains only a few percent of the hard X-ray emissivity. Theequivalent width of the Fe-K line blend varies by more than an order of magnitude over the remnant, it is hard toexplain this as Fe-emission from the reverse shock heated ejecta given the ejecta temperature and the age of theremnant. The uniquely high wavelength-dispersive RGS-spectrometer has allowed, for the first time, to extractmonochromatic images in several highly ionised element species with high spectral resolution. We present here apreliminary result on the measurement of the O viii Ly-α and Ly-β brightness distribution and brightness ratios.The large observed decrease of the Ly-α/Ly-β ratio going from the N to the SE can be explained by small-scale(10′′) variations in the NH column over the remnant and the potential presence of resonance scattering of theO viii Ly-α photons in the limb brightened shell.

Key words. ISM: supernova remnants – ISM: individual objects: Cas A – X-rays: ISM

1. Introduction

The young galactic supernova remnant (SNR)Cassiopeia A (Cas A) is widely believed to be theresult of the core collapse of a massive star, probablyan early type Wolf-Rayet star (Fesen et al. 1987). CasA is classified as an oxygen rich remnant since opticalspectroscopic observations (Chevalier & Kirshner 1979)show the supernova ejecta (in the form of fast movingknots) to contain mostly oxygen and oxygen burningproducts such as sulphur, argon and calcium. At allwavelengths Cas A has the appearance of a broken shellwith a radius varying between 1.6′ to 2.5′.

One of the outstanding problems in the study of Cas Ais the origin of a recently discovered hard X-ray contin-uum in the spectrum of this remnant. From a theoreti-cal point of view both synchrotron emission from shock

Send offprint requests to: J. A. M. Bleeker,e-mail: J.A.M.Bleeker@sron.nl

accelerated electrons and non-thermal Bremsstrahlungfrom electrons accelerated from the tail of the thermalemission could be possible explanations for the hard con-tinuum. In addition, high resolution spatially resolvedspectroscopy of the remnant in the X-ray domain is a pow-erful tool to study in detail the distribution and physicalproperties of the (reverse) shock heated plasma. The band-width (up to 15 keV) of XMM-Newton and the presence ofhigh wavelength-dispersive spectrometers (RGS) offers anunique opportunity to address these questions. In this re-spect XMM-Newton strongly complements Chandra, andthis paper gives a first highlight of this capability.

2. Observations

A description of the instrument is given by Jansen et al.(2001). The data were obtained in July 2000 with a netexposure time of 30 ks. The telescope was pointed at thecentre of the remnant (α = 23h23m25s δ = 58◦48′20′′) and

Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20000048

L226 J. A. M. Bleeker et al.: Cas A

the telescope roll angle was such that the RGS dispersionaxis was aligned at 45◦ (NE) on the sky.

The raw data were processed with the develop-ment version of the XMM-Newton Science AnalysisSystem (SAS). The RGS spectra were extracted by ap-plying spatial filters to the spectral image and the spec-tral order (m = −1) is selected applying the appropriatepulseheight intervals to the CCD spectral camera.

3. The hard X-ray data

A high-energy tail has been observed in the X-ray spec-trum of the supernova remnant Cassiopeia A by theCompton Gamma Ray Observatory (The et al. 1996),BeppoSAX (Favata et al. 1997) and the Rossi X-rayTiming Explorer (Allen et al. 1997). Previous hard X-rayimaging observations, for example using BeppoSAX (Vinket al. 1999) indicated that the hard continuum radiationoriginated predominantly in the W region of the remnant,however this result was based on deconvolved images froma typically 1′ resolution telescope with moderate effectivearea. The combination of large collecting area in the en-ergy band 4.0 to 12.0 keV and angular resolution of a fewarcseconds of XMM-Newton provides us with a unique op-portunity to search for the distribution and origin of thishard “tail”.

3.1. The X-ray spectrum above 4.0 keV

Below 4.0 keV the observed spectrum is a complicatedcombination of spectral lines, continuum emission and theeffects of interstellar absorption but above 4.0 keV it isdominated by a smooth continuum and the Fe-K emissionline complex. A joint spectral fit of the EPIC MOS and PNspectra was performed in the energy band 4.0–12.0 keVfor the complete remnant. The model included a singleBremsstrahlung component plus a power law continuumwith photon index fixed at the previously observed value(Allen et al. 1997) of 1.8 below a break energy of 16 keV.Gaussian line features were used to fit the obvious Fe andNi line blends. The resulting fit is shown in Fig. 1. Thebest fit Bremsstrahlung temperature is 2.56 ± 0.05 keVwith emission measure 1.99 ± 0.04 1059 cm−3 for a dis-tance of 3.4 kpc and the power law flux at 1 keV is0.0416± 0.003 photons cm−2 s−1 keV−1. The Fe line en-ergy is 6.603±0.001 keV in the MOS and 6.623±0.002 keVin the PN. The errors quoted are statistical only andindicate the high quality of the data, the difference be-tween the MOS and PN values may be attributed to sys-tematic uncertainties which are at present of the orderof 10 eV. We estimated the effect of potential pile-upin the PN camera to be not larger than ∼3% for thebrightest regions. Figure 1 indeed shows that the MOSspectra, which do not suffer from pile-up, and the PNspectrum are fully compliant. The equivalent width ofthe Fe line is 1.02 keV for the MOS and 1.11 keV forthe PN. The Ni line energy is 7.740 ± 0.015 keV in theMOS and 7.783± 0.015 keV in the PN corresponding to

Fig. 1. Joint spectral fit of the hard tail from the completeremnant, MOS 1 red, MOS 2 green and PN blue. The dashedand dotted line indicate the Bremsstrahlung and power lawcomponents contributing to the MOS continuum

Ni xxvi 7.687 keV and/or Ni xxvii 7.799 keV. The equiv-alent width of the Ni line was 0.130 keV for the MOS and0.179 keV for the PN. Allen et al. (1997) fitted the compos-ite spectrum of Cas A over a much broader energy rangeextending out to 100 keV using two Raymond-Smith col-lisional ionisation equilibrium (CIE) thermal componentswith kT1 = 0.6 and kT2 = 2.9 keV to fit the soft band be-low 6 keV and a broken power law (Eb = 15.9 keV) withphoton index 1.8 below Eb and 3.0 above Eb with a fluxof 0.038 photons cm−2 s−1 keV−1 at 1 keV. Favata et al.(1997), from the BeppoSAX observations, obtained a con-tinuum parameterized by two non-equilibrium ionisation(NEI) components with kT1 = 1.25 keV, kT2 = 3.8 keVand a power law with photon index 2.95 with a normalisa-tion of 0.74 photons cm−2 s−1 keV−1 at 1 keV. It is mostimportant to note that, irrespective of the exact parame-terization of these fits, they all share the fact that the X-ray flux above 8 keV becomes dominated by the power lawcomponent. More quantitatively, our spectral fit predicts aratio between the power law and the Bremsstrahlung com-ponent in the MOS cameras of 0.29 in the interval 4–6 keV,1.04 in the 8–10 keV band and 2.00 in the 10–12 keV hard-est band. We shall come back to this point in the nextsection on the X-ray images.

J. A. M. Bleeker et al.: Cas A L227

Fig. 2. Continuum 4.06–6.07 keV (left) and Fe K equivalentwidth (right) black 0 keV to white 4 keV and above

Fig. 3. Continuum 8.10–15.0 keV (left) and hardness ratio(10.0–15.0 MOS+PN)/(8.10–10.0 MOS+PN) 0.15 black to0.45 white

3.2. X-ray images above 4.0 keV

The MOS and PN images are practically identical.Figure 2 shows MOS images and Fig. 3 shows combinedMOS and PN images. The lefthand panel of Fig. 2 is thecontinuum map for 4.07–6.07 keV an energy band in whichno bright spectral lines are visible in either the EPIC MOSor PN spectra. The peak of the hard continuum occurs inthe large knot to the W but there is also extended emis-sion throughout the volume of the remnant. The outer rim(presumably just behind the blast wave) is visible but isvery faint compared with the central volume.

The righthand image of Fig. 2 is the Fe K lineequivalent width derived from the narrow energy band6.20–6.92 keV. The continuum used to estimate the equiv-alent width was estimated by interpolation from the bands4.06–6.07 keV and 8.10–15.0 keV. The Fe line is verybright relative to the continuum on the outer extremi-ties of the SE knots, i.e. region marked P, but is ratherweaker in the W where the continuum is bright, i.e. re-gion marked L. The lefthand panel of Fig. 3 is the hardcontinuum above the Fe K line, 8.10–12.0 keV, which isvery similar to the softer continuum image in Fig. 2. Inorder to try and locate where the hard X-ray tail origi-nates we calculated the hardness ratio (10.0–15.0)/(8.10–10.0) using both the MOS and PN cameras to give thebest possible statistics. The result is shown in the right-hand panel of Fig. 3. The contour superimposed in greenis taken from the hard continuum map 8.10–15.0 keV andcontains 37% of the total counts in this energy band, thehardest region in the W contains less than 3% of the total.

Table 1. Results of joint spectral fits using a singleBremsstrahlung and Gaussian line 4.0–12.0 keV. Listed are thetemperature kT , line centroid E`, equivalent width EW andline sigma width σ. The first values for E`, EW and σ are forthe MOS cameras and the second from the PN

Parameter all L P

kT (keV) 3.53± 0.02 3.41 ± 0.14 3.15 ± 0.34E` (keV) 6.60 6.60 6.67

6.62 6.61 6.68EW (keV) 0.95± 0.02 0.57 ± 0.08 6.19 ± 0.56

1.01± 0.02 0.43 ± 0.18 5.28 ± 0.53σ (keV) 0.083 0.079 0.047

0.078 0.042 0.031

The remaining 63% of the hard flux is spread out over therest of the remnant. There are significant changes in hard-ness over the remnant but all the bright features within thegreen contour have a remarkably similar spectrum above8.0 keV.

From the spectral fits described in the previous section,it is clear that the photon flux observed in the continuumimage in Fig. 3 is dominated by the hard tail (power law)component. XMM-Newton clearly detects the hard X-raytail from the remnant but the hard X-ray image and thehardness ratio indicate that this flux does not predominatein a few localised regions, but pervades the whole remnantin a distribution similar to the softer thermal components.

3.3. The Fe K emission

Simpler spectral fits using a single Bremsstrahlung andone gaussian line were used to quantify the variation of thecontinuum and Fe K emission over the remnant. Region Lin Fig. 2 corresponds to the brightest hard continuum anda rather low Fe equivalent width and region P to ratherweak continuum with the highest Fe K equivalent width.The same model was also used to fit the composite spec-trum. The results are summarised in Table 1. The appar-ent temperature in these fits is a measure of hardness andis higher than the value reported above because the powerlaw continuum has been omitted for simplicity. It is clearthat the temperature for the composite fit is similar tothat of the small regions (within the statistical accuracy),again indicating that the hard flux is distributed through-out the remnant. As shown in Fig. 2 and quantified inTable 1 the variation in equivalent width of the iron lineis very large. We attempted to fit the composite spectrumfrom MOS 1 + 2+ PN 4.07–12.0 keV with a simple NEImodel (see for example Borkowski 1994) plus a power law.The power law was again fixed at index 1.8 and the tem-perature kT fixed at 2.6 keV. This gave a good fit to thecontinuum with the Fe line blanked out.

A reasonable fit was obtained for an ionisationtimescale parameter of 5.0 1011 s cm−3 or greater. Foran age of 340 years, (1010 s) this implies a density of>50 cm−3 which is rather large. This fit gave about thecorrect equivalent width for the Fe line (assuming solar

L228 J. A. M. Bleeker et al.: Cas A

abundances), but did not fit the line profile very well, pre-sumably because the line is Doppler shifted and/or broad-ened. It also gave a reasonable fit to the Ni line featureat around 7.7 keV. Lower more reasonable values of thetime scale 5.0 1010 s cm−3 corresponding to a density of5 cm−3 predict a much larger equivalent width (a factorof 4 too large), a line energy which is too low and no Niline at 7.7 keV.

Using the same model on region P in the SE wherethe equivalent width of Fe is very high gave a poor fit. Nocombination of temperature or time scale could producea strong enough line at the correct energy. The E knotis clearly anomalous with abnormally high Fe abundanceand/or extreme non-thermal equilibrium ionisation.

3.4. Discussion

The spectral form of the non-thermal high-energy “tail” isnot inconsistent with a simple model of synchrotron emis-sion from SNRs Reynolds (1998). However in such a modelthe electrons need to be accelerated to energies of tens ofTeV at the primary shock and the associated synchrotronemission would be expected to be concentrated in the com-pressed magnetic field just inside the shock front. The hardX-ray continuum maps from XMM-Newton indicates thatthe 8.0–15.0 keV flux, predominantly due to the previouslyreported high-energy tail, does not originate from a fewlocalized regions such as X-ray bright knots and filaments,nor does it originate from a limb brightened (fractionary)shell close to the shock front generated by the primaryblast wave. In fact a low brightness outer ring structure,presumably associated with the primary shock, can be dis-cerned in the hard X-ray image but it contains only a fewpercent of the total hard X-ray flux. Therefore the hardX-ray image is morphologically inconsistent with the sim-ple synchrotron model developed by Reynolds (1998). Analternative explanation for the observation of hard X-raytails in the spectra of supernova remnants is the presenceof non-thermal Bremsstrahlung generated by a populationof suprathermal electrons (Asvarov et al. 1989; Laming2000). One might expect in this case some degree of corre-lation with the strong line emitting regions, however abun-dance variations make this potential correlation ambigu-ous. A search of Cas A for regions with spectra devoid ofline emission, as a possible tracer of synchrotron emission,was unsuccesful although Chandra observations indicatedthe presence of such a region just beyond the westernmosttip of the N rim of the remnant (Hughes et al. 2000). Wespecifically checked this region and still confirm the pres-ence of, relatively weak, line emission, with for exampleequivalent widths of 200–300 eV for the He-like triplet ofSi, S and Fe. We cannot exclude some contamination byline emission from neighbouring regions due to the wingsof the XMM psf, on the other hand the Chandra datafor this region also show line residuals above the contin-uum fit presented by Hughes et al. In summary we havenot spotted any positive evidence, either morphologically

Fig. 4. First order (m = −1) RGS spectra of Cas A for thethree regions. Line identifications of the principle lines areprovided

or spectrally, for the presence of synchrotron emission inCas A and, by implication, the associated TeV electrons.Consequently, whether the hard X-ray tail is thermal ornon-thermal remains an open question.

4. High-resolution spectroscopic data

4.1. Analysis and results

Figure 4 displays spectra from three extraction regionsover the remnant situated in the N, NE and SE of theremnant. The data from RGS1 and RGS2 have been com-bined in order to increase the statistical weight of the ob-servation. Several lines of highly ionized species of Si, Mg,Ne, Fe L and O are detected in the spectrum. Due to inter-stellar absorption no features can be measured long-wardof ∼20 A. The analysis and interpretation of these spectrawill be the subject of a forthcoming paper.

As a first result for this paper we extracted images ofthe O viii Ly-α and Ly-β lines to probe small scale vari-ations in absorption effects over this part of the remnantand to investigate the potential presence of resonance scat-tering in the limb brightened shells viewed edge-on. Thetemperature range relevant for Cas A has no influence onthe Ly-α/Ly-β ratio. Since RGS is a slitless spectrometerit is possible to extract dispersed monochromatic imagesof Cas A. These images were converted from wavelength tospatial coordinates using the equation ∆λ = d sin(α)∆φ,where ∆φ is the offset along the dispersion direction, ∆λthe wavelength shift, α the angle of incidence on the grat-ings and d the line distance of the gratings. However, anyDoppler broadening is also convolved along the disper-sion direction and, depending on its magnitude, Dopplerbroadening could distort the RGS images. The O viii Ly-αand Ly-β maps are presented in Fig. 5. The oxygen emis-sion is seen to originate from three blobs on the E andN rim of the remnant. The emission is faintest in the SEblob where there also seems to be a Ly-α deficit compared

J. A. M. Bleeker et al.: Cas A L229

Fig. 5. RGS images of O viii Ly-β (top left) and Ly-α (topright), the dispersion direction runs from top-left to lower-rightat 45 degrees with the vertical. Emission is seen in three blobsin the E and N rim. The ratio map of the Ly-β to Ly-α imagesis shown at the bottom left panel. The enhanced spot on the SErim of the image indicates a Ly-α emission deficit in this region.For comparison we show a soft band MOS image (0.5–1.2 keV)in the lower right panel

to Ly-β. To investigate this we produced cross-dispersionimage profiles of the Ly-α and Ly-β lines by integratingover the dispersion direction. The cross-dispersion profilestogether with the Ly-α to Ly-β ratio have been plottedin Fig. 6 (note that this is the inverse of the ratio plot-ted in the brightness distribution in Fig. 5 because of thelow Ly-α/Ly-β ratio). The ratio plot seems to steepengradually from a value of ∼0.5 to ∼0.15–0.20, at a cross-dispersion above +1′. This decrease seems to indicate aLy-α emission deficit in this region. The same effect isseen in the ratio image presented in Fig. 6.

4.2. Discussion

Three possible causes can be identified for explaining theO viii Ly-α deficit in the SE part of Cas A, i.e. lineblending, NH variation and resonance scattering.

(1) A line blending effect due to the reduced spectral res-olution arising from the finite source extent causes theLy-β line to be “polluted” by neighbouring lines fromthe Fe-L complex. Given the limited angular extent ofthe bright emission region in the dispersion direction,i.e. <1′, and the fact that Cas A is an oxygen-richremnant, this effect can be neglected, i.e. the oxygenemission lines strongly dominate the Fe-L lines and thespectral resolving power is still adequate (R ∼130);

(2) The potential influence of variations in the columndensity NH in this region can be assessed by usingthe column densities given by Keohane (1998). Thesedata take into account both the H i and molecular ab-sorption and have an average resolution of 30′′. In theE part of Cas A they centre at 1.10 1022 cm−2 witha full-width spread ranging from 1.00–1.20 1022 cm−2

Fig. 6. Upper panel: cross dispersion profiles of O viii Ly-β(thick line) and O viii Ly-α (thin line). Lower panel: ratio ofLy-α to Ly-β. Numbers indicate estimated NH values fromKeohane (1998), in units of 1022 cm−2

(only the bright spot Cas A W shows a larger columnof about 1.4 1022 cm−2). Applying this full range goingfrom N (least absorbed) to the SE bright rim (most ab-sorbed) yields a decrease of the O viii Ly-α/Ly-β ratioof 1.4. The observed decrease of the Ly-α/Ly-β ratio,displayed in Fig. 6, appears to be substantially larger,i.e. a factor ≥ 3 from about 0.45 to 0.15. Smaller-scalevariations could be caused by small scale structure inthe NH value on a ∼10′′ angular scale. To fully explainthe decrease observed a column density variation of6 1021 cm−2 (i.e. from 1.0–1.6 1022 cm−2) would haveto be present, which seems rather unlikely;

(3) Alternatively a decrease in the Ly-α/Ly-β ratio couldalso be introduced by the presence of resonance scat-tering of the O viii Ly-α photons in the X-ray brightrims if viewed edge-on (Kaastra & Mewe 1995). A fac-tor of 2–3 reduction in Ly-α intensity can be explainedby an optical depth in the Ly-α line of 2 or more (fol-lowing the discussion in Kaastra & Mewe). For typicalconditions in the rims of Cas A, an optical depth largerthan 2 is expected to be present if the micro turbulentvelocity is smaller than ∼700 km s−1.

Acknowledgements. The results presented are based on obser-vations obtained with XMM-Newton, an ESA science mission

L230 J. A. M. Bleeker et al.: Cas A

with instruments and contributions directly funded by ESAMember States and the USA. We thank the referee J. Balletfor his helpful comments and suggestions.

References

Allen, G. E., Keohane, J. W., Gotthelf, E. V., et al. 1997, ApJ,487, L97

Asvarov, A. I., Guseinov, O. H., Kasumov, F. K., & Dogel’, V.A. 1990, A&A, 229, 196

Borkowski, K. J., Sarazin, C. L., & Blondin, J. M. 1994, ApJ,429, 710

Chevalier, R. A., & Kirshner, R. P. 1979, ApJ, 233, 154

Favata, F., Vink, J., dal Fiume, D., et al. 1997, A&A, 324, L49Fesen, R. A., Becker, R. H., & Blair, W. P. 1987, ApJ, 313,

378Hughes, J. P., Rakowski, C. E., Burrows, D. N., & Slane, P. O.

2000, ApJ, 528, L109Jansen, F., Lumb, D., Altieri, B., et al. 2001, A&A, 365, L1Kaastra, J. S., & Mewe, R. 1995, A&A, 302, L13Keohane, J. W. 1998, Ph.D. Thesis, NASA/CR-1998-206893Laming, J. M. 2000, accepted for ApJ [astro-ph/0008426]Reynolds, S. P., 1998, ApJ, 493, 375The L.-S., Leising, M. D., Kurfess, J. D., et al. 1996, A&AS,

120, 357Vink, J., Maccarone, M. C., Kaastra, J. S., et al. 1999, A&A,

344, 289