Mem. S.A.It. Suppl. Vol. 9, 223c© SAIt 2006

Memorie della

Supplementi

X-raying the interstellar medium: the study ofSNR shells at the OAPa

F. Bocchino1, M. Miceli1,2, F. Reale2, A. Maggio1, S. Orlando1, and G. Peres2

1 INAF - Osservatorio Astronomico ”G.S.Vaiana”, Palermo, Italy2 Sezione di Astronomia, Dipartimento Scienze Fisiche ed Astronomiche, Universita’ di

Palermo e-mail: bocchino@astropa.unipa.it

Abstract. The shock wave of shell supernova remnant (SNR) offers the unique opportunityto detect directly the structures of the interstellar medium, both at large and small scale.Unfortunately, the several physical effects at work in the interaction between the shockand the medium, along with the superposition effects along a given light of sight, requiredetailed modeling and high resolution observations of the systems in order to understandits physical behavior At the Osservatorio Astronomico di Palermo, we have started a long-term project which will lead, for the first time, to a self-consistent methodological approachfor comparison of new accurate numerical models and high resolution multi-wavelengthobservations. In this poster, we introduce the observation we have already analyzed,whilein the talk of S. Orlando the new numerical models we have developed are presented

Key words. ISM: clouds - ISM: general - ISM: individual objects: Vela SupernovaRemnant - ISM: structure - ISM: supernova remnants - X-rays: ISM

1. Introduction: evolved SNR shellsas tracers of the ISM structuresand energetic

Evolved supernova remnant shells traces smallans large scale structure of the interstel-lar medium. The interpretation of multi-wavelength data may seem straightforward at afirst glance. However, high resolution observa-tions of post-shock regions reveal a great com-plexity which is difficult to understand in termsof simple models. It seems that superpositioneffects as long as real physical effects (hydro-dynamics of shock-cloud interactions, radia-tive losses, thermal conduction, amplitude andorientation of magnetic fields) must be taken

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into account to model the observed emission.Energy exchange between the shock and theISM also critically depends on the character-istic of the clouds. Several efforts have beenmade in trying to detect simple regions wherethe shock is interacting with relatively smalland isolated clouds since these regions are thebest candidates to study all the physical pro-cesses involved in shock-ISM interactions.

For instance, the Cygnus Loop is the pro-totypical shell SNR. Patnaude et al. (2002) hasargued that the filament in the southwest maybe interpreted as the very early stage of a blast-wave interaction with an isolated interstellarcloud. The Vela SNR is the shell located near-est to us at only 250 pc. Its morphology sug-gests large-scale density enhancements towardthe galactic plane, but small scale structures are

224 Bocchino: SNR shells at OAPa

Fig. 1. Multiwavelength images of the Vela SNR and the Filament D region.

seen in the X-ray and optical bands down tofew arcminutes. Bocchino et al. (1999) haveidentified a bright X-ray spot (Filament D)with an associated arc-like optical filaments,subsequently studied by Miceli et al. (2005).G272.2-3.2 is small shell discovered in theROSAT All-Sky Survey. It has both a brightinterior and a bright X-ray limb. Egger et al.(1996) has identified a region of interactionwith a small isolated cloud in the eastern partof the shell.

2. Shock-cloud interaction in the VelaSNR shell

We have already analyzed the Filament D(“FilD”) region in the Vela SNR shell. TheMean Photon Energy map (MPE) of “FilD andsurroundings allow us to identify thermally ho-mogeneous regions for subsequent analysis,

0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012count rate

0

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mea

n ph

oton

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rgy

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maximum errorin the count rate

Fig. 2. Scatter plot of median energy and count ratein the Filament D region of Vela SNR.

while the relation between MPE and count-rate straightforwardly inform us about pressureequilibrium and different

Bocchino: SNR shells at OAPa 225

0.0003 0.0004 0.0005 0.0006 0.0007mean count rate

1.0

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Fig. 3. Best-fit value of cool and hot thermal com-ponent in the Filament D region.

N

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Fig. 4. 3D representation of shocked ISM plasmain the Filament D Vela SNR region.

volumes of the emitting plasma. Spectralfits of selected regions yields very mild varia-tion of best-fit temperatures of the two thermalcomponent, and large variation in the emissionmeasures. This in turns implies that the ob-served X-ray structures are mostly due to dif-ferent extension along the line of sight of thetwo emitting components. There seems to beno signs of overpressured zones (i.e. reflectedshocks). A three-dimensional representation ofthe shocked ISM is derived extrapolating thefittings results to the whole field of view ob-served in Fig. 2, bottom panels. The hot and di-luted X-ray component (transparent blue) is as-sumed to sit on top of the cold and dense X-raycomponent (opaque white). A simple model

for the “FilD. For the derived densities andtemperatures, thermal conduction was foundto be dominant over radiative losses in shap-ing the observed morphology. Both the coronaand the core of the inhomogeneities shouldbe evaporating (yellow arrows) with significantenergy and mass exchange with the surround-ings.

3. Open questions

In spite of the recent results, there are stillmany open questions in the field of SNRshock-ISM interactions. In particular, the sizeand mass of the clouds are difficult to esti-mate. Moreover, do we see plasma instabilitiesin real observations? Or are they suppressed bythermal conduction? Why temperature varia-tions are so mild? Why we do not see reflectedshocks? Why the plasma is in equilibrium ofionization and the post-shock Neon abundanceis above solar? Are optical filaments due topristine dense cloud cores or are they the re-sult of radiative instabilities developed in theevolution of a relatively rarefied ISM cloud?Finally, the heating efficiency (HE) of ISM bySN shocks is a key parameter which plays acrucial role in a number of astrophysical sit-uations. ISM clouds may, in principle, largelyreduce the HE. What are the models and theobservational constraints to estimate the HE ofevolved galactic SNR shells in realistic condi-tions?

References

Bocchino, F., Maggio, A., & Sciortino, S.1999, A&A, 342, 839

Egger, R., Greiner, J., & Aschenbach, B. 1996,in Roentgenstrahlung from the Universe,247–248

Miceli, M., Bocchino, F., Maggio, A., & Reale,F. 2005, Advances in Space Research, 35,1012

Patnaude, D. J., Fesen, R. A., Raymond, J. C.,et al. 2002, AJ, 124, 2118