Shock–cloud interactions in the Vela SNR: preliminary results of an XMM-Newton observation F. Bocchino a, * , M. Miceli b , A. Maggio a a Instituto Nazionale di Astrofisica – Osservatorio Astronomico di Palermo, 90134 Palermo, Italy b Dipartimento di Scienze Fisiche ed Astronomiche, Universit a di Palermo, Italy Received 3 December 2002; received in revised form 17 July 2003; accepted 6 August 2003 Abstract The study of the clumpy and irregular features in the X-ray emission of middle-aged supernova remnants shells allows us to shed light on the various characteristic of the interstellar medium, like its structure and composition. We have observed with XMM- Newton a small knot in the Vela SNR, which previous ROSAT studies have indicated as one of the best examples of an interaction between the SNR shock and an isolated cloud. We present preliminary results of this study. Thanks to the combination of good spectral and spatial resolution of the EPIC camera, we have realized maps of the X-ray emission in three different bands, pinpointing the contribution from different spatial regions. We have also studied the relation between the mean photon energy in a 10 arcsec/ pixel image and the pixel count rate, arguing that at least two kinds of isobaric structures (a small isolated cloud and a larger feature) can be identified, both of which have interacted with a shock and both with their own thermal structure. Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: X-rays: ISM; ISM: supernova remnants; ISM: structure; ISM: individual objects; Vela supernova remnant; ISM: clouds 1. The study of shell SNR shock regions High spatial and spectral resolution X-ray and opti- cal observations of supernova remnants (SNRs) offer a unique opportunity to study the interaction of inter- stellar shocks with the local environment, and then to get crucial information on the structure of the inter- stellar medium (ISM). The gross mechanism responsible for the X-ray and optical emission of the middle-aged SNRs is well-understood: optical filaments arise from slow radiative shocks propagating in a dense and cool environment (typical density and temperature values are n J 1 cm 3 and T K 10 5 K), whereas, X-ray emission originates from zones with higher temperature (T > 10 5 K) and lower density (usually <1 cm 3 ). However, the proper interpretation of what we see in high spatial resolution observations is still not straightforward, es- sentially because the presence of clumps in the ISM disrupts the idealized spherical symmetry of the super- nova explosion and its remnant, and determines local variations of the thermodynamical parameters. In this perspective multi-wavelength observations of SNRs shock regions (e.g., Graham et al., 1995 on Cygnus Loop, or Bocchino et al., 2000 on Vela) may be quite a powerful tool to clarify the relevant physical scenario, but, if the X-ray observations lack sufficient high spatial resolution, they can be subject to several interpretations. The first scenario points out the importance of evaporation of ISM cloudlets engulfed by the main blast wave. The evaporation is caused by thermal conduction between the cooler cloud and the hotter intercloud me- dium (ICM) behind the shock. This model typically re- quires the clouds to be rather small, and this yields practical constraints on cloud filling factors and the small scale structure of the ISM. The evaporative model has been applied in the past to observation of SNRs (Charles et al., 1985; Fesen et al., 1982), and more re- cently to describe the composite SNR G327.1-1.1 (Bo- cchino and Bandiera, 2003), but a rigorous comparison between the data and the model, including effects like * Corresponding author. Tel.: +39-091-233444; fax: +39-91-233444. E-mail address: bocchino@astropa.unipa.it (F. Bocchino). 0273-1177/$30 Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.08.016 Advances in Space Research 33 (2004) 381–385 www.elsevier.com/locate/asr