Multi-wavelength observations and modelling of shock-cloud interaction regions in the Vela Supernova Remnant

We present optical photometric and spectrophotometric observations of two selected filaments near the North rim of the Vela supernova remnant also observed in UV with the International Ultraviolet Explorer, and in X-rays with the ROSAT PSPC detector. From the optical data, we have derived spatially-resolved (∼ 3′′) profiles of several emission lines across the filaments, which allowed us to resolve the post-shock region. Starting from model-independent line ratio diagnostics, we have estimated densities (n = 3.2 − 8.2 cm−3), temperatures (T = 3− 10× 104 K), pressures (P = 2.8× 10−10 dyne cm−2) and ambient magnetic fields (B = 50 − 85 μG) in the post-shock flow. We have also found clear evidence of a relative displacement between the X-ray emission, the emission in the Hα line and the emission in the [O III] line. We have compared the measured displacement, the width of the filaments in Hα and [O III], and the peak values of selected line ratios with the predictions of a radiative shock model which describes in detail the structure of the post-shock region. Shock models with velocities around vs = 110 km sec−1 are shown to be consistent with the optical and UV observations if we assume that the direction of propagation is tilted with respect to the line of sight. The layout of the X-ray and optical emission of the filament and their thermodynamical properties strongly suggest that the shock is travelling along increasing density gradient.