The supernova remnant G78.2+2.1: New optical and X-ray observations

New optical and X-ray observations of the supernova remnant (SNR) G78.2+2.1 are presented. CCD H alpha observations with a Fabry-Perot interferometer attached to the 125-cm reflector at the Crimean Station of the Sternberg As tronomical Institute are used to obtain the radial velocity field toward th e SNR and in its vicinity. The brightness distribution and X-ray spectrum o f the SNR are obtained from archival ROSAT and ASCA X-ray data. The X-ray i mage of G78.2+2.1 exhibits a shell structure (Delta R/R similar or equal to 0.3) and is generally similar to its radio image; a comparison with the ra dio map at nu = 1.4 GHz constructed from archival VLA data reveals the coin cidence of features on scales of several arcminutes at the eastern boundary of G78.2+2.1. Weak X-ray emission tan outer shell or a halo of size simila r or equal to 2 degrees) has been identified for the first time far outside G78.2+2.1. The X-ray emission from G78.2+2.1 is shown to characterize a yo ung adiabatic SNR [Mx-ray similar or equal to 100M(.), V-s similar or equal to 10(3) km s(-1), t similar or equal to (5-6) x 10(3) years], which proba bly expands inside the cavity swept up by the progenitor's stellar wind. Se arches for the corresponding radio structure are required to elucidate the nature of the outer Xray shell or halo. (C) 2000 MAIK "Nauka/Interperiodica ".