The Warm Interstellar Medium around the Cygnus Loop

Observations of the oxygen lines [OII]3729 and [OIII]5007 in the mediumimmediately beyond the Cygnus Loop supernova remnant were carried out with thescanning Fabry-P\'erot spectrophotometer ESOP. Both lines were detected inthree different directions - east, northeast and southwest - and up to adistance of 15 pc from the shock front. The ionized medium is in the immediatevicinity of the remnant, as evinced by the smooth brightening of both lines asthe adiabatic shock transition (defined by the X-ray perimeter) is crossed.These lines are usually brighter around the Cygnus Loop than in the generalbackground in directions where the galactic latitude is above 5 degrees. Thereis also marginal (but significant) evidence that the degree of ionization issomewhat larger around the Cygnus Loop. We conclude that the energy necessaryto ionize this large bubble of gas could have been supplied by an O8 or O9 typeprogenitor or the particles heated by the expanding shock front. The secondpossibility, though highly atractive, would have to be assessed by extensivemodelling.Comment: 18 pages, 8 figures, ApJ 512 in pres