ROSAT DETECTION OF DIFFUSE HOT GAS IN THE EDGE-ON GALAXY NGC-4631

We present our ROSAT observation of the edge-on spiral galaxy NGC 4631 , a nearby Sc/SBd galaxy best known for its extended radio halo. Becau se of the low foreground Galactic X-ray-absorbing gas column density, N-HI approximate to 1.4 x 10(20) cm(-2), this observation is sensitive to gas of temperature greater than or similar to a few x 10(5) K. We detected soft (similar to 0.25 keV) X-ray radiation out to more than 8 kpc above the midplane of the galaxy. The strongest X-ray emission in the halo is above the central disk, a region of about 3 kpc radius wh ich shows high star formation activity. The X-ray emission in the halo is bordered by two extended filaments of radio continuum emission. We also found diffuse X-ray emission from hot gas in the galaxy's disk. The spectrum of the radiation can be characterized by a thermal plasma with a temperature of 3 x 10(6) K and a radiative cooling rate of sim ilar to 8 x 10(39) ergs s(-1). This rate is only a few percent of the estimated supernova energy release in the interstellar medium of the g alaxy. Analysis of the X-ray spectrum shows evidence for the presence of a cooler (several times 10(5) K) halo gas component that could cons ume a much larger fraction of the supernova energy. We found strong ev idence for disk/halo interaction. Hot gas apparently blows out from su pershells in the galaxy's disk at a rate of similar to 1 M. yr(-1). Th is outflow of hot gas drags magnetic field lines up in the halo and fo rms a magnetized gaseous halo. If the magnetic field lines are still a nchored to the disk gas at large disk radii, the outflowing gas may be confined high above the disk by magnetic pressure. We have identified a strong X-ray source which coincides spatially with an H I supershel l. However, the source is likely an extremely luminous X-ray binary wi th L(x)(0.1-2 keV) approximate to 5 x 10(39) ergs s(-1), which makes i t a stellar mass black hole candidate.