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. Because of the low foreground Galactic X-ray-absorbing gas column density, N_HI_ ~ 1.4 x 10^20^ cm^-2^, this observation is sensitive to gas of temperature ~> a few x 10^5^ K. We detected soft (~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 which 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 ~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 galaxy. Analysis of the X-ray spectrum shows evidence for the presence of a cooler (several times 10^5^ K) halo gas component that could consume a much larger fraction of the supernova energy. We found strong evidence for disk/halo interaction. Hot gas apparently blows out from supershells in the galaxy's disk at a rate of ~1 M_sun_ yr^-1^. This outflow of hot gas drags magnetic field lines up in the halo and forms a magnetized gaseous halo. If the magnetic field lines are still anchored 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 supershell. However, the source is likely an extremely luminous X-ray binary with L_x_(0.1-2 keV)~5 x 10^39^ ergs s^-1^, which makes it a stellar mass black hole candidate.