EXTENDED SOFT X-RAY EMISSION IN SEYFERT GALAXIES: ROSAT HRI OBSERVATIONS OF NGC 3516, NGC 4151, AND MARKARIAN 3

摘要

We have used the ROSAT High Resolution Imager (HRI) to examine the distribution of soft X-rays in three nearby Seyfert galaxies with ～4″-5″ FWHM spatial resolution. A feature of our analysis is an attempt to remove errors in the aspect solution using a method developed by one of us (J. M.). NGC 4151 shows resolved X-ray emission that is spatially correlated with the optical extended narrow-line region (ENLR), confirming the results obtained with the Einstein HRI by Elvis, Briel, & Henry. Image decon-volutions allow us to trace the extended X-rays along a position angle of ～50°/230° as far as ～1.5 kpc southwest and ～0.5 kpc northeast of the nucleus (assuming a distance of 20 Mpc with H_0 = 50 km s~(-1) Mpc~(-1)). When a point source is subtracted from the nucleus of NGC 4151, the extended, bipolar X-rays peak in brightness ～425 pc southwest of the nucleus and ～280 pc northeast of the nucleus. The extended emission accounts for at least 31% of the total 0.1-2 keV ROSAT HRI flux (19% from the southwest quadrant, 12% from the northeast and constitutes roughly half of the total soft X-ray excess emission observed with other X-ray detectors. The soft X-ray excess in NGC 4151 has been modeled recently as containing both variable and constant flux components. We suggest that the constant flux component of the soft excess emission originates in the spatially extended regions we have resolved. If the extended X-rays result from electron-scattering of nuclear X-rays, the central source must emit anisotropically, and preferentially toward the extended X-rays and the ENLR. Alternatively, the extended X-rays may represent thermal emission from a hot (T ～ 10~7 K), outflowing wind which is in rough pressure equilibrium with the optical narrow-line-emitting clouds observed over the same spatial scale. NGC 3516 is elongated along a position angle of ～40°/220°, similar to the direction of the Z-shaped narrow-line region. However, the azimuthally averaged radial brightness profile inside a radius of 10″ is not distinguishable from a calibration point source. Much or all of the elongation may result from residual errors in the aspect solution, although an extended component associated with the ENLR is possible. Mrk 3 is very faint in our HRI image and is probably spatially unresolved. We detect the faint X-ray source ～2′ west of the Mrk 3 nucleus previously found by Turner, Urry, & Mushotzk'y. It is not known whether this companion is physically associated with Mrk 3 although it does lie along a direction that is within approx= 1° of the axis of the 2″ nuclear radio jet. We also detected the BL Lac object BL 1207 + 39 ～5′ north-northwest of NGC 4151. This object appears spatially unresolved, but some excess X-ray emission may be observed in the azimuthally averaged radial brightness profile of BL 1207 + 39 between radii of 10″ and 30″ when compared to a calibration source. A much deeper image is necessary to confirm this result.