Richness class 0 clusters of galaxies typically have luminosities with Lx ~ 1043-45 ergs s-1 ([Ebeling et al. 1998]), while X-ray AGN range from Lx ~ 1040-44 ergs s-1 ([Green, Anderson & Ward 1992]), so a cluster or bright AGN will easily be seen with a medium length exposure with ROSAT. NVSS 2146+82 was observed with the ROSAT High Resolution Imager (HRI) between 1998 February 24 and 1998 March 13 for a duration of 30.3 ksec to search for any hot gas that might be associated with the apparent overdensity of galaxies or for an X-ray luminous AGN.
The data were analyzed with the IRAF Post-Reduction Off-line Software (PROS). The HRI data were filtered for periods of high background and corrected for non-X-ray background, vignetting, and exposure using the computer programs developed by Snowden ([Plucinsky et al. 1993]; [Snowden 1998]). After filtering, the live exposure was 29.8 ksec. The resulting X-ray image was convolved with a gaussian beam with to recover diffuse X-ray emission. The contours of the image are shown superposed on the DSS image in Figure 15.
A few sources were visible near the edge of the field, but there seem to be no significant sources of X-ray emission associated with any optical or radio sources within the 20' extent of NVSS 2146+82 (Figure 15). We derived upper limits on both the AGN or cluster emission by extracting the X-ray counts from the corrected X-ray image using circular regions centered on the host galaxy of 20'' and 2.25', respectively. The region sizes were chosen simply because 20'' represents the size of a typical HRI point source and 2.25' is roughly 1-2 times the typical size of a cluster core at the distance of the radio galaxy. The X-ray background was determined by extracting the X-ray counts from an annulus of 2.25-5' centered on the nucleus of the radio host and removing 3 point sources using 20'' circular regions. We used PIMMS ([Mukai 1993]) to convert the HRI count rate into an unabsorbed flux in the 0.1-2.0 keV band, assuming an emission model and a Galactic photoelectric absorption column of 1.058 × 1021 cm2 ([Stark et al. 1992]). For the AGN, we assumed a power law with a photon index, , of 2.0 and derived an upper limit at the 90% confidence level of 3.52 × 10-14 ergs cm-2 s-1 , or 3.63 × 1042 h50-2 ergs s-1 at the distance of the radio galaxy. Similarly for the cluster, we assumed a Raymond-Smith thermal emission spectrum characterized by kT=1 keV which yielded an upper limit of 1.33 × 10-13 ergs cm-2 s-1 , or 3.63 × 1042 h50-2 ergs s-1 .
Unfortunately, our limit on the X-ray emission from the radio galaxy is not very stringent. [Fabbiano et al. (1984)] studied the X-ray properties of several 3CR radio galaxies with the Einstein Observatory. They found that the FR II's radio and X-ray luminosities are strongly correlated. Thus with a radio flux of 6.8 mJy at 5 GHz, NVSS 2146+82 should have a nuclear X-ray flux of a few times 1042 ergs s-1 . This flux is comparable to our upper limit. Taking into account the intrinsic scatter in the radio/X-ray correlation, our non-detection of the AGN is quite reasonable.
Our upper limit on the X-ray emission from hot cluster gas provides a much stronger constraint. Most Abell richness class 0 clusters have X-ray luminosities of ~ 1042-45 erg/s ([Ebeling et al. 1998]). Therefore any cluster of galaxies associated with the radio galaxy must be either intrinsically weak in X-rays or must be poorer than our optical estimate. [Wan & Daly (1996)] studied the X-ray emission of low-redshift FR II galaxies and found that poor clusters that contain FR II sources are underluminous in X-rays compared to similar clusters that do not contain FR IIs. The median X-ray luminosity for low-z clusters with FR IIs was found to be 1.3 × 1042 h50-2 ergs s-1 while it is 1.33 × 1043 h50-2 ergs s-1 for a sample of low-z clusters without FR IIs (Wan & Daly 1996). Assuming that the group surrounding NVSS 2146+82 is similar to that of other clusters found around low-z FR IIs and is underluminous in X-rays, the optical richness estimate is probably correct.