Astron. Astrophys. 327, 479-482 (1997)

## 3. The X-ray identifications

Because 3C 345 is bright in X-rays and variable, a number of short exposures (up to 4 ks) were taken with ROSAT. Those optically identified quasars listed in Table 1 and shown in Fig. 2 which are detected in the archived ROSAT X-ray exposures are plotted in Fig. 4. Their broad band counts per kilosecond (cts ks-1) are listed in the last column of Table 1 and written to the upper right of each quasar in Fig. 4.

Table 1. Quasars near NGC 6212/3C345

 Fig. 4. Those quasars which are detected as X-ray sources in archived ROSAT observations are marked with cts ks-1 written to upper right. Apparent V magnitude is written below.

Fig. 4 shows that the quasars close to 3C 345 are very bright X-ray sources and are well aligned across it. We can roughly compute the probability of finding such bright X-ray sources so close to 3C 345 by halving the broad band fluxes listed in Table 1 and consulting the average background density for (.5 to 1.0 keV) X-ray sources from Hasinger et al.(1993) or Hasinger et al. (1997 unpublished).

Let us pick the two brightest X-ray quasars across 3C 345 at C = 37 cts/ks and C = 62 cts/ks. This represents the same kind of pairing of X-ray quasars as reported across active Seyfert galaxies by Pietsch et al. (1994), E.M. Burbidge (1995) and Arp (1996; 1997). The chance of accidently finding such bright X-ray sources this close to a given point in the sky is .06 and .2, giving a probability of finding two such sources so close as p = .01. But now we have to multiply by the chance they would be accidently aligned to within 7 deg. out of 180 or p = .04. This gives a total accidental probability that two such bright sources would be so well aligned across 3C 345 as . This is quite in line with the probabilities of the pairs of X-ray sources aligned across the Seyfert galaxies mentioned above. But we have more X-ray quasars aligned in this case, and considering just the three of C = 13, 12 and 24 cts/ks we have three more cases of which produces an overall probability of the alignment of the five strongest X-ray quasars of . This partial calculation of probability would seem to strongly confirm the visual conclusion that these quasars are physically associated with the central, active 3C 345.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998