**Table 2:** Data on X-ray nuclear emission used for the SED (see the Appendix for details).
Source	$L^{a}_{\nu {\rm X}}$	$\alpha_{\rm X}$	Instrum. (refer.)
3C 29	$5.5^{\star}$	1.0	PSPC (1)
3C 31	0.46	$\rm0.5^{+0.3}_{-0.2}$	Chandra (2)
3C 66B	$\rm 9.5^{+0.2}_{-0.9}$	$\rm0.3^{+0.3}_{-0.4}$	PSPC (1)
3C 78	$\rm 32^{+10}_{-17}$	1.0	SAX (3)
3C 83.1	$3.6^{\star}$	1.0	PSPC (4)
3C 189	14	$\rm 1.1^{+0.1}_{-0.4}$	Chandra (5)
3C 272.1	$\rm0.051^{+0.003}_{-0.003}$	$\rm 1.3^{+0.1}_{-0.1}$	Chandra (6)
3C 277.3	<5.3	1.0	PSPC (1)
3C 288	<2100	1.0	PSPC (4)
3C 296	$6.0^{\star}$	1.0	HRI (4)
3C 310	$16^{\star}$	1.0	HRI (4)
3C 317	$\rm 30^{+21}_{-23}$	$\rm0.1^{+0.3}_{-0.7}$	PSPC (1)
3C 338	$\rm0.53^{+0.11}_{-0.11}$	$\rm0.5^{+0.5}_{-0.4}$	Chandra (7)
3C 346	$\rm 618^{+110}_{-160}$	$\rm0.7^{+0.2}_{-0.2}$	ASCA (8)
3C 348	234	1.0	PSPC, ASCA, SAX (9)
3C 442	<1.9	1.0	HRI (4)
3C 449	$0.89^{\star}$	1.0	PSPC (4)

^a $\times 10^{16}$ W Hz^-1 (at 1 keV). For each object $N_{\rm H}$ is fixed to its galactic value, except for 3C 272.1 where it is deduced from the fit ( $N_{\rm H} = 2.7^{+0.3}_{-0.3} \times 10^{21}$ cm^-2). In the sources marked with $^{\star}$ contamination from thermal emission may be present.
References: (1) this work; (2) Hardcastle et al. (2002); (3) Trussoni et al. (1999); (4) HW99; (5) Worral et al. (2001); (6) Finoguenov & Jones (2001); (7) Di Matteo et al. (2000); (8) Worrall & Birkinshaw (2001); (9) Siebert et al. (1999), Trussoni et al. (2001).

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