Two X-ray clusters close to line of sight of the luminous QSO HS 1700+6416.

We report the serendipitous discovery with ROSAT of two X-ray clusters close to the line of sight of the QSO HS 1700+6416 (z=2.72) which is itself one of the most luminous QSOs known. Cluster A (1'40" northwest of the QSO) is Abell 2246, while cluster B (3' northeast) is a hitherto unknown distant cluster. We have determined the redshifts of clusters A and B as z=0.25 and z=0.44 respectively. Although both clusters appear barely resolved by ROSAT, the X-ray surface brightness distribution can be represented well by β-models I(θ)=I_o_(1+(θ/θ_c_)^2^)^-3β+0.5^ where θ_c_ is the cluster core radius (cf. Henry et al. 1993) if the detector point spread function is taken into account. Assuming β=2/3 we find core radii θ_c_(A)=14"+/-2" and θ_c_(B)=18"+/-2". Gas temperatures are T(A)=6+/-3x10^7^K and T(B)=3+/-1.5x10^7^K, X-ray luminosities are L_x_(0.4-2keV rest frame)=4.1x10^43^erg/s and 8.45x10^43^erg/s respectively (H_0_=50, q_0_=1/2). Cluster B shows a giant luminous arc located approximately 15'' (=~1 core radius) NW of the cluster center. We show that magnification of HS 1700+6416 by cluster lensing by the two X ray clusters can be excluded as reason for the high apparent luminosity of the QSO.