2000MNRAS.315L...8F

The powerful combination of the Chandra X-ray telescope, the SCUBA submillimetre-wave camera and the gravitational lensing effect of the massive galaxy clusters A2390 and A1835 has been used to place stringent X-ray flux limits on six faint submillimetre SCUBA sources and deep submillimetre limits on three Chandra sources which lie in fields common to both instruments. One further source is marginally detected in both the X-ray and submillimetre bands. For all the SCUBA sources our results are consistent with starburst-dominated emission. For two objects, including SMMJ 14011+0252 at z=2.55, the constraints are strong enough that they can only host powerful active galactic nuclei if they are both Compton-thick and any scattered X-ray flux is weak or itself absorbed. The lensing amplification for the sources is in the range 1.5-7, assuming that they lie at z≳1. The brightest detected X-ray source has a faint extended optical counterpart (I~22) with colours consistent with a galaxy at z≃1. The X-ray spectrum of this galaxy is hard, implying strong intrinsic absorption with a column density of about 10²³cm^–2 and an intrinsic (unabsorbed) 2-10keV luminosity of 3x10⁴⁴erg.s^–1. This source is therefore a Type II quasar. The weakest detected X-ray sources are not detected in Hubble Space Telescope images down to I≃26.