High-redshift quasar Q 1745+624 observed in the ROSAT all-sky survey.

By inspecting high ecliptic latitude ROSAT survey sky fields we detected a point like X-ray source at a position, where recently the radio-loud quasar Q1745+624 with a redshift of z = 3.87 was discovered. The quasar was claimed to be the optical and the radio counterpart of a 2.10 σ fluctuation in an X-ray image taken with the telescope aboard the Einstein Observatory. The ROSAT source is coincident well within the limits of the survey's positional accuracy with the VLA position of the quasar. It shows a flat X-ray spectrum with a likely spectral steepening below 1 keV in the quasar's rest frame. There are no indications of intrinsic absorption. If we suppose that the ROSAT source can be identified with Q1745+624, the spectral characteristics of this distant quasar are strikingly similar to those of well studied nearby QSOs. It is contrary to the strongly absorbed, high-redshift quasar PKS 0438-436 with its likely steeper spectrum, thus demonstrating the non-uniformity of the early quasar population.