The extreme X-ray luminosity of the z=4.72 radio-loud quasar GB 1428+4217

We report on a ROSAT High Resolution Imager observation of the radio-loud quasar GB 1428+4217, which is at a redshift z = 4.72. The quasar was well detected, with a flux in the observed 0.1-2.4 keV band of similar to 10(-12) erg cm(-2) s(-1). Our result confirms that the object in the catalogue of serendipitous X-ray sources, WGACAT, is indeed the quasar. GB 1428+4217 is therefore the brightest X-ray source above a redshift of 4 and has an extremely high isotropic X-ray luminosity, which exceeds that in other (observed) bands. The observed spectral energy distribution of the object therefore peaks at X-ray, or even shorter, wavelengths. We speculate that the emission in the object is beamed toward us.