The correlation function of the 4-12 keV X-ray background intensity measured with the GINGA LAC

The angular autocorrelation function of the X-ray background in the energy band 4–12 keV has been explored on scales of $$2^{\circ} -25^{\circ} $$ using 132 exposures obtained with the Large Area Counter (LAC) on board the GINGA satellite. No significant signal is found over the whole range. An upper limit at $$2^{\circ} \,\text{of}\,{10}^{-4}$$ is found for the autocorrelation function at 95 per cent confidence level, significantly lower than previous values obtained in the 2–10 keV band at 3° separation. This upper limit is shown to constrain clustering on scales about $$\sim 10-300$$ Mpc in the high-redshift $$(z\sim 0.1-5)$$ universe, and in particular the cluster–cluster and AGN–AGN correlation functions.