The ROSAT X-ray background dipole

We estimate the dipole of the diffuse 1.5-keV X-ray background from the ROSAT all-sky survey map of Snowden et al. We first subtract the diffuse Galactic emission by fitting an exponential scale height, finite-radius, disc model to the data. We further exclude regions of low galactic latitudes, of local X-ray emission (e.g. the North Polar Spur) and model them using two different methods. We find that the ROSAT X-ray background dipole points towards (l, b) approximate to (288 degrees, 25 degrees) +/- 19 degrees in consistency with the cosmic microwave background (within similar to 30 degrees); its direction is also in good agreement with the HEAO-1 X-ray dipole at harder energies. The normalized amplitude of the ROSAT XRB dipole is similar to 1.7 per cent. Subtracting from the ROSAT map the expected X-ray background dipole resulting from the reflex motion of the observer with respect to the cosmic rest frame (Compton-Getting effect) we find the large-scale dipole of the X-ray emitting extragalactic sources having an amplitude D-LSS similar to 0.9 D-XRB, in general agreement with the predictions of Lahav et al. We finally estimate that the Virgo cluster is responsible for similar to 20 per cent of the total measured XRB dipole amplitude.