M. Plionis, I. Georgantopoulos
Monthly Notices of the Royal Astronomical Society, Volume 306, Issue 1, June 1999, Pages 112–116
Publication year: 1999


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 scaleheight, 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) ≈ (288° 25°) ± 19° in consistency with the cosmic microwave background (within ∼ 30°); 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 ∼ 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 DLSS∼ 0.9 DXRB, in general agreement with the predictions of Lahav et al. We finally estimate that the Virgo cluster is responsible for ∼ 20 per cent of the total measured XRB dipole amplitude.