Results from a deep RXTE /PCA scan across the Galactic plane

We present the results of deep scans across the Galactic plane that were performed with RXTE /PCA in 2010–2011. We studied the stripe from b II = 0° to b II = −30° at l II ≈ 18.°5. We show that the variance of the cosmic X‐ray background (CXB) at angular scales ∼1 deg 2 is approximately 5 per cent, which is compatible with the variance expected from Poisson variations of a number of discrete sources within the considered field of view. We estimate the cumulative fractional emissivity of the Galactic ridge X‐ray emission (GRXE) as (3.0 ± 0.3) × 10 27  erg s −1 M− 1in the energy band 2–10 keV. We do not detect significant variations of the GRXE energy spectrum over the studied latitudes, which might be expected if the GRXE consists of contributions of sources with very different characteristic ages and scaleheights. Making use of ROSAT /PSPC/R7 data, collected within the stripe of our study, we estimate the cumulative emissivity of GRXE sources in the energy band 0.5–2 keV as ∼8 × 10 27  erg s −1 M− 1 ; however, this estimate is subject to uncertainties owing to low‐accuracy correction for interstellar photo‐absorption. At Galactic latitudes −12° to −18° we detect ∼6–8 degrees size contributionfrom the hot plasma component, with the effective temperature ∼1.6 keV significantly exceeding that usually known for the soft X‐ray Galactic background. Its surface brightness is higher than that of any other Galactic components at energies 1–2 keV and gives up to ∼20 per cent of the CXB at these energies. Accounting for such emission is of vital importance, for example, in studies of the emission of clusters of galaxies at their virial radii.