AN IMAGING STUDY OF THE GALACTIC DIFFUSE SOFT X-RAY BACKGROUND

Abstract

ABSTRACT In the soft energy regime < 1 keV, the Galactic background is a significant component of the celestial diffuse X-ray radiation and carries important information about hot gas in the disk and in the corona of the Galaxy. We have conducted a study of the Galactic diffuse soft X-ray background with the data collected with the Imaging Proportional Counter on board the Einstein X-ray Observatory. The high sensitivity and spatial resolution of the data enable us to detect and subtract discrete sources to a flux level < 1013 erg cm-2 s-1, to measure the background with minmized confusion between various components in the radiation, and to study arcmin-scale fluctuation of the background. The results reported here demonstrate the value of imaging data in studying the background. At high Galactic latitudes, the Galactic background, indicated by the global enhancement of the X-ray radiation intensity toward the Galactic center, could arise in the corona of the galaxy. We have measured the intensity of the radiation in the 0.2-0.8 keV band in the latitude interval 30 degrees < |b|< 70 degrees and have compared the distribution of the intensity with a simple emission model of the Galactic corona. This comparison suggests that the corona probably accounts for more than half of the radiation in the energy band. The estimated density and pressure in the coronal gas are consistent with both theoretical arguments and UV observations made with IUE, HST, and rocket experiments. The intensity and spectrum of the Galactic background at the Galactic plane are measured with a deep image of the plane. The spectrum in the 0.2-3.5 keV band can be characterized by the emission from optically-thin, thermal coronal gas of a temperature at 4 x 106 K, and the M-band intensity is Ix(0.5 - 1 keV) 3 X 10-8 erg cm-2 s-1 sr-1. An angular autocorrelation analysis of the imaging data leads to the conclusion that no known stellar population is likely responsible for the bulk of the background. A hot gas model seems consistent with the observation; but a satisfactory fit to the observed spectrum requires X-ray emission from hot gas beyond the 106 K Local Bubble. We further point out that the X-ray background, enhanced in the M-band at the Galactic plane, is apparently not related to the Galactic ridge emission inferred from observations in the higher energy 2-10 keV regime.