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Register a new userLimits to the 1/4 keV Extragalactic X-ray Background
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Wei Cui
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We observed several nearby face-on spiral galaxies with the ROSAT PSPC. The apparent deficiency in soft X-ray surface brightness observed at the outer portion of their disks is consistent with the absorption of the extragalactic soft X-ray background by material associated with these galaxies, and allows us to place a lower limit on the intensity of this cosmologically important background. From the depth of the soft X-ray shadow observed in NGC 3184, a 95% confidence lower limit was derived to be $32 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV. This was obtained by assuming that there is no unresolved 1/4 keV X-ray emission from the outer region of the galaxy which may otherwise partially fill in the shadow: any such emission, or any unresolved structure in the absorbing gas, would imply a larger value. In the deepest exposure to date in this energy range, Hasinger et al. (1993) resolved about $30 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV into discrete sources; our current limit is therefore consistent with an extragalactic origin for all of these sources. Our results can also be directly compared with the corresponding upper limit derived from the ROSAT PSPC detection of soft X-ray shadows cast by high-latitude clouds in Ursa Major, $simeq 65 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV. The lower and upper limits are only a factor of 2 apart, and begin to provide a reasonable measurement of the intensity of the 1/4 keV extragalactic X-ray background.
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The information content of the autocorrelation function (ACF) of intensity fluctuations of the X-ray background (XRB) is analyzed. The tight upper limits set by ROSAT deep survey data on the ACF at arcmin scales imply strong constraints on clustering properties of X-ray sources at cosmological distances and on their contribution to the soft XRB. If quasars have a clustering radius r_0=12-20 Mpc (H_0=50), and their two point correlation function, is constant in comoving coordinates as indicated by optical data, they cannot make up more 40-50% of the soft XRB (the maximum contribution may reach 80% in the case of stable clustering, epsilon=0). Active Star-forming (ASF) galaxies clustered like normal galaxies, with r_0=10-12 Mpc can yield up to 20% or up to 40% of the soft XRB for epsilon=-1.2 or epsilon=0, respectively. The ACF on degree scales essentially reflects the clustering properties of local sources and is proportional to their volume emissivity. The upper limits on scales of a few degrees imply that hard X-ray selected AGNs have r_0<25 Mpc if epsilon=0 or r_0<20 Mpc if epsilon=-1.2. No significant constraints are set on clustering of ASF galaxies, due to their low local volume emissivity. The possible signal on scales >6 deg, if real, may be due to AGNs with r_0=20 Mpc; the contribution from clusters of galaxies with r_0~50 Mpc is a factor 2 lower.
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