Search for X-Ray Emission Associated with the Shapley Supercluster with Suzaku

Abstract

Suzaku performed observations of 3 regions in and around the Shapley supercluster, a region located between A 3558 and A 3556, at $\sim\ $0.9 times the virial radii of both clusters, and two other regions at 1$^\circ$ and 4$^\circ$ away from the first pointing. The 4$^\circ$-offset observation was used to evaluate the Galactic foreground emission. We did not detect significant redshifted Oxygen emission lines (O VII and O VIII) in the spectra of all three pointings, after subtracting the contribution of foreground and background emission. The upper limit for the redshifted O VIII K$\alpha$ line intensity of the warm-hot intergalactic medium (WHIM) is 1.5 $\times$ 10$^{-7}$ photons s$^{-1}$ cm$^{-2}$ arcmin$^{-2}$, which corresponds to an overdensity of $\sim\ $380 ($Z/$0.1$\ Z_{\odot}$)$^{-1/2}$ ($L/$3 Mpc)$^{-1/2}$, assuming $T$$=$ 3 $\times$ 10$^{6}$ K. We found excess continuum emission in the 1$^\circ$-offset and on-filament regions, represented by thermal models with $ kT\sim 1$ keV and $\sim\ $2 keV, respectively. The redshifts of both 0 and that of the supercluster (0.048) are consistent with the observed spectra. The $\sim\ $1 keV emission can also be fitted with Ne-rich Galactic (zero redshift) thin thermal emission. The radial intensity profile of the 2 keV component suggests contributions from A 3558 and A 3556, but with significant steepening of the intensity slope in the outer region of A 3558. Finally, we summarized the previous Suzaku search for the WHIM, and discussed the feasibility of constraining the WHIM. An overdensity of $\lt\ $400 could be detectable using O VII and O VIII emission lines in a range of 1.4 $\times$ 10$^{6}$ K $\lt\ $$T$$\lt\ $ 5 $\times$ 10$^{6}$ K or a continuum emission in a relatively high-temperature range of $T$$\gt$ 5 $\times$ 10$^{6}$ K with the Suzaku XIS. The non-detection with Suzaku suggests that a typical line-of-sight average overdensity is $\lt\ $400.