Dark matter and baryon fraction at the virial radius in Abell 2256

We combine high-quality ASCA and ROSAT X-ray data to constrain the radial dark matter distribution in the primary cluster of Abell 2256, free from the assumption of gas isothermality. Both instruments indicate that the temperature declines with radius. The region including the central galaxy has a multicomponent spectrum, which results in a wide range of allowed central gas temperatures. We find that the secondary subcluster has a temperature and luminosity typical of a rich cluster; however, the ASCA temperature map shows no signs of an advanced merger in this double system. It is therefore assumed that the primary cluster is in hydrostatic equilibrium. The data then require dark matter density profiles steeper than p proportional to r(-2.5) in the cluster outer part. Acceptable models have a total mass within r = 1.5 h(-1) Mpc (approximately the virial radius) of 6.0 +/- 1.5 x 10(14) h(-1) M. at the 90% confidence level. This is about 1.6 times smaller than the mass derived assuming isothermality. The gas fraction is correspondingly higher and is 0.08 +/- 0.02 h(-3/2). A lower limit on the fraction of gas in the total local density at the same radius is 0.09 h(-3/2), which is twice the isothermal value. Near the center, dark matter profiles with and without central cusps are consistent with the data. Our inferred total mass inside the X-ray core (r = 0.26 h(-1) Mpc) is 1.28 +/- 0.08 x 10(14) h(-1) M., which exceeds the isothermal value by a factor of 1.4. Although the confidence intervals above may be underestimates, since they do not include uncertainties arising from asymmetry and departures from hydrostatic equilibrium, the behavior of the mass distribution, if applicable to other clusters, can bring X-ray and lensing mass estimates into better agreement but aggravate the "baryon catastrophe." The observed considerable increase in the gas content with radius, not anticipated by simulations, may imply that a significant fraction of thermal gas energy comes from sources other than gravity and merger shocks, such as supernovae-driven galactic winds, for example.