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 \rho \propto 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 \pm 1.5 \times 10 ^ { 14 } h ^ { -1 } M _ { \odot } at the 90 % confidence .
This is about 1.6 times smaller than the mass derived assuming isothermality .
The gas fraction is correspondingly higher and is 0.08 \pm 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 \pm 0.08 \times 10 ^ { 14 } h ^ { -1 } M _ { \odot } , 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 into better agreement X-ray and lensing mass estimates , 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 .