We combine spatially resolved ASCA temperature data with ROSAT imaging data
to constrain the total mass distribution in the cluster A401, assuming tha
t the cluster is in hydrostatic equilibrium, but without the assumption of
gas isothermality. We obtain a total mass within the X-ray core (290 h(50)(
-1) kpc) of 1.2(-0.5)(+0.1) x 10(14) h(50)(-1) M-circle dot at the 90% conf
idence level, 1.3 times larger than the isothermal estimate. The total mass
within r(500) (1.7 h(50)(-1) Mpc) is M-500 = 0.91(-0.2)(+0.3) x 10(15) h(5
0)(-1) M-circle dot at 90% confidence, in agreement with the optical virial
mass estimate, and 1.2 times smaller than the isothermal estimate. Our M-5
00 value is 1.7 times smaller than that estimated using the mass-temperatur
e scaling law predicted by simulations. The best-fit dark matter density pr
ofile scales as r-(3.1) at large radii, which is consistent with the Navarr
o, Frenk & White (NFW) "universal profile" as well as the King profile of t
he galaxy density in A401. From the imaging data, the gas density profile i
s shallower than the dark matter profile, scaling r(-2.1) at large radii, l
eading to a monotonically increasing gas mass fraction with radius. Within
r(500) asr the gas mass fraction reaches a value of f(gas) = 0.21(-0.05)(+0
.06) h(50)(-3/2) (90% confidence errors). Assuming that f(gas) (plus an est
imate of the stellar mass) is the universal value of the baryon fraction, w
e estimate the 90% confidence upper limit of the cosmological matter densit
y to be Omega(m) < 0.31, in conflict with an Einstein-deSitter universe. Ev
en though the NFW dark matter density profile is statistically consistent w
ith the temperature data, its central temperature cusp would lead to convec
tive instability at the center, because the gas density does not have a cor
responding peak. One way to reconcile a cusp-shaped total mass profile with
the observed gas density profile, regardless of the temperature data, is t
o introduce a significant nonthermal pressure in the center. Such a pressur
e must satisfy the hydrostatic equilibrium condition without inducing turbu
lence. Alternately, significant mass drop-out from the cooling flow would m
ake the temperature less peaked and the NFW profile acceptable. However, th
e quality of data is not adequate to test this possibility.