EP-FXT observations of the cool-core cluster Abell 478 out to R200: Thermodynamic properties and azimuthal asymmetry

Abstract

We use deep observations from the Einstein Probe Follow-up X-ray Telescope (EP-FXT) to investigate the gas distribution and thermodynamic properties of the cool-core galaxy cluster Abell 478 (A478) from the center out to $R_{200}$, and to examine its azimuthal asymmetry and the influence of local dynamical disturbances on hydrostatic mass estimates. We derive the surface-brightness, temperature, electron-density, pressure, entropy, and total mass profiles. The surface-brightness distribution is well described by a double-$β$ model, while the temperature profile shows the characteristic cool-core behavior, with a cool center, a rise toward intermediate radii, and a gradual decline in the outskirts. The hydrostatic mass profile is well fitted by a Navarro-Frenk-White model, yielding $R_{200}=2082\pm95$ kpc and $M_{200}=(1.12\pm0.15)\times10^{15}\,M_\odot$, indicating that the cluster is close to quasi-static equilibrium on global scales. Despite the globally regular structure, clear azimuthal asymmetry is present. The SW sector shows lower temperatures, higher densities, and lower entropies at intermediate and large radii, together with signatures consistent with a cold-front candidate. Combined with the two-dimensional temperature distribution, these features suggest local dynamical disturbance, likely associated with gas sloshing. A478 is therefore globally relaxed but locally disturbed, demonstrating that nonequilibrium structures can still affect thermodynamic measurements and hydrostatic mass estimates even in an apparently regular cool-core cluster.