ROSAT X-Ray Observations of the Cooling Flow Cluster A2597

The cluster A2597 was observed in X-rays with the ROSAT PSPC and HRI detectors. The X-ray emission from the cluster extends out to a radius of at least 2.37 Mpc. The X-ray isophotes are oriented similarly to the optical isophotes of the central cD galaxy and to the isopleths of the galaxy distribution in the cluster but are otherwise quite regular, which suggests that this cluster is reasonably relaxed and in hydrostatic equilibrium. The merged HRI and PSPC surface brightness profile is not adequately fitted by a beta model because of the central X-ray surface brightness peak, which indicates the presence of a cooling flow. If the central 108" in radius are excluded, an acceptable fit is found that gives β = 0.64^+0.08_-0.03 but only an upper limit to the core radius, r_core < 78". The gas density and accumulated mass were derived as a function of radius, and the hydrostatic equilibrium condition was used to determine the total gravitational mass, also as a function of radius. Within a radius of 2 Mpc, we found masses of M_gas = 1.2 × 10¹⁴ M_⊙ and M_tot = 5.6 × 10¹⁴ M_⊙, and a gas mass fraction of about 21%. However, our poor knowledge of the spatial variation of the gas temperature makes the total mass values uncertain by at least a factor of 2.

The ROSAT PSPC X-ray spectrum of the cluster was determined. Both the overall cluster spectrum and the spatially resolved spectra within 300 kpc require the presence of both hot gas and a cooling flow in the spectrum. The spectrally determined total cooling rate of Ṁ = 344⁺⁷⁵_-67 M_⊙ yr^-1 is in good agreement with those derived from analyses of the X-ray surface brightness profile from the Einstein IPC and the ROSAT HRI images. The ROSAT spatially resolved X-ray spectra indicate that the cooling component is distributed over the inner ~300 kpc in radius of the cluster. The presence of the cooling flow in the central regions and poorer statistics in the outer regions prevent us from deriving an accurate profile for the variation of the ambient cluster gas temperature.

We do not detect any significant excess X-ray absorption toward the center of A2597, and we set a very conservative upper limit on the excess column in front of the cooling flow region of ΔN_H < 1.72 × 10²⁰ cm^-2. A2597 is one of the few cooling flows toward which a large column has been detected in radio observations (O'Dea et al.). A total column of N_H ~ 1 × 10²¹ cm^-2 was seen in 21 cm absorption against the very small central radio source. Our X-ray upper limit is not inconsistent with the 21 cm detection if the absorber covers only the small region (~6") occupied by the radio source. However, we can rule out an absorber with a uniform column that covers the entire cooling flow or even just the smaller region of the extended optical emission-line nebula.