Introduction

The classic ``cooling flow problem'' (Fabian 1994), the discrepancy between the observed radiative cooling rate of the intracluster medium (ICM) in rich galaxy clusters and the amount of cold gas or star formation actually observed, has obvious connections to galaxy formation. One can rephrase the cooling flow issue into the question ``Why are the cD galaxies in rich clusters not still in the process of forming?'' It seems clear that some agent must be heating the ICM core of rich clusters in order to (on average) balance the radiative cooling, and an obvious candidate for that agent is AGN activity. In other words, radio-galaxy/ICM interactions may well be the agent that terminates the formation of the most massive galaxies (Benson et al. 2003; Binney 2004).

While a few examples of radio-galaxy/ICM interactions were known in the Einstein and ROSAT days, it took the superior imaging capability of Chandra to reveal the full complexity and ubiquity of this phenomenon. For example, Perseus A (Fabian et al. 2000, 2003), Hydra A (McNamara et al. 2000; David et al. 2001; Nulsen et al. 2002), Abell 2052 (Blanton et al. 2001), and Cygnus A (Smith et al. 2002) all show well defined cavities in the X-ray emitting gas which are coincident with the current radio lobes of the central radio galaxy. In these sources, it is clear that the radio lobes have displaced the X-ray emitting gas producing the observed X-ray/radio anti-coincidence. Bîrzan et al. (2004) have used an analysis of 16 clusters with cavities to find a significant correlation between the mechanical energy required to create the cavities and the radio power level of the central radio source; the existence of this correlation is strong evidence that the radio source is responsible for evacuating these cavities. Additionally, in at least 50% of the cases studied, the mechanical energy estimated to be present in the cavities is sufficient to offset the estimated cooling of the cluster core. Chandra has also revealed the presence of ``ghost'' cavities, i.e., X-ray cavities that are not coincident with the active radio lobes. In the Bîrzan et al. (2004) study the ``ghost cavities'' have significantly larger estimated mechanical energies than cavities containing current-outburst radio source lobes. Examples include the outer cavity of Perseus A (Fabian et al. 2000, 2003), Abell 2597 (McNamara et al. 2001), NGC 4636 (Jones et al. 2002), and Abell 4059 (Heinz et al. 2002; Choi et al. 2004). In these sources, it is believed that the cavities are associated with old radio lobes (related to previous cycles of AGN activity). The low-frequency (74MHz) synchrotron radio emission expected within this scenario has been observed from the ghost cavity of Perseus-A (Fabian et al. 2002).

If one is interested in assessing the wider importance of radio-galaxy/ICM interactions (e.g., to galaxy formation processes), it is of obvious use to extend the study to powerful radio-galaxies beyond our local universe. In this paper, we describe an observation of the moderately powerful ($P_{\rm 20cm}= 5\times10^{26}\,{\rm W}\,{\rm Hz}^{-1}$) radio-galaxy 3C 401 ($z=0.201$) by the Chandra X-ray Observatory. Despite having the radio power of an FR II, this source has been classified as intermediate between an FR I and FR II morphologically (Harvanek & Stocke 2000), raising the possibility that it might be an example of a fading radio source. Specifically, 3C 401 contains no highly concentrated ``hot spots'' at the leading edges of its two lobes and the brightest portion of its extended structure is a luminous jet in the southern lobe. These characteristics are more similar to those of typical FR Is than FR IIs. 3C 401 is also much broader compared to its length than a typical FR II; thus, the nickname given to sources with this morphology: ``fat doubles''. See Harvanek & Stocke (2002) for an identification and discussion of other ``fat doubles'', including Hercules A. 3C 401 is surrounded by a cluster of galaxies with optical galaxy density (with the galaxy-galaxy two point correlation function B$_{gg} \approx$ 1100 Mpc$^{1.77}$; Harvanek et al. 2001), equivalent to Abell richness class I - II. Harvanek et al. (2001) found that ``fat doubles'' are found exclusively in clusters of galaxies at intermediate redshift.

The goal of the Chandra observation reported in this paper was to search for and characterize any interaction between this radio-galaxy and the ICM of its host galaxy cluster. Clear signatures of this interaction were found. Section 2 describes the observation and our data reduction. Both our spectral and imaging results are presented in Section 3, and placed into a wider context in Section 4. Our conclusions are drawn in Section 5. Assuming the WMAP Cosmology (flat universe with $\Omega_{\Lambda}=0.73$; $H_0=71\hbox{$\hbox{${\rm\thinspace km}{\rm\thinspace s}^{-1}\,$}{\rm\thinspace Mpc}^{-1}$}$; Spergel et al. 2003) gives a luminosity distance of 976Mpc, an angular size distance of 678Mpc, and a look-back time of 2.42Gyr. At this distance, 1arcsec subtends a linear distance of 3.38kpc.