The nuclear region

A point source is clearly detected coincident with the radio core of 3C 401. Although the spatial resolution of Chandra/ACIS is approximately 2kpc at the distance of 3C 401, it seems likely that we are detecting the X-ray emission from the active galactic nucleus itself. Using a smaller than usual extraction region (1arcsec radius) in order to separate this source from the surrounding ICM emission, we detect 385 photons in this source, approximately 10-20% of which are accounted for by a reasonable extrapolation of the underlying ICM surface brightness profile. We have searched for but fail to detect temporal variability of this central source. However, our limits on temporal variability are weak due to the limited photon statistics -- inspection of the light curve suggests that the source has not varied by more than 30% on the timescale of the total observation length ($1.2\times 10^5{\rm\thinspace s}$), and has not varied by more than a factor of two on a timescale of $10^4{\rm\thinspace s}$.

Using our small extraction radius, we have extracted a time-average spectrum of this central source. We produce a ``background'' spectrum from a region offset by 3arcsec in the western direction. Therefore, in an attempt to isolate the pure nuclear spectrum, the background spectrum also includes thermal emission from the ICM immediately neighboring the nuclear source. The source spectrum was then binned so as to contain at least 15 photons per energy bin, hence allowing the use of $\chi^2$ statistics.

The 0.5-6keV nuclear spectrum can be adequately fit with either a power-law or thermal plasma model modified by the effect of Galactic absorption ($N_H=6.8\times 10^{20}\hbox{${\rm\thinspace cm}^{-2}\,$}$) and absorption by contaminants on the ACIS filter (modeled with the acisabs model). Note that there are inadequate numbers of photons above 6keV to allow meaningful extension of these fits to higher energies. Best fit parameters for the power-law fits are; photon index $\Gamma=1.89^{+0.27}_{-0.21}$, intrinsic (redshifted) absorption $N_{Hz}<1.0\times 10^{21}\hbox{${\rm\thinspace cm}^{-2}\,$}$, observed 0.5-10keV flux $F_{0.5-10}=5.5\times 10^{-14}\hbox{${\rm\thinspace erg}{\rm\thinspace cm}^{-2}{\rm\thinspace s}^{-1}\,$}$, and intrinsic (unabsorbed) 0.5-10keV luminosity $L_{0.5-10}=7.1\times 10^{42}\hbox{${\rm\thinspace erg}{\rm\thinspace s}^{-1}\,$}$, with a goodness of fit parameter $\chi^2/{\rm dof}=18.4/19$.

The power-law model will be appropriate if we are, indeed, observing AGN emission. However, the possibility remains that this central source could be compact (unresolved) core of thermal emission from the hot gas halo. Indeed, fitting such a model to the data using the mekal model encoded in the XSPEC spectral fitting package (version 11.3.0; Mewe, Gronenschild & van den Oord 1985; Mewe, Lemen & van den Oord 1986; Kaastra 1992; Liedahl, Osterheld & Goldstein 1995) results in a fit which is almost as good as (and statistically indistinguishable from) the power-law model. Best fit parameters for this thermal plasma model are; plasma temperature $kT=4.9^{+2.5}_{-1.8}{\rm\thinspace keV}$, plasma abundance $Z<2.6\,Z_\odot$, intrinsic (redshifted absorption) $N_{Hz}<5\times 10^{20}\hbox{${\rm\thinspace cm}^{-2}\,$}$, observed 0.5-10keV flux $F_{0.5-10}=5.1\times 10^{-14}\hbox{${\rm\thinspace erg}{\rm\thinspace cm}^{-2}{\rm\thinspace s}^{-1}\,$}$, and intrinsic (unabsorbed) 0.5-10keV luminosity $L_{0.5-10}=6.7\times 10^{42}\hbox{${\rm\thinspace erg}{\rm\thinspace s}^{-1}\,$}$, with a goodness of fit parameter $\chi^2/{\rm dof}=21.3/18$. The emission measure suggested by this spectral fit is $EM=4.1\times 10^{65}\hbox{${\rm\thinspace cm}^{-3}\,$}$. If we make the simple and conservative approximation that this thermal plasma uniformly fills a sphere of the size of our extraction region, this emission measure implies a plasma density of $n>0.25\hbox{${\rm\thinspace cm}^{-3}\,$}$, and a bremsstrahlung cooling timescale of $t_{\rm brems}<1-2\times 10^8{\rm\thinspace yr}$. Thus, if this central source is indeed a dense gaseous core, it possesses a cooling timescale which is much shorter than any realistic age of this cluster.