We report Chandra ACIS and quasi-simultaneous RXTE observations of the nearby , powerful radio galaxy Cygnus A , with the present paper focusing on the properties of the active nucleus .
In the Chandra observation , the hard ( > a few keV ) X-ray emission is spatially unresolved with a size \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } \hss } \raise 2.0 pt \hbox { $ < $ } } 1 \arcsec ( 1.5 kpc , H _ { 0 } = 50 km s ^ { -1 } Mpc ^ { -1 } ) and coincides with the radio and near infrared nuclei .
In contrast , the soft ( < 2 { \thinspace keV } ) emission exhibits a bi-polar nebulosity that aligns with the optical bi-polar continuum and emission-line structures and approximately with the radio jet .
In particular , the soft X-ray emission corresponds very well with the [ O iii ] \lambda 5007 and H \alpha + [ N ii ] \lambda \lambda 6548 , 6583 nebulosity imaged with HST .
At the location of the nucleus there is only weak soft X-ray emission , an effect that may be intrinsic or result from a dust lane that crosses the nucleus perpendicular to the source axis .
The spectra of the various X-ray components have been obtained by simultaneous fits to the 6 detectors .
The compact nucleus is detected to 100 keV and is well described by a heavily absorbed power law spectrum with \Gamma _ { h } = 1.52 ^ { +0.12 } _ { -0.12 } ( similar to other narrow line radio galaxies ) and equivalent hydrogen column N _ { H } ( { nuc } ) = 2.0 ^ { +0.1 } _ { -0.2 } \times 10 ^ { 23 } \hbox { $ { \thinspace cm } ^ { -2 } % $ } .
This column is compatible with the dust obscuration to the near infrared source for a normal gas to dust ratio .
The soft ( < 2 { \thinspace keV } ) emission from the nucleus may be described by a power law spectrum with the same index ( i.e . \Gamma _ { l } = \Gamma _ { h } ) , though direct fits suggests a slightly larger value for \Gamma _ { l } .
Narrow emission lines from highly ionized neon and silicon , as well as a “ neutral ” Fe K \alpha line , are detected in the nucleus and its vicinity ( r \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } \hss } \raise 2.0 pt \hbox { $ < $ } } % 2 { \thinspace kpc } ) .
The equivalent width ( EW ) of the Fe K \alpha line ( 182 ^ { +40 } _ { -54 } eV ) is in good agreement with theoretical predictions for the EW versus N _ { H } ( { nuc } ) relationship in various geometries .
An Fe K edge is also seen .
The RXTE observations indicate a temperature of kT = 6.9 ^ { +0.3 } _ { -1.0 } keV for the cluster gas ( discussed in Paper III of this series ) and cluster emission lines of Fe K \alpha and Fe K \beta and / or Ni K \alpha .

We consider the possibility that the extended soft X-ray emission is electron-scattered nuclear radiation .
Given that 1 % of the unabsorbed 2 – 10 keV nuclear radiation would have to be scattered , the necessary gas column ( N _ { H } ( { scattering } ) \simeq 3.5 \times 10 ^ { 22 } \hbox { $ { \thinspace cm } ^ { -2 } % $ } ) would absorb the X-rays rather than scatter them if the gas is cold .
Thus the scattering plasma must be highly ionized .
If this ionization is achieved through photoionization by the nucleus , the ionization parameter \xi > 1 erg cm s ^ { -1 } and the electron density n _ { e } \simeq 6 \hbox { $ { \thinspace cm } ^ { -3 } $ } given the observed distance of the soft X-ray emission from the nucleus .
The electron column density inferred from the X-ray observations is much too low to account for the extended optical scattered light , strongly suggesting that the polarized optical light is scattered by dust .
The presence of highly ionized Ne lines in the soft X-ray spectrum requires 20 \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } \hss } \raise 2.0 pt \hbox { $ < $ } % } \xi \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } \hss } \raise 2.0 pt \hbox { $ < % $ } } 300 erg cm s ^ { -1 } ; these lines may originate closer to the nucleus than the extended soft continuum or in a lower density gas .
A collisionally-ionized thermal model of the extended soft X-rays can not be ruled out , but is unattractive in view of the low metal abundance required ( Z = 0.03 Z _ { \odot } ) .

The hard X-ray to far-infrared ratio for the nucleus of Cygnus A is similar to that seen in Seyfert 1 and unobscured radio galaxies .
By means of the correlation between hard X-ray luminosity and nuclear optical absolute magnitude for these classes of object , we estimate M _ { B } = -22.4 for Cygnus A , near the borderline between Seyferts and QSOs .