X-ray Spectrum

The X-ray spectrum of the hot spot has been modelled in various ways. A hot plasma with solar abundances and in collisional equilibrium (``Raymond-Smith'' model) provides a poor fit to the data (Tab. 1) and can be ruled out. If the abundances of all elements heavier than helium are allowed to vary, we find that an abundance of $\lesssim$ 10 per cent solar is needed for this thermal model to provide an acceptable description of the data. However, this model (Tab. 1) requires a hydrogen column well below the Galactic value of N$_{H}$ = 4.2 $\times$ 10$^{20}$ atoms cm$^{-2}$ towards Pictor A (Heiles & Cleary 1979), and is thus implausible. Fixing the column at the Galactic value while allowing the abundances to vary leads to a poor fit (Tab. 1). Further, we shall argue in Section 3.2.3 that a thermal model can be excluded on grounds that the required gas density is so high that the radio emission would be Faraday depolarized, contrary to observation. Lastly, a power law of photon index $\Gamma$ = 2.07 $^{+0.11}_{-0.11}$ absorbed by a column density of solar abundance gas N$_{H}$ = (7.1 $^{+2.3}_{-2.3}$) $\times$ 10$^{20}$ atoms cm$^{-2}$ provides an excellent description of the spectrum (Tab. 1, Fig. 5). This column density is only slightly greater than the Galactic column density towards Pictor A, suggesting that the interstellar medium of our Galaxy is responsible for most of the observed absorption. The unabsorbed flux and luminosity of the western hot spot in the 2 - 10 keV band are 3.1 $\times$ 10$^{-13}$ erg cm$^{-2}$ s$^{-1}$ and 1.7 $\times$ 10$^{42}$ erg s$^{-1}$, respectively.