Introduction

X-ray observations of radio galaxies are of value for several reasons. Low brightness radio lobes may be thermally confined by the hot, extended atmospheres in elliptical galaxies or clusters of galaxies and X-ray observations can provide the density, temperature and pressure of this gas, thus constraining the pressures of the relativistic gas and magnetic field in a lobe (e.g. Worrall & Birkinshaw 2000). High brightness regions, such as the hot spots in Fanaroff-Riley class II (FRII) radio galaxies, cannot be statically confined by thermal pressure and must be expanding and/or advancing away from the nucleus. If the motion is supersonic, such a feature must be preceded by a bow shock in the surrounding medium. The effect of bow shocks on the ambient gas may be identifiable in thermal X-ray emission, depending on the run of temperature and density in the post-shock gas (e.g. Clarke, Harris & Carilli 1997). Thermal X-rays may also be detectable from radio jets and lobes themselves if they entrain and shock surrounding gas (e.g. de Young 1986). The relativistic particles responsible for the synchrotron radio emission must also radiate through inverse Compton scattering of ambient photons (such as the microwave background and the starlight of the galaxy) or the synchrotron photons themselves. This inverse Compton radiation is expected to be spread over a wide range of frequencies, including the X-ray band. Detection of both synchrotron and inverse Compton radiation provides a direct measurement of the magnetic field in the emitting region (e.g. Harris, Carilli & Perley 1994), allowing a check on the common assumption of equipartition of energy between cosmic rays and magnetic fields. Lastly, synchrotron X-ray emission may be observable if electrons and/or positrons of sufficiently high energy are present, a finding which would provide important clues about the origin of the relativistic particles in these objects. For these reasons, we have begun a program of imaging and spectroscopy of radio galaxies with the Chandra X-ray Observatory.

Pictor A is the seventh brightest extragalactic radio source in the sky at 408 MHz (Robertson 1973), and the most powerful radio galaxy (P$_{408 MHz}$ = 6.7 $\times$ 10$^{26}$ W Hz$^{-1}$ for z = 0.035, q$_{0}$ = 0 and H$_{0}$ = 50 km s$^{-1}$ Mpc$^{-1}$) with z $<$ 0.04. The best radio maps (Perley, Röser & Meisenheimer 1997, hereafter PRM) reveal two round, diffuse lobes with very much brighter radio hot spots on the sides away from the nucleus, corresponding to an FRII classification. The total angular diameter of the radio emission is about 7.'6 (430 kpc). The western hot spot, some 4.'2 (240 kpc) from the nucleus, is a remarkable object, being amongst the brightest of radio hot spots in radio galaxies and quasars. PRM have found a very faint radio jet extending from the compact, flat spectrum (cf. Jones, McAdam & Reynolds 1994) nuclear core to the western hot spot. Recently, VLBI observations (Tingay et al. 2000) have revealed a milli arc second- (pc-) scale nuclear radio jet which is approximately aligned with the large-scale radio jet. Simkin et al. (1999) have reported an optical continuum extension $\simeq$ 0.''1 (95 pc) to the W of the nucleus and the direction to this extension is also roughly aligned with the larger scale radio jet.

Röser & Meisenheimer (1987) discovered optical emission from the western hot spot. This emission has a featureless continuum, is strongly polarized and thus of synchrotron origin. These authors also found a marginal detection of the hot spot at X-ray wavelengths in an Einstein IPC observation. Further observations of the hot spot in the infrared have been reported by Meisenheimer, Yates & Röser (1997). The overall spectrum of the western hot spot conforms to a power law with index $\alpha$ = 0.74 (S $\propto$ $\nu^{-\alpha}$) from 327 MHz to $\sim$ 10$^{14}$ Hz, where the spectrum turns down (see Fig. 2i of Meisenheimer, Yates & Röser 1997). The morphologies of the hot spot are very similar at radio and optical wavelengths, with a bright, compact, leading ``core'' and a fainter, following ``filament'', which extends $\simeq$ 15 $^{\prime\prime}$ (14 kpc) more or less perpendicular to a line joining the hot spot to the nucleus (PRM). Fainter radio emission associated with the hot spot is also seen to the E and NE of the optical and radio filament, the overall angular extent of the radio hot spot being $\simeq$ 25 $^{\prime\prime}$ (24 kpc). The radio to optical spectrum of the ``filament'' is somewhat steeper ( $\alpha_{opt}^{3.6cm}$ = 0.98) than that of the bright, compact region ( $\alpha_{opt}^{3.6cm}$ = 0.87). Optical imaging and polarimetry of the bright part of the hot spot with the Hubble Space Telescope (Thomson, Crane & Mackay 1995) confirms the high polarization ($\gtrsim$ 50%) and resolves the hot spot into highly polarized ``wisps'' elongated nearly perpendicular to the nucleus - hot spot line.

The nucleus of Pictor A has long (Schmidt 1965; Danziger, Fosbury & Penston 1977) been known for its strong emission-line spectrum, with broad wings to both forbidden and permitted lines. Double-peaked, very broad Balmer lines were discovered in 1993/1994 (Halpern & Eracleous 1994; Sulentic et al. 1995); these double-peaked lines were not present in 1983. Such line profiles are commonly interpreted as emission from an accretion disk (e.g. Storchi-Bergmann et al. 1997). The nucleus of Pictor A is a strong X-ray source; the spatially integrated emission observed with ASCA has a power-law continuous spectrum and no evidence for an Fe K$\alpha$ line (Eracleous & Halpern 1998). Further observations of the X-ray spectrum of Pictor A have been reported recently by Padovani et al. (1999) and Eracleous, Sambruna & Mushotzky (2000).

We selected Pictor A for observation with Chandra because of the X-ray emission from the western hot spot, tentatively detected with the Einstein IPC and confirmed by us through an inspection of an archival ROSAT PSPC observation. Further, the optical synchrotron emission from the hot spot is of sufficient extent ($\sim$ 15 $^{\prime\prime}$) to be well resolved by Chandra, making this object a prime candidate for the study of high energy processes in the hot spots of powerful radio galaxies. In addition to the Chandra observations, we also report analysis of archival Hubble Space Telescope (HST) optical and near uv imaging observations of the western hot spot.

Section 2 describes the observations and their reduction while Section 3 presents the results. In Section 4, we discuss the orientation of the radio source and the processes responsible for the X-ray emission of the western hot spot and jet. Concluding remarks are given in Section 5.