0 M ay 2 00 9 Origin of the X - rays and Possible GeV - TeV Emission from the Western Hot Spot Of Pictor A

Pictor A is a nearby Fanaroff-Riley class II (FR II) radio galaxy with a bright hot spot, the western hot spot. Observation of high polarization in the optical emission of the hot spot indicates that the optical emission could be synchrotron radiation of relativistic electrons in the hot spot. These electrons may be able to produce high energy γ-ray photons through inverse Compton (IC) scattering. We use single-zone and multi-zone synchrotron + synchrotron-self-Compton (SSC) models to fit the observed spectral energy distribution (SED) from the radio to the X-ray band of the hot spot. Our results show that in the case of a much weaker magnetic field strength than the equipartition magnetic field, both the single-zone and multi-zone models can fit the SED, but the multi-zone model significantly improves the fit. The two models predict the hot spot as a GeVTeV source, which might be marginally detectable with Fermi/LAT and HESS. The inverse Compton scattering of cosmic microwave background (IC/CMB) is also considered, but its contribution to GeV-TeV emission is negligible. Note that under the equipartition condition, the SED can also be fit with the multizone model, but the predicted flux at > 10 Hz is too weak to be detectable. The detection of TeV γ-rays from this FR II radio galaxy, if confirmed, would establish a new subclass of extragalactic source in this energy regime since most of the AGNs detected to date at TeV energies are high-energy-peaked BL Lac objects. Subject headings: FR II radio galaxies: individual: Pictor A—gamma rays: theory—radiation mechanisms: nonthermal—X-rays: hot spot National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, P. O. Box 110, Kunming, Yunnan, 650011, China; College of Physics and Electronic Engineering, Guangxi Teachers Education University, Nanning, Guangxi, 530001, China; The Graduate School of Chinese Academy of Sciences