Abstract
New images from the Hubble Space Telescope of the FRII radio galaxy Pictor A
reveal a previously undiscovered tidal tail, as well as a number of jet knots
coinciding with a known X-ray and radio jet. The tidal tail is approximately 5"
wide (3 kpc projected), starting 18" (12 kpc) from the center of Pictor A, and
extends more than 90" (60 kpc). The knots are part of a jet observed to be
about 4' (160 kpc) long, extending to a bright hotspot. These images are the
first optical detections of this jet, and by extracting knot flux densities
through three filters we set constraints on emission models. While the radio
and optical flux densities are usually explained by synchrotron emission, there
are several emission mechanisms which might be used to explain the X-ray flux
densities. Our data rule out Doppler boosted inverse Compton scattering as a
source of the high energy emission. Instead, we find that the observed emission
can be well described by synchrotron emission from electrons with a low energy
index (\$p\sim2\$) that dominates the radio band, while a high energy index
(\$p\sim3\$) is needed for the X-ray band and the transition occurs in the
optical/infrared band. This model is consistent with a continuous electron
injection scenario.
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