Optical Detection of the Pictor A Jet and Tidal Tail: Evidence against an IC/CMB jet

TL;DR

This study presents optical images of Pictor A revealing a tidal tail and jet knots, constraining emission mechanisms and ruling out Doppler boosted IC/CMB as the source of high-energy emission, favoring synchrotron models.

Contribution

First optical detection of the Pictor A jet and tidal tail, providing new constraints on emission mechanisms and challenging previous IC/CMB interpretations.

Findings

Optical images reveal a tidal tail and jet knots in Pictor A.

Data exclude Doppler boosted inverse Compton scattering as the high-energy emission source.

Synchrotron emission from electrons with different energy indices explains the multi-wavelength data.

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.