Chandra Imaging of the Western Hotspot in the Radio Galaxy Pictor A: Image Deconvolution and Variability Analysis

TL;DR

This paper analyzes Chandra X-ray observations of the Western hotspot in Pictor A, revealing its structure, variability over 15 years, and the offset between optical and radio emission regions, enhancing understanding of jet termination shocks.

Contribution

It provides detailed image deconvolution and variability analysis of the hotspot, revealing its extended structure and flux changes over time, which were not previously characterized.

Findings

Hotspot's brightest X-ray segment is a 3 kpc-long feature perpendicular to the jet.

The hotspot's X-ray flux decreased by about 30% over 15 years.

The optical peak aligns with the X-ray front but is offset from the radio peak.

Abstract

Here we present an analysis of the X-ray morphology and flux variability of the particularly bright and extended Western hotspot in the nearest powerful (FR II-type) radio galaxy, Pictor A, based on data obtained with the Chandra X-ray Observatory. The hotspot marks the position where the relativistic jet, that originates in the active nucleus of the system, interacts with the intergalactic medium, at hundreds-of-kiloparsec distances from the host galaxy, forming a termination shock that converts jet bulk kinetic energy to internal energy of the plasma. The hotspot is bright in X-rays due to the synchrotron emission of electrons accelerated to ultra-relativistic energies at the shock front. In our analysis, we make use of several Chandra observations targeting the hotspot over the last decades with various exposures and off-axis angles. For each pointing, we study in detail the PSF, which allows us to perform the image deconvolution, and to resolve the hotspot structure. In particular, the brightest segment of the X-ray hotspot is observed to be extended in the direction perpendicular to the jet, forming a thin, $\sim$3 kpc-long, feature that we identify with the front of the reverse shock. The position of this feature agrees well with the position of the optical intensity peak of the hotspot, but is clearly off-set from the position of the radio intensity peak, located $\sim$1 kpc further downstream. In addition, we measure the net count rate on the deconvolved images, finding a gradual flux decrease by about 30% over the 15-year timescale of the monitoring.