Two-Component gamma-ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A

TL;DR

This study analyzes 16 years of Fermi-LAT data and IXPE observations of Pictor A, revealing a two-component gamma-ray spectrum with variability and providing insights into the emission mechanisms of the nucleus and hotspot.

Contribution

It presents the first evidence of a two-component gamma-ray spectrum in Pictor A and links spectral features to different emission regions, supported by polarization constraints.

Findings

Evidence of two gamma-ray components with spectral hardening at 2.46 GeV.

Gamma-ray variability driven by the low-energy component.

X-ray polarization upper limits consistent with SSC emission for the nucleus.

Abstract

Pictor A is a $\gamma$-ray emitting radio galaxy and has a bright hotspot called WHS, located $\sim$4 arcmin away from the nucleus. In this work, we present an analysis of its 16-year Fermi-LAT data and report the Imaging X-ray Polarimetry Explorer (IXPE) observations for this source. Our analysis of the Fermi-LAT observations reveals evidence of two components in the average $\gamma$-ray spectrum of Pictor A, exhibiting a statistically significant hardening from $\Gamma_{\rm \gamma,1}=3.25\pm0.15$ to $\Gamma_{\rm \gamma,2}=1.81\pm0.07$ at a break energy of $2.46\pm0.09$ GeV. Notably, variability of $\gamma$-rays is evident in Pictor A, predominantly driven by the component below the break energy, while the component above the break energy remains stable. Furthermore, our analysis reveals that a power-law function provides an adequate fit for the high-flux-state spectrum, while a broken power-law function remains necessary to accurately model the low-flux-state spectrum. We suggest that the low-energy component originates from the nucleus, while the high-energy component primarily stems from WHS. The broadband spectral energy distributions of both nucleus and WHS can be well represented by a simple leptonic model, with both $\gamma$-ray components attributed to the synchrotron-self-Compton (SSC) process. Analysis of IXPE data provides upper limits on the polarization degree of $\Pi_{\rm X}<$6.6% for the nucleus and $\Pi_{\rm X}<$56.4% for the WHS within the 2--8 keV band. For the nucleus, this result aligns with X-ray emission originating from the SSC process. However, the upper limit of $\Pi_{\rm X}<$56.4% for WHS is insufficient to conclusively determine the X-ray emission mechanism in this region.