Hard X-ray emission in Centaurus A

TL;DR

This study analyzes 13 years of Swift/BAT data to understand the origin of hard X-ray emission in Centaurus A, revealing variability, spectral changes, and a jet-based emission mechanism.

Contribution

It provides the first detailed long-term analysis linking hard X-ray variability to jet emission, ruling out ADAF models for Centaurus A.

Findings

X-ray variability shows a red-noise process with slope -1.3.

Significant correlation between X-ray and radio fluxes with no delay over 30 days.

Spectral variations indicate emission from the inner radio jet.

Abstract

We used 13 years of Swift/BAT observations to probe the nature and origin of hard X-ray (14-195 KeV) emission in Centaurus A. Since the beginning of the Swift operation in 2004, significant X-ray variability in the 14-195 KeV band is detected, with mild changes in the source spectrum. Spectral variations became more eminent after 2013, following a softer-when-brighter trend. Using the power spectral density method, we found that the observed hard X-ray photon flux variations are consistent with a red-noise process of slope, $-1.3$ with no evidence for a break in the PSD. We found a significant correlation between hard X-ray and 230 GHz radio flux variations, with no time delay longer than 30 days. The temporal and spectral analysis rules out the ADAF (advection-dominated accretion flow) model, and confirms that the hard X-ray emission is produced in the inner regions of the radio jet.