Centaurus A: Exploring the Nature of the Hard X-ray/Soft Gamma-ray Emission with INTEGRAL

TL;DR

This study analyzes INTEGRAL data of Centaurus A, revealing that its hard X-ray/soft gamma-ray emission likely originates from the corona, with jet contributions becoming significant at higher energies, challenging previous assumptions about the emission source.

Contribution

The paper provides the first comprehensive analysis combining INTEGRAL, NuSTAR, and Fermi data to distinguish between corona and jet origins of Cen A's high-energy emission.

Findings

Spectral fits suggest a corona electron temperature near 550 keV, close to pair-production threshold.

The spectrum can be explained by a combination of corona emission and jet synchrotron self-Compton processes.

A power-law excess above 3 GeV indicates additional jet-related emission components.

Abstract

The question of the origin of the hard X-ray/soft gamma-ray emission in Centaurus A (Cen A) persists despite decades of observations. Results from X-ray instruments suggest a jet origin since the implied electron temperature (kT_e) would cause pair production runaway in the corona. In contrast, instruments sensitive to soft gamma-rays report electron temperatures indicating a corona origin may be possible. In this context, we analyzed archival INTEGRAL/IBIS-ISGRI and SPI data and observations from a 2022 Cen A monitoring program. Our analysis did not find any spectral variability. Thus we combined all observations for long-term average spectra, which were fit with a NuSTAR observation to study the 3.5 keV - 2.2 MeV spectrum. Spectral fits using a CompTT model found kT_e ~ 550 keV, near pair-production runaway. The spectrum was also well described by a log-parabola to model synchrotron self-Compton emission from the jet. Additionally, a spectral fit with the 12-year catalog Fermi/LAT spectrum using a log-parabola can explain the data up to ~ 3 GeV. Above ~ 3 GeV, a power-law excess is present, which has been previously reported in LAT/H.E.S.S. analysis. However, including a corona spectral component can also describe the data well. In this scenario, the hard X-rays/soft gamma-rays are due the corona and the MeV to GeV emission is due to the jet.