Linnaeus Physics Colloquium: The nearest radio galaxy, Centaurus A, at very high energies

X-ray: NASA/CXC/CfA/R.Kraft et al

Title: The nearest radio galaxy, Centaurus A, at very high energies
Lecturer: Dmitry Prokhorov, University of Amsterdam, the Netherlands

Abstract

Centaurus A (Cen A) is the nearest radio galaxy discovered as a very-high-energy (VHE; 100 GeV-100 TeV) gamma-ray source by the High Energy Stereoscopic System (H.E.S.S.). It is a faint VHE gamma-ray emitter, though its VHE flux exceeds both the extrapolation from early Fermi-LAT observations as well as expectations from a (misaligned) single-zone synchrotron-self Compton (SSC) description. The latter satisfactorily reproduces the emission from Cen A at lower energies up to a few GeV.

New observations with H.E.S.S., comparable in exposure time to those previously reported, were performed and eight years of Fermi-LAT data were accumulated to clarify the spectral characteristics of the gamma-ray emission from the core of Cen A. The results allow us for the first time to achieve the goal of constructing a representative, contemporaneous gamma-ray core spectrum of Cen A over almost five orders of magnitude in energy. Advanced analysis methods allow detection in the VHE range of the core with a statistical significance of 12 sigma on the basis of 213 hours of total exposure time. The spectrum in the energy range of 250 GeV-6 TeV is compatible with a power-law function with a photon index Γ = 2.52. An updated Fermi-LAT analysis provides evidence for spectral hardening by ∆Γ=0.4 at gamma-ray energies above 2.8 GeV at a level of 4.0 sigma.

The fact that the spectrum hardens at GeV energies and extends into the VHE regime disfavour a single-zone SSC interpretation for the overall spectral energy distribution (SED) of the core and is suggestive of a new gamma-ray emitting component connecting the high-energy emission above the break energy to the one observed at VHE energies. The presence of a new spectral component together with the absence of significant variability at both GeV and TeV energies shows that it is of importance to spatially resolve the gamma-ray core of Cen A, which includes the kiloparsec-scale jet.

The detection of extended gamma-ray emission, directly probing the presence of ultra-relativistic electrons, could distinguish between the scenarios of origin of X-rays coming from the kiloparsec jet, but instruments have hitherto been unable to resolve the relevant structures in gamma rays. H.E.S.S. observations of Cen A at TeV energies resolve its large-scale jet. It allows us to interpret the data as evidence for the acceleration of ultra-relativistic electrons in the jet, and favour the synchrotron explanation for the X-rays.

About Dmitry Prokhorov

Dmitry Prokhorov received a PhD degree in theoretical physics from the theoretical physics and astrophysics group based in Igor Tamm's theoretical department of Physical Institute in Moscow and in astronomy from Institute for Astrophysics of Paris. In 2010-2011 he held postdoctoral positions in South Korea (Korea Institute for Astronomy and Space Science) and Japan (Yukawa Institute for Theoretical Physics) before starting his two-year postdoctoral research in gamma-ray astronomy in the group led by Prof. Michelson, who is the Principal Investigator of the Fermi-LAT gamma-ray telescope (Stanford University, US). In 2012, Dmitry started the postdoctoral position in the high-energy astrophysics group led by Prof. Rashid Sunyaev at Max Planck Institute for Astrophysics in Garching/Munich. Since 2014, Dmitry's research interests include gamma-ray astronomy at very-high energies with modern Cherenkov Telescopes, High Energy Stereoscopic System (H.E.S.S.). Given that the first head of the theoretical department of Physical Institute in Moscow, Prof. Igor Tamm, proposed a physical explanation of Cherenkov radiation, Dmitry was always fascinating about applications of Cherenkov radiation in astronomy. Dmitry Prokhorov started to work on science with the H.E.S.S. Cherenkov telescopes in the astroparticle research group led by Associate Prof. Yvonne Becherini in Linnaeus University. After two years of his postdoctoral position in Vaxjo, Dmitry continues to work on science with the H.E.S.S. telescopes in Johannesburg (2017-2019) and in Prof. Jacco Vink's group in University of Amsterdam since 2019.