23/06/2020: Nature paper: Resolving acceleration to very high energies along the jet of Centaurus A

We know now that most, if not all, galaxies have supermassive black holes in their centers. Under the right circumstances these black holes swallow mass from their surroundings. If this is the case they often eject part of the mass they gathered in a pair of very fast jet streams. In the jets  gas  has velocities close to the speed of light. These jets have been known  for decades and were discovered initially due to the bright radiation emitted in the radio bands.  Galaxies with these jets are called radio galaxies.
The H.E.S.S. Imaging Atmospheric Cherenkov Telescope system in Namibia has now found evidence that inside jets in a nearby galaxy, Centaurus A, electrons are being accelerated to very-high energies, around 50 Tera-electronVolt; higher than the energies protons can obtain at the CERN LHC facility! Scientists of the University of Amsterdam (GRAPPA & Anton Pannekoek Institute) were involved in this new study that just appeared in the Nature scientific journal.
Centaurus A is a radio galaxy at a distance of 12 million light years from Earth. The jets launched by its black hole are also detected in X-rays. But a question has been for a long time, what causes this radiation. For radio emission the mechanism is electrons that are rotating in the magnetic fields of the jets. This is called synchrotron radiation. In X-rays the same radiation mechanism could be happening, but only if the electrons causing it have much more energy than the electrons causing radio radiation. The energies needed are of the order of 50 Tera-electronVolt. However, since these electrons radiate so much, they also lose energy very fast. So the presence of Tera-electronVolt electrons throughout the jet is only possible if somehow the electrons keep on gaining energy within the jet through some acceleration mechanism.
An alternative explanation is that the X-ray emission is from electrons with much less energy, which can scatter light from the surroundings. The scattering process itself can boost the energy of the scattered light, making it visible in X-rays. This process is called Compton scattering.
One way to test which mechanism is responsible for X-rays is to observe the jets in gamma-rays. These gamma-rays can also be caused by Compton scattering, but in this case by bouncing off electrons with 10-100 Tera-electronVolt energies, causing gamma-ray radiation with energies of 1-10 Tera-electronVolt.

Shown are the observed and modelled spectral energy distribution (SED) from radio to gamma-ray energies for the kiloparsec-scale jet of Centaurus A. The VHE emission is dominated by relativistic electrons with energies above 10 TeV inverse Compton up-scattering dust photons to high energies (solid blue curve, ‘IC total’). This emission from the kiloparsec-scale jet makes a major contribution to the unexpected spectral hardening above a few GeV as seen by Fermi-LAT (red points). The lower-energy part of the gamma-ray spectrum (red points) is attributed to emission from the core (grey dashed line referring to a core model. The green curve (‘Sync.’) designates the synchrotron emission of the inferred broken power-law electron distribution in a magnetic field of characteristic strength B = 23 μG. The blue ‘butterfly’ corresponds to the H.E.S.S. spectra, while green data points mark radio, infrared and X-ray measurements and reported uncertainties (error bars) from the inner region of the Centaurus A jet. A breakdown is provided of the full IC contribution, from the scattering of: the cosmic microwave background (CMB), the starlight emission of the host galaxy, infrared emission from dust, and the low-energy synchrotron jet emission (synchrotron self Compton, SSC). 

H.E.S.S. already detected gamma-ray radiation from Centaurus A, but it was not clear whether the radiation originated from the jet, or from near the black hole. But in the new publication in Nature the H.E.S.S. collaboration announced that they have been able to make an image of the Centaurus A gamma-ray emission, which clearly shows that the gamma-ray radiation is coming from the jets. The jet system detected in gamma-rays has a length of several thousands of light years. Among the authors of the new publication are also scientists from the University of Amsterdam, D. Prokhorov, R. Simoni, and local H.E.S.S. group leader Jacco Vink. Dr. Dmitry Prokhorov was actively involved in the analysis of the observations and and led the efforts of a spectral analysis of H.E.S.S. and Fermi-LAT data presented in Figure 2 of this paper. He is very enthusiastic about the results, saying that the results shows that “the surprising implication is that ultrarelativistic electrons may be commonplace in the large-scale jets of radio-loud active galaxies!“

H.E.S.S. is a system of Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma rays in the energy range from 10s of GeV to 10s of TeV. The instrument allows scientists to explore gamma-ray sources with intensities at a level of a few thousandths of the flux of the Crab nebula. H.E.S.S. is located in in the Khomas Highland in Namibia, a country in Southern Africa. The area is well known for its excellent optical quality. The four telescopes of Phase I of the H.E.S.S. project were operational in December 2003, while a much larger fifth telescope – H.E.S.S. II – is operational since July 2012, extending the energy coverage towards lower energies and further improving sensitivity. The H.E.S.S. data reported in this paper were accumulated during the Phase I. Scientists from Jacco Vink’s group at University of Amsterdam are leading a new H.E.S.S. observation campaign on Centaurus A with participation of all five telescopes of H.E.S.S.. and collaborating on their task with Namibian and Swedish colleagues. The new study will result in an even better understanding of the spectral characteristics of the Centaurus A jet especially below a few hundred GeV energy, given the participation of a fifth telescope outfitted with a new camera.