Quasar jets thousands of light-years long

The very-high-energy gamma ray emission from quasars are not concentrated in the region close to their central black holes but in fact extend over several thousand light-years along jets of plasma, new research reveals.

Writing in the journal Nature, a team of more than 200 astrophysicists from 13 countries describes its observations using the HESS observatory in Namibia which, it says, shake up current scenarios for the behaviour of these jets.

The work was carried out as part of the international High Energy Stereoscopic System (HESS) collaboration, and led by the CNRS and CEA in France, the Max Planck Society and other German research institutions and universities.

They studied the highly luminous Centaurus A – the closest radio galaxy to Earth – at unparalleled resolution for more than 200 hours, enabling them to identify the region emitting the very high-energy radiation while studying the trajectory of the plasma jets.

In recent years, scientists have observed the Universe using gamma rays, which form part of the cosmic rays that constantly bombard the Earth. They originate from regions where particles are accelerated to huge energies.

Gamma rays are emitted by a wide range of cosmic objects, including quasars, which are active galaxies with a highly energetic nucleus. The intensity of the radiation emitted can vary over very short timescales of up to one minute.

It has therefore been assumed that the source of this radiation is very small and located in the vicinity of a supermassive black hole.

However, the researchers say they have shown that the gamma ray source extends over several thousand light-years. This in turn indicates that particle acceleration does not take place solely in the vicinity of the black hole, but along the entire length of the plasma jets.

Based on these results, the authors suggest the particles are reaccelerated by stochastic processes along the jet, and that many radio galaxies with extended jets accelerate electrons to extreme energies and might emit gamma-rays. This might explain the origins of a substantial fraction of the diffuse extragalactic gamma background radiation.

“Here we report observations of Centaurus A at teraelectronvolt energies that resolve its large-scale jet,” the authors write in their paper.

“We interpret the data as evidence for the acceleration of ultrarelativistic electrons in the jet and favour the synchrotron explanation for the X-rays.

“Given that this jet is not exceptional in terms of power, length or speed, it is possible that ultrarelativistic electrons are commonplace in the large-scale jets of radio-loud active galaxies.”