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  Blazars shed light on black hole physics
Fast-acting astronomers have captured blazars in the act of spewing out material, providing new insights into these powerful phenomena.

Blazars are found in the middle of galaxies where supermassive black holes emit powerful jets of charged particles, at velocities close to the speed of light, directly towards the Earth.

Italian astronomers using the gamma ray imaging detector on the Astro-rivelatore Gamma a Immagini Leggero (AGILE) satellite noticed marked changes in the behaviour of eight blazars.

The team then trained a number of ground and space-based telescopes from other observatories on each blazer, enabling them to observe the jets in a range of energies from gamma ray to radio wavelengths.

Their results appear on electronic preprint website arXiv.
Mysterious jets

Blazars typically emit energy in a wide range of frequencies, ejected in two narrow, oppositely-directed beams.

When a large amount of matter falls into the central black hole, the blazer's jets increase in intensity.

The physics of these jets is not well understood, and the new observations go some way towards narrowing down how the process operates.

"The key to obtaining a more complete understanding of the physics behind the emission mechanisms acting in the blazars is the simultaneous multi-wavelength observations over the entire electromagnetic spectrum," says lead author Filippo D'Ammando, a PhD student at the University of Rome working at the Istituto di Astrofisica Spaziale e Fisica Cosmica in Rome, Italy.

It is thought the jets rely on a mechanism called 'inverse Compton scattering', in which photons gain energy as they interact with matter.

Where the matter comes from, and what is the precise source of photons, remains a mystery. It's also not certain if inverse Compton scattering is the only mechanism at work.

The researchers' results showed that while different inverse Compton scattering models explain the processes operating in several blazars, in others, the story was more complex.

"The spectral energy distribution of some blazars may require us to take into account new components and a more refined treatment of the emission mechanisms acting in the blazars" said D'Ammando.

Commenting on the research, Associate Professor Scott Croom, an astrophysicist at the University of Sydney, says this research will feed into our understanding of jet physics, but astronomers were still a "way off a full understanding of what's going on."

"We have a lot of information about the radiation coming off the jets, but we don't know simple things, like the mass or spin of the black holes in these systems, because these are difficult to detect," says Croom. "Fundamental things like that need to be measured and that's actually quite hard."
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