Fermi telescope: 'Violent cosmos' map gets more extreme

Fermi >10GeV all-sky map

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The Fermi space telescope has updated its catalogue of the Universe's most violent neighbourhoods.

Fermi catches gamma rays, the most energetic light, spewing from nature's most extreme physical processes.

The new catalogue, posted on the Arxiv server, lists the sources of the highest-energy gamma rays that Fermi has yet seen: 514 of them.

Some 65 of them are "unassociated" sources that may turn out to be completely new astronomical objects.

The catalogue is to be published in the Astrophysical Journal.

It will help outline which of the known sources of other kinds of electromagnetic radiation - such as visible light, or radio waves, or X-rays - are also emitting at this higher range of gamma-ray energies.

That will help astrophysicists unpick exactly what is happening in these violent corners of the cosmos, where processes are going on that we will never be able to mimic in Earth-bound laboratories.

Bridge catalogue

Fermi scans in every direction, gathering gamma rays from the whole cosmos every three hours.

What is an electronvolt?

Particle interaction simulation (SPL)
  • Charged particles tend to speed up in an electric field, defined as an electric potential - or voltage - spread over a distance
  • One electronvolt (eV) is the energy gained by a single electron as it accelerates through a potential of one volt
  • It is a convenient unit of measure for particle accelerators, which speed particles up through much higher electric potentials
  • The Large Hadron Collider, for example, can accelerate particles up to an energy of several trillion electronvolts (TeV)
  • That is about equal to the energy in the motion of a flying mosquito

The "all-sky map" accompanying the publication represents all of the gamma-ray detections above 10 gigaelectronvolts that the Fermi telescope has seen in three years' worth of data.

The prior catalogues produced by Fermi contain literally thousands of gamma-ray emitters in the far-flung cosmos, corresponding to objects such as blazars and active galactic nuclei - both associated with black holes snacking at the centres of galaxies - or pulsars, fast-spinning neutron stars that flash our neighbourhood like a distant lighthouse.

But even in the most recent catalogue, the energy range above 10 gigaelectronvolts has been poorly explored.

This is largely untrodden ground in energy terms - the amount of light that comes towards us at these energies is far lower than that associated with lower-energy sources. But it is a range that Fermi is uniquely placed to catch.

For energy ranges above about 10 times that high, we have relied on ground-based telescopes, which do not catch the gamma rays directly.

At these highest imaginable energies, incoming gamma rays streak through the atmosphere, creating a shower of secondary particles that then emit light that telescopes on the ground can see.

This "Cerenkov" light is the focus of instruments such as the Veritas and Hess telescopes in the US and Namibia, respectively, and the Magic facility on the island of Palma. A similar process at the Earth's surface is what the newly commissioned Hawc telescope searches for.

"The idea is to have some sort of bridge catalogue between the typical catalogue done by Fermi... which contains thousands of sources, and the domain of the Cerenkov telescopes that have been operating over 20 years," lead author of the new catalogue, David Paneque of the Max Planck Institute for Physics in Munich, Germany, told BBC News.

But, as with prior Fermi catalogues, Dr Paneque said that there are "unassociated sources" in the new catalogue.

Fermi space telescope

Fermi telescope
  • Telescope has initial 5-year mission, but expected to last for a decade
  • Looks at the Universe in highest-energy form of light - gamma rays
  • Spacecraft is 2.8m (9.2ft) high and 2.4m (8.2ft) in diameter
  • Mission is a team-up between Nasa and US Department of Energy

"What that means is that we know it's a gamma-ray source, but we don't know what kind of source," he explained.

"We can't associate it with a radio object, with an optical object. It might be actually a new class of object - something that only emits in gamma rays."

One tantalising possibility is that these mystery sources are dwarf galaxies, rich with "dark matter" - the stuff believed to make up a majority of the Universe's mass but which has resolutely evaded detection so far.

"If we see a source that only emits gamma rays and not radio and optical, of course that would be a good candidate for dark matter," Dr Paneque said.

Nasa's deputy project scientist on the Fermi mission, Dave Thompson, said the new catalogue release was "exciting".

"A lot of work has gone into it," he told BBC News. "It represents another step in what we can do with Fermi, extending our reach in energy in more than an order of magnitude, and being able to see an entirely different picture of the sky," he said.

And although the number of unassociated sources is a smaller fraction of the total compared to prior catalogues, Dr Thompson said that "there are enough to be intriguing, and certainly following up on them will be one of the things we'll be doing - and we hope other people will too".

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