Powerful Jet Pointing Directly At Earth
– They are among the most fascinating cosmic phenomena. Astronomers know them as blazars and each of them harbors a supermassive black hole and jets emanating in opposite directions from near its poles. These sources are unique evidence of the most extreme speeds and energies known in the extreme Universe.
The blazar’s jet is pointing in the general direction of the Earth. Only one blazar would be enough to endanger our existence so it is good that none of them is located too close to our planet Earth.
Their strong, oppositely directed jets can travel outward into large regions of intergalactic space. Blazars emit over the widest range of frequencies, demonstrate very strong variations in brightness and spew huge amounts of energy from their centers, far more than ordinary galaxies.
Powerful blazars have weak emission lines or even none at all, which may be caused by very little or no gas at all in their cores. The shortage of gas means that there are few if any atoms to produce these emission lines.
Another possibility is that the blazars have emission lines of normal strength, but very strong and continuous radiation, makes it very difficult to detect them.
A few years ago, a team of scientists from Boston University, led by Alan Marscher watched the phenomenon, using the National Radio Astronomy Observatory’s VLBA composed of ten powerful radio telescopes spread across the US.
“We have gotten the clearest look yet at the innermost portion of the jet, where the particles actually are accelerated, and everything we see supports the idea that twisted, coiled magnetic fields are propelling the material outward,” said Alan Marscher.
“This is a major advance in our understanding of a remarkable process that occurs throughout the Universe.”
Marscher’s team studied a blazar at the centre of BL Lacertae, a galaxy some 950 million light years from Earth.
The blazar jet points almost directly at Earth and it fires powerful bursts of radiation that last for several days and occur about once or twice a year.
In 2009, Fermi, which detected a large number of blazars during it first year of operation, observed a blazar, PKS 2155-304. It is located 1.5 billion light-years away in the southern constellation of Piscis Austrinus and is usually a detectable but faint gamma-ray source.
The blazar’s jet contributed to the brightest source in the sky at the highest gamma-ray energies scientists can detect — up to 50 trillion times the energy of visible light.
Even from strong sources, only about one gamma ray this energetic strikes a square yard at the top of Earth’s atmosphere each month, reported scientists.
Also in 2009, NASA’s Fermi Gamma-ray Space Telescope paid attention to the object as 3C 454.3, an active galaxy (the brightest source in the gamma-ray sky, at that time) located 7.2 billion light-years away in the constellation Pegasus.
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What makes a blazar so bright in gamma rays is its orientation: One of the jets happens to be aimed straight at us.
Many blazars have apparent superluminal features within the first few parsecs of their jets, and outshines the entire galaxy!
We all know that the brightest persistent source in the gamma-ray sky is the Vela pulsar, at a distance of only 1,000 light-years from Earth.
This time, Vela has got a competitor – the object 3C 454.3. When the object is undergoing a flaring episode, 3C 454.3 is spectacularly bright.
“3C 454.3 is millions of times farther away, yet the current flare makes it twice as bright as Vela,” said Lise Escande at the Center for Nuclear Studies in Gradignan, near Bordeaux, France.
“That represents an incredible energy release, and one the source can’t sustain for very long.”
First version of this article was originally published on March 9, 2012
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