Sunday, April 23, 2006
The Legend of the Rabbit and the Kookaburra
This time, we start in space, go to the Australian outback, and
then to Africa! Hunting rabbits while a Kookaburra laughs at us.
Yes indeedy....
Our story starts with an EGRET. Not the bird, but the gamma-ray telescope that flew on board the Compton Gamma-Ray Observatory satellite. When that bird fell into the ocean in 1999, it left a legacy of about a hundred unidentified high-energy gamma-ray sources in our Galaxy. By high-energy, I mean 100-10,000 MeV photons, or about a billion times the energy of photons that make up visible light. This is not to be confused with very high-energy gamma-rays, which are around TeV energies, or a trillion times as energetic as what our eyes see. We'll get to those energies when we go to Africa. In fact, we will also get to radio waves which are about 500,000 times less energetic than optical in Australia, to mid-infrared which is about 10 times less energetic than optical, and to X-rays detected by Japanese, US, and European satellites which are about 1000 times more energetic than optical light. Everything but optical light.
Back in 1997, the Japanese X-ray satellite ASCA (which sounds similar to the word Asuka, which means flying bird in Japanese...I think there is a theme here....) pointed at a region of the sky way in the south about the size of the full moon. Somewhere within that region is one of the brightest of the sources detected by EGRET. It was looking for indirect evidence of a powerful pulsar. The radio and X-ray beams of an energetic pulsar can miss the Earth while the gamma-ray beam hits it. But such an energetic pulsar will often have a highly relativistic wind blowing a bubble in it's surroundings at supersonic speeds. In other words, it can make the sky around it glow at all wavelengths of light. And I do mean all, as we will show. These glowing regions are known simply as pulsar wind nebulae.
Well, ASCA didn't detect a single X-ray blob. It detected TWO. But how can you tell if they are pulsar wind nebulae? Well one way is to detect a radio pulsar. And, in fact, a powerful radio pulsar was detected later on in one of the blobs during the Parkes Multibeam Pulsar Survey. But, as I mentioned, if the beam doesn't pass near the Earth, you look for the nebula. In radio, these nebulae have distinctive properties Although they tend to look just like blobs, how their brightness changes in wavelength, their tendency to be highly polarized, and their relatively low infrared emission distinguish them from other radio emitting blobs.
And so, to Narrabri, New South Wales, Australia; home of numerous grey kangaroos and not much else. This is why the Australia Telescope Compact Array is situated near there. These six radio dishes looked towards our X-ray blobs 3 times for twelve hours each time over a period of a few months in 1998. An image was made of the region, and was compared to another image made at another radio telescope in Molonglo NSW, Australia. These saw a complex of sources made up of a ring with two wings coming off of it that late at night with kangaroos hopping all around can look like a fat Australian bird....i.e. a Kookabura
(laugh, Kookaburra, laugh!!!!!). In the lower armpit of the bird is a bright blob with ears
pointing up, a source near its butt, a nose and a paw....More hopping creatures! Obviously a rabbit! (Find the radio rabbit! The above image shows the lower wing of the Kookaburra, where red is radio, green is polarized radio, and blue is X-rays.) And this Rabbit has the spectrum and the polarization of a pulsar wind nebula, holding the brightest part of the X-ray blob in its front paw! Further X-ray observations showed a little point which seems to have some sort of bow shock in front of it.
In the other wing (picture to the left) is the aforementioned energetic pulsar, with its own X-ray blob. But are either or both of these sources the gamma-ray source??
Well, the answer to that will have to wait for the next generation of gamma-ray satellites, i.e. GLAST. But in the high plains of Namibia, a different kind of gamma-ray telescope was looking for our bird. Very high-energy gamma-rays (greater than 100 GeV or so, i.e. a factor of 100 higher than those detected by
EGRET) are so energetic that when they hit our upper atmosphere, they create a shower of charged particles that streak downward emitting a large cone of light for a tiny fraction of a second. Large optical telescopes specially designed to catch these flashes of light are used to determine where these extreme gamma-rays are coming from. The latest and greatest of these is the HESS telescope, operated by a European/African consortium. This month, they announced that BOTH wings of the Kookaburra were detected in very high-energy gamma-rays.
The wings of the Kookaburra have now been detected from long wavelength radio (a few millionths of an electron Volt) to few ten trillion electron Volts, using telescopes from opposite sides of the Earth and out of this world and involving researchers from 5 continents. But do we know exactly how and why these nebulae glow the way they do, especially at
the highest and lowest energies? Nope. That's what's so cool! And maybe, just maybe, GLAST will be able to give us some clues. Or at least tell us if the Rabbit, the Kookaburra pulsar, or both are the EGRET source.
then to Africa! Hunting rabbits while a Kookaburra laughs at us.
Yes indeedy....
Our story starts with an EGRET. Not the bird, but the gamma-ray telescope that flew on board the Compton Gamma-Ray Observatory satellite. When that bird fell into the ocean in 1999, it left a legacy of about a hundred unidentified high-energy gamma-ray sources in our Galaxy. By high-energy, I mean 100-10,000 MeV photons, or about a billion times the energy of photons that make up visible light. This is not to be confused with very high-energy gamma-rays, which are around TeV energies, or a trillion times as energetic as what our eyes see. We'll get to those energies when we go to Africa. In fact, we will also get to radio waves which are about 500,000 times less energetic than optical in Australia, to mid-infrared which is about 10 times less energetic than optical, and to X-rays detected by Japanese, US, and European satellites which are about 1000 times more energetic than optical light. Everything but optical light.
Back in 1997, the Japanese X-ray satellite ASCA (which sounds similar to the word Asuka, which means flying bird in Japanese...I think there is a theme here....) pointed at a region of the sky way in the south about the size of the full moon. Somewhere within that region is one of the brightest of the sources detected by EGRET. It was looking for indirect evidence of a powerful pulsar. The radio and X-ray beams of an energetic pulsar can miss the Earth while the gamma-ray beam hits it. But such an energetic pulsar will often have a highly relativistic wind blowing a bubble in it's surroundings at supersonic speeds. In other words, it can make the sky around it glow at all wavelengths of light. And I do mean all, as we will show. These glowing regions are known simply as pulsar wind nebulae.
Well, ASCA didn't detect a single X-ray blob. It detected TWO. But how can you tell if they are pulsar wind nebulae? Well one way is to detect a radio pulsar. And, in fact, a powerful radio pulsar was detected later on in one of the blobs during the Parkes Multibeam Pulsar Survey. But, as I mentioned, if the beam doesn't pass near the Earth, you look for the nebula. In radio, these nebulae have distinctive properties Although they tend to look just like blobs, how their brightness changes in wavelength, their tendency to be highly polarized, and their relatively low infrared emission distinguish them from other radio emitting blobs.
And so, to Narrabri, New South Wales, Australia; home of numerous grey kangaroos and not much else. This is why the Australia Telescope Compact Array is situated near there. These six radio dishes looked towards our X-ray blobs 3 times for twelve hours each time over a period of a few months in 1998. An image was made of the region, and was compared to another image made at another radio telescope in Molonglo NSW, Australia. These saw a complex of sources made up of a ring with two wings coming off of it that late at night with kangaroos hopping all around can look like a fat Australian bird....i.e. a Kookabura
(laugh, Kookaburra, laugh!!!!!). In the lower armpit of the bird is a bright blob with ears
pointing up, a source near its butt, a nose and a paw....More hopping creatures! Obviously a rabbit! (Find the radio rabbit! The above image shows the lower wing of the Kookaburra, where red is radio, green is polarized radio, and blue is X-rays.) And this Rabbit has the spectrum and the polarization of a pulsar wind nebula, holding the brightest part of the X-ray blob in its front paw! Further X-ray observations showed a little point which seems to have some sort of bow shock in front of it.
In the other wing (picture to the left) is the aforementioned energetic pulsar, with its own X-ray blob. But are either or both of these sources the gamma-ray source??
Well, the answer to that will have to wait for the next generation of gamma-ray satellites, i.e. GLAST. But in the high plains of Namibia, a different kind of gamma-ray telescope was looking for our bird. Very high-energy gamma-rays (greater than 100 GeV or so, i.e. a factor of 100 higher than those detected by
EGRET) are so energetic that when they hit our upper atmosphere, they create a shower of charged particles that streak downward emitting a large cone of light for a tiny fraction of a second. Large optical telescopes specially designed to catch these flashes of light are used to determine where these extreme gamma-rays are coming from. The latest and greatest of these is the HESS telescope, operated by a European/African consortium. This month, they announced that BOTH wings of the Kookaburra were detected in very high-energy gamma-rays.
The wings of the Kookaburra have now been detected from long wavelength radio (a few millionths of an electron Volt) to few ten trillion electron Volts, using telescopes from opposite sides of the Earth and out of this world and involving researchers from 5 continents. But do we know exactly how and why these nebulae glow the way they do, especially at
the highest and lowest energies? Nope. That's what's so cool! And maybe, just maybe, GLAST will be able to give us some clues. Or at least tell us if the Rabbit, the Kookaburra pulsar, or both are the EGRET source.
