Gamma ray signal

It is pretty well known that stars can brighten and dim on fairly short time-scales. These stars are called variables and thousands of amateur and professional astronomers monitor them regularly. But it isn't just in optical light that astronomical objects vary. It is almost 40 years since the first pulsar was discovered by Jocelyn Bell at Cambridge using a radio telescope. Pulsars are like cosmic clocks with a pretty regular cycle of 'ticks' as they rotate on their axes and the beams of radio waves they emit sweep past us. Now, for the first time, a regular clock has been found through observations with a gamma-ray telescope named H.E.S.S. in Namibia.

The High Energy Stereoscopic System (H.E.S.S) consists of four individual telescopes that are quite unusual. Rather than observing space, they are actually looking at our atmosphere. They look for small flashes of blue light known as Cherenkov radiation emitted by particle showers that occur when a gamma ray hits our atmosphere. By observing with several telescopes in different locations you can trace the flashes back to the point where the gamma ray hit and recover the direction that the gamma ray originally came from. So, you can make gamma ray images of the sky.

H.E.S.S. discovered a modulated gamma ray signal coming from a binary star system known as LS5039. I say binary star system but although one of the stars is a blue giant (20 times the mass of the Sun), the other is thought to be a black hole. It appears that these objects have orbits which take them from between 0.2 to 0.4 AU (1AU is the separation of the Earth and the Sun) of one another. The compact object interacts with the wind from the blue giant in different ways as it changes its separation and this changes the number of gamma rays that are produced. The number of gamma rays we detect on the Earth also depends on how the blue giant and companion are orientated with more gamma rays heading our way when the compact object is between us and the blue giant.

The best way to explore the Universe is to look in as many different ways as you can. I wish the ground-based gamma ray telescopes much success in the future.

Posted in astro blog by Stuart on Saturday 16th Dec 2006 (10:25 CET) | Permalink
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