Something out there in our Galaxy is blinking 45 times per second and is also blinking about once every three seconds. What is it, and why are astronomers so excited about this discovery?

As you might have guessed, things in space that blink regularly are usually pulsars. These are rotating neutron stars with beams of radiation that sweep past the Earth like lighthouse beams. What is so exciting is that for the first time radio astronomers have found two pulsars circling each other in a binary system. PSR J0737-3039A (the one that flashes 45 times per second) and PSR J0737-3039B (which flashes every 2.8 seconds) are their names, based on their location in the sky.

This binary system with two pulsars is a strange place, indeed. Each spinning neutron star has a mass greater than that of our Sun but is only about 20 km in diameter, and they are close enough to each other that they revolve around their common center of gravity in just 2.4 hours. All this is taking place in a region of space smaller than our own Sun.

The real excitement comes from realizing that spinning pulsars are excellent clocks, and now we have an astrophysical laboratory in which two clocks are moving rapidly in a strong gravitational field. That is precisely the setting for testing Einstein's general relativity predictions. The first pulsar in a binary system, with just one pulsar, provided enough information about relativity to win a Nobel Prize for Joseph Taylor and Russell Hulse. Now with two pulsars, there are even more measurements that can be made, and a number of predictions of relativistic effects in this double pulsar system have already been made. One feature is that these two pulsars are spiraling in toward each other, giving off gravitational radiation in the process. In 85 million years, short on astronomical time scales, the two pulsars will merge to form a black hole, giving off a burst of gravitational radiation in the process. Knowing that such systems are out there is another stimulus to ongoing programs to measure gravitational radiation, like LIGO and LISA.

The discovery of the double pulsar system was made at the Australia Telescope National Facility, and followed up at the Jodrell Bank Observatory in England. You can find more information, including some animations of the double pulsar system at these Web sites:

http://www.atnf.csiro.au/news/press/double_pulsar/

http://www.jb.man.ac.uk/news/doublepulsar/

This week's question is provided by Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Laboratory for High Energy Astrophysics. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006. His particular scientific interest is gamma-ray pulsars.