Radio pulsars are extremely accurate clocks. They are more accurate than atomic clocks for measurements extending years. In the right setting, they provide us with an unparalleled tool. For instance, theorists always explain the effects of general relativity in terms of clocks: if we could see a clock near a black hole, we would see it count the seconds more slowly than our own clock; and if we moved a star between ourselves and a clock, the clock would appear to slow because of a time delay in the propagation of light. In the radio pulsar we have our clock. Do we ever have it in the right setting?
The binary pulsar provides the perfect setting for testing general relativity with radio pulsars. These systems come in two flavors: a radio pulsar orbiting a degenerate (white) dwarf, and a radio pulsar orbiting another neutron star. The second type of binary, the double-neutron-star binary pulsar, is an ideal apparatus for testing many effects of general relativity. In particular, the periastron drift expected under general relativity and seen weakly in the orbit of Mercury is very apparent in the orbit of a binary pulsar, and the loss of energy through gravitational radiation is evident in the shortening of a binary pulsar's orbital period.
Beyond its usefulness as a scientific apparatus, the binary pulsar is interesting in itself. The pulsars we find in these systems resemble an unusual class of isolated radio pulsars, the millisecond pulsars. The pulsars in both classes are ancient, being of order 1 billion years old. This is a striking contrast to most other radio pulsars, which are younger than 100,000 years, and younger than 1,000 years in the case of the Crab Pulsar (PSR 0531+21). We only see the millisecond pulsars and the binary pulsars today because these pulsars were rejuvenated by their companions, long before their companions became compact stars. In earlier times, the binary pulsars were x-ray binaries, and the millisecond pulsars were members of x-ray binaries.
The binary pulsars and their usefulness in testing general relativity are discussed in the pages added this week to the web site. A new page on the “Stars” topical path describes the characteristics and evolution of binary pulsars and millisecond pulsars. A new page on the “General Relativity” topical path describes how binary pulsars are used to test general relativity.
Next Issue: The next issue of The Astrophysics Spectator is scheduled for February 8.
Binary Pulsars and Millisecond Pulsars. A handful of pulsars are found in binary systems. These systems, which are very old, appear to be dead x-ray binaries. While the companion stars were once fusion-powered stars spilling their atmospheres onto the neutron star in the system, they are now compact stars, either degenerate dwarfs or neutron stars. The pulsars in these systems are recycled, having been spun-up during the x-ray binary phase, so that the pulsar can generate a current and radiate radio waves despite possessing relatively weak magnetic fields. A handful of isolated pulsars have characteristics similar to the pulsars in binary systems; these isolated pulsars are called millisecond pulsars. These old stars were once members of x-ray binary systems, but were kick into the Galaxy when their companions exploded in supernovae. (continue)
Tests of General Relativity with Binary Pulsars. A double-neutron-star binary pulsar is the ideal apparatus for testing general relativity. Because a pulsar is an accurate clock, we can measure the tiny timing fluctuations in the arrival of radio pulses created by the binary's gravitational field. In particular, the binary pulsars provide us with our best evidence for the existence of gravitational waves. (continue)