New structure seen in the Crab pulsar
Mar 12, 2003
The Crab pulsar emits periodic, exceptionally bright radio pulses. Now, astronomers at New Mexico Tech and the National Radio Astronomy Observatory in New Mexico have discovered that it also emits sub-pulses within these “giant” pulses. The researchers believe that the structures responsible for such emissions must be less than one metre across, which makes them the smallest objects ever to be detected outside the Solar System and the brightest radio sources in the sky (T H Hankins et al. 2003 Nature 422 141).
The Crab pulsar is a rotating neutron star in the constellation of Taurus, some 6520 light years from Earth. Formed as the result of a supernova explosion that occurred in 1054, it is known as a “cosmic lighthouse” because it emits extremely bright radio pulses thirty times a second as it rotates. It also emits radiation in the optical, X-ray and gamma-ray parts of the spectrum, but astronomers are still unsure about how this emission mechanism works.
Tim Hankins of New Mexico Tech and co-workers looked at the radio spectrum of the pulsar with the 305-m telescope at the Arecibo Observatory. They used a new detection method that was able to resolve down to the nanosecond time scale. “Because of the propagation through the plasma from the pulsar to the Earth, the short-wavelength parts of the pulsar signals arrive before the long wavelength parts, so the signals are ‘smeared’ across the receiver band,” Hankins told Physics Web. “We developed a technique for removing this smearing.”
The researchers selected a sequence of six giant pulses that were recorded over a period of a few minutes. The time interval between two giant pulses can typically vary from a fraction of a second to over a minute. Based on the very regular rotation rate of the pulsar, the giant pulses "jitter" by several hundred microseconds, relative to the time one would expect them to arrive if they resulted from a narrow beam rotating with the pulsar.
Occasionally, the researchers recorded a pulse that was composed of extremely short, isolated and non-overlapping pulses which lasted only tens of microseconds. Some of the sub-pulses lasted less than two nanoseconds.
The team believes that a process known as “plasma turbulence” - the conversion of kinetic energy in the pulsar’s magnetic atmosphere to radio energy - could explain how such short radio pulses are produced. They hope that their “dispersion removal” technique will help in the search for extra-solar radio bursts and pulsars in other galaxies.
About the author
Belle Dumé is Science Writer at PhysicsWeb