Megan's Blog

In the news this month: pulsar irregularities

After massive stars like Eta Carinae explode, the object left behind is thought to be either a neutron star or a black hole, depending on the final mass of the progenitor star. Pulsars are neutron stars that have strong magnetic fields and behave somewhat like cosmic lighthouses, projecting beams of radio emission into space as they spin. Studying the pulses of radiation as the beams sweep past the Earth can provide valuable information on the physics of these extreme objects, allowing astronomers to probe physics under conditions which are not possible to create in a terrestrial laboratory. While pulsars are known to be extremely accurate natural clocks, their pulse rates are very stable over time, there are however unexplained deviations from the expected spin rate, a phenomenon known as timing noise. Now, a team led by Andrew Lyne at the University of Manchester, have uncovered a mechanism which could explain this noise.

Over long timescales, the rate at which a pulsar spins (known as the spin down rate) decreases slowly in a predictable way due to the conversion of rotational energy into photons. By studying a large number of pulsars repeatedly over 40 years, the team found that the deviations from the expected spin down rate were actually quasi-periodic on timescales between one and ten years, and that several other pulsar characteristics may be linked to the same phenomenon. One particular pulsar, known as B1931+24, only displays radio pulses intermittently, and long term study showed that it also had two different spin down rates: its spin rate decreased faster when the radio signal was detectable.

The team analysed the data on a large sample of pulsars and found a further seventeen which show evidence of quasi-periodic spin down rates, many of which also show variations in the shape of the pulse profile. The authors suggest that the likely explanation is that the pulsar's magnetosphere is switching between two distinct states. Exactly what causes the pulsar to switch between states is not yet known, but if the changes can be accurately modeled then the timing noise can be reduced, and astronomers will find it easier to compensate for errors in pulsar “clocks” in highly sensitive experiments designed to detect gravitational waves.

---

This blog post is a news story from the Jodcast, aired in the July 2010 edition.

Lyne, A., Hobbs, G., Kramer, M., Stairs, I., & Stappers, B. (2010). Switched Magnetospheric Regulation of Pulsar Spin-Down Science, 329 (5990), 408-412 DOI: 10.1126/science.1186683