Thursday, February 16, 2006

RRATs - where normal pulsars go to die?

Maura McLaughlin et al have an interesting paper in Nature today

They have found RRATs - or Rotating RAdio Transients - current sample is 11 sources found in the Parkes multibeam survey over several years of observing.
They are rotating neutron stars, with rotation periods ranging from 0.4 to 7 seconds, and those with measured rotation period derivatives have large period derivatives, indicative of rapid spin-down and therefore large (over 5 × 1013 Gauss) magnetic fields. They seem to be single (certainly not in a close binary)

These are clearly an extension of the "normal" young pulsar population, probable ages of hundreds of thousands to millions of years, and they are characterised by relatively sharp (few millisecond duration) emission, which then "nulls" for minutes or hours.
So total time spent emitting is about 1 second per day, or about one part in hundred thousand.

They are easy to miss in a survey, and if observed, likely to be dismissed as noise or anthropic transients.

The emission is bright when it is on, but very irregular. The sources are consistent with being in, or near, the galactic plane.

50% of them have periods over 4 seconds, compared with 0.5% of the "normal" pulsar population.

Since the duty cycle is low, and they are easy to miss in radio surveys (where each patch of the sky is typically observed for minutes or tens of minutes - less than the characteristic interval between bursts), the observation of these few imply a much larger underlying population.
How big is hard to estimate with confidence, but it is likely to be much larger than the population of "normal" pulsars, where we know of about 2000 pulsars and estimate a total population of about 100,000.

So, there are considerably more youngish neutron stars out there than we knew about, and this population seems to be somewhat distinct from the normal pulsars - possibly these are dead, or retired, magnetars, or possibly they are some sort of intermediate population between the magnetars and the normally pulsing pulsars.

Hopefully observations of this population will both further constrain models for the origin of the neutron stars, in particular what range of masses of main sequence stars really becomes a neutron star, and the role of anomalous velocity impulses on neutron star formation, and possible the mechanism for the actual radio emission.

Anyway, nice new result.

RRAT press release with graphics from Jodrell Bank
Jodrell Bank Pulsar Group


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