As the remains of the explosion of massive stars, pulsars are very small neutrons stars of the order of 20km in diameter with a rapid rotation and a mass slightly higher than our Sun. Produced by the high magnetic field, the collimated beam of radiation is sweeping away and periodic radio pulses are received on Earth. The pulsar B1937+21 was by far the fastest one at the time of its discovery at Arecibo in 1982 by Don Backer (with a period of rotation of only 1.55ms) and was immediately identified as a recycled object. We now think that this class of pulsar is usually produced through accretion of matter from a companion transfering orbital angular momentum and spinning it up to short periods. When the lower mass companion evolves, it overflows its Roche lobe and donates material to the neutron star, transforming it from a radio-quiet object to a millisecond pulsar with a large particle wind. During or after the recycling, the particle wind ablates the companion and the disk formation starts. While most of the material from the disrupted companion is accreted from the disk into the pulsar, conservation of angular momentum requires dispersal of a fraction of the material over a wide region around the pulsar.
The recycled pulsar B1937+21 is observed with the Nançay radiotelescope since the beginning of the pulsar observation program initiated in 1987. After several generations of intrumentations used to remove the interstellar dispersion, there are now more than 100 pulsars regularly observed at Nançay along with B1937+21 (a recent observation is shown Fig.1).
Collecting data since late 1984 from different radiotelescopes, a study led by Ryan Shannon, from Cornell, characterized the times of arrival residuals (obtained after having removed all the known effects) using maximum entropy power spectrum. After a careful study about the plausibility to have a stable asteroid belt orbiting such a pulsar, many simulations over a wide range of configurations (inner and outer radius, law mass distribution) were conducted. After having defined all the asteroids belonging to a given belt, the cumulative orbital effect was calculated. Times of arrival residuals were then obtained by substracting a least-squares fit comprising quadratic terms that account for the pulsar spin down. Two of the best-matching asteroid belt configurations, obtained using comparaisons of maximum entropy power spectra, are shown in Fig.2. The first row corresponds to a small numbers of asteroids and a flat mass distribution, while the second one shows a much larger number of objects and a steep mass distribution.
Observational tests of this asteroid belt include : identifying periodicities from individual asteroids, which is difficult given the current timing precision ; testing for statistical stationarity that become possible when observations are conducted over a longer observing span ; and searching for reflected radio emission.
Reference
An Asteroid Belt Interpretation for the Timing Variations of the Millisecond Pulsar B1937+21
Shannon, Cordes, Metcalfe, Lazio, Cognard, Desvignes, Janssen, Jessner, Kramer, Lazaridis, Purver, Stappers, and Theureau, (2013)
Astrophysical Journal, in press