Long-term Radio Observations of the Intermittent Pulsar B1931+24
  We present an analysis of approximately 13-yr of observations of the intermittent pulsar B1931+24 to further elucidate its behaviour. We find that while the source exhibits a wide range of nulling (~4-39 d) and radio-emitting (~1-19 d) timescales, it cycles between its different emission phases over an average timescale of approximately 38 d, which is remarkably stable over many years. On average, the neutron star is found to be radio emitting for 26 +- 6 % of the time. No evidence is obtained to suggest that the pulsar undergoes any systematic, intrinsic variations in pulse intensity during the radio-emitting phases. In addition, we find no evidence for any correlation between the length of consecutive emission phases. An analysis of the rotational behaviour of the source shows that it consistently assumes the same spin-down rates, i.e. nudot = -16 +- 1 x 10^-15 s^-2 when emitting and nudot = -10.8 +- 0.4 x 10^-15 s^-2 when not emitting, over the entire observation span. Coupled with the stable switching timescale, this implies that the pulsar retains a high degree of magnetospheric memory, and stability, in spite of comparatively rapid (~ms) dynamical plasma timescales. While this provides further evidence to suggest that the behaviour of the neutron star is governed by magnetospheric-state switching, the underlying trigger mechanism remains illusive. This should be elucidated by future surveys with next generation telescopes such as LOFAR, MeerKAT and the SKA, which should detect similar sources and provide more clues to how their radio emission is regulated. 
N. J. Young, B. W. Stappers, A. G. Lyne, P. Weltevrede, M. Kramer and   I. Cognard
astro-ph.HE