DUAL SPIN-DOWN STATES OF THE PULSAR J1001-5507

Quasi-regular timing observations obtained, using the 26 m Hartebeesthoek Radio Astronomy Observatory radiotelescope near center frequencies of 1.668 and 2.272 GHz, between 1985 July and 2005 August have given a further insight into the long-term spin-down behavior of the pulsar J1001–5507 (B0959–54). Here, we show that the spin-down of the pulsar during ∼20 years is dominated by dual spin-down modes characterized by two discrete and nearly stable spin-down rates (ν-dotlow=-0.24871x10-13 s–2 and ν-dothigh=-0.25195x10-13 s–2). Our analysis demonstrates that the switching of the pulsar's spin-down rate from ν-dotlow to ν-dothigh modes occurred over about 800 days (between about MJD 48500 and MJD 49300) at a steady rate of ∼–4.6 × 10–24 s–3, after which the pulsar was found to be slowing down 1.3% faster. It is observed that the extreme pulse profile shapes at the two spin-down modes are notably different, while the interval of steady switching between the spin-down modes is coincident with evidence for some correlated pulse shape variations. We discuss these results in light of the current understanding of various mechanisms for radio pulsar spin-down modulation.