Reduced spin-down rate of PSR J0738-4042 explained as due to an asteroid disruption event

Abstract

Long term observations by Brook et al. reveal that the derivative of rotational frequency of PSR J0738-4042 changed abruptly in 2005. Originally, the spin-down rate was relatively stable, with the rotational frequency derivative being -1.14-10-14 s-2. After September 2005, the derivative began to rise. About 1000 days later, it arrived at another relatively stable value of about -0.98 10-14 s-2, indicating that the pulsar is spinning-down relatively slowly. To explain the observed change in spin-down rate, we resort to an asteroid disrupted by PSR J0738-4042. In our model, the orbital angular momentum of the asteroid is assumed to be parallel to that of the rotating pulsar, so that the pronounced reduction in the spin-down rate can be naturally explained as due to the transfer of angular momentum from the disrupted material to the central pulsar. The derived magnetospheric radius is about 7.0 109 cm, which is smaller than the tidal disruption radius (8.7 1010 cm). Our model is self-consistent. It is shown that the variability in the spin-down rate of PSR J0738-4042 can be quantitatively accounted for by accretion from the asteroid disrupted by the central pulsar.