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 of $-1.14 \times 10^{-14}~\rm s^{-2}$. After September 2005, the derivative began to rise up. About 1000 days later, it arrived at another relatively stable value of about $-0.98 \times 10^{-14}~\rm s^{-2}$, indicating that the pulsar is spinning-down relatively slowly. To explain the observed spin-down rate change, 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 the angular momentum from the disrupted material to the central pulsar. The derived magnetospheric radius is about $4.0 \times 10^{9}$ cm, which is smaller than the tidal disruption radius ($4.9 \times 10^{10}$ cm). Our model is self-consistent. It is shown that the variability of the spin-down rate of PSR J0738-4042 can be quantitatively accounted for by the accretion from the asteroid disrupted by the central pulsar.