cm. Possible accretion of matter into a pulsar’s magnetosphere requires $r_{\rm m}<r_{\rm c}<r_{\rm l}$ (see, e.g., [@henri83] and [@bv91]). The constraint of $r_{\rm c} < r_{\rm l}$ gives $$P>3.1\times 10^{-5} M_1 ~{\rm s},$$ which could be satisfied for all known pulsars. The requirement that the Alfven radius is smaller than the corotating radius ($r_{\rm m} < r_{\rm c}$), however, is very tight, which reads $$P > 3.2\times 10^3 B_{12}^{6/7} R_6^{18/7}
M_1^{-5/7}\dot M_{10}^{-3/7} ~{\rm s},
\label{accretion-2}$$ since the Alfven radius could not get too low unless the accretion rate is very high, e.g., $\dot M_{10} \gg 1$. A similar discussion is presented. Note that in [@bv91] the maximum accretion rate (Eddington luminosity) is adopted.

Let’s give a rough estimate on the accretion rate of a solitary strange star. Consider a strange star which is moving with a velocity $V$ through a gas with a density $\rho$. The critical radius within which the dynamics of gas is dominated by the stellar gravity is $r_{\rm g} \sim {GM\over V^2}\sim 1.3\times 10^{12}M_1V_7^{-2}$, where $V_7 = V/(10^7 {\rm cm/s})$. Thus according to Bondi & Hoyle[@bh44], the accretion rate is roughly $$\begin{array}{lll}
\dot M \sim 4\pi r_{\rm g}^2 \rho V
& \sim & 2.2 \times 10^8 