1 Introduction

PSR B1509-58 (Seward & Harnden 1982, Manchester et al. 1982) is situated not far from the geometrical centre of the SNR MSH15-52 (or G 320.4-01.2), and their association is beyond any doubt. However, this association causes a number of difficulties for the understanding and interpretation of the observational data. The problem is that the size and the general appearance of the SNR suggest that it should be much older than it follows from the pulsar age estimates.

It is usually assumed that the rotational frequency $\Omega$ of a pulsar decreases according to the relation ${\dot \Omega} \,=\, -\,K\Omega ^n$, where K depends upon the physics of the slow-down mechanism, and $n = \Omega {\ddot \Omega}/{\dot \Omega}^2$ is the braking index. Assuming constant K and n, and provided that the initial spin period $P_{\rm i}$ of the pulsar was much smaller than the current period $P=2\pi /\Omega$, one can estimate the characteristic spin-down age $\tau \,=\, P/(n-1){\dot P}$. For $P\simeq 0.15$ s, ${\dot P} \simeq 1.49 \times 10^{-12} \, {\rm s\,s}^{-1}$ and n=2.84 (Weisskopf et al. 1983; Manchester et al. 1985; Kaspi et al. 1994), one derives an age of PSR B1509-58 of $\simeq 1700$ years, i.e. it is nearly as young as the Crab pulsar. The spin-down age could be even less by a factor of $[1-(P_{\rm i} /P)^{n-1}]^{-1}$, if the pulsar was born with $P_{\rm i}$ of $\simeq$0.1 s (see e.g. Spruit & Phinney 1998). These estimates are at odds with the age estimates for MSH15-52 (Seward et al. 1983; van den Berg & Kamper 1984; Kamper et al. 1995), which show that the SNR is a much older object.

To reconcile the ages of the pulsar and the SNR, Seward et al. (1983) considered two possibilities: 1) MSH15-52 is a young SNR, and 2) PSR B1509-58 is an old pulsar. The first one implies (in the framework of the Sedov-Taylor model) that the SN explosion was very energetic and occured in a tenuous medium (see also Bhattacharya 1990). This point of view is generally accepted (e.g. Kaspi et al. 1994; Greiveldinger et al. 1995; Trussoni et al. 1996; Gaenzler et al. 1999). The second possibility implies that $\tau$ is at least few times shorter than the "true" age (Seward et al. 1983). This possibility was re-examinated by Blandford & Romani (1988). Assuming that the pulsar spin-down is mostly due to the electromagnetic torque, they suggested that the torque grew within the last $\simeq$10³ years due to the growth of the pulsar's magnetic field (see also Muslimov & Page 1996). In this case, the coefficient K is an increasing function of time and therefore the "true" age of the pulsar could be as large as follows from the age estimates for the SNR. In this paper we offer an alternative explanation for the increase of the braking torque (Sect. 2), viz. we suggest that it could be episodically enhanced due to the interaction of the pulsar's magnetosphere with dense clumps of circumstellar matter (Sect. 3). Section 4 deals with some issues related to our suggestion.