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Astron. Astrophys. 321, 305-310 (1997)

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1. Introduction

The study of the interstellar magnetic field has great interest for the understanding of the physical processes involved in the evolution and structure of the interstellar medium. Pulsars are powerful instruments for such studies, since they have highly linearly polarized radioemission which, when propagating through the magnetoactive interstellar medium, experiences a rotation of the plane of linear polarization - the well known Faraday effect. This rotation is the physical foundation for the measurement of the average magnetic field projected in the direction of pulsars [FORMULA]. In this work we use a new method for the measurement of [FORMULA] (Smirnova 1991) which is based on the measurement of the period of the temporal modulation of pulsar radioemission observed in the wide bandpass of a receiver. This method has a higher accuracy than the standard method, and is experimentally much simpler.

The main purpose of this work consists of the high accuracy measurement of [FORMULA] in the direction of pulsars, and, in a following study, the monitoring of its time variations. Since pulsars are fast moving objects ([FORMULA] hundreds of km/sec) we may be able to detect small-scale variations by monitoring [FORMULA] in a reasonably short span of time. This may be particularly true for pulsars associated with SNR's, where in the interface of the expanding shock wave and the interstellar medium we should expect such variations. Except for the SNR's surrounding the Crab and Vela pulsars no monitoring of this type was done up to now. However, there are measurements of extragalactic sources located behind SNR's (e.g. Kim et al. 1988) which show a substantially larger RM than that of nearby background sources. For PSR 0531 [FORMULA] 21 (Crab) and PSR 0833-45 (Vela), both inside SNR's, a change of the RM with time was noted (Rankin et al. 1988, Hamilton et al. 1977), in both cases these changes were attributed to the environment local to the pulsar, not to the interstellar medium.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998