1 Introduction

Compared with other types of radio sources, pulsars are known to have strong polarization, even up to 100% if one observes them with high time resolutions. Pulsar polarization would be smeared somehow if they are observed as continuum point sources over a duration much longer than a pulsar period, mainly because of the fast swing of polarization angle across a pulse profile. However, we will show in this paper that is not so serious as generally believed.

Pulsars have high (birth) velocities, on average 450 km s^-1 (Lyne & Lorimer 1994) and maybe up to 1600 km s^-1 for individuals (e.g. Cordes & Chernoff 1998), much faster than that of other types of stars (typically a few tens km s^-1). The high velocity was probably caused by the asymmetric kick during supernova explosion when a pulsar was born. This leads to a large proper motion for (nearby) pulsars. However, measuring the proper motion is not an easy task since the precise positions of a pulsar at well-separated epochs have to be measured. Up to now, there are 96 pulsars with proper motion measurements (e.g. Taylor et al. 1993; Fomalont et al. 1997).

Recently, the National Radio Astronomical Observatory (NRAO) Very Large Array (VLA) Sky Survey (NVSS) has been finished, which covers the sky north of Dec(J2000) $= -40\hbox{$^\circ$}$ at 1.4 GHz (Condon et al. 1998). The survey detected more than 1.8 million sources, with polarization measurements, down to a flux density limit about 2.5 mJy. Observations have a resolution of 45'', but the positional accuracy is a few arcsec for weak sources, and much better for strong sources. The observations were made with two IF channels at 1.365 and 1.435 GHz with an effective bandwidth of 42 MHz each. Most sources in the NVSS were observed in three pointings of 23 s each. The final sky map is the weighted sum from these pointings (Condon et al. 1998).

We had tried to identify the pulsars from the NVSS catalog, and then to investigate the pulsar polarization properties and proper motions from continuum observations. In the sky region covered by the NVSS, there are 520 known pulsars according to the updated pulsar catalog of Taylor et al. (1993). Updated catalog was kindly provided by Manchester. Using the latest version of the NVSS catalog (with 1814748 entries), we identified 97 strong pulsars according to positional coincidence. During revising this paper for publication, we noticed that similar identification work has been done by Kaplan et al. (1998), but they emphasized the other aspects, such as position accuracy, scintillation effects and completeness of detections. Comparing to Kaplan et al. (1998), we got 24 further new identifications. In the following, we will not repeat their work, but present our results in Sect. 2. We discuss briefly in Sect. 3 about scintillations (Sect. 3.1), pulsar polarization properties (Sect. 3.2), and proper motions (Sect. 3.3). We compared the pulsar positions with those from the pulsar catalog if the epochs were separated over more than 5 years, and got the upper limits of proper motion of 18 pulsars, including one pulsar which has had no proper motion measurements previously.