Profile instabilities of the millisecond pulsar PSR J1022+1001

We present evidence that the integrated profiles of some millisecond pulsar s exhibit severe changes that are inconsistent with the moding phenomenon a s known from slowly rotating pulsars. We study these profile instabilities in particular for PSR J1022+1001 and show that they occur smoothly, exhibit ing longer time constants than those associated with moding. In addition, t he profile changes of this pulsar seem to be associated with a relatively n arrowband variation of the guise shape. Only parts of the integrated profil e participate in this process, which suggests that the origin of this pheno menon is intrinsic to the pulsar magnetosphere and unrelated to the interst ellar medium. A polarization study rules out profile changes due to geometr ical effects produced by any sort of precession. However, changes are obser ved in the circularly polarized radiation component. In total we identify f our recycled pulsars that also exhibit instabilities in the total power or polarization profiles due to an unknown phenomenon (PSRs J1022 + 1001, J173 0-2304, B1821-24, and J2145-0750). The consequences for high-precision puls ar timing are discussed in view of the standard assumption that the integra ted profiles of millisecond pulsars are stable. As a result we present a ne w method to determine pulse times of arrival that involves an adjustment of relative component amplitudes of the template profile. Applying this metho d to PSR J1022 + 1001, we obtain an improved timing solution with a proper- motion measurement of -17 +/- 2 mas yr(-1) in ecliptic longitude. Assuming a distance to the pulsar as inferred from the dispersion measure, this corr esponds to a one-dimensional space velocity of 50 km s(-1).