Unusual increase in the 325 MHz flux density of PSR B0655+64.

We report on the detection of a large amplification of the flux density of PSR B0655+64 at 325MHz (a factor of ~43) that lasted about one hour. To the best of our knowledge such a large amplification has not been reported before. The phenomenon is restricted to a very narrow bandwidth (50% decorrelation half-width of 3.6+/-0.4MHz). The decorrelation bandwidth and characteristic time scale of the variation are in agreement with the values expected for diffractive interstellar scintillation. However, the observed amplification of the flux density of PSR B0655+64 is rather large. The observations suggest that amplifications of a factor up to 8 are not uncommon for PSR 0655+64 at 92cm. The extreme flux density amplification of PSR 0655+64 might be explained in terms of caustics. We consider it unlikely that this event can be explained by intrinsic variability.