Simulation of the interstellar scintillation and the extreme scattering events of pulsars

The rare and conspicuous flux density variations of some radio sources (extragalactic and pulsars) for periods of weeks to months have been denoted Extreme Scattering Events (ESE's) by Fiedler et al. (1987, Nature, 362, 675). Presently, there is no astrophysical mechanism that satisfactorily produces this phenomenon. In this paper, we conjecture that inhomogeneities of the electronic density in the turbulent interstellar medium might be the origin of this phenomenon. We have tested this conjecture by a simulation of the scintillation of the pulsar B1937+21 at 1.4 GHz and 1.7 GHz for a period of six months. To this end, we have constructed a large square Kolmogorov phase screen made of 131k x 131k pixels with electron inhomogeneity scales ranging from 6 x 10(6) m to 10(12) m and used the Kirchho.-Fresnel integral to simulate dynamic spectra of a pulsar within the framework of Physical Optics. The simulated light curves exhibit a 10 day long variation simultaneously at 1.41 and 1.7 GHz that is alike the "ESE" observed with the Nancay radiotelescope toward the pulsar B1937+21 in October 1989. Thus, we conclude that the "ESE" toward pulsars can be caused naturally by the turbulence in the ionized interstellar medium. This is instead of discrete over pressured ionized clouds crossing the line of sight as in the model of Fiedler et al. (1987, Nature, 362, 675). We suggest that longer events could occur in a simulation of scintillation, if larger electron inhomogeneities (> 10(12) m) were included in the construction of the Kolmogorov phase screen. This next step requires a supercomputer.