Interstellar modulation of the flux density and arrival time of pulses from pulsar

Observations of the millisecond pulsar B1937+214 made at Nançay over 6 years show 30% rms flux variations over 13± 4 days due to Refractive Interstellar Scintillations. The arrival times (TOA) also show variations over a similar time scale 16 ± 10 days with an rms amplitude of about 0.3μsecs. These “rapid” TOA variations are anti-correlated (∼ −40%) with the flux and so are also caused by propagation through the ionized interstellar medium. The correlation is such that weak pulses tend to arrive late. While TOA modulations due to changing geometric delay should be positively correlated with flux, those due to small scale variations in the dispersive delay should be negatively correlated with the flux and so are presumed to be responsible in our observations. The level and time scales are shown to be consistent with expectations based on the Kolmogorov model of the interstellar density spectrum. However, in the data there is a sequence of about 5 discrete events, in which the flux remains low over 10-30 days and the TOA is on average late but also shows rapid variations. Assuming that these are indeed discrete events, we interpret them as due to isolated regions of enhanced plasma density crossing the line of sight. Such “Extreme Scattering Events” make a major contribution to the TOA variations and their anti-correlations with the observed flux. They are seen against a background of the normal refractive scintillation. A model is proposed in which discrete sheets of plasma cross the line of sight and cause a “de-focussing” event when aligned parallel to the line of sight. The statistics of the events imply a surprisingly large space density of the sheets; an alternative is that by chance we view PSR B1937+214 tangentially through a supernova shell which is fragmented and so causes multiple events.