  We present observational evidence for "propeller" effects in two X-ray
pulsars, GX 1+4 and GROJ1744-28. Both sources were monitored regularly by the
Rossi X-ray Timing Explorer (RXTE) throughout a decaying period in the X-ray
brightness. Quite remarkably, strong X-ray pulsation became unmeasurable when
total X-ray flux had dropped below a certain threshold. Such a phenomenon is a
clear indication of the propeller effects which take place when pulsar
magnetosphere grows beyond the co-rotation radius as a result of the decrease
in mass accretion rate and centrifugal force prevents accreting matter from
reaching the magnetic poles. The entire process should simply reverse as the
accretion rate increases. Indeed, steady X-ray pulsation was reestablished as
the sources emerged from the non-pulsating faint state. These data allow us to
directly derive the surface polar magnetic field strength for both pulsars:
3.1E+13 G for GX 1+4 and 2.4E+11 G for GROJ1744-28. The results are likely to
be accurate to within a factor of 2, with the total uncertainty dominated by
the uncertainty in estimating the distances to the sources. Possible mechanisms
for the persistent emission observed in the faint state are discussed in light
of the extreme magnetic properties of the sources.
