Evidence for “Propeller” Effects In X-ray Pulsars GX 1+4 And GRO J1744-28

We present observational evidence for “propeller” effects in two X–ray pulsars, GX 1+4 and GRO J1744–28. 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: 3.1× 1013 G for GX 1+4 and 2.4× 1011 G for GRO J1744-28. 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. Subject headings: accretion, accretion disks — stars: pulsars: individual (GX 1+4, GRO J1744-28) — X-rays: stars