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-raypulsars, GX 1+4 and GROJ1744-28. Both sources were monitored regularly by theRossi X-ray Timing Explorer (RXTE) throughout a decaying period in the X-raybrightness. Quite remarkably, strong X-ray pulsation became unmeasurable whentotal X-ray flux had dropped below a certain threshold. Such a phenomenon is aclear indication of the propeller effects which take place when pulsarmagnetosphere grows beyond the co-rotation radius as a result of the decreasein mass accretion rate and centrifugal force prevents accreting matter fromreaching the magnetic poles. The entire process should simply reverse as theaccretion rate increases. Indeed, steady X-ray pulsation was reestablished asthe sources emerged from the non-pulsating faint state. These data allow us todirectly 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 tobe accurate to within a factor of 2, with the total uncertainty dominated bythe uncertainty in estimating the distances to the sources. Possible mechanismsfor the persistent emission observed in the faint state are discussed in lightof the extreme magnetic properties of the sources.Comment: 12 pages including 3 ps figures. To appear in ApJ Letters Vol. 48