We present a method for determining the B-field around neutron stars based on observed kHz and viscous QPO frequencies used in combination with the best-fit optical depth and temperature of a Comptonization model .
In the framework of the transition layer QPO model , we analyze magnetoacoustic wave formation in the layer between a neutron star surface and the inner edge of a Keplerian disk .
We derive formulas for the magnetoacoustic wave frequencies for different regimes of radial transition layer oscillations .
We demonstrate that our model can use the QPO as a new kind of probe to determine the magnetic field strengths for 4U 1728-42 , GX 340+0 , and Sco X-1 in the zone where the QPOs occur .
Observations indicate that the dependence of the viscous frequency on the Keplerian frequency is closely related to the inferred dependence of the magnetoacoustic wave frequency on the Keplerian frequency for a dipole magnetic field .
The magnetoacoustic wave dependence is based on a single parameter , the magnetic moment of the star as estimated from the field strength in the transition layer .
The best-fit magnetic moment parameter is about ( 0.5 - 1 ) \times 10 ^ { 25 } G cm ^ { 3 } for all studied sources .
From observational data , the magnetic fields within distances less 20 km from neutron star for all three sources are strongly constrained to be dipole fields with the strengths 10 ^ { 7 - 8 } G on the neutron star surface .