ROSAT X-ray sources and exponential field decay in isolated neutron stars

The influence of exponential magnetic field decay on the spin evolution of isolated neutron stars is studied. ROSAT observations have revealed several X-ray sources which may be accreting old isolated neutron stars. Assuming that this interpretation is correct, the observations can be used to constr ain parameters of the exponential field decay. We show that the range of minimum value of magnetic moment mu(b) and the ch aracteristic decay time, t(d), similar to 10(29.5) greater than or equal to mu(b) greater than or equal to 10(28) G cm(3), similar to 10(8) greater th an or equal to t(d) greater than or equal to 10(7) yrs are excluded assumin g the standard initial magnetic momentum, mu(0) = 10(30) G cm(3) For these parameters, neutron stars would never reach the stage of accretion from the interstellar medium even for a low space velocity of the stars and a densi ty of the ambient plasma. The range of excluded parameters increases for lo wer values of mu(0).