The diagram of the magnetic field versus spin period for pulsars. The spin up line represents the minimum spin period to which such a spin up may proceed in an Eddington-limited accretion, while the “death line” corresponds to a polar cap voltage below which the pulsar activity is likely to switch off (Bhattacharya & van den Heuvel 1991). The evolutionary tracks in B − P diagram are plotted with different initial magnetic field strengths, but with same initial spin period (P_o = 100 s) and the same accretion rate ; the solid, dashed, and dotted lines denote the numerical solutions for Eqs. (1), (4)–(6) with initial field B_o = 5 × 10¹³ G, 5 × 10¹² G and 5 × 10¹¹ G, respectively.