r-Mode oscillations and spin-down of young rotating magnetic neutron stars

Recent work has shown that a young, rapidly rotating neutron star loses ang ular momentum to gravitational waves generated by unstable r-mode oscillati ons. We study the spin evolution of a young, magnetic neutron star includin g the effects of both gravitational radiation and magnetic braking (modeled as magnetic dipole radiation). Our phenomenological description of nonline ar r-modes is similar to, but distinct from, that given by Owen and colleag ues in 1998, in that our treatment is consistent with the principle of adia batic invariance in the limit when direct driving and damping of the mode a re absent. We show that while magnetic braking tends to increase the r-mode amplitude by spinning down the neutron star, it nevertheless reduces the e fficiency of gravitational wave emission from the star. For B greater than or similar to 10(14)(nu (s)/300 Hz)(2) G, where nu (s) is the spin frequenc y, the spin-down rate and the gravitational waveforms are significantly mod ified by the effect of magnetic braking. We also estimate the growth rate o f the r-mode due to electromagnetic (fast magnetosonic) wave emission and d ue to Alfven wave emission in the neutron star magnetosphere. The Alfven wa ve driving of the r-mode becomes more important than the gravitational radi ation driving when B greater than or similar to 10(13)(nu (s)/150 Hz)(3) G; the electromagnetic wave driving of the r-mode is much weaker. Finally, we study the properties of local Rossby-Alfven waves inside the neutron star and show that the fractional change of the r-mode frequency due to the magn etic held is of order 0.5(B/10(16)G)(2)(nu (s)/100 Hz)(-2).