Evolution of newborn rapidly rotating magnetars: Effects ofR-mode and fall-back accretion

Abstract

In this paper we investigate effects of the r-mode instability on a newborn rapidly-rotating magnetar with fall-back accretion. Such a magnetar could usually occur in core-collapse supernovae and gamma-ray bursts. We find that the magnetar's spin and r-mode evolution are influenced by accretion. If the magnetar is sufficiently spun up to a few milliseconds, gravitational radiation leads to the growth of the r-mode amplitude significantly. The maximum r-mode amplitude reaches an order ∼ 0.001 when the damping due to the growth of a toroidal magnetic field balances the growth of the r-mode amplitude. If such a sufficiently spun-up magnetar was located at a distance less than 1 Mpc, then gravitational waves would be detectable by the Einstein Telescope but would have an extremely low event rate. However, if the spin-up is insufficient, the growth of the r-mode amplitude is mainly due to the accretion torque. In this case, the maximum r-mode amplitude is of the order of ∼ 10 −6 − 10 −5 .

The braking index of PSR B0540−69 and the associated pulsar wind nebula emission after spin-down rate transition

“…The aforementioned surface field growth may occur even when the interior magnetic field decays in a neutron star (Goldreich & Reisenegger 1992). Alternatively, the magnetic field in a neutron star may be enhanced by magnetorotational instability (Balbus & Hawley 1991 and R-mode (Andersson 1998;Rezzolla et al 2000;Arras et al 2003;Wang & Dai 2017), although these effects may be very small for a neutron star as old as PSR B0540-69.…”

Gravitational Wave Radiation from Newborn Accreting Magnetars

“…The presence of a fall-back disk may complicate the evolution of the newborn magnetar, thus further affecting its GW radiation. Some previous studies have only focused on the effect of fall-back accretion on the magnetar's spin evolution (Bernardini et al 2013;Gompertz et al 2014;Gibson et al 2017), while others considered a more realistic case with its effect on the gravitational mass evolution involved simultaneously (Piro & Ott 2011;Dai & Liu 2012;Wang & Dai 2017;Metzger et al 2018). Depending on the values of the corotation radius R c , magnetospheric radius R m , and light cylinder radius R l , the fall-back disk may exert a spin-up torque (the accretion phase), or a spin-down torque (the propeller phase), or even no torque (the ejector phase) on the newborn magnetar.…”

The r-mode instability windows of strange stars

“…The r-mode instability only happens in a range of spin frequencies and temperatures, which is determined by a competition between the effects of gravitational radiation and viscous dissipation damping on modes (Lindblom et al 1998). Therefore, r-mode instability is an important primary physical mechanism that can prevent pulsars from spinning up to their Kepler frequency, which also affects the spinning evolutions of pulsars (Madsen 1998;Andersson et al 1999;Wang & Dai 2017). Meanwhile, gravitational waves emitted during the instability process could be detected (Owen et al 1998;Andersson et al 2002;Mahmoodifar & Such r-modes are damped by viscous dissipation mechanisms (Lindblom et al 1998), and therefore are connected with microscopic properties of matter inside the stars, which depend on the low energy degrees of freedom and the equation of state (EOS), thus affecting the macroscopic and observable properties of the star.…”