The viscosity-driven “ spin-flip ” instability in newly born magnetars with interior toroidal magnetic fields is re-examined .
We calculate the bulk viscosity coefficient ( \zeta ) of cold , npe \mu matter in neutron stars ( NS ) , for selected values of the nuclear symmetry energy and in the regime where \beta -equilibration is slower than characteristic oscillation periods .
We show that : i ) \zeta is larger than previously assumed and the instability timescale correspondingly shorter ; ii ) for a magnetically-induced ellipticity \epsilon _ { B } \lesssim 4 \times 10 ^ { -3 } , typically expected in newborn magnetars , spin-flip occurs for initial spin periods \lesssim 2 - 3 ms , with some dependence on the NS equation of state ( EoS ) .

We then calculate the detectability of GW signals emitted by newborn magnetars subject to “ spin-flip ” , by accounting also for the reduction in range resulting from realistic signal searches .
For an optimal range of \epsilon _ { B } \sim ( 1 - 5 ) \times 10 ^ { -3 } , and birth spin period \lesssim 2 ms , we estimate an horizon of \gtrsim 4 Mpc , and \gtrsim 30 Mpc , for Advanced and third generation interferometers at design sensitivity , respectively .
A supernova ( or a kilonova ) is expected as the electromagnetic counterpart of such GW events .

Outside of the optimal range for GW emission , EM torques are more efficient in extracting the NS spin energy , which may power even brighter EM transients .