We have discovered rapid quasi-periodic oscillations (QPOs) in RXTE/PCA measurements of the pulsating tail of the 2004 December 27 giant flare of SGR 1806Ϫ20. QPOs at ∼92.5 Hz are detected in a 50 s interval starting 170 s after the onset of the giant flare. These QPOs appear to be associated with increased emission by a relatively hard unpulsed component and are seen only over phases of the 7.56 s spin period pulsations away from the main peak. QPOs at ∼18 and ∼30 Hz are also detected ∼200-300 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected in the flux of a soft gamma-ray repeater or any other magnetar candidate. We interpret the highest QPOs in terms of the coupling of toroidal seismic modes with Alfvén waves propagating along magnetospheric field lines. The lowest frequency QPO might instead provide indirect evidence on the strength of the internal magnetic field of the magnetar.

The primary components of two new candidate events (GW190403 051519 and GW190426 190642) fall in the mass gap predicted by pair-instability supernova theory. We also expand the population of binaries with significantly asymmetric mass ratios reported in GWTC-2 by an additional two events (q < 0.61 and q < 0.62 at 90% credibility for GW190403 051519 and GW190917 114630 respectively), and find that 2 of the 8 new events have effective inspiral spins χ eff > 0 (at 90% credibility), while no binary is consistent with χ eff < 0 at the same significance.

We present a state-of-the-art scenario for newly born magnetars as strong sources of gravitational waves (GWs) in the early days after formation. We address several aspects of the astrophysics of rapidly rotating, ultra-magnetized neutron stars (NSs), including early cooling before transition to superfluidity, the effects of the magnetic field on the equilibrium shape of NSs, the internal dynamical state of a fully degenerate, oblique rotator and the strength of the electromagnetic torque on the newly born NS. We show that our scenario is consistent with recent studies of supernova remnant surrounding Anomalous X-ray Pulsars (AXPs) and Soft Gamma-Ray Repeaters (SGRs) in the Galaxy that constrains the electromagnetic energy input from the central NS to be < 1051 erg. We further show that if this condition is met, then the GW signal from such sources is potentially detectable with the forthcoming generation of GW detectors up to Virgo cluster distances where an event rate similar to 1 yr-1 can be estimated. Finally, we point out that the decay of an internal magnetic field in the 1016 G range couples strongly with the NS cooling at very early stages, thus significantly slowing down both processes: the field can remain this strong for at least 103 yr, during which the core temperature stays higher than several times 108 K

There is growing evidence that two classes of high-energy sources, the soft gamma repeaters and the anomalous X-ray pulsars, contain slowly spinning "magnetars," i.e., neutron stars whose emission is powered by the release of energy from their extremely strong magnetic fields (110 15 G). We show here that the enormous energy liberated in the 2004 December 27 giant flare from SGR 1806Ϫ20 (∼ ergs), together with the likely recurrence 46 5 # 10 time of such events, requires an internal field strength of տ10 16 G. Toroidal magnetic fields of this strength are within an order of magnitude of the maximum fields that can be generated in the core of differentially rotating neutron stars immediately after their formation, if their initial spin period is on the order of a few milliseconds. A substantial deformation of the neutron star is induced by these magnetic fields and, provided the deformation axis is offset from the spin axis, a newborn fast-spinning magnetar would radiate for a few weeks a strong gravitational wave signal, the frequency of which (∼0.5-2 kHz range) decreases in time. The signal from a newborn magnetar with internal field 110 16.5 G could be detected with Advanced LIGO-class detectors up to the distance of the Virgo Cluster (characteristic amplitude ). Magnetars are expected to form in Virgo at Ϫ21 h ∼ 10 c a rate of ≥1 yr Ϫ1 . If a fraction of these have sufficiently high internal magnetic fields, then newborn magnetars constitute a promising new class of gravitational wave emitters.