Originally, it was thought that . In such a case, we can estimate the GW signal from stellar
collapse. We expect multiple GW bursts to occur as material falls back onto the neutron star and results in
repeat episodes of r-mode growth (note that a single r-mode episode can have multiple amplitude
peaks [180
]). Using Equation (10
), FHH [106
] calculate that the characteristic amplitude of the
GW emission from this r-mode evolution tracks from 6 – 1 × 10–22, over a frequency range of
103 – 102 Hz for a source driven by fallback at 10 Mpc. They estimate the emitted energy to exceed
1052 erg.
Later work indicated that may be
3 [180, 294, 268, 181
]. But most recent research
suggests that magnetic fields, hyperon cooling, and hyperon bulk viscosity may limit the growth of
the r-mode instability, even in nascent neutron stars [159, 158, 248, 249, 181, 178, 132, 5]
(significant uncertainties remain regarding the efficacy of these dissipative mechanisms). One way to
reduce this viscosity is to invoke non-standard physics in the dense equation of state, e.g., quark
material, anti-kaon core [53, 71]. Even with this more exotic physics, the reduction in viscosity is
limited to specific regions in the neutron star and in the spin/temperature phase space. More
work is needed to determine if such modifications can allow r-modes to make a detectable
signal.
In addition, a study of a simple barotropic neutron star model by Arras et al. [8] argue
that multimode couplings could limit to values
. If
is indeed
(see
also [125, 27, 26, 28, 252, 24]), GW emission from r-modes in collapsed remnants is likely undetectable.
From Equation (10
), we can see that the GW signal is proportional to the mode amplitude, so a decrease
in the maximum amplitude by an order of magnitude corresponds to an order of magnitude
decrease in the GW strain. If correct, and much of the community believes the maximum of the
mode amplitude may be even smaller than 0.1, the GW signal from r-modes is much lower
than any other GW source and will not contribute significantly to the observed signal in stellar
collapse. Because of this, r-mode sources are omitted from figures comparing source strengths
and detector sensitivities and from discussions of likely detectable sources in the concluding
section.
r-modes may still produce signals in accreting systems such as low-mass X-ray binaries. Work continues in this subject for these systems, but this is beyond the subject of this review.
http://www.livingreviews.org/lrr-2011-1 |
Living Rev. Relativity 14, (2011), 1
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