Upper bounds on r-mode amplitudes from observations of LMXB neutron stars. (arXiv:1302.1204v1 [astro-ph.HE])

Upper bounds on r-mode amplitudes from observations of LMXB neutron stars. (arXiv:1302.1204v1 [astro-ph.HE]):
The r-mode oscillations of neutron stars can be potentially powerful probes
of cold ultra-dense matter. In this paper we present upper limits on the
amplitude of r-mode oscillations, and their gravitational-radiation-induced
spin-down rates, in low mass X-ray binary (LMXB) neutron stars under the
assumption that the quiescent neutron star luminosity is powered by dissipation
from a steady-state r-mode. We compute results for neutron star models
constructed with the APR equation of state for masses of 1.4, 2 and 2.21
M_{sun}. For the lower mass models (1.4 and 2 M_{sun}) we find dimensionless
r-mode amplitudes in the range from about 1x10^{-8} to 1.5x10^{-6}. For the
accreting millisecond X-ray pulsar (AMXP) sources with known quiescent
spin-down rates these limits suggest that about 1% of the observed rate can be
due to an unstable r-mode. Interestingly, the AMXP with the highest amplitude
limit, NGC 6640, could have an r-mode spin-down rate comparable to the
observed, quiescent rate for SAX J1808-3658. Thus, quiescent spin-down
measurements for this source would be particularly interesting. For all the
sources considered here our amplitude limits suggest that their gravitational
wave signals are likely too weak for detection with Advanced LIGO. Our highest
mass model (2.21 M_{sun}) can support enhanced, direct Urca neutrino emission
in the core and thus can have higher r-mode amplitudes. Indeed, the inferred
r-mode spin-down rates at these higher amplitudes are inconsistent with the
observed spin-down rates for some of the sources, such as IGR J00291+5934 and
XTE J1751-305. This can be used to place an upper limit on the masses of these
sources if they are made of normal nuclear matter, or alternatively it could be
used to probe the existence of exotic matter in them if their masses were
known.