32 research outputs found
Where Are the r-modes? Chandra Observations of Millisecond Pulsars
We present the results of {\it Chandra} observations of two non-accreting
millisecond pulsars, PSRs J16402224 (J1640) and J17092313 (J1709), with
low inferred magnetic fields and spin-down rates in order to constrain their
surface temperatures, obtain limits on the amplitude of unstable -modes in
them, and make comparisons with similar limits obtained for a sample of
accreting low-mass X-ray binary (LMXB) neutron stars. We detect both pulsars in
the X-ray band for the first time. They are faint, with inferred soft X-ray
fluxes ( keV) of and erg
cm s for J1640 and J1709, respectively. Spectral analysis
assuming hydrogen atmosphere emission gives global effective temperature upper
limits ( confidence) of K for J1640 and K for J1709, where the low end of the range corresponds to
canonical neutron stars (), and the upper end corresponds to
higher-mass stars (). Under the assumption that -mode
heating provides the thermal support, we obtain dimensionless -mode
amplitude upper limits of and for J1640 and J1709, respectively, where again the low end of the
range corresponds to lower-mass, canonical neutron stars ().
These limits are about an order of magnitude lower than those we derived
previously for a sample of LMXBs, except for the accreting millisecond X-ray
pulsar (AMXP) SAX J1808.43658, which has a comparable amplitude limit to
J1640 and J1709.Comment: 9 pages, 4 figures, published in Ap
X-ray Burst Oscillations: From Flame Spreading to the Cooling Wake
Type I X-ray bursts are thermonuclear flashes observed from the surfaces of
accreting neutron stars (NSs) in Low Mass X-ray Binaries. Oscillations have
been observed during the rise and/or decay of some of these X-ray bursts. Those
seen during the rise can be well explained by a spreading hot spot model, but
large amplitude oscillations in the decay phase remain mysterious because of
the absence of a clear-cut source of asymmetry. To date there have not been any
quantitative studies that consistently track the oscillation amplitude both
during the rise and decay (cooling tail) of bursts. Here we compute the light
curves and amplitudes of oscillations in X-ray burst models that realistically
account for both flame spreading and subsequent cooling. We present results for
several such "cooling wake" models, a "canonical" cooling model where each
patch on the NS surface heats and cools identically, or with a
latitude-dependent cooling timescale set by the local effective gravity, and an
"asymmetric" model where parts of the star cool at significantly different
rates. We show that while the canonical cooling models can generate
oscillations in the tails of bursts, they cannot easily produce the highest
observed modulation amplitudes. Alternatively, a simple phenomenological model
with asymmetric cooling can achieve higher amplitudes consistent with the
observations.Comment: 8 pages, 7 figures, Accepted for publication in ApJ, Additional
calculations and discussion compared to v
Non-linear viscous saturation of r-modes
Pulsar spin frequencies and their time evolution are an important source of
information on compact stars and their internal composition. Oscillations of
the star can reduce the rotational energy via the emission of gravitational
waves. In particular unstable oscillation modes, like r-modes, are relevant
since their amplitude becomes large and can lead to a fast spin-down of young
stars if they are saturated by a non-linear saturation mechanism. We present a
novel mechanism based on the pronounced large-amplitude enhancement of the bulk
viscosity of dense matter. We show that the enhanced damping due to non-linear
bulk viscosity can saturate r-modes of neutron stars at amplitudes appropriate
for an efficient spin-down.Comment: 3 pages, contribution to the proceedings of the conference "Quark
Confinement and the Hadron Spectrum IX", August 30 - September 3, 2010,
Madrid; typos correcte
Probing the phases of cold ultra-dense matter using neutron star physics
Matter at very high densities and low temperatures is predicted to be in a “color superconducting” phase. At high enough densities, quark matter is in the Color-Flavor-Locked: CFL) phase, but the possible phases of matter at intermediate densities are unknown. Since the density at the core of a neutron star can be as high as a few times the nuclear saturation density, it is the most likely place to find these exotic forms of matter in the real world. The main goal of this thesis is to probe the phases of cold dense matter using neutron star physics. Studying the transport properties of different phases of dense matter that may occur in a compact star is particularly important because transport properties such as viscosity, in addition to depending on the equation of state of matter, also depend on the low-energy degrees of freedom and therefore can discriminate between different phases of dense matter more efficiently. In the first part of this thesis we calculate the mean free path and kaonic contribution to the shear viscosity of kaon-condensed color-flavor-locked: CFL-K0) phase of quark matter. In the second part we calculate the large-amplitude enhancement of the bulk viscosity of dense matter. We obtain general analytic solutions as well as numerical solutions for the amplitude-dependent bulk viscosity of dense matter which are valid for any equations of state where equilibration occurs via fermions. In the third and fourth parts, we use our general results for the bulk viscosity to calculate the damping timescales of r-mode oscillations of neutron stars due to small-amplitude and large-amplitude bulk viscosity, the instability window of the r-modes and the saturation amplitude due to “supra-thermal” enhancement of the bulk viscosity for different cases of strange quark stars, hadronic stars and hybrid stars
Large amplitude behavior of the bulk viscosity of dense matter
We study the bulk viscosity of dense matter, taking into account non-linear
effects which arise in the large amplitude "supra-thermal" region where the
deviation of the chemical potentials from chemical equilibrium
fulfills . This regime is relevant to unstable modes such as
r-modes, which grow in amplitude until saturated by non-linear effects. We
study the damping due to direct and modified Urca processes in hadronic matter,
and due to nonleptonic weak interactions in strange quark matter. We give
general results valid for an arbitrary equation of state of dense matter and
find that the viscosity can be strongly enhanced by supra-thermal effects. Our
study confirms previous results on quark matter and shows that the non-linear
enhancement is even stronger in the case of hadronic matter. Our results can be
applied to calculations of the r-mode-induced spin-down of fast-rotating
neutron stars, where the spin-down time will depend on the saturation amplitude
of the r-modeComment: 15 pages, 11 figure
Impact of r-modes on the cooling of neutron stars
Studying the frequency and temperature evolution of a compact star can give
us valuable information about the microscopic properties of the matter inside
the star. In this paper we study the effect of dissipative reheating of a
neutron star due to r-mode oscillations on its temperature evolution. We find
that there is still an impact of an r-mode phase on the temperature long after
the star has left the instability region and the r-mode is damped completely.
With accurate temperature measurements it may be possible to detect this trace
of a previous r-mode phase in observed pulsars.Comment: 7 pages, 5 figures, Proceedings of QCD@work 2012 International
Workshop on QCD Theory and Experimen
Suprathermal viscosity of dense matter
Motivated by the existence of unstable modes of compact stars that eventually
grow large, we study the bulk viscosity of dense matter, taking into account
non-linear effects arising in the large amplitude regime, where the deviation
mu_Delta of the chemical potentials from chemical equilibrium fulfills mu_Delta
> T. We find that this supra-thermal bulk viscosity can provide a potential
mechanism for saturating unstable modes in compact stars since the viscosity is
strongly enhanced. Our study confirms previous results on strange quark matter
and shows that the suprathermal enhancement is even stronger in the case of
hadronic matter. We also comment on the competition of different weak channels
and the presence of suprathermal effects in various color superconducting
phases of dense quark matter.Comment: 8 page
Upper Bounds on r-Mode Amplitudes from Observations of Low-Mass X-Ray Binary Neutron Stars
We present upper limits on the amplitude of r-mode oscillations and gravitational-radiation-induced spin-down rates in low-mass X-ray binary neutron stars, under the assumption that the quiescent neutron star luminosity is powered by dissipation from a steady-state r-mode. For masses <2M solar mass we find dimensionless r-mode amplitudes in the range from about 110(exp8) to 1.510(exp6). For the accreting millisecond X-ray pulsar sources with known quiescent spin-down rates, these limits suggest that approx. less than 1% of the observed rate can be due to an unstable r-mode. Interestingly, the source with the highest amplitude limit, NGC 6440, could have an r-mode spin-down rate comparable to the observed, quiescent rate for SAX J18083658. Thus, quiescent spin-down measurements for this source would be particularly interesting. For all sources considered here, our amplitude limits suggest that gravitational wave signals are likely too weak for detection with Advanced LIGO. Our highest mass model (2.21M solar mass) 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 J1751305. In the absence of other significant sources of internal heat, these results could be used to place an upper limit on the masses of these sources if they were made of hadronic matter, or alternatively it could be used to probe the existence of exotic matter in them if their masses were known