Constraints on physics of neutron stars from X-ray observations. (arXiv:1303.0317v1 [astro-ph.HE])

Constraints on physics of neutron stars from X-ray observations. (arXiv:1303.0317v1 [astro-ph.HE]):
I summarize some constraints on the physics of neutron stars arising from
X-ray observations of the surfaces of neutron stars, focusing on using models
of low-magnetic-field neutron star atmospheres to interpret their X-ray
spectra. I discuss observations of spectral lines, pulsation profiles, X-ray
bursts, radius measurements of transiently accreting neutron stars in
quiescence, crust and core cooling measurements of transiently accreting
neutron stars, and cooling of young neutron stars. These observations have
constrained the neutron star mass and radius (and thus the internal
composition, and dense matter equation of state), the superfluidity and
neutrino emissivity properties of the core, and the composition and superfluid
state of the crust.

Nuclear Masses and Neutron Stars. (arXiv:1303.1343v1 [nucl-th])

Nuclear Masses and Neutron Stars. (arXiv:1303.1343v1 [nucl-th]):
Precision mass spectrometry of neutron-rich nuclei is of great relevance for
astrophysics. Masses of exotic nuclides impose constraints on models for the
nuclear interaction and thus affect the description of the equation of state of
nuclear matter, which can be extended to describe neutron-star matter. With
knowledge of the masses of nuclides near shell closures, one can also derive
the neutron-star crustal composition. The Penning-trap mass spectrometer
ISOLTRAP at CERN-ISOLDE has recently achieved a breakthrough measuring the mass
of 82Zn, which allowed constraining neutron-star crust composition to deeper
layers (Wolf et al., PRL 110, 2013). We perform a more detailed study on the
sequence of nuclei in the outer crust of neutron stars with input from
different nuclear models to illustrate the sensitivity to masses and the
robustness of neutron-star models. The dominant role of the N=50 and N=82
closed neutron shells for the crustal composition is confirmed.

Updates of the nuclear equation of state for core-collapse supernovae and neutron Stars: effects of 3-body forces, QCD, and magnetic fields. (arXiv:1302.5875v3 [astro-ph.HE] UPDATED)

Updates of the nuclear equation of state for core-collapse supernovae and neutron Stars: effects of 3-body forces, QCD, and magnetic fields. (arXiv:1302.5875v3 [astro-ph.HE] UPDATED):
We summarize several new developments in the nuclear equation of state for
supernova simulations and neutron stars. We discuss an updated and improved
Notre-Dame-Livermore Equation of State (NDL EoS) for use in supernovae
simulations. This Eos contains many updates. Among them are the effects of 3-
body nuclear forces at high densities and the possible transition to a QCD
chiral and/or super-conducting color phase at densities. We also consider the
neutron star equation of state and neutrino transport in the presence of strong
magnetic fields. We study a new quantum hadrodynamic (QHD) equation of state
for neutron stars (with and without hyperons) in the presence of strong
magnetic fields. The parameters are constrained by deduced masses and radii.
The calculated adiabatic index for these magnetized neutron stars exhibit rapid
changes with density. This may provide a mechanism for star-quakes and flares
in magnetars. We also investigate the strong magnetic field effects on the
moments of inertia and spin down of neutron stars. The change of the moment of
inertia associated with emitted magnetic flares is shown to match well with
observed glitches in some magnetars. We also discuss a perturbative calculation
of neutrino scattering and absorption in hot and dense hyperonic neutron-star
matter in the presence of a strong magnetic field. The absorption
cross-sections show a remarkable angular dependence in that the neutrino
absorption strength is reduced in a direction parallel to the magnetic field
and enhanced in the opposite direction. The pulsar kick velocities associated
with this asymmetry comparable to observed pulsar velocities and may affect the
early spin down rate of proto-neutron star magnetars with a toroidal field
configuration.

The NDL Equation of State for Supernova Simulations. (arXiv:1303.0064v1 [astro-ph.HE])

The NDL Equation of State for Supernova Simulations. (arXiv:1303.0064v1 [astro-ph.HE]):
We present an updated and improved equation of state (which we call the NDL
EoS) for use in neutron-star structure and supernova simulations. This EoS is
based upon a framework originally developed by Bowers & Wilson, but there are
numerous changes. Among them are: (1) a reformulation in the context of density
functional theory; (2) the possibility of the formation of material with a net
proton excess (Ye > 0.5); (3) an improved treatment of the nuclear statistical
equilibrium and the transition to heavy nuclei as the density approaches
nuclear matter density; (4) an improved treatment of the effects of pions in
the regime above nuclear matter density including the incorporation of all the
known mesonic and baryonic states at high temperature; (5) the effects of
3-body nuclear forces at high densities; and (6) the possibility of a
first-order or crossover transition to a QCD chiral symmetry restoration and
deconfinement phase at densities above nuclear matter density. This paper
details the physics of, and constraints on, this new EoS and describes its
implementation in numerical simulations. We show comparisons of this EoS with
other equations of state commonly used in supernova collapse simulations.

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.

To differentiate neutron star models by X-ray polarimetry. (arXiv:1302.1328v1 [astro-ph.HE])

To differentiate neutron star models by X-ray polarimetry. (arXiv:1302.1328v1 [astro-ph.HE]):
The nature of pulsar is still unknown because of non-perturbative effects of
the fundamental strong interaction, and different models of pulsar inner
structures are then suggested, either conventional neutron stars or quark
stars. Additionally, a state of quark-cluster matter is conjectured for cold
matter at supranuclear density, as a result pulsars could thus be quark-cluster
stars. Besides understanding different manifestations, the most important issue
is to find an effective way to observationally differentiate those models.
X-ray polarimetry would play an important role here. In this letter, we focus
on the thermal X-ray polarization of quark/quark-cluster stars. While the
thermal X-ray linear polarization percentage is typically higher than ~10% in
normal neutron star models, the percentage of quark/quark-cluster stars is
almost zero. It could then be an effective method to identify
quark/quark-cluster stars by soft X-ray polarimetry. We are therefore expecting
to detect thermal X-ray polarization in the coming decades.

High density matter. (arXiv:1302.1928v1 [astro-ph.SR])

High density matter. (arXiv:1302.1928v1 [astro-ph.SR]):
The microscopic composition and properties of matter at super-saturation
densities have been the subject of intense investigation for decades. The
scarcity of experimental and observational data has lead to the necessary
reliance on theoretical models. However, there remains great uncertainty in
these models, which, of necessity, have to go beyond the over-simple assumption
that high density matter consists only of nucleons and leptons. Heavy strange
baryons, mesons and quark matter in different forms and phases have to be
included to fulfil basic requirements of fundamental laws of physics. In this
review the latest developments in construction of the Equation of State (EoS)
of high-density matter at zero and finite temperature assuming different
composition of the matter are surveyed. Critical comparison of model EoS with
available observational data on neutron stars, including gravitational masses,
radii and cooling patterns is presented. The effect of changing rotational
frequency on the composition of neutron stars during their lifetime is
demonstrated. Compatibility of EoS of high-density, low temperature compact
objects and low density, high temperature matter created in heavy-ion
collisions is discussed.

Measurement of the Radius of Neutron Stars with High S/N Quiescent Low-mass X-ray Binaries in Globular Clusters. (arXiv:1302.0023v1 [astro-ph.HE])

Measurement of the Radius of Neutron Stars with High S/N Quiescent Low-mass X-ray Binaries in Globular Clusters. (arXiv:1302.0023v1 [astro-ph.HE]):
This paper presents the measurement of the neutron star (NS) radius using the
thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) inside globular
clusters (GCs). Recent observations of NSs have presented evidence that cold
ultra dense matter -- present in the core of NSs -- is best described by
"normal matter" equations of state (EoSs). Such EoSs predict that the radii of
NSs, Rns, are quasi-constant (within measurement errors, of ~10%) for
astrophysically relevant masses (Mns > 0.5 Msun). The present work adopts this
theoretical prediction as an assumption, and uses it to constrain a single Rns
value from five qLMXB targets with available high signal-to-noise X-ray
spectroscopic data. Employing a Markov-Chain Monte-Carlo approach, we produce
the marginalized posterior distribution for Rns, constrained to be the same
value for all five NSs in the sample. An effort was made to include all
quantifiable sources of uncertainty into the uncertainty of the quoted radius
measurement. These include the uncertainties in the distances to the GCs, the
uncertainties due to the Galactic absorption in the direction of the GCs, and
the possibility of a hard power-law spectral component for count excesses at
high photon energy, which are observed in some qLMXBs in the Galactic plane.
Using conservative assumptions,we found that the radius, common to the five
qLMXBs and constant for a wide range of masses, lies in the low range of
possible NS radii, Rns=9.1(+1.3)(-1.5) km (90%-confidence). Such a value is
consistent with low-res equations of state. We compare this result with
previous radius measurements of NSs from various analyses of different types of
systems. In addition, we compare the spectral analyses of individual qLMXBs to
previous works.

A link between measured neutron star masses and lattice QCD data. (arXiv:1212.5907v1 [astro-ph.SR])

A link between measured neutron star masses and lattice QCD data. (arXiv:1212.5907v1 [astro-ph.SR]):
We study the hadron-quark phase transition in neutron star matter and the
structural properties of hybrid stars using an equation of state (EOS) for the
quark phase derived with the Field Correlator Method (FCM). We make use of
measured neutron star masses, and particularly the mass of PSR J1614-2230, to
constrain the values of the gluon condensate $G_2$ which is one of the EOS
parameter within the FCM. We find that the values of $G_2$ extracted from the
mass measurement of PSR J1614-2230 are fully consistent with the values of the
same quantity derived, within the FCM, from recent lattice QCD calculations of
the deconfinement transition temperature at zero baryon chemical potential. The
FCM thus provides a powerful tool to link numerical calculations of QCD on a
space-time lattice with neutron stars physics.

Hadron-Quark Crossover and Massive Hybrid Stars. (arXiv:1212.6803v1 [nucl-th])

Hadron-Quark Crossover and Massive Hybrid Stars. (arXiv:1212.6803v1 [nucl-th]):
On the basis of the percolation picture from the hadronic phase with hyperons
to the quark phase with strangeness, we construct a new equation of state (EOS)
with the pressure interpolated as a function of the baryon density. The maximum
mass of neutron stars can exceed $2M_{\odot}$ if the following two conditions
are satisfied; (i) the crossover from the hadronic matter to the quark matter
takes place at around three times the normal nuclear matter density, and (ii)
the quark matter is strongly interacting in the crossover region. This is in
contrast to the conventional approach assuming the first order phase transition
in which the EOS becomes always soft due to the presence of the quark matter at
high density. Although the choice of the hadronic EOS does not affect the above
conclusion on the maximum mass, the three-body force among nucleons and
hyperons plays an essential role for the onset of the hyperon mixing and the
cooling of neutron stars.

Review of Multi-messenger observations of neutron rich matter. (arXiv:1212.6405v1 [nucl-th])

Review of Multi-messenger observations of neutron rich matter. (arXiv:1212.6405v1 [nucl-th]):
At very high densities, electrons react with protons to form neutron rich
matter. This material is central to many fundamental questions in nuclear
physics and astrophysics. Moreover, neutron rich matter is being studied with
an extraordinary variety of new tools such as the Facility for Rare Isotope
Beams (FRIB) and the Laser Interferometer Gravitational Wave Observatory
(LIGO). We describe the Lead Radius Experiment (PREX) that uses parity
violating electron scattering to measure the neutron radius of 208Pb. This has
important implications for neutron stars and their crusts. We discuss X-ray
observations of neutron star radii. These also have important implications for
neutron rich matter. Gravitational waves (GW) open a new window on neutron rich
matter. They come from sources such as neutron star mergers, rotating neutron
star mountains, and collective r-mode oscillations. Using large scale molecular
dynamics simulations, we find neutron star crust to be very strong. It can
support mountains on rotating neutron stars large enough to generate detectable
gravitational waves. Finally, neutrinos from core collapse supernovae (SN)
provide another, qualitatively different probe of neutron rich matter.
Neutrinos escape from the surface of last scattering known as the
neutrino-sphere. This is a low density warm gas of neutron rich matter.
Neutrino-sphere conditions can be simulated in the laboratory with heavy ion
collisions. Observations of neutrinos can probe nucleosyntheses in SN. We
believe that combing astronomical observations using photons, GW, and
neutrinos, with laboratory experiments on nuclei, heavy ion collisions, and
radioactive beams will fundamentally advance our knowledge of compact objects
in the heavens, the dense phases of QCD, the origin of the elements, and of
neutron rich matter.

Hyperons and Condensed Kaons in Compact Stars. (arXiv:1301.0067v1 [nucl-th])

Hyperons and Condensed Kaons in Compact Stars. (arXiv:1301.0067v1 [nucl-th]):
Using the Callan-Klebanov bound state model for hyperons simulated on crystal
lattice to describe strange baryonic matter, we argue that to ${\cal O} (1)$ in
the large $N_c$ counting to which the theory is robust, hyperons can figure
only when -- or after -- kaons condense in compact-star matter. We also discuss
how the skyrmion-half-skyrmion topological transition affects the equation of
state (EoS) of dense baryonic matter. The observations made in this note open
wide the issue of how to theoretically access the EoS of compact stars.

Structure of neutron stars in R-squared gravity. (arXiv:1301.5189v1 [astro-ph.CO])

Structure of neutron stars in R-squared gravity. (arXiv:1301.5189v1 [astro-ph.CO]):
The effects implied for the structure of compact objects by the modification
of General Relativity produced by the generalization of the Lagrangian density
to the form f(R)=R+\alpha R^2, where R is the Ricci curvature scalar, have been
recently explored. It seems likely that this squared-gravity may allow heavier
Neutron Stars (NSs) than GR. In addition, these objects can be useful to
constrain free parameters of modified-gravity theories. The differences between
alternative gravity theories is enhanced in the strong gravitational regime. In
this regime, because of the complexity of the field equations, perturbative
methods become a good choice to treat the problem. Following previous works in
the field, we performed a numerical integration of the structure equations that
describe NSs in f(R)-gravity, recovering their mass-radius relations, but
focusing on particular features that arise from this approach in the profiles
of the NS interior.

We show that these profiles run in correlation with the second-order
derivative of the analytic approximation to the Equation of State (EoS), which
leads to regions where the enclosed mass decreases with the radius in a
counter-intuitive way. We reproduce all computations with a simple polytropic
EoS to separate zeroth-order modified gravity effects.

Mass/Radius Constraints on the Quiescent Neutron Star in M13 Using Hydrogen and Helium Atmospheres. (arXiv:1301.3768v1 [astro-ph.HE])

Mass/Radius Constraints on the Quiescent Neutron Star in M13 Using Hydrogen and Helium Atmospheres. (arXiv:1301.3768v1 [astro-ph.HE]):
The mass and radius of the neutron star (NS) in low-mass X-ray binaries can
be obtained by fitting the X-ray spectrum of the NS in quiescence, and the mass
and radius constrains the properties of dense matter in NS cores. A critical
ingredient for spectral fits is the composition of the NS atmosphere: hydrogen
atmospheres are assumed in most prior work, but helium atmospheres are possible
if the donor star is a helium white dwarf. Here we perform spectral fits to
XMM, Chandra, and ROSAT data of a quiescent NS in the globular cluster M13.
This NS has the smallest inferred radius from previous spectral fitting.
Assuming an atmosphere composed of hydrogen, we find a significantly larger
radius, more consistent with those from other quiescent NSs. With a helium
atmosphere (an equally acceptable fit), we find even larger values for the
radius.

Constraints on the quark matter equation of state from astrophysical observations. (arXiv:1301.4060v1 [nucl-th])

Constraints on the quark matter equation of state from astrophysical observations. (arXiv:1301.4060v1 [nucl-th]):
We calculate the structure of neutron star interiors comprising both the
hadronic and the quark phases. For the hadronic sector we employ a microscopic
equation of state involving nucleons and hyperons derived within the
Brueckner-Hartree-Fock many-body theory with realistic two-body and three-body
forces. For the description of quark matter, we use several different models,
e.g. the MIT bag, the Nambu--Jona-Lasinio (NJL), the Color Dielectric (CDM),
the Field Correlator method (FCM), and one based on the Dyson-Schwinger model
(DSM). We find that a two solar mass hybrid star is possible only if the
nucleonic EOS is stiff enough.

Momentum dependent mean-field dynamics of compressed nuclear matter and neutron stars. (arXiv:1206.4821v2 [nucl-th] CROSS LISTED)

Momentum dependent mean-field dynamics of compressed nuclear matter and neutron stars. (arXiv:1206.4821v2 [nucl-th] CROSS LISTED):
Nuclear matter and compact neutron stars are studied in the framework of the
non-linear derivative (NLD) model which accounts for the momentum dependence of
relativistic mean-fields. The generalized form of the energy-momentum tensor is
derived which allows to consider different forms of the regulator functions in
the NLD Lagrangian. The thermodynamic consistency of the NLD model is
demonstrated for arbitrary choice of the regulator functions. The NLD approach
describes the bulk properties of the nuclear matter and compares well with
microscopic calculations and Dirac phenomenology. We further study the high
density domain of the nuclear equation of state (EoS) relevant for the matter
in $\beta$-equilibrium inside neutron stars. It is shown that the low density
constraints imposed on the nuclear EoS and by the momentum dependence of the
Schr\"odinger-equivalent optical potential lead to a maximum mass of the
neutron stars around $M \simeq 2 M_{\odot}$ which accommodates the observed
mass of the J1614-2230 millisecond radio pulsar.

Dense QCD and phenomenology of compact stars. (arXiv:1301.2675v1 [astro-ph.HE])

Dense QCD and phenomenology of compact stars. (arXiv:1301.2675v1 [astro-ph.HE]):
I discuss three topics in physics of massive (two solar-mass and larger)
neutron stars containing deconfined quark matter: (i) the equation of state of
deconfined dense quark matter and its color superconducting phases, (ii) the
thermal evolution of stars with quark cores, (iii) color-magnetic flux tubes in
type-II superconducting quark matter and their dynamics driven by Aharonov-Bohm
interactions with unpaired fermions.

Structure of Spin Polarized Strange Quark Star in the Presence of Magnetic Field at Finite Temperature. (arXiv:1301.0899v1 [astro-ph.SR])

Structure of Spin Polarized Strange Quark Star in the Presence of Magnetic Field at Finite Temperature. (arXiv:1301.0899v1 [astro-ph.SR]):
In this paper, we have calculated the thermodynamic properties of spin
polarized strange quark matter at finite temperature in the presence of a
strong magnetic field using MIT bag model. We have also computed the equation
of state of spin polarized strange quark matter in the presence of strong
magnetic field and finally, using this equation of states we have investigated
the structure of spin polarized strange quark star at different temperatures
and magnetic fields.

The mass and the radius of the neutron star in the transient low mass X-ray binary SAX J1748.9-2021. (arXiv:1301.0831v1 [astro-ph.HE])

The mass and the radius of the neutron star in the transient low mass X-ray binary SAX J1748.9-2021. (arXiv:1301.0831v1 [astro-ph.HE]):
We use time resolved spectroscopy of thermonuclear X-ray bursts observed from
SAX J1748.9-2021 to infer the mass and the radius of the neutron star in the
binary. Four X-ray bursts observed from the source with RXTE enable us to
measure the angular size and the Eddington limit on the neutron star surface.
Combined with a distance measurement to the globular cluster NGC 6440, in which
SAX J1748.9-2021 resides, we obtain two solutions for the neutron star radius
and mass, R = 8.18 +/- 1.62 km and M = 1.78 +/- 0.3 M_\sun or R = 10.93 +/-
2.09 km and M = 1.33 +/- 0.33 M_\sun.

Unified description of dense matter in neutron stars and magnetars. (arXiv:1301.2438v1 [astro-ph.HE])

Unified description of dense matter in neutron stars and magnetars. (arXiv:1301.2438v1 [astro-ph.HE]):
We have recently developed a set of equations of state based on the nuclear
energy density functional theory providing a unified description of the
different regions constituting the interior of neutron stars and magnetars. The
nuclear functionals, which were constructed from generalized Skyrme effective
nucleon-nucleon interactions, yield not only an excellent fit to essentially
all experimental atomic mass data but were also constrained to reproduce the
neutron-matter equation of state as obtained from realistic many-body
calculations.