156 research outputs found
The mass and the radius of the neutron star in the transient low mass X-ray binary SAX J1748.9−2021
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
Tidal deformability from GW170817 as a direct probe of the neutron star radius
Gravitational waves from the coalescence of two neutron stars were recently
detected for the first time by the LIGO-Virgo collaboration, in event GW170817.
This detection placed an upper limit on the effective tidal deformability of
the two neutron stars and tightly constrained the chirp mass of the system. We
report here on a new simplification that arises in the effective tidal
deformability of the binary, when the chirp mass is specified. We find that, in
this case, the effective tidal deformability of the binary is surprisingly
independent of the component masses of the individual neutron stars, and
instead depends primarily on the ratio of the chirp mass to the neutron star
radius. Thus, a measurement of the effective tidal deformability can be used to
directly measure the neutron star radius. We find that the upper limit on the
effective tidal deformability from GW170817 implies that the radius cannot be
larger than ~13km, at the 90% level, independent of the assumed masses for the
component stars. The result can be applied generally, to probe the stellar
radii in any neutron star-neutron star merger with a measured chirp mass. The
approximate mass-independence disappears for neutron star-black hole mergers.
Finally, we discuss a Bayesian inference of the equation of state that uses the
measured chirp mass and tidal deformability from GW170817 combined with nuclear
and astrophysical priors and discuss possible statistical biases in this
inference.Comment: Submitted to ApJ Letter
Confronting Models of Massive Star Evolution and Explosions with Remnant Mass Measurements
The mass distribution of compact objects provides a fossil record that can be
studied to uncover information on the late stages of massive star evolution,
the supernova explosion mechanism, and the dense matter equation of state.
Observations of neutron star masses indicate a bimodal Gaussian distribution,
while the observed black hole mass distribution decays exponentially for
stellar-mass black holes. We use these observed distributions to directly
confront the predictions of stellar evolution models and the neutrino-driven
supernova simulations of Sukhbold et al. (2016). We find excellent agreement
between the black hole and low-mass neutron star distributions created by these
simulations and the observations. We show that a large fraction of the stellar
envelope must be ejected, either during the formation of stellar-mass black
holes or prior to the implosion through tidal stripping due to a binary
companion, in order to reproduce the observed black hole mass distribution. We
also determine the origins of the bimodal peaks of the neutron star mass
distribution, finding that the low-mass peak (centered at ~1.4 M_sun)
originates from progenitors with M_zams ~ 9-18 M_sun. The simulations fail to
reproduce the observed peak of high-mass neutron stars (centered at ~1.8 M_sun)
and we explore several possible explanations. We argue that the close agreement
between the observed and predicted black hole and low-mass neutron star mass
distributions provides new promising evidence that these stellar evolution and
explosion models are accurately capturing the relevant stellar, nuclear, and
explosion physics involved in the formation of compact objects.Comment: Typos in fit coefficients corrected, results unchanged. 13 pages, 10
figures. Submitted to Ap
From Neutron Star Observables to the Equation of State. I. An Optimal Parametrization
The increasing number and precision of measurements of neutron star masses,
radii, and, in the near future, moments of inertia offer the possibility of
precisely determining the neutron star equation of state. One way to facilitate
the mapping of observables to the equation of state is through a
parametrization of the latter. We present here a generic method for optimizing
the parametrization of any physically allowed EoS. We use mock equations of
state that incorporate physically diverse and extreme behavior to test how well
our parametrization reproduces the global properties of the stars, by
minimizing the errors in the observables mass, radius, and the moment of
inertia. We find that using piecewise polytropes and sampling the EoS with five
fiducial densities between ~1-8 times the nuclear saturation density results in
optimal errors for the smallest number of parameters. Specifically, it
recreates the radii of the assumed EoS to within less than 0.5 km for the
extreme mock equations of state and to within less than 0.12 km for 95% of a
sample of 42 proposed, physically-motivated equations of state. Such a
parametrization is also able to reproduce the maximum mass to within 0.04 M_sun
and the moment of inertia of a 1.338 M_sun neutron star to within less than 10%
for 95% of the proposed sample of equations of state.Comment: Minor changes made to match published ApJ versio
Mapping the Surface of the Magnetar 1E 1048.1-5937 in Outburst and Quiescence Through Phase Resolved X-ray Spectroscopy
We model the pulse profiles and the phase resolved spectra of the anomalous
X-ray pulsar 1E 1048.1-5937 obtained with XMM-Newton to map its surface
temperature distribution during an active and a quiescent epoch. We develop and
apply a model that takes into account the relevant physical and geometrical
effects on the neutron star surface, magnetosphere, and spacetime. Using this
model, we determine the observables at infinity as a function of pulse phase
for different numbers and sizes of hot spots on the surface. We show that the
pulse profiles extracted from both observations can be modeled with a single
hot spot and an antipodal cool component. The size of the hot spot changes from
in 2007, 3 months after the onset of a dramatic flux
increase, to during the quiescent observation in 2011,
when the pulsed fraction returned to the pre-outburst 65\% level. For
the 2007 observation, we also find that a model consisting of a single 0.4 keV
hot spot with a magnetic field strength of G accounts for
the spectra obtained at three different pulse phases but under predicts the
flux at the pulse minimum, where the contribution to the emission from the
cooler component is non-negligible. The inferred temperature of the spot stays
approximately constant between different pulse phases, in agreement with a
uniform temperature, single hot spot model. These results suggest that the
emitting area grows significantly during outbursts but returns to its
persistent and significantly smaller size within a few year timescale.Comment: Accepted for publication in The Astrophysical Journa
X-ray Lightcurves from Realistic Polar Cap Models: Inclined Pulsar Magnetospheres and Multipole Fields
Thermal X-ray emission from rotation-powered pulsars is believed to originate
from localized "hotspots" on the stellar surface occurring where large-scale
currents from the magnetosphere return to heat the atmosphere. Lightcurve
modeling has primarily been limited to simple models, such as circular
antipodal emitting regions with constant temperature. We calculate more
realistic temperature distributions within the polar caps, taking advantage of
recent advances in magnetospheric theory, and we consider their effect on the
predicted lightcurves. The emitting regions are non-circular even for a pure
dipole magnetic field, and the inclusion of an aligned magnetic quadrupole
moment introduces a north-south asymmetry. As the aligned quadrupole moment is
increased, one hotspot grows in size before becoming a thin ring surrounding
the star. For the pure dipole case, moving to the more realistic model changes
the lightcurves by for millisecond pulsars, helping to quantify the
systematic uncertainty present in current dipolar models. Including the
quadrupole gives considerable freedom in generating more complex lightcurves.
We explore whether these simple dipole+quadrupole models can account for the
qualitative features of the lightcurve of PSR J04374715.Comment: 12 pages, 9 figure
Principal Component Analysis as a Tool for Characterizing Black Hole Images and Variability
We explore the use of principal component analysis (PCA) to characterize
high-fidelity simulations and interferometric observations of the millimeter
emission that originates near the horizons of accreting black holes. We show
mathematically that the Fourier transforms of eigenimages derived from PCA
applied to an ensemble of images in the spatial-domain are identical to the
eigenvectors of PCA applied to the ensemble of the Fourier transforms of the
images, which suggests that this approach may be applied to modeling the sparse
interferometric Fourier-visibilities produced by an array such as the Event
Horizon Telescope (EHT). We also show that the simulations in the spatial
domain themselves can be compactly represented with a PCA-derived basis of
eigenimages allowing for detailed comparisons between variable observations and
time-dependent models, as well as for detection of outliers or rare events
within a time series of images. Furthermore, we demonstrate that the spectrum
of PCA eigenvalues is a diagnostic of the power spectrum of the structure and,
hence, of the underlying physical processes in the simulated and observed
images.Comment: 16 pages, 17 figures, submitted to Ap
Narrow Atomic Features from Rapidly Spinning Neutron Stars
Neutron stars spinning at moderate rates (~300-600Hz) become oblate in shape
and acquire a nonzero quadrupole moment. In this paper, we calculate profiles
of atomic features from such neutron stars using a ray-tracing algorithm in the
Hartle-Thorne approximation. We show that line profiles acquire cores that are
much narrower than the widths expected from pure Doppler effects for a large
range of observer inclinations. As a result, the effects of both the oblateness
and the quadrupole moments of neutron stars need to be taken into account when
aiming to measure neutron star radii from rotationally broadened lines.
Moreover, the presence of these narrow cores substantially increases the
likelihood of detecting atomic lines from rapidly spinning neutron stars.Comment: 7 pages, 8 figures, accepted to Ap
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