42 research outputs found
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
Jets and Rings in Images of Spinning Black Holes
We develop a "dual cone" model for millimeter wavelength emission near a
spinning black hole. The model consists of optically thin, luminous cones of
emission, centered on the spin axis, which are meant to represent jet walls.
The resulting image is dominated by a thin ring. We first consider the effect
of black hole's spin on the image, and show that the dominant effect is to
displace the ring perpendicular to the projection of the spin axis on the sky
by . This effect is lower order in
than changes in the shape and size of the photon ring itself, but is
undetectable without a positional reference. We then show that the centerline
of the jet can provide a suitable reference: its location is exactly
independent of spin if the observer is outside the cone, and nearly independent
of spin if the observer is inside the cone. If astrophysical uncertainties can
be controlled for, then spin displacement is large enough to be detectable by
future space VLBI missions. Finally, we consider ring substructure in the dual
cone model and show that features in total intensity are not universal and
depend on the cone opening angle.Comment: 9 pages, 5 figures, submitted to Ap
A Detection of Sgr A* in the far infrared
We report the first detection of the Galactic Centre massive black hole,
Sgr~A*, in the far infrared. Our measurements were obtained with PACS on board
the \emph{Herschel} satellite at and .
While the warm dust in the Galactic Centre is too bright to allow for a direct
detection of Sgr~A*, we measure a significant and simultaneous variation of its
flux of and during one observation. The significance level of
the band variability is and the corresponding
band variability is significant at . We find
no example of an equally significant false positive detection. Conservatively
assuming a variability of in the FIR, we can provide upper limits to the
flux. Comparing the latter with theoretical models we find that 1D RIAF models
have difficulties explaining the observed faintness. However, the upper limits
are consistent with modern ALMA and VLA observations. Our upper limits provide
further evidence for a spectral peak at and
constrain the number density of electrons in the accretion
disk and or outflow.Comment: accepted for publication in AP
What stellar orbit is needed to measure the spin of the Galactic center black hole from astrometric data?
Astrometric and spectroscopic monitoring of individual stars orbiting the
supermassive black hole in the Galactic Center offer a promising way to detect
general relativistic effects. While low-order effects are expected to be
detected following the periastron passage of S2 in Spring 2018, detecting
higher-order effects due to black hole spin will require the discovery of
closer stars. In this paper, we set out to determine the requirements such a
star would have to satisfy to allow the detection of black hole spin. We focus
on the instrument GRAVITY, which saw first light in 2016 and which is expected
to achieve astrometric accuracies as. For an observing campaign
with duration years, total observations, astrometric precision
and normalized black hole spin , we find that
is needed. For and a potential observing
campaign with as, 30 observations/year and duration 4-10
years, we expect star with satisfying this constraint based
on the current knowledge about the stellar population in the central 1". We
also propose a method through which GRAVITY could potentially measure radial
velocities with precision km/s. If the astrometric precision can be
maintained, adding radial velocity information increases the expected number of
stars by roughly a factor of two. While we focus on GRAVITY, the results can
also be scaled to parameters relevant for future extremely large telescopes.Comment: Accepted to MNRA
A Ray-Tracing Algorithm for Spinning Compact Object Spacetimes with Arbitrary Quadrupole Moments. II. Neutron Stars
A moderately spinning neutron star acquires an oblate shape and a spacetime
with a significant quadrupole moment. These two properties affect its apparent
surface area for an observer at infinity, as well as the lightcurve arising
from a hot spot on its surface. In this paper, we develop a ray-tracing
algorithm to calculate the apparent surface areas of moderately spinning
neutron stars making use of the Hartle-Thorne metric. This analytic metric
allows us to calculate various observables of the neutron star in a way that
depends only on its macroscopic properties and not on the details of its
equation of state. We use this algorithm to calculate the changes in the
apparent surface area, which could play a role in measurements of neutron star
radii and, therefore, in constraining their equation of state. We show that
whether the spinning neutron star appears larger or smaller than its
non-rotating counterpart depends primarily on its equatorial radius. For
neutron stars with radii ~10 km, the corrections to the Schwarzschild spacetime
cause the apparent surface area to increase with spin frequency. In contrast,
for neutron stars with radii ~15 km, the oblateness of the star dominates the
spacetime corrections and causes the apparent surface area to decrease with
increasing spin frequency. In all cases, the change in the apparent geometric
surface area for the range of observed spin frequencies is < 5% and hence only
a small source of error in the measurement of neutron star radii.Comment: 9 pages, 6 figures, published in Ap
First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass
In this paper we quantify the temporal variability and image morphology of the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that exceeds what can be explained by the uncertainties in the data or by the effects of interstellar scattering. The magnitude of this variability can be a substantial fraction of the correlated flux density, reaching ∼100% on some baselines. Through an exploration of simple geometric source models, we demonstrate that ring-like morphologies provide better fits to the Sgr A* data than do other morphologies with comparable complexity. We develop two strategies for fitting static geometric ring models to the time-variable Sgr A* data; one strategy fits models to short segments of data over which the source is static and averages these independent fits, while the other fits models to the full data set using a parametric model for the structural variability power spectrum around the average source structure. Both geometric modeling and image-domain feature extraction techniques determine the ring diameter to be 51.8 ± 2.3 μas (68% credible intervals), with the ring thickness constrained to have an FWHM between ∼30% and 50% of the ring diameter. To bring the diameter measurements to a common physical scale, we calibrate them using synthetic data generated from GRMHD simulations. This calibration constrains the angular size of the gravitational radius to be 4.8−0.7+1.4 μas, which we combine with an independent distance measurement from maser parallaxes to determine the mass of Sgr A* to be 4.0−0.6+1.1×106 M ⊙
Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI
The extraordinary physical resolution afforded by the Event Horizon Telescope has opened a window onto the astrophysical phenomena unfolding on horizon scales in two known black holes, M87* and Sgr A*. However, with this leap in resolution has come a new set of practical complications. Sgr A* exhibits intraday variability that violates the assumptions underlying Earth aperture synthesis, limiting traditional image reconstruction methods to short timescales and data sets with very sparse (u, v) coverage. We present a new set of tools to detect and mitigate this variability. We develop a data-driven, model-agnostic procedure to detect and characterize the spatial structure of intraday variability. This method is calibrated against a large set of mock data sets, producing an empirical estimator of the spatial power spectrum of the brightness fluctuations. We present a novel Bayesian noise modeling algorithm that simultaneously reconstructs an average image and statistical measure of the fluctuations about it using a parameterized form for the excess variance in the complex visibilities not otherwise explained by the statistical errors. These methods are validated using a variety of simulated data, including general relativistic magnetohydrodynamic simulations appropriate for Sgr A* and M87*. We find that the reconstructed source structure and variability are robust to changes in the underlying image model. We apply these methods to the 2017 EHT observations of M87*, finding evidence for variability across the EHT observing campaign. The variability mitigation strategies presented are widely applicable to very long baseline interferometry observations of variable sources generally, for which they provide a data-informed averaging procedure and natural characterization of inter-epoch image consistency
The Event Horizon Telescope Image of the Quasar NRAO 530
We report on the observations of the quasar NRAO 530 with the Event Horizon Telescope (EHT) on 2017 April 5−7, when NRAO 530 was used as a calibrator for the EHT observations of Sagittarius A*. At z = 0.902, this is the most distant object imaged by the EHT so far. We reconstruct the first images of the source at 230 GHz, at an unprecedented angular resolution of ∼20 μas, both in total intensity and in linear polarization (LP). We do not detect source variability, allowing us to represent the whole data set with static images. The images reveal a bright feature located on the southern end of the jet, which we associate with the core. The feature is linearly polarized, with a fractional polarization of ∼5%–8%, and it has a substructure consisting of two components. Their observed brightness temperature suggests that the energy density of the jet is dominated by the magnetic field. The jet extends over 60 μas along a position angle ∼ −28°. It includes two features with orthogonal directions of polarization (electric vector position angle), parallel and perpendicular to the jet axis, consistent with a helical structure of the magnetic field in the jet. The outermost feature has a particularly high degree of LP, suggestive of a nearly uniform magnetic field. Future EHT observations will probe the variability of the jet structure on microarcsecond scales, while simultaneous multiwavelength monitoring will provide insight into the high-energy emission origin
First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric
Astrophysical black holes are expected to be described by the Kerr metric. This is the only stationary, vacuum, axisymmetric metric, without electromagnetic charge, that satisfies Einstein’s equations and does not have pathologies outside of the event horizon. We present new constraints on potential deviations from the Kerr prediction based on 2017 EHT observations of Sagittarius A* (Sgr A*). We calibrate the relationship between the geometrically defined black hole shadow and the observed size of the ring-like images using a library that includes both Kerr and non-Kerr simulations. We use the exquisite prior constraints on the mass-to-distance ratio for Sgr A* to show that the observed image size is within ∼10% of the Kerr predictions. We use these bounds to constrain metrics that are parametrically different from Kerr, as well as the charges of several known spacetimes. To consider alternatives to the presence of an event horizon, we explore the possibility that Sgr A* is a compact object with a surface that either absorbs and thermally reemits incident radiation or partially reflects it. Using the observed image size and the broadband spectrum of Sgr A*, we conclude that a thermal surface can be ruled out and a fully reflective one is unlikely. We compare our results to the broader landscape of gravitational tests. Together with the bounds found for stellar-mass black holes and the M87 black hole, our observations provide further support that the external spacetimes of all black holes are described by the Kerr metric, independent of their mass