209 research outputs found
ALMA Observations of the Terahertz Spectrum of Sagittarius A*
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations at 233, 678, and 870 GHz of the Galactic Center black hole, Sagittarius A*. These observations reveal a flat spectrum over this frequency range with spectral index α ≈ −0.3, where the flux density S ∝ ν α . We model the submillimeter and far-infrared spectrum with a one-zone synchrotron model of thermal electrons. We infer electron densities n = (2–5) × 106 cm−3, electron temperatures T e = (1–3) × 1011 K, and magnetic field strength B = 10–50 G. The parameter range can be further constrained using the observed quiescent X-ray luminosity. The flat submillimeter spectrum results in a high electron temperature and implies that the emitting electrons are efficiently heated. We also find that the emission is most likely optically thin at 233 GHz. These results indicate that millimeter and submillimeter wavelength very long baseline interferometry of Sgr A* including those of the Event Horizon Telescope should see a transparent emission region down to event horizon scales.Alexander von Humboldt foundation; NWO VICI grant [639.043.513]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
Detection of intrinsic source structure at ~3 Schwarzschild radii with Millimeter-VLBI observations of SAGITTARIUS A*
We report results from very long baseline interferometric (VLBI) observations
of the supermassive black hole in the Galactic center, Sgr A*, at 1.3 mm (230
GHz). The observations were performed in 2013 March using six VLBI stations in
Hawaii, California, Arizona, and Chile. Compared to earlier observations, the
addition of the APEX telescope in Chile almost doubles the longest baseline
length in the array, provides additional {\it uv} coverage in the N-S
direction, and leads to a spatial resolution of 30 as (3
Schwarzschild radii) for Sgr A*. The source is detected even at the longest
baselines with visibility amplitudes of 4-13% of the total flux density.
We argue that such flux densities cannot result from interstellar refractive
scattering alone, but indicate the presence of compact intrinsic source
structure on scales of 3 Schwarzschild radii. The measured nonzero
closure phases rule out point-symmetric emission. We discuss our results in the
context of simple geometric models that capture the basic characteristics and
brightness distributions of disk- and jet-dominated models and show that both
can reproduce the observed data. Common to these models are the brightness
asymmetry, the orientation, and characteristic sizes, which are comparable to
the expected size of the black hole shadow. Future 1.3 mm VLBI observations
with an expanded array and better sensitivity will allow a more detailed
imaging of the horizon-scale structure and bear the potential for a deep
insight into the physical processes at the black hole boundary.Comment: 11 pages, 5 figures, accepted to Ap
Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86 GHz
Observations of the Galactic Center supermassive black hole Sagittarius A*
(Sgr A*) with very long baseline interferometry (VLBI) are affected by
interstellar scattering along our line of sight. At long radio observing
wavelengths (cm), the scattering heavily dominates image
morphology. At 3.5 mm (86 GHz), the intrinsic source structure is no longer
sub-dominant to scattering, and thus the intrinsic emission from Sgr A* is
resolvable with the Global Millimeter VLBI Array (GMVA). Long-baseline
detections to the phased Atacama Large Millimeter/submillimeter Array (ALMA) in
2017 provided new constraints on the intrinsic and scattering properties of Sgr
A*, but the stochastic nature of the scattering requires multiple observing
epochs to reliably estimate its statistical properties. We present new
observations with the GMVA+ALMA, taken in 2018, which confirm non-Gaussian
structure in the scattered image seen in 2017. In particular, the ALMA-GBT
baseline shows more flux density than expected for an anistropic Gaussian
model, providing a tight constraint on the source size and an upper limit on
the dissipation scale of interstellar turbulence. We find an intrinsic source
extent along the minor axis of as both via extrapolation of
longer wavelength scattering constraints and direct modeling of the 3.5 mm
observations. Simultaneously fitting for the scattering parameters, we find an
at-most modestly asymmetrical (major-to-minor axis ratio of )
intrinsic source morphology for Sgr A*.Comment: 18 pages, 10 figures, submitted to Ap
Selected 'Starter kit' energy system modelling data for selected countries in Africa, East Asia, and South America (#CCG, 2021)
Energy system modeling can be used to develop internally-consistent quantified scenarios. These provide key insights needed to mobilise finance, understand market development, infrastructure deployment and the associated role of institutions, and generally support improved policymaking. However, access to data is often a barrier to starting energy system modeling, especially in developing countries, thereby causing delays to decision making. Therefore, this article provides data that can be used to create a simple zero-order energy system model for a range of developing countries in Africa, East Asia, and South America, which can act as a starting point for further model development and scenario analysis. The data are collected entirely from publicly available and accessible sources, including the websites and databases of international organisations, journal articles, and existing modeling studies. This means that the datasets can be easily updated based on the latest available information or more detailed and accurate local data. As an example, these data were also used to calibrate a simple energy system model for Kenya using the Open Source Energy Modeling System (OSeMOSYS) and three stylized scenarios (Fossil Future, Least Cost and Net Zero by 2050) for 2020–2050. The assumptions used and the results of these scenarios are presented in the appendix as an illustrative example of what can be done with these data. This simple model can be adapted and further developed by in-country analysts and academics, providing a platform for future work
Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope
The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009-2017 to be consistent with a persistent asymmetric ring of ∼40 μas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin
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