56 research outputs found
Incidence, Clinical Characteristics, and Natural History of Pediatric IBD in Wisconsin: a Population-based Epidemiological Study
Epidemiological studies of pediatric inflammatory bowel diseases (IBD) are needed to generate etiological hypotheses and inform public policy; yet, rigorous population-based studies of the incidence and natural history of Crohn’s disease (CD) and ulcerative colitis (UC) in the United States are limited
First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 \ub1 2.3 μas (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4
7 106 M☉, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i > 50\ub0), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of 103-105 gravitational radii to event-horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87* shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass
A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows
We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT
Resolving the inner parsec of the blazar J1924–2914 with the Event Horizon Telescope
Rest of authors: Ikeda, Shiro; Impellizzeri, C. M. Violette; Inoue, Makoto; James, David J.; Jannuzi, Buell T.; Jeter, Britton; Jiang, Wu; Jimenez-Rosales, Alejandra; Johnson, Michael D.; Joshi, Abhishek, V; Jung, Taehyun; Karami, Mansour; Karuppusamy, Ramesh; Kawashima, Tomohisa; Keating, Garrett K.; Kettenis, Mark; Kim, Dong-Jin; Kim, Jae-Young; Kim, Jongsoo; Kim, Junhan; Kino, Motoki; Koay, Jun Yi; Kocherlakota, Prashant; Kofuji, Yutaro; Koch, Patrick M.; Koyama, Shoko; Kramer, Carsten; Kramer, Michael; Kuo, Cheng-Yu; La Bella, Noemi; Lauer, Tod R.; Lee, Daeyoung; Lee, Sang-Sung; Leung, Po Kin; Levis, Aviad; Li, Zhiyuan; Lindahl, Greg; Lindqvist, Michael; Liu, Kuo; Liuzzo, Elisabetta; Lo, Wen-Ping; Lobanov, Andrei P.; Lonsdale, Colin; Mao, Jirong; Marchili, Nicola; Markoff, Sera; Marrone, Daniel P.; Marscher, Alan P.; Matsushita, Satoki; Matthews, Lynn D.; Medeiros, Lia; Menten, Karl M.; Michalik, Daniel; Mizuno, Izumi; Mizuno, Yosuke; Moran, James M.; Mueller, Cornelia; Mus, Alejandro; Musoke, Gibwa; Myserlis, Ioannis; Nadolski, Andrew; Nagai, Hiroshi; Nagar, Neil M.; Nakamura, Masanori; Narayan, Ramesh; Narayanan, Gopal; Natarajan, Iniyan; Nathanail, Antonios; Neilsen, Joey; Neri, Roberto; Ni, Chunchong; Noutsos, Aristeidis; Nowak, Michael A.; Oh, Junghwan; Okino, Hiroki; Olivares, Hector; Ortiz-Leon, Gisela N.; Oyama, Tomoaki; Ozel, Feryal; Palumbo, Daniel C. M.; Paraschos, Georgios Filippos; Park, Jongho; Parsons, Harriet; Patel, Nimesh; Pen, Ue-Li; Pietu, Vincent; Plambeck, Richard; PopStefanija, Aleksandar; Porth, Oliver; Potzl, Felix M.; Prather, Ben; Preciado-Lopez, Jorge A.; Psaltis, Dimitrios; Pu, Hung-Yi; Rao, Ramprasad; Rawlings, Mark G.; Raymond, Alexander W.; Rezzolla, Luciano; Ricarte, Angelo; Ripperda, Bart; Roelofs, Freek; Rogers, Alan; Ros, Eduardo; Romero-Canizales, Cristina; Roshanineshat, Arash; Rottmann, Helge; Roy, Alan L.; Ruiz, Ignacio; Ruszczyk, Chet; Rygl, Kazi L. J.; Sanchez, Salvador; Sanchez-Arguelles, David; Sanchez-Portal, Miguel; Sasada, Mahito; Satapathy, Kaushik; Savolainen, Tuomas; Schloerb, F. Peter; Schuster, Karl-Friedrich; Shao, Lijing; Shen, Zhiqiang; Small, Des; Sohn, Bong Won; SooHoo, Jason; Souccar, Kamal; Sun, He; Tazaki, Fumie; Tetarenko, Alexandra J.; Tilanus, Remo P. J.; Titus, Michael; Torne, Pablo; Trent, Tyler; Trippe, Sascha; van Bemmel, Ilse; van Langevelde, Huib Jan; van Rossum, Daniel R.; Vos, Jesse; Wagner, Jan; Ward-Thompson, Derek; Wardle, John; Weintroub, Jonathan; Wex, Norbert; Wharton, Robert; Wiik, Kaj; Witzel, Gunther; Wondrak, Michael; Wong, George N.; Wu, Qingwen; Yamaguchi, Paul; Yoon, Doosoo; Young, Andre; Young, Ken; Younsi, Ziri; Yuan, Feng; Yuan, Ye-Fei; Zensus, J. Anton; Zhang, Shuo; Zhao, Shan-Shan.The blazar J1924–2914 is a primary Event Horizon Telescope (EHT) calibrator for the Galactic center’s black hole
Sagittarius A*. Here we present the first total and linearly polarized intensity images of this source obtained with
the unprecedented 20 μas resolution of the EHT. J1924–2914 is a very compact flat-spectrum radio source with
strong optical variability and polarization. In April 2017 the source was observed quasi-simultaneously with the
EHT (April 5–11), the Global Millimeter VLBI Array (April 3), and the Very Long Baseline Array (April 28),
giving a novel view of the source at four observing frequencies, 230, 86, 8.7, and 2.3 GHz. These observations
probe jet properties from the subparsec to 100 pc scales. We combine the multifrequency images of J1924–2914 to
study the source morphology. We find that the jet exhibits a characteristic bending, with a gradual clockwise
rotation of the jet projected position angle of about 90° between 2.3 and 230 GHz. Linearly polarized intensity
images of J1924–2914 with the extremely fine resolution of the EHT provide evidence for ordered toroidal
magnetic fields in the blazar compact core.We thank the anonymous reviewer for their thoughtful and
helpful comments. The Event Horizon Telescope Collaboration
thanks the following organizations and programs: the Academy
of Finland (projects 274477, 284495, 312496, 315721); the
Agencia Nacional de Investigación y Desarrollo (ANID), Chile
via NCN19_058 (TITANs) and Fondecyt 3190878, the
Alexander von Humboldt Stiftung; an Alfred P. Sloan Research
Fellowship; Allegro, the European ALMA Regional Centre
node in the Netherlands, the NL astronomy research network
NOVA and the astronomy institutes of the University of
Amsterdam, Leiden University and Radboud University; the
black hole Initiative at Harvard University, through a grant
(60477) from the John Templeton Foundation; the China Scholarship Council; Consejo Nacional de Ciencia y Tecnología
(CONACYT, Mexico, projects U0004-246083, U0004-
259839, F0003-272050, M0037-279006, F0003-281692,
104497, 275201, 263356); the Delaney Family via the Delaney
Family John A. Wheeler Chair at Perimeter Institute; Dirección
General de Asuntos del Personal Académico-Universidad
Nacional Autónoma de México (DGAPA-UNAM, projects
IN112417 and IN112820); the European Research Council
Synergy Grant “BlackHoleCam: Imaging the Event Horizon of
Black Holes” (grant 610058); the Generalitat Valenciana
postdoctoral grant APOSTD/2018/177 and GenT Program
(project CIDEGENT/2018/021); MICINN Research Project
PID2019-108995GB-C22; the Gordon and Betty Moore
Foundation (grant GBMF-3561); the Istituto Nazionale di
Fisica Nucleare (INFN) sezione di Napoli, iniziative specifiche
TEONGRAV; the International Max Planck Research School
for Astronomy and Astrophysics at the Universities of Bonn
and Cologne; Joint Princeton/Flatiron and Joint Columbia/
Flatiron Postdoctoral Fellowships, research at the Flatiron
Institute is supported by the Simons Foundation; the Japanese
Government (Monbukagakusho: MEXT) Scholarship; the
Japan Society for the Promotion of Science (JSPS) Grant-in-
Aid for JSPS Research Fellowship (JP17J08829); the Key
Research Program of Frontier Sciences, Chinese Academy of
Sciences (CAS, grants QYZDJ-SSW-SLH057, QYZDJSSWSYS008,
ZDBS-LY-SLH011); the Leverhulme Trust Early
Career Research Fellowship; the Max-Planck-Gesellschaft (MPG); the Max Planck Partner Group of the MPG and the
CAS; the MEXT/JSPS KAKENHI (grants 18KK0090,
JP18K13594, JP18K03656, JP18H03721, 18K03709,
18H01245, 25120007); the Malaysian Fundamental Research
Grant Scheme (FRGS) FRGS/1/2019/STG02/UM/02/6; the
MIT International Science and Technology Initiatives (MISTI)
Funds; the Ministry of Science and Technology (MOST) of
Taiwan (105-2112-M-001-025-MY3, 106-2112-M-001-011,
106-2119- M-001-027, 107-2119-M-001-017, 107-2119-M-
001-020, 107-2119-M-110-005, 108-2112-M-001-048, and
109-2124-M-001-005); the National Aeronautics and Space
Administration (NASA, Fermi Guest Investigator grant
80NSSC20K1567, NASA Astrophysics Theory Program grant
80NSSC20K0527, NASA NuSTAR award 80NSSC20K0645);
the National Institute of Natural Sciences (NINS) of Japan; the
National Key Research and Development Program of China
(grant 2016YFA0400704, 2016YFA0400702); the National
Science Foundation (NSF, grants AST-0096454, AST-
0352953, AST-0521233, AST-0705062, AST-0905844, AST-
0922984, AST-1126433, AST-1140030, DGE-1144085, AST-
1207704, AST-1207730, AST-1207752, MRI-1228509, OPP-
1248097, AST-1310896, AST-1555365,AST-1615796, AST-
1715061, AST-1716327, AST-1903847,AST-2034306); the
Natural Science Foundation of China (grants 11573051,
11633006, 11650110427, 10625314, 11721303, 11725312,
11933007, 11991052, 11991053); a fellowship of China
Postdoctoral Science Foundation (2020M671266); the Natural Sciences and Engineering Research Council of Canada
(NSERC, including a Discovery Grant and the NSERC
Alexander Graham Bell Canada Graduate Scholarships-Doctoral
Program); the National Youth Thousand Talents Program
of China; the National Research Foundation of Korea (the
Global PhD Fellowship Grant: grants NRF-
2015H1A2A1033752, 2015- R1D1A1A01056807, the Korea
Research Fellowship Program: NRF-2015H1D3A1066561, Basic Research Support Grant 2019R1F1A1059721); the
Netherlands Organization for Scientific Research (NWO) VICI
award (grant 639.043.513) and Spinoza Prize SPI 78-409; the
New Scientific Frontiers with Precision Radio Interferometry
Fellowship awarded by the South African Radio Astronomy
Observatory (SARAO), which is a facility of the National
Research Foundation (NRF), an agency of the Department of
Science and Technology (DST) of South Africa; the Onsala
Space Observatory (OSO) national infrastructure, for the
provisioning of its facilities/observational support (OSO
receives funding through the Swedish Research Council under
grant 2017-00648) the Perimeter Institute for Theoretical
Physics (research at Perimeter Institute is supported by the
Government of Canada through the Department of Innovation,
Science and Economic Development and by the Province of
Ontario through the Ministry of Research, Innovation and
Science); the Spanish Ministerio de Economía y Competitividad
(grants PGC2018-098915-B-C21, AYA2016-80889-P,
PID2019-108995GB-C21); the State Agency for Research of
the Spanish MCIU through the “Center of Excellence Severo
Ochoa” award for the Instituto de Astrofísica de Andalucía
(SEV-2017-0709); the Toray Science Foundation; the Consejería
de Economía, Conocimiento, Empresas y Universidad of the
Junta de Andalucía (grant P18-FR-1769), the Consejo Superior
de Investigaciones Científicas (grant 2019AEP112); the US
Department of Energy (USDOE) through the Los Alamos
National Laboratory (operated by Triad National Security, LLC,
for the National Nuclear Security Administration of the USDOE
(Contract 89233218CNA000001); the European Unionʼs Horizon
2020 research and innovation program under grant
agreement No 730562 RadioNet; ALMA North America
Development Fund; the Academia Sinica; Chandra DD7-
18089X and TM6-17006X; the GenT Program (Generalitat
Valenciana) Project CIDEGENT/2018/021. This work used the
Extreme Science and Engineering Discovery Environment
(XSEDE), supported by NSF grant ACI-1548562, and CyVerse,
supported by NSF grants DBI-0735191, DBI-1265383, and
DBI-1743442. XSEDE Stampede2 resource at TACC was
allocated through TG-AST170024 and TG-AST080026N.
XSEDE JetStream resource at PTI and TACC was allocated
through AST170028. The simulations were performed in part on
the SuperMUC cluster at the LRZ in Garching, on the LOEWE
cluster in CSC in Frankfurt, and on the HazelHen cluster at the
HLRS in Stuttgart. This research was enabled in part by support
provided by Compute Ontario (http://computeontario.ca),
Calcul Quebec (http://www.calculquebec.ca) and Compute
Canada (http://www.computecanada.ca). We thank the staff at
the participating observatories, correlation centers, and institutions
for their enthusiastic support. This paper makes use of the
following ALMA data: ADS/JAO.ALMA#2016.1.01154.V
and ADS/JAO.ALMA2016.1.00413.V. ALMA is a partnership
of the European Southern Observatory (ESO; Europe, representing
its member states), NSF, and National Institutes of Natural
Sciences of Japan, together with National Research Council
(Canada), Ministry of Science and Technology (MOST;
Taiwan), Academia Sinica Institute of Astronomy and Astrophysics
(ASIAA; Taiwan), and Korea Astronomy and Space
Science Institute (KASI; Republic of Korea), in cooperationwith
the Republic of Chile. The Joint ALMA Observatory is operated
by ESO, Associated Universities, Inc. (AUI)/NRAO, and the
National Astronomical Observatory of Japan (NAOJ). The
NRAO is a facility of the NSF operated under cooperative
agreement by AUI. APEX is a collaboration between the Max-
Planck-Institut für Radioastronomie (Germany), ESO, and the
Onsala Space Observatory (Sweden). The SMA is a joint project
between the SAO and ASIAA and is funded by the Smithsonian
Institution and the Academia Sinica. The JCMT is operated by
the East Asian Observatory on behalf of the NAOJ, ASIAA, and
KASI, as well as the Ministry of Finance of China, Chinese
Academy of Sciences, and the National Key R&D Program (No.
2017YFA0402700) of China. Additional funding support for the
JCMT is provided by the Science and Technologies Facility
Council (UK) and participating universities in the UK and
Canada. The LMT is a project operated by the Instituto Nacional
de Astrófisica, Óptica, y Electrónica (Mexico) and the University
of Massachusetts at Amherst (USA). The IRAM 30 m telescope
on Pico Veleta, Spain is operated by IRAM and supported by
CNRS (Centre National de la Recherche Scientifique, France),
MPG (Max-Planck- Gesellschaft, Germany) and IGN (Instituto
Geográfico Nacional, Spain). The SMT is operated by the
Arizona Radio Observatory, a part of the Steward Observatory
of the University of Arizona, with financial support of operations
from the State of Arizona and financial support for instrumentation
development from the NSF. Support for SPT participation in
the EHT is provided by the National Science Foundation
through award OPP-1852617 to the University of Chicago.
Partial support is also provided by the Kavli Institute of
Cosmological Physics at the University of Chicago. The SPT
hydrogen maser was provided on loan from the GLT, courtesy
of ASIAA. The EHTC has received generous donations of
FPGA chips from Xilinx Inc., under the Xilinx University
Program. The EHTC has benefited from technology shared under open-source license by the Collaboration for Astronomy
Signal Processing and Electronics Research (CASPER). The
EHT project is grateful to T4Science and Microsemi for their
assistance with Hydrogen Masers. This research has made use of
NASAʼs Astrophysics Data System. We gratefully acknowledge
the support provided by the extended staff of the ALMA, both
from the inception of the ALMA Phasing Project through the
observational campaigns of 2017 and 2018. We would like to
thank A. Deller and W. Brisken for EHT-specific support with
the use of DiFX. We acknowledge the significance that
Maunakea, where the SMA and JCMT EHT stations are located,
has for the indigenous Hawaiian people.
We also thank Alexandra Elbakyan for her contributions to
the open science initiative. This research has made use of data
obtained with the Global Millimeter VLBI Array (GMVA),
coordinated by the VLBI group at the Max-Planck-Institut für
Radioastronomie (MPIfR). The GMVA consists of telescopes
operated by MPIfR, IRAM, Onsala, Metsahovi, Yebes, the
Korean VLBI Network, the Green Bank Observatory, and the
Very Long Baseline Array (VLBA). The VLBA and the GBT
are facilities of the National Science Foundation under
cooperative agreement by Associated Universities, Inc. The
data were correlated at the DiFX correlator of the MPIfR in
Bonn, Germany. We thank the National Science Foundation
(awards OISE-1743747, AST-1816420, AST-1716536, AST-
1440254, AST-1935980) and the Gordon and Betty Moore
Foundation (GBMF-5278) for financial support of this work.
Support for this work was also provided by the NASA Hubble
Fellowship grant HST-HF2-51431.001-A awarded by the
Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc.,
for NASA, under contract NAS5-26555.http://iopscience.iop.org/0004-637Xam2023Physic
Selective dynamical imaging of interferometric data
Recent developments in very long baseline interferometry (VLBI) have made it possible for the Event Horizon
Telescope (EHT) to resolve the innermost accretion flows of the largest supermassive black holes on the sky. The
sparse nature of the EHT’s (u, v)-coverage presents a challenge when attempting to resolve highly time-variable
sources. We demonstrate that the changing (u, v)-coverage of the EHT can contain regions of time over the course
of a single observation that facilitate dynamical imaging. These optimal time regions typically have projected
baseline distributions that are approximately angularly isotropic and radially homogeneous. We derive a metric of
coverage quality based on baseline isotropy and density that is capable of ranking array configurations by their
ability to produce accurate dynamical reconstructions. We compare this metric to existing metrics in the literature
and investigate their utility by performing dynamical reconstructions on synthetic data from simulated EHT
observations of sources with simple orbital variability. We then use these results to make recommendations for
imaging the 2017 EHT Sgr A* data sethttp://iopscience.iop.org/2041-8205Physic
The Polarized Image of a Synchrotron-emitting Ring of Gas Orbiting a Black Hole
Abstract: Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black hole. By using an approximate expression for the null geodesics derived by Beloborodov and conservation of the Walker–Penrose constant, we provide analytic estimates for the image polarization. We test this model using currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions, such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent with the clockwise rotation inferred from total intensity images
Polarimetric Properties of Event Horizon Telescope Targets from ALMA
We present the results from a full polarization study carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) during the first Very Long Baseline Interferometry (VLBI) campaign, which was conducted in 2017 April in the λ3 mm and λ1.3 mm bands, in concert with the Global mm-VLBI Array (GMVA) and the Event Horizon Telescope (EHT), respectively. We determine the polarization and Faraday properties of all VLBI targets, including Sgr A*, M87, and a dozen radio-loud active galactic nuclei (AGNs), in the two bands at several epochs in a time window of 10 days. We detect high linear polarization fractions (2%–15%) and large rotation measures (RM > 103.3–105.5 rad m−2), confirming the trends of previous AGN studies at millimeter wavelengths. We find that blazars are more strongly polarized than other AGNs in the sample, while exhibiting (on average) order-of-magnitude lower RM values, consistent with the AGN viewing angle unification scheme. For Sgr A* we report a mean RM of (−4.2 ± 0.3) × 105 rad m−2 at 1.3 mm, consistent with measurements over the past decade and, for the first time, an RM of (–2.1 ± 0.1) × 105 rad m−2 at 3 mm, suggesting that about half of the Faraday rotation at 1.3 mm may occur between the 3 mm photosphere and the 1.3 mm source. We also report the first unambiguous measurement of RM toward the M87 nucleus at millimeter wavelengths, which undergoes significant changes in magnitude and sign reversals on a one year timescale, spanning the range from −1.2 to 0.3 × 105 rad m−2 at 3 mm and −4.1 to 1.5 × 105 rad m−2 at 1.3 mm. Given this time variability, we argue that, unlike the case of Sgr A*, the RM in M87 does not provide an accurate estimate of the mass accretion rate onto the black hole. We put forward a two-component model, comprised of a variable compact region and a static extended region, that can simultaneously explain the polarimetric properties observed by both the EHT (on horizon scales) and ALMA (which observes the combined emission from both components). These measurements provide critical constraints for the calibration, analysis, and interpretation of simultaneously obtained VLBI data with the EHT and GMVA
Selective Dynamical Imaging of Interferometric Data
Recent developments in very long baseline interferometry (VLBI) have made it possible for the Event Horizon Telescope (EHT) to resolve the innermost accretion flows of the largest supermassive black holes on the sky. The sparse nature of the EHT's (u, v)-coverage presents a challenge when attempting to resolve highly time-variable sources. We demonstrate that the changing (u, v)-coverage of the EHT can contain regions of time over the course of a single observation that facilitate dynamical imaging. These optimal time regions typically have projected baseline distributions that are approximately angularly isotropic and radially homogeneous. We derive a metric of coverage quality based on baseline isotropy and density that is capable of ranking array configurations by their ability to produce accurate dynamical reconstructions. We compare this metric to existing metrics in the literature and investigate their utility by performing dynamical reconstructions on synthetic data from simulated EHT observations of sources with simple orbital variability. We then use these results to make recommendations for imaging the 2017 EHT Sgr A* data set
The polarized image of a synchrotron-emitting ring of gas orbiting a black hole
Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is
determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of
the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore
these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black
hole. By using an approximate expression for the null geodesics derived by Beloborodov and conservation of the
Walker–Penrose constant, we provide analytic estimates for the image polarization. We test this model using
currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by
the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces
the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions,
such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can
reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under
the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent
with the clockwise rotation inferred from total intensity images.http://iopscience.iop.org/0004-637Xam2023Physic
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