In Preparation
Galison, Peter, and Caroline A. Jones. Invisibilities: Seeing and Unseeing the Anthropocene, In Preparation.
Galison, Peter, Juliusz Doboszewski, Jamee Elder, Niels C.M. Martens, Abhay Ashtekar, Jonas Enander, Marie Gueguen, and et al. “The Next Generation Event Horizon Telescope Collaboration: History, Philosophy, and Culture.” Galaxies (Submitted).
Galison, Peter, and Moritz Neumüller. “Visualisation as a Political Act.” In The Routledge Companion to Photography, Representation and Social Justice, edited by Moritz Neumüller. London: Routledge, Forthcoming.
Galison, Peter. Dream of a Shadow. USA, 2023.Abstract
My world was jolted by two shadow images: one, thrilling, the other terrifying.  After a years-long effort—with 200 other scientists—we made the first image of a black hole, its shadow of no return.  Then I fell into debilitating pain. A deadly shadow blot appeared on an MRI of my spine.  Faced with emergency surgery and no assurance of success, I sought comfort in memory images from the past and, from collaborative work that engrossed me: images of the whole visible universe could be stored in light circling a black hole.  It was a universal memory. An experimental back and forth between the innermost-personal and the astronomical, where shadows and consolation cross.
Galison, Peter, Michael D. Johnson, Paul Tiede, and Daniel C. M. Palumbo. “What is in a Detection? Space VLBI Photon Ring Hunt,” 2022.
Galison, Peter. “Foreword: What Are Technical Lands?” In Technical Lands: A Critical Primer, edited by Jeffrey S. Nesbit and Charles Waldheim, 18-27. JOVIS, 2022. Publisher's Version
Kurczynski, Peter, Michael D. Johnson, Sheperd S. Doeleman, Kari Haworth, Eliad Peretz, Tirupati Kumara Sridharan, Byran Bilyeu, and et al. “The Event Horizon Explorer mission concept.” In Space Telescopes and Instrumentation 2022: Optical, Infrared, and Millimeter Wave, edited by Laura E. Coyle, Shuji Matsuura, and Marshall D. Perrin. Montréal, Québec, Canada: Proc. of SPIE 12180, 2022. Publisher's VersionAbstract
The Event Horizon Explorer (EHE) is a mission concept to extend the Event Horizon Telescope via an additional space-based node. We provide highlights and overview of a concept study to explore the feasibility of such a mission. We present science goals and objectives, which include studying the immediate environment around supermassive black holes, and focus on critical enabling technologies and engineering challenges. We provide an assessment of their technological readiness and overall suitability for a NASA Medium Explorer (MIDEX) class mission.
Broderick, Avery E., Dominic W. Pesce, Roman Gold, Paul Tiede, Hung-Yi Pu, Richard Anantua, Silke Britzen, and et al. “The Photon Ring in M87*.” The Astrophysical Journal 935 (2022): 61. Publisher's VersionAbstract
We report measurements of the gravitationally lensed secondary image—the first in an infinite series of so-called “photon rings”—around the supermassive black hole M87* via simultaneous modeling and imaging of the 2017 Event Horizon Telescope (EHT) observations. The inferred ring size remains constant across the seven days of the 2017 EHT observing campaign and is consistent with theoretical expectations, providing clear evidence that such measurements probe spacetime and a striking confirmation of the models underlying the first set of EHT results. The residual diffuse emission evolves on timescales comparable to one week. We are able to detect with high significance a southwestern extension consistent with that expected from the base of a jet that is rapidly rotating in the clockwise direction. This result adds further support to the identification of the jet in M87* with a black hole spin-driven outflow, launched via the Blandford–Znajek process. We present three revised estimates for the mass of M87* based on identifying the modeled thin ring component with the bright ringlike features seen in simulated images, one of which is only weakly sensitive to the astrophysics of the emission region. All three estimates agree with each other and previously reported values. Our strongest mass constraint combines information from both the ring and the diffuse emission region, which together imply a mass-to-distance ratio of m -+ 4.20 as 0.06 0.12 and a corresponding black hole mass of (7.13 ± 0.39) × 109 Me,wherethe error on the latter is now dominated by the systematic uncertainty arising from the uncertain distance to M87*.
Farah, Joseph, Peter Galison, Kazunori Akiyama, Katherine L. Bouman, Geoffrey C. Bower, Andrew Chael, Antonio Fuentes, and et al. “Selective Dynamical Imaging of Interferometric Data.” The Astrophysical Journal Letters 930 (2022): L18. Publisher's VersionAbstract
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.
Tiede, Paul, Michael D. Johnson, Dominic W. Pesce, Daniel C. M. Palumbo, Dominic O. Chang, and Peter Galison. “Measuring Photon Rings with the ngEHT.” Galaxies 10, no. 6 (2022): 111. Publisher's VersionAbstract
General relativity predicts that images of optically thin accretion flows around black holes should generically have a “photon ring”, composed of a series of increasingly sharp subrings that correspond to increasingly strongly lensed emission near the black hole. Because the effects of lensing are determined by the spacetime curvature, the photon ring provides a pathway to precise measurements of the black hole properties and tests of the Kerr metric. We explore the prospects for detecting and measuring the photon ring using very long baseline interferometry (VLBI) with the Event Horizon Telescope (EHT) and the next-generation EHT (ngEHT). We present a series of tests using idealized self-fits to simple geometrical models and show that the EHT observations in 2017 and 2022 lack the angular resolution and sensitivity to detect the photon ring, while the improved coverage and angular resolution of ngEHT at 230 GHz and 345 GHz is sufficient for these models. We then analyze detection prospects using more realistic images from general relativistic magnetohydrodynamic simulations by applying “hybrid imaging”, which simultaneously models two components: a flexible raster image (to capture the direct emission) and a ring component. Using the Bayesian VLBI modeling package Comrade.jl, we show that the results of hybrid imaging must be interpreted with extreme caution for both photon ring detection and measurement—hybrid imaging readily produces false positives for a photon ring, and its ring measurements do not directly correspond to the properties of the photon ring.
Collaboration, Event Horizon Telescope. “First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric.” The Astrophysical Journal Letters 930 (2022): L17. Publisher's VersionAbstract
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.
Collaboration, Event Horizon Telescope. “First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole.” The Astrophysical Journal Letters 930 (2022): L16. Publisher's VersionAbstract
In this paper we provide a first physical interpretation for the Event Horizon Telescopeʼs (EHT) 2017 observations of Sgr A*. Our main approach is to compare resolved EHT data at 230 GHz and unresolved non-EHT observations from radio to X-ray wavelengths to predictions from a library of models based on time-dependent general relativistic magnetohydrodynamics simulations, including aligned, tilted, and stellar-wind-fed simulations; radiative transfer is performed assuming both thermal and nonthermal electron distribution functions. We test the models against 11 constraints drawn from EHT 230 GHz data and observations at 86 GHz, 2.2 μm,andintheX-ray.Allmodelsfailatleast one constraint. Light-curve variability provides a particularly severe constraint, failing nearly all strongly magnetized (magnetically arrested disk (MAD)) models and a large fraction of weakly magnetized models. A number of models fail only the variability constraints. We identify a promising cluster of these models, which are MAD and have inclination i „ 30°. They have accretion rate (5.2–9.5) × 10−9 Me yr−1, bolometric luminosity (6.8–9.2) × 1035 erg s−1,and outflow power (1.3–4.8) × 1038 erg s−1.Wealsofind that all models with i … 70° fail at least two constraints, as do all models with equal ion and electron temperature; exploratory, nonthermal model sets tend to have higher 2.2 μm flux density; and the population of cold electrons is limited by X-ray constraints due to the risk of bremsstrahlung overproduction. Finally, we discuss physical and numerical limitations of the models, highlighting the possible importance of kinetic effects and duration of the simulations.
Collaboration, Event Horizon Telescope. “First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass.” The Astrophysical Journal Letters 930 (2022): L15. Publisher's VersionAbstract
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 ringlike 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 10 0.6 1.1 6 Me.
Collaboration, Event Horizon Telescope. “First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole.” The Astrophysical Journal Letters 930 (2022): L14. Publisher's VersionAbstract
We present the first event-horizon-scale images and spatiotemporal analysis of Sgr A* taken with the Event Horizon Telescope in 2017 April at a wavelength of 1.3 mm. Imaging of Sgr A* has been conducted through surveys over a wide range of imaging assumptions using the classical CLEAN algorithm, regularized maximum likelihood methods, and a Bayesian posterior sampling method. Different prescriptions have been used to account for scattering effects by the interstellar medium toward the Galactic center. Mitigation of the rapid intraday variability that characterizes Sgr A* has been carried out through the addition of a “variability noise budget” in the observed visibilities, facilitating the reconstruction of static full-track images. Our static reconstructions of Sgr A* can be clustered into four representative morphologies that correspond to ring images with three different azimuthal brightness distributions and a small cluster that contains diverse nonring morphologies. Based on our extensive analysis of the effects of sparse (u, v)-coverage, source variability, and interstellar scattering, as well as studies of simulated visibility data, we conclude that the Event Horizon Telescope Sgr A* data show compelling evidence for an image that is dominated by a bright ring of emission with a ring diameter of ∼50 μas, consistent with the expected “shadow” of a 4 × 106 Me black hole in the Galactic center located at a distance of 8 kpc.
Collaboration, Event Horizon Telescope. “First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multiwavelength Observations, Data Processing, and Calibration.” The Astrophysical Journal Letters 930 (2022): L13. Publisher's VersionAbstract
We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio source located at the position of the supermassive black hole Sagittarius A* (Sgr A*), collected during the 2017 April 5–11 campaign. The observations were carried out with eight facilities at six locations across the globe. Novel calibration methods are employed to account for Sgr A*ʼs flux variability. The majority of the 1.3 mm emission arises from horizon scales, where intrinsic structural source variability is detected on timescales of minutes to hours. The effects of interstellar scattering on the image and its variability are found to be subdominant to intrinsic source structure. The calibrated visibility amplitudes, particularly the locations of the visibility minima, are broadly consistent with a blurred ring with a diameter of ∼50 μas, as determined in later works in this series. Contemporaneous multiwavelength monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near-infrared and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by Chandra, one at low significance jointly with Swift on 2017 April 7 and the other at higher significance jointly with NuSTAR on 2017 April 11. The brighter April 11 flare is not observed simultaneously by the EHT but is followed by a significant increase in millimeter flux variability immediately after the X-ray outburst, indicating a likely connection in the emission physics near the event horizon. We compare Sgr A*’s broadband flux during the EHT campaign to its historical spectral energy distribution and find that both the quiescent emission and flare emission are consistent with its long-term behavior.
Collaboration, Event Horizon Telescope. “First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way.” The Astrophysical Journal Letters 930 (2022): L12. Publisher's VersionAbstract
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 ± 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 × 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°), 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.
Shattering Stars
Galison, Peter. Shattering Stars. USA, 2021.Abstract
1930, Madras, India. On a steamship to England, 19-year-old Indian physics prodigy, Subrahmanyan Chandrasekhar makes a paradigm-shattering discovery about the life cycle of stars: a discipline pioneered by Sir Arthur Eddington, the world’s most famous astronomer and Chandra’s new advisor. After years of work under his mentor's watchful eye, Chandra presents at a public forum, only for Eddington to denounce his protégé and his work. A half-century later, after a more conservative career, Chandra is awarded the Nobel Prize for the work begun on board that ship to England. Grounded in his recorded memories, “Shattering Stars” alternates between realistic and imaginary animation.
Galison, Peter. “The Great Speakers of the Vienna Circle: Rudolph Carnap, Herbert Feigl, Otto Neurath, Hans Reichenbach.” In Hannes Meyer's New Bauhaus Pedagogy, edited by Philipp Oswalt, 311-323. Leipzig: Spector Books, 2021. the_great_speakers_of_the_vienna_circle_galison.pdf
Galison, Peter. “How do you photograph a black hole?MoMA Magazine, 2021. Publisher's Version how_do_you_photograph_a_black_hole_magazine_moma.pdf
Galison, Peter, and Winifred Elysse Newman. “Interview with Peter Galison: On Method.” Technology | Architecture + Design 5, no. 1 (2021): 5-9. Publisher's Version interview_with_peter_galison_on_method.pdf