Search for: All records

Abstract The Event Horizon Telescope (EHT) is preparing to produce time sequences of black hole images, or videos. In anticipation, we previously developed an autocorrelation technique to measure apparent rotational motion using the image-domain pattern speed Ω_p. Here, we extend this technique to the visibility domain and introduce the visibility amplitude pattern speed Ω_VA. We show that in the Illinois v3 library of EHT source models, Ω_VAdepends on the source inclination, black hole mass, black hole spin, accretion state (MAD or SANE), and baseline length, and then provide approximate fits for this dependence. We show that Ω_VAis particularly sensitive to baseline length for MAD (strongly magnetized) models, and that the slope of this dependence can be used to constrain black hole spin. As with Ω_p, models predict that Ω_VAis well below the Keplerian frequency in the emission region for all model parameters. This is consistent with the idea that Ω_VAmeasures an angular phase speed for waves propagating through the emission region. Finally, we identify the information that would be provided by space-based millimeter very long baseline interferometry such as the proposed BHEX mission. 

Abstract We continue our previous work, H.-S. Chan et al., to investigate how variations in the electron temperature prescription parameter,R_Low, influence the 3 hr variability at 230 GHz,M_ΔT, in magnetically arrested disk (MAD) models of Sagittarius A* (Sgr A*), through analyzing a series of general-relativistic magnetohydrodynamics and ray-tracing simulations. For models with a black hole spina > 0, we discovered that increasingR_Lowrenders the photon ring more optically thick, obscuring the varying accretion flows that contribute to the variability. However, asR_Lowincreases further, MAD flux eruptions become more pronounced, compensating for the decrease inM_ΔT. For models with spina < 0, although a higherR_Lowalso increases the optical thickness of the fluid, voids within the optically thick gas fail to cover the entire photon ring. Similarly, flux eruptions become more prominent asR_Lowincreases further, contributing to the observed rise inM_ΔTrelative toR_Low. For black holes with spina= 0, although the effect of increasing optical depth is still present, their 230 GHz light curves, and henceM_ΔT, are insensitive to changes inR_Low. Furthermore, we found that the variability of the 230 GHz light curves atR_Low = 1 might correlate with fluctuations in the internal energy of the gas near the black hole, and we listed potential causes and solutions to the over-variability problem. Our findings highlight potential approaches for refiningM_ΔTto better align with observations when modeling Sgr A*. 

Abstract We introduceMahakala, aPython-based, modular, radiative ray-tracing code for curved spacetimes. We employ Google’sJAXframework for accelerated automatic differentiation, which can efficiently compute Christoffel symbols directly from the metric, allowing the user to easily and quickly simulate photon trajectories through non-Kerr spacetimes.JAXalso enablesMahakalato run in parallel on both CPUs and GPUs.Mahakalanatively uses the Cartesian Kerr–Schild coordinate system, which avoids numerical issues caused by the pole in spherical coordinate systems. We demonstrateMahakala’s capabilities by simulating 1.3 mm wavelength images (the wavelength of Event Horizon Telescope observations) of general relativistic magnetohydrodynamic simulations of low-accretion rate supermassive black holes. The modular nature ofMahakalaallows us to quantitatively explore how different regions of the flow influence different image features. We show that most of the emission seen in 1.3 mm images originates close to the black hole and peaks near the photon orbit. We also quantify the relative contribution of the disk, forward jet, and counterjet to 1.3 mm images. 

Abstract Models of the resolved Event Horizon Telescope (EHT) sources Sgr A* and M87* are constrained by observations at multiple wavelengths, resolutions, polarizations, and time cadences. In this paper, we compare unresolved circular polarization (CP) measurements to a library of models, where each model is characterized by a distribution of CP over time. In the library, we vary the spin of the black hole, the magnetic field strength at the horizon (i.e., both SANE and magnetically arrested disk or MAD models), the observer inclination, a parameter for the maximum ion–electron temperature ratio assuming a thermal plasma, and the direction of the magnetic field dipole moment. We find that Atacama Large Millimeter/submillimeter Array (ALMA) observations of Sgr A* are inconsistent with all edge-on (i= 90°) models. Restricting attention to the MAD models favored by earlier EHT studies of Sgr A*, we find that only models with magnetic dipole moment pointing away from the observer are consistent with ALMA data. We also note that in 26 of the 27 passing MAD models, the accretion flow rotates clockwise on the sky. We provide a table of the means and standard deviations of the CP distributions for all model parameters, along with their trends. 

Models for the observational appearance of astrophysical black holes rely critically on accurate general-relativistic ray tracing and radiation transport to compute the intensity measured by a distant observer. In this paper, we illustrate how the choice of coordinates and initial conditions affect this process. In particular, we show that propagating rays from the camera to the source leads to different solutions if the spatial part of the momentum of the photon points towards the horizon or away from it. In doing this, we also show that coordinates that are well suited for numerical general-relativistic magnetohydrodynamic (GRMHD) simulations are typically not optimal for generic ray tracing. We discuss the implications for black hole images and show that radiation transport in optimal and nonoptimal spacetime coordinates lead to the same images up to numerical errors and algorithmic choices. 

Abstract Direct imaging of supermassive black holes (SMBHs) at event horizon-scale resolutions, as recently done by the Event Horizon Telescope, allows for testing alternative models to SMBHs such as Kerr naked singularities (KNSs). We demonstrate that the KNS shadow can be closed, open, or vanishing, depending on the spins and observational inclination angles. We study the critical parameters where the KNS shadow opens a gap, a distinctive phenomenon that does not happen with the black hole shadow. We show that the KNS shadow can only be closed for dimensionless spina≲ 1.18 and vanishing fora≳ 1.18 for certain ranges of inclination angles. We further analyze the effective angular momentum of photon orbits to demonstrate the fundamental connections between light geodesics and the KNS shadow geometry. We also perform numerical general relativistic ray-tracing calculations, which reproduce the analytical topological change in the KNS shadow, and illustrate other observational features within the shadow due to the lack of an event horizon. By comparing the geometric features of the KNS shadow with black hole shadow observations, the topological change in the shadow of KNSs can be used to test the cosmic censorship hypothesis and KNSs as alternative models to SMBHs. 

Abstract The Event Horizon Telescope (EHT) has produced images of M87* and Sagittarius A*, and will soon produce time sequences of images, or movies. In anticipation of this, we describe a technique to measure the rotation rate, or pattern speed Ω_p, from movies using an autocorrelation technique. We validate the technique on Gaussian random field models with a known rotation rate and apply it to a library of synthetic images of Sgr A* based on general relativistic magnetohydrodynamics simulations. We predict that EHT movies will have Ω_p≈ 1° perGMc⁻³, which is of order 15% of the Keplerian orbital frequency in the emitting region. We can plausibly attribute the slow rotation seen in our models to the pattern speed of inward-propagating spiral shocks. We also find that Ω_pdepends strongly on inclination. Application of this technique will enable us to compare future EHT movies with the clockwise rotation of Sgr A* seen in near-infrared flares by GRAVITY. Pattern speed analysis of future EHT observations of M87* and Sgr A* may also provide novel constraints on black hole inclination and spin, as well as an independent measurement of black hole mass. 

Abstract The Event Horizon Telescope recently captured images of the supermassive black hole in the center of the M87 galaxy, which shows a ring-like emission structure with the south side only slightly brighter than the north side. This relatively weak asymmetry in the brightness profile along the ring has been interpreted as a consequence of the low inclination of the observer (around 17° for M87), which suppresses the Doppler beaming and boosting effects that might otherwise be expected due to the nearly relativistic velocities of the orbiting plasma. In this work, we use a large suite of general relativistic magnetohydrodynamic simulations to reassess the validity of this argument. By constructing explicit counterexamples, we show that low inclination is a sufficient but not necessary condition for images to have low brightness asymmetry. Accretion flow models with high accumulated magnetic flux close to the black hole horizon (the so-called magnetically arrested disks) and low black hole spins have angular velocities that are substantially smaller than the orbital velocities of test particles at the same location. As a result, such models can produce images with low brightness asymmetry even when viewed edge on.