Abstract In this paper we provide a first physical interpretation for the Event Horizon Telescope's (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 μ m, and in the X-ray. All models fail at least 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 M ⊙ yr −1 , bolometric luminosity (6.8–9.2) × 10 35 erg s −1 , and outflow power (1.3–4.8) × 10 38 erg s −1 . We also find 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 μ m 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.
more »
« less
The Effects of Tilt on the Time Variability of Millimeter and Infrared Emission from Sagittarius A*
Using a combination of general-relativistic magnetohydrodynamics simulations and ray tracing of synchrotron emission, we study the effect of modest (24°) misalignment between the black hole spin and plasma angular momentum, focusing on the variability of total flux, image centroids, and image sizes. We consider both millimeter and infrared (IR) observables motivated by Sagittarius A* (Sgr A*), though our results apply more generally to optically thin flows. For most quantities, tilted accretion is more variable, primarily due to a significantly hotter and denser
- NSF-PAR ID:
- 10362948
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 926
- Issue:
- 2
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 136
- Size(s):
- ["Article No. 136"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
ABSTRACT Recent observations with mm very long baseline interferometry (mm-VLBI) and near-infrared (NIR) interferometry provide mm images and NIR centroid proper motion for Sgr A*. Of particular interest are the NIR flares that have more than an order of magnitude higher flux density than the quiescent state. Here, we model the flares using time-dependent, axisymmetric, general relativistic magnetohydrodynamic (GRMHD) simulations with an electron distribution function that includes a small, variable, non-thermal component motivated by magnetic reconnection models. The models simultaneously match the observed mm mean flux density, mm image size, NIR quiescent flux density, NIR flare flux density, and NIR spectral slope. They also provide a better fit to the observed NIR flux density probability density function than previously reported models by reproducing the power-law tail at high flux density, though with some discrepancy at low flux density. Further, our modelled NIR image centroid shows very little movement: centroid excursions of more than 10 μas (the resolution of GRAVITY) are rare and uncorrelated with flux.more » « less
-
Abstract 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 μ as (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 μ as, 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 × 10 6 M ⊙ .more » « less
-
more » « less
Abstract Sgr A* is the variable electromagnetic source associated with accretion onto the Galactic center supermassive black hole. While the near-infrared (NIR) variability of Sgr A* was shown to be consistent over two decades, unprecedented activity in 2019 challenges existing statistical models. We investigate the origin of this activity by recalibrating and reanalyzing all of our Keck Observatory Sgr A* imaging observations from 2005–2022. We present light curves from 69 observation epochs using the NIRC2 imager at 2.12
μ m with laser-guide star adaptive optics. These observations reveal that the mean luminosity of Sgr A* increased by a factor of ∼3 in 2019, and the 2019 light curves had higher variance than in all time periods we examined. We find that the 2020–2022 flux distribution is statistically consistent with the historical sample and model predictions, but with fewer bright measurements above 0.6 mJy at the ∼2σ level. Since 2019, we have observed a maximumK s (2.2μ m) flux of 0.9 mJy, compared to the highest pre-2019 flux of 2.0 mJy and highest 2019 flux of 5.6 mJy. Our results suggest that the 2019 activity was caused by a temporary accretion increase onto Sgr A*, possibly due to delayed accretion of tidally stripped gas from the gaseous object G2 in 2014. We also examine faint Sgr A* fluxes over a long time baseline to search for a quasi-steady quiescent state. We find that Sgr A* displays flux variations over a factor of ∼500, with no evidence for a quiescent state in the NIR. -
more » « less
Abstract A complete census of dusty star-forming galaxies (DSFGs) at early epochs is necessary to constrain the obscured contribution to the cosmic star formation rate density (CSFRD); however, DSFGs beyond
z ∼ 4 are both rare and hard to identify from photometric data alone due to degeneracies in submillimeter photometry with redshift. Here, we present a pilot study obtaining follow-up Atacama Large Millimeter Array (ALMA) 2 mm observations of a complete sample of 39 850μ m-bright dusty galaxies in the SSA22 field. Empirical modeling suggests 2 mm imaging of existing samples of DSFGs selected at 850μ m—1 mm can quickly and easily isolate the “needle in a haystack” DSFGs that sit atz > 4 or beyond. Combining archival submillimeter imaging with our measured ALMA 2 mm photometry (1σ ∼ 0.08 mJy beam−1rms), we characterize the galaxies’ IR spectral energy distributions (SEDs) and use them to constrain redshifts. With available redshift constraints fit via the combination of six submillimeter bands, we identify 6/39 high-z candidates each with >50% likelihood to sit atz > 4, and find a positive correlation between redshift and 2 mm flux density. Specifically, our models suggest the addition of 2 mm to a moderately constrained IR SED will improve the accuracy of a millimeter-derived redshift from Δz /(1 +z ) = 0.3 to Δz /(1 +z ) = 0.2. Our IR SED characterizations provide evidence for relatively high-emissivity spectral indices (〈β 〉 = 2.4 ± 0.3) in the sample. We measure that especially bright (S 850μ m > 5.55 mJy) DSFGs contribute ∼10% to the cosmic-averaged CSFRD from 2 <z < 5, confirming findings from previous work with similar samples.
