arXiv subjects

Jason Dexter

Publications and source records attributed to Jason Dexter.

At least 19 recordsLinked to original sources

arXiv2026

Dynamics and geometry of the inner sub-parsec-scale jet in 3C 279 observed with the Event Horizon Telescope

The 2021 Event Horizon Telescope observations resolve the innermost jet region of the blazar 3C279 with unprecedented detail. The reconstructed images consistently reveal a compact core elongated nearly orthogonal to the large-scale jet axis. This rarely observed morphology recurs across multiple epochs and from 22-230 GHz and is therefore intrinsic rather than an imaging artifact. Geometric model fitting identifies several components with apparent speeds up to 10c, requiring bulk Lorentz factors greater than 10.3 and constraining viewing angles to extremely small values (smaller than one degree). Rest-frame brightness temperatures are systematically low (between 10^9 and 10^10 K), consistent with optically thin emission at 230 GHz. These results suggest that the jet bends toward the observer on sub-parsec scales, producing strong relativistic beaming. Possible drivers of the observed jet bending and temporal evolution include the jet's interaction with the interstellar medium, kink or Kelvin--Helmholtz instabilities, magnetic reconnection near the horizon, or binary-induced precession. However, the current temporal coverage of VLBI data remains insufficient to distinguish between these mechanisms. Continued multifrequency VLBI monitoring will be essential to constraining the dynamics and geometry of the jet base in 3C279.

astro-ph.GA↗

arXiv2026

Broadband multiwavelength properties of the archetypal blazar 3C 279 during the 2017 Event Horizon Telescope campaign

The archetypal blazar 3C 279 hosts a prominent relativistic jet and exhibits strong broadband variability across the electromagnetic spectrum. In April 2017, the Event Horizon Telescope (EHT) observed 3C 279, alongside one of the most extensive quasi-simultaneous multiwavelength (MWL) campaigns ever conducted. With the aim of investigating the physical processes governing 3C 279, we analyzed individual observations and multiband light curves, and constructed a new quasi-simultaneous MWL spectrum. We also performed phenomenological modeling using the turbulent extreme multi-zone (TEMZ) model to constrain the fundamental physical properties of the source. The EHT observations reveal a clear flux increase in the innermost core between April 5 and 11, 2017. Over a broader timescale, radio measurements at longer wavelengths show concurrent enhancements in core flux and polarization around mid-April, coinciding with the ejection of a superluminal knot. Record UV-optical flares with strong polarization variability occurred in late March, followed by gamma-ray activity that declined before the end of the EHT observing period. During this interval, the source remained in a low X-ray state and showed no detectable VHE emission. The TEMZ modeling suggests that the broadband spectrum and variability of 3C 279 can be explained within a jet scenario in which turbulent plasma cells are compressed by a stationary conical shock. However, alternative interpretations, such as magnetic reconnection or a moving shock-in-jet event, remain plausible. This coordinated MWL campaign advances our understanding of the origin of jet and gamma-ray emission in 3C 279, while also providing a comprehensive publicly available dataset that will serve as a valuable reference for future studies.

astro-ph.HE↗

arXiv2026

Constraints on the Gas Geometry Surrounding Little Red Dots through Narrow-Line Diagnostics

Little Red Dots (LRDs) are a recently identified population of high-redshift sources, with a common interpretation being accreting black holes embedded within a spherical, optically thick gas envelope. Within this framework, some models propose that the continuum arises from the dense-gas envelope, where hard ionizing radiation from the central engine is reprocessed into a stellar-like photosphere with an effective temperature of $\sim$5000 K. This implies that both the UV continuum and narrow-line emission are then powered by the host galaxy rather than an exposed central engine. To test whether this is consistent with the observed narrow-line ratios, we analyze multiple line diagnostics for a sample of $\sim$20 LRDs with high signal-to-noise NIRSpec grating spectra. We find that at least 40\% of the LRDs have line ratios pointing toward high ionization parameter and electron temperature, with a further 15\% also falling in the AGN regime for the O\textsc{i}/H$\alpha$ diagnostic, indicative of harder ionizing radiation. These line ratios are incompatible with stellar photoionization from a star-forming host alone. This suggests lower density channels within the gas envelope through which high energy photons can escape and excite the surrounding narrow-line emitting gas. At the same time, most LRDs lack strong high-ionization line emission, with He\,\textsc{ii}/H$\beta$ $\lesssim0.1$, consistent with an ionizing spectrum softer than that of a standard AGN. Together, these results disfavour a uniform gas envelope with a covering fraction of unity, and instead point to a more complex geometry that gives rise to anisotropic ionizing radiation.

astro-ph.GA↗

arXiv2026

Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign

Sagittarius A* (Srg A*), the supermassive black hole at the center of the Milky Way, provides a unique laboratory to study accretion dynamics and plasma processes near the event horizon. We investigated the variability and polarization properties of Srg A* using ALMA observations during the 2018 Event Horizon Telescope campaign. We analyzed high-cadence full-polarization light curves from ALMA at millimeter wavelengths, performed time-series analysis, and investigated the temporal behavior during an X-ray flare observed by Chandra on 2018 April 24. The variability characteristics are compared with expectations from standard accretion flow models. We find low variability in total intensity ($\sigma/\mu < 10\%$), but significantly higher variability in linear and circular polarization (~ 30% and ~ 50%, respectively). A time-series analysis reveals red-noise variability, with power spectral densities between -2 and -3 across all Stokes parameters. Polarized intensity shows stable intra-day timescales, while total intensity exhibits more variable timescales, suggesting distinct emission regions, with polarization likely arising from a coherent structure. On April 24, a statistically significant inter-band delay in polarized intensity coincides with a near-simultaneous X-ray and millimeter peak that deviates from the typical delayed flare scenario. This event also features enhanced millimeter variability and coherent polarization loop evolution. The observed simultaneity challenges standard models of transient synchrotron emission with cooling delays, favoring instead a scenario of continuous energy injection in an optically thin region. Our results offer new constraints on the physical mechanisms driving variability in Srg A*, and provide key observational input for refining theoretical models of accretion and plasma behavior in the vicinity of supermassive black holes.

astro-ph.GA↗

arXiv2026

Electron-positron Pair Production in Global GRMHD Simulations of Black Hole Accretion Flows

We present global, three-dimensional general relativistic magnetohydrodynamic simulations of accreting black holes that incorporate pair physics. Pairs are modeled as a passive scalar that maintains a constant temperature. For high accretion rate models, we observe a maximum pair fraction of $\sim \mathcal{O}(0.01)$, consistent with those inferred from some X-ray binaries, and identify a `pair void' extending to a few gravitational radii from the black hole. Pair fractions peak in the midplane just outside the plunging region and within a thin strip at the base of the corona. For moderate to high accretion rate models, pairs are near equilibrium close to the disk midplane, where the scattering optical depth is high and pair equilibrium timescales are short, and could be comparable to the Coulomb collision timescale. This suggests the possibility of a pair-regulated coronal temperature. In contrast, the upper corona and jets, where the scattering optical depth is relatively low and pair equilibrium timescales are long, are populated with pairs that may exceed their equilibrium value by orders of magnitude. These pairs are transported by advection from the disk, which dominates over local pair processes. This result highlights advection as a significant source of pair injection, which may be relevant for certain X-ray binaries exhibiting $\gamma$-ray signatures. The pair density along the magnetically dominated poles exceeds the Goldreich-Julian density in some models.

astro-ph.HE↗

arXiv2026

Hot, Retrograde Tilted MADs: Misaligned, Precessing, and Shaped by Electromagnetic Torques

Tilted accretion disks in the magnetically arrested (MAD) state may be present in X-ray binaries and active galactic nuclei such as Sgr A* and M87. We have carried out 3D global GRMHD simulations to study the evolution of these accretion flows as a function of black hole spin and misalignment angle. Prograde MADs align with the spin through a two-stage process: an initial rapid alignment phase that operates on the magnetic flux saturation timescale, followed by a slower, spin-independent phase. In contrast, retrograde MADs remain persistently misaligned regardless of the black hole spin, displaying solid-body precession at rates four times higher than weakly magnetized flows at the same spin magnitude. By deriving torque equations in ideal GRMHD and evaluating them in a frame aligned with instantaneous disk orientation, we demonstrate that electromagnetic (EM) torques always act to align the disk with the BH spin, but are countered by opposing hydrodynamic fluxes in retrograde flows. We further develop a preliminary empirical model to explain the cause of two-stage prograde alignment and discuss the possibility of alignment in the retrograde MAD. Strongly magnetized, retrograde, misaligned accretion disks provide a candidate scenario for the low-frequency quasi-periodic oscillations in black hole X-ray binaries.

astro-ph.HE↗

arXiv2026

Locating the missing large-scale emission in the jet of M87* with short EHT baselines

In Very-Long Baseline Interferometric arrays, nearly co-located stations probe the largest scales and typically cannot resolve the observed source. In the absence of large-scale structure, closure phases constructed with these stations are zero and, since they are independent of station-based errors, they can be used to probe data issues. Here, we show with an expansion about co-located stations, how these trivial closure phases become non-zero with brightness distribution on smaller scales than their short baseline would suggest. When applied to sources that are made up of a bright compact and large-scale diffuse component, the trivial closure phases directly measure the centroid relative to the compact source and higher-order image moments. We present a technique to measure these image moments with minimal model assumptions and validate it on synthetic Event Horizon Telescope (EHT) data. We then apply this technique to 2017 and 2018 EHT observations of M87* and find a weak preference for extended emission in the direction of the large-scale jet. We also apply it to 2021 EHT data and measure the source centroid about 1 mas northwest of the compact ring, consistent with the jet observed at lower frequencies.

astro-ph.HE↗

arXiv2026

Ring Asymmetry and Spin in M87*

Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring's peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin $a_{*}$. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor $|a_{*}| \lesssim 0.2$. This is consistent with the Blandford-Znajek model for M87's jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.

astro-ph.HE↗

arXiv2025

A Convolutional Neural Network for the Recovery of Transfer Functions From Velocity-Resolved Reverberation Mapping Data

One of the hallmarks of active galactic nuclei are that they are highly variable with time. In watching the spectra vary it has been observed that the emission-lines often appear to "reverberate" -- that is they vary in response to continuum variations assumed to originate close to the black hole. This critical observation underlies the reverberation mapping technique, an elegant physics experiment that has allowed us to characterize the environment around many supermassive black holes in nearby active galactic nuclei. Recent observations are of such quality that the response can be measured as a function of velocity across the emission-line, and in doing so we can construct velocity-delay maps that show the structure and physics of the gas in the broad-line region better than any other measurement to date. Unfortunately constructing such maps requires a deconvolution, and given that the data are often noisy and with gaps such deconvolutions are non-trivial. Here we present a novel deconvolution method for the recovery of velocity-delay maps using a custom convolutional neural network architecture, showcasing that such methods have great promise for the deconvolution of reverberation mapping data products. While we have designed this new method with the BLR in mind, in principle this technique could be applied to any reverberation deconvolution problem, including in the accretion disk and torus.

astro-ph.GA↗

arXiv2025

Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations

We investigate the presence and spatial characteristics of the jet base emission in M87* at 230 GHz, enabled by the enhanced uv coverage in the 2021 Event Horizon Telescope (EHT) observations. The addition of the 12-m Kitt Peak Telescope and NOEMA provides two key intermediate-length baselines to SMT and the IRAM 30-m, giving sensitivity to emission structures at scales of $\sim250~\mu$as and $\sim2500~\mu$as (0.02 pc and 0.2 pc). Without these baselines, earlier EHT observations lacked the capability to constrain emission on large scales, where a "missing flux" of order $\sim1$ Jy is expected. To probe these scales, we analyzed closure phases, robust against station-based gain errors, and modeled the jet base emission using a simple Gaussian offset from the compact ring emission at separations $>100~\mu$as. Our analysis reveals a Gaussian feature centered at ($\Delta$RA $\approx320~\mu$as, $\Delta$Dec $\approx60~\mu$as), a projected separation of $\approx5500$ AU, with a flux density of only $\sim60$ mJy, implying that most of the missing flux in previous studies must arise from larger scales. Brighter emission at these scales is ruled out, and the data do not favor more complex models. This component aligns with the inferred direction of the large-scale jet and is consistent with emission from the jet base. While our findings indicate detectable jet base emission at 230 GHz, coverage from only two intermediate baselines limits reconstruction of its morphology. We therefore treat the recovered Gaussian as an upper limit on the jet base flux density. Future EHT observations with expanded intermediate-baseline coverage will be essential to constrain the structure and nature of this component.

astro-ph.HE↗

arXiv2025

Demystifying flux eruptions: Magnetic flux transport in magnetically arrested disks

Magnetically arrested disks (MADs) are a compelling model for explaining variability in low-luminosity active galactic nuclei (AGN), including horizon-scale outbursts like those observed in Sagittarius A*. MADs experience powerful flux eruptions-episodic ejections of magnetic flux from the black hole horizon-that may drive the observed luminosity variations. In this work, we develop and validate a new formalism describing large-scale magnetic field transport in general relativistic magnetohydrodynamic simulations of MADs with geometrical thicknesses of $h/R=0.1$ and $h/R=0.3$. We introduce a net flux transport velocity, $v_\Phi$, which accounts for both advective and diffusive processes. We show that MADs maintain a statistical quasi-steady state where advection and diffusion nearly balance. Flux eruptions appear as small deviations from this equilibrium, with $v_\Phi/V_k\ll1$, where $V_k$ is the local Keplerian velocity. Using this framework, we analytically derive a recurrence timescale for flux eruptions, $t_{\rm rec}\sim1500\, r_g/c$. This timescale closely matches simulation results. The smallness of $v_\Phi$ explains the long recurrence times of flux eruptions compared to other system timescales. We also take a closer look at the diffusion of the magnetic field by performing the first measurement of turbulent resistivity in MADs. We then estimate the turbulent magnetic Prandtl number, defined as the ratio of turbulent viscosity to turbulent resistivity. We find $\mathcal{P}_m\sim3$, consistent with shearing-box simulations of magneto rotational instability-driven turbulence. While flux eruptions excite large-scale non-axisymmetric modes and locally enhance turbulent resistivity, magnetic field diffusion is dominated by smaller-scale turbulent motions. These results provide new insight into the nature of AGN variability and the fundamental physics of magnetic field transport.

astro-ph.HE↗

arXiv2025

Little Red Dots As Late-stage Quasi-stars

We argue that the "Little Red Dots" (LRDs) discovered with the James Webb Space Telescope are quasi-stars in their late stages of evolution. Quasi-stars are hypothetical objects predicted to form following the core collapse of supermassive stars, and consist of black holes accreting from massive envelopes at a super-Eddington rate. We show that models of late-stage quasi-stars, with black hole masses exceeding $\sim 10\%$ of the total, predict thermal and radiative properties that are insensitive to both black hole and envelope mass, and spectrally resemble LRDs. Specifically, we show that they are likely to exhibit reddish colors, a strong Balmer break, and possess conditions favorable to the production of Balmer lines that are broadened by electron scattering. Their huge electron column densities suppress any X-rays. Late-stage quasi-stars, with black hole masses $\gtrsim 10^6 M_\odot$, should dominate the overall quasi-star population. Their short predicted lifetimes (tens of Myr), coupled with the high observed comoving density of LRDs, suggest that most or all supermassive black holes go through a quasi-star/LRD phase during their formation and growth.

astro-ph.GA↗

arXiv2025

Reverberation Mapping Data of NGC 5548 Imply a Multicomponent Broad-Line Region

Line broadening and variability are observational hallmarks of active galactic nuclei which allow us to measure supermassive black hole masses as well as constrain the geometry and kinematics of the emitting gas, with the most precise measurements requiring a degree of modeling. Two popular models of the broad-line region describe the emitting gas as either a distribution of puffed up clouds or a thin disk with strong velocity gradients. As we show in this work, key features in the reverberation mapping dataset obtained by the AGN STORM team in NGC 5548 cannot be accounted for by either simple model. In several emission lines the observed broad-line profile has a single peak yet the delay profile has a distinct double peak, strongly motivating a BLR with emission from multiple components. We demonstrate a few possibilities that may alleviate the tension and better represent the true nature of the broad-line emitting gas in NGC 5548 and beyond.

astro-ph.GA↗

arXiv2025

Long-Term Astrometric Monitoring of the Galactic Center Magnetar PSR J1745--2900

We present new astrometric observations of the Galactic Center magnetar, PSR J1745-2900, with the Very Long Baseline Array (VLBA). Combined with previously published measurements in 10 epochs that spanned 477 days, the complete data set consists of 25 epochs and 41 independent measurements that span 1984 days. These data constrain the proper motion to an accuracy of $\lesssim 2\%$ and set an upper limit on the absolute value of the magnetar's acceleration of $\lesssim (0.4, 0.2)\, {\rm mas\,y^{-2}}$ in the two celestial coordinates, consistent with the maximum value of $\sim 0.03\,{\rm mas\,y^{-2}}$ expected for an orbit around Sgr A*. Future measurements have the potential to detect the acceleration of PSR J1745-2900 due to Sgr A* should PSR J1745-2900 re-brighten. We consider several potential sources of systematic variations in the astrometric residuals after fitting for standard parameters, including refractive wander, changes in the structure of Sgr A*, and the presence of an unseen binary companion. While a stellar companion model can be fit to the astrometric data, pulse period measurements are inconsistent with that model. No changes in the apparent image size of the magnetar were detected over the duration of these observations, indicating a lack of change in the properties of the line-of-sight scattering during this period. We also show that the upper limit to the mean core shift of Sgr A* is consistent with expectations for a compact jet or symmetric accretion flow.

astro-ph.HE↗

arXiv2025

Origin of the ring ellipticity in the black hole images of M87*

We investigate the origin of the elliptical ring structure observed in the images of the supermassive black hole M87*, aiming to disentangle contributions from gravitational, astrophysical, and imaging effects. Leveraging the enhanced capabilities of the Event Horizon Telescope (EHT) 2018 array, including improved $(u,v)$-coverage from the Greenland Telescope, we measure the ring's ellipticity using five independent imaging methods, obtaining a consistent average value of $\tau = 0.08_{-0.02}^{+0.03}$ with a position angle $\xi = 50.1_{-7.6}^{+6.2}$ degrees. To interpret this measurement, we compare against General Relativistic Magnetohydrodynamic (GRMHD) simulations spanning a wide range of physical parameters including thermal or non-thermal electron distribution function, spins, and ion-to-electron temperature ratios in both low and high-density regions. We find no statistically significant correlation between spin and ellipticity in GRMHD images. Instead, we identify a correlation between ellipticity and the fraction of non-ring emission, particularly in non-thermal models and models with higher jet emission. These results indicate that the ellipticity measured from the \m87 emission structure is consistent with that expected from simulations of turbulent accretion flows around black holes, where it is dominated by astrophysical effects rather than gravitational ones. Future high-resolution imaging, including space very long baseline interferometry and long-term monitoring, will be essential to isolate gravitational signatures from astrophysical effects.

astro-ph.HE↗

arXiv2025

Polarimetric Signatures of Bulk Comptonization from within the Plunging Region of Accreting Black Holes

Inverse Compton scattering by the thermal motions of electrons is believed to produce polarized hard X-rays in active galactic nuclei and black-hole binaries. Meanwhile, plasma within the plunging region of the black hole free falls into the event horizon with a bulk relativistic speed, which could also imprint polarization on up-scattered photons but has not been discussed in detail. To examine this, we computed polarimetric signatures via general relativistic ray-tracing of a toy model consisting of an accreting, geometrically thin plasma with moderate optical depth, falling onto the black hole with a bulk relativistic speed within the plunging region. We show that the maximum spatially unresolved linear polarization could be as large as approximately $7 - 8$ percent when the black hole is viewed near edge-on, while the corresponding resolved linear polarization could be roughly $50$ percent. The large discrepancy between the two is due to 1) dilution from the radiation outside the plunging region and 2) substantial cancellations of the Stokes $Q$ and $U$ fluxes. The resultant polarization contributed by bulk Comptonization could nevertheless exceed that of thermal electron scattering in a Novikov-Thorne disk. Our results thus suggest a new model for imprinting considerable polarization on the electromagnetic observables of accreting black holes. Measurements of X-ray polarization from black-hole binaries and the central black hole of active galactic nuclei could provide direct detection of the plunging region and help constrain plasma properties in the immediate vicinity of the event horizon.

astro-ph.HE↗

arXiv2025

The Impact of Plasma Angular Momentum on Magnetically Arrested Flows and Relativistic Jets in Hot Accretion Flows Around Black Holes

In certain scenarios, the accreted angular momentum of plasma onto a black hole could be low; however, how the accretion dynamics depend on the angular momentum content of the plasma is still not fully understood. We present three-dimensional, general relativistic magnetohydrodynamic simulations of low angular momentum accretion flows around rapidly spinning black holes (with spin $a = +0.9$). The initial condition is a Fishbone-Moncrief (FM) torus threaded by a large amount of poloidal magnetic flux, where the angular velocity is a fraction $f$ of the standard value. For $f = 0$, the accretion flow becomes magnetically arrested and launches relativistic jets but only for a very short duration. After that, free-falling plasma breaks through the magnetic barrier, loading the jet with mass and destroying the jet-disk structure. Meanwhile, magnetic flux is lost via giant, asymmetrical magnetic bubbles that float away from the black hole. The accretion then exits the magnetically arrested state. For $f = 0.1$, the dimensionless magnetic flux threading the black hole oscillates quasi-periodically. The jet-disk structure shows concurrent revival and destruction while the gas outflow efficiency at the event horizon changes accordingly. For $f \geq 0.3$, we find that the dynamical behavior of the system starts to approach that of a standard accreting FM torus. Our results thus suggest that the accreted angular momentum is an important parameter that governs the maintenance of a magnetically arrested flow and launching of relativistic jets around black holes.

astro-ph.HE↗

arXiv2025

Revealing the Accretion Flow in M87*: Insights from Faraday Rotation

The Faraday rotation measure (RM) is a commonly used tool to trace electron number density and magnetic fields in hot accretion flows, particularly in low-luminosity accreting supermassive black holes. We focus on the nuclear region of M87, which was observed at 230 GHz (1.3 mm) by the Event Horizon Telescope in 2019. It remains unclear whether this emission originates from the accretion flow, the jet base, or both. To probe the presence of an accretion flow, we explore the scenario where the linearly polarized emission from the counter-jet, visible at 43 GHz (7 mm), is Faraday-rotated by the accretion flow. We calculate theoretical predictions for counter-jet polarization using analytical and numerical models. In all cases, we find a Faraday-thick flow at 43 GHz (7 mm), with $\mathrm{RM} \sim 10^6$ rad m$^{-2}$, and a polarization angle that follows a linear relationship with wavelength squared, consistent with external Faraday rotation. The more realistic model, which includes turbulence and magnetic field fluctuations, predicts that the polarization pattern should be time-dependent, and that the counter-jet emission is depolarized due to Faraday depth fluctuations across the accretion flow. Despite the Faraday thick regime and strong depolarization, the linear relationship persists, enabling us to constrain the flow's physical properties. Comparing the counter-jet and forward-jet linear polarization states should enable detection of M87's accretion flow and provide lower limits on electron density, magnetic field strength, and mass accretion rate.

astro-ph.HE↗