Mitchell C. Begelman

We present results from a comprehensive multiwavelength monitoring campaign of the changing-look active galactic nucleus 1ES 1927+654 during the onset and evolution of a relativistic radio jet ∼(2022 May–2025 August), using observations from XMM-Newton, Swift, Telescopio Nazionale Galileo (TNG), the Zwicky Transient Facility (ZTF), Very Large Array (VLA), and Very Long Baseline Array (VLBA). The soft X-ray emission lines at ∼0.56 keV and ∼1 keV have appeared with variable strength and width during the formation of the nascent jet. We note that the ∼1 keV feature has been persisting since the post-2017 flare phase. We also report the detection of a broad (∼800 eV) FeK emission feature at (6–7) keV in the ∼70 ks stacked EPIC-pn spectra, marking the first such detection, which historically was lacking in this source. The joint spectral fitting of XMM-Newton EPIC-pn and Reflection Grating Spectrometer (RGS) data reveals the presence of ionized absorbers in 2022 ( log ξ / erg cm s − 1 ∼ 1.5 ± 0.3 , N _H ∼ 2.5 ± 0.9 × 10 ²⁰ cm ⁻² ), but weaker than that detected during the high accretion state in 2018 (Eddington ratio, λ _Edd > 1). The absorption features further weakened in 2023–2025 and were marginally detectable ( N _H ≤ 10 ²⁰ cm ⁻² ). The entire scenario is suggestive of a real-time transition of the accretion flow (from λ _Edd > 1 to λ _Edd ∼ 0.3) during which the winds become weaker and the jet starts to form and evolve. Furthermore, both the soft X-ray (0.3–2) keV and 5 GHz radio fluxes, which increased by factors of ∼10 and ∼60, respectively, since 2022, have recently plateaued at elevated levels, indicating a stabilized accretion disk, corona, and jet configuration.

Magnetically arrested disks (MADs) are a compelling model for explaining variability in low-luminosity active galactic nuclei (AGN), including horizon-scale outbursts. 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 _Φ , 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 _Φ / V _k ≪ 1, where V _k is the local Keplerian velocity. Using this framework, we analytically derive a recurrence timescale for flux eruptions, t _rec ≃ 1500 r _g / c . This timescale closely matches simulation results. The smallness of v _Φ 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, P m , defined as the ratio of turbulent viscosity to turbulent resistivity. We find P m ≃ 3 , consistent with shearing-box simulations of magnetorotational instability-driven turbulence. While flux eruptions excite large-scale nonaxisymmetric 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.

The combined University of Michigan Radio Astronomy Observatory and Owens Valley Radio Observatory blazar monitoring programs at 14.5/15 GHz provide uninterrupted light curves of ∼ 46–50 yr duration for 83 blazars, selected from among the brightest and most rapidly flaring blazars north of declination −20°. In a search for supermassive black hole binary (SMBHB) candidates, we carried out tests for periodic variability using generalized Lomb–Scargle (GLS), weighted wavelet-Z, and sine-wave fitting analyses of this sample. We used simulations to test the effects of the power-law spectrum of the power spectral density (PSD) on our findings, and show that the irregular sampling in the observed light curves has very little effect on the GLS spectra. Apparent periodicities and putative harmonics appear in all 83 of the GLS spectra of the blazars in our sample. We tested the reality of these apparent periodicities and harmonics with simulations, and found that in the overwhelming majority of cases, they must be due to the steep slope of the PSD and the random nature of blazar flares, implying that, in general, apparent periodicities and harmonics in blazar light curves observed in any energy band should be treated with great caution. We find one new SMBHB candidate: PKS 1309+1154, which exhibits a 17.9 yr periodicity. The fraction of SMBHB candidates in our sample is 2 . 4 − 0.8 + 3.2 % .

The JWST discovery of “little red dots” (LRDs) has revealed a population of compact, red sources at z ∼ 5–10 that likely host supermassive black holes. Recent observations of the gravitationally lensed LRD R2211-RX1 reveal century-scale photometric variability and a hysteresis loop in the luminosity–temperature plane, strongly suggesting that the optical emission originates from a pulsating, stellar-like photosphere rather than an accretion disk. This supports the “quasi-star” hypothesis, where a rapidly growing black hole seed is embedded within a massive, radiation-pressure supported envelope. Here, we investigate the pulsational stability of these envelopes using the stellar evolution code MESA coupled with the nonadiabatic oscillation code GYRE . We identify a theoretical “quasi-star instability strip” with a blue edge at T _eff ≈ 5000 − 5200 K. Models hotter than this threshold are stable, consistent with the nonvariable LRD R2211-RX2 ( T _eff ≈ 5000 K), while cooler models are unstable to radial pulsations driven by the κ -mechanism in helium and hydrogen ionization zones. For quasi-star masses in the range M _⋆ ∼ 10 ⁴ –10 ⁵ M _⊙ , we find that the unstable fundamental radial modes ( ℓ = 0, n _p = 1) have periods in the range ∼20–180 yr. The first overtone ( ℓ = 0, n _p = 2) is also unstable or marginally stable in some of our models, with typical pulsation timescales ∼10–30 yr. These oscillations match the comoving frame variability timescale of RX1. We argue that these violent pulsations likely drive enhanced mass loss analogous to super-AGB winds, which could affect the duration of the quasi-star phase and regulate the final mass of the seeded black hole.

Magnetic effects have long been considered a possible factor in stabilizing the outer regions of active galactic nuclei accretion disks against gravitational instability (GI). However, the computational demands of testing this hypothesis have prevented comprehensive study of this problem. Here, we present results from a suite of six isothermal magnetohydrodynamics (MHD) shearing box simulations, three initialized with strong magnetization ( β 0 mid = p gas / p mag = 1 0 2.5 ) and three initialized with weak magnetization ( β 0 mid = 1 0 4 ). For each magnetization, we performed simulations with both strong ( Q ₀ = 1.0) and weak ( Q ₀ = 10.0) self-gravity, where Q 0 = c s Ω π G Σ 0 is the Toomre stability parameter; we also performed pure MHD simulations for comparison. We find that our strongly magnetized disk stabilized against GI after initialization to critical stability against GI, while our corresponding weakly magnetized disk did not. We show that the strongly magnetized, strongly self-gravitating disk became dominated by magnetic pressure, which led to its stabilization.

Observations by JWST have confirmed the presence of supermassive black holes (BHs) at redshifts z ≳ 10, lending support to scenarios in which BHs experience rapid growth through intense gas accretion. Here we investigate the growth of a BH embedded at the center of a quasi-star, a theoretically predicted object formed via direct collapse. In a quasi-star, the central BH accretes at a highly super-Eddington rate, while the excess energy is transported outward by convection and radiated at approximately the Eddington luminosity of the entire star. We employ the open-source stellar evolution code MESA to construct quasi-star models and follow the time-dependent growth of the central BH under different prescriptions for the accretion rate at the inner boundary R _i , and further considering the effect of winds. For the case R _i = NR _B , where N is a constant and R _B is the Bondi radius corresponding to the mass of the BH and the gas infalling onto it, our models terminate when the BH mass reaches a critical value M crit ( N ) = c s , i 3 / ( 12 N 3 G 3 π ρ i ) (where c _{s , i} and ρ _i are the sound speed and density at R _i , respectively), a limit we also derive analytically. Models that feature an inner convective region matched to an outer adiabatic envelope exhibit BH growth up to approximately M _BH / M _⋆ ≃ 0.33, largely independent of the stellar mass M _⋆ itself. This ratio is approximately preserved even in the presence of mass loss, as several properties of the model are independent of the quasi-star’s total mass.

Supermassive black hole binary systems (SMBHBs) are thought to emit the recently discovered nHz gravitational wave background; however, not a single individual nHz source has been confirmed to date. Long-term radio-monitoring at the Owens Valley Radio Observatory has revealed two potential SMBHB candidates: blazars PKS 2131-021 and PKS J0805-0111. These sources show periodic flux density variations across the electromagnetic spectrum, signaling the presence of a good clock. To explain the emission, we propose a generalizable jet model, where a mildly relativistic wind creates an outward-moving helical channel, along which the ultrarelativistic jet propagates. The observed flux variation from the jet is mostly due to aberration. The emission at a lower frequency arises at a larger radius, and its variation is consequently delayed, as observed. Our model reproduces the main observable features of both sources and can be applied to other sources as they are discovered. We make predictions for radio polarization, direct imaging, and emission line variation, which can be tested with forthcoming observations. Our results motivate future numerical simulations of jetted SMBHB systems and have implications for the fueling, structure, and evolution of blazar jets.

We recently discovered a supermassive black hole binary (SMBHB) candidate, PKS J1309+1154, in the combined 46 yr University of Michigan Radio Astronomy Observatory (UMRAO) plus Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz. The light curve of PKS 1309+1154 exhibits a 17.9 yr periodicity. We also reported a hint of a first harmonic with a 9 yr periodicity in this object. Further analysis of the PKS J1309+1154 light curve provides compelling evidence that both the fundamental and the harmonic are real, confirming the existence of real periodicities in blazar light curves. This is the first case, to our knowledge, of watertight evidence for a fundamental periodicity and a harmonic periodicity in a blazar light curve. It makes PKS J1309+1154 a strong SMBHB candidate and thus the third such candidate to be revealed through long-term radio monitoring, the other two being PKS J0805–0111 and PKS 2131–021, both discovered through the OVRO 40 m Telescope monitoring program. It is argued that hundreds of SMBHB candidates will be discovered by the combination of the South Pole Telescope, the Vera Rubin Observatory, and the Simons Observatory. Coherent searches for gravitational waves from a network of SMBHB candidates, starting immediately, are strongly motivated.

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 (BHs) accreting from massive envelopes at a super-Eddington rate. We show that models of late-stage quasi-stars, with BH masses exceeding ∼10% of the total, predict thermal and radiative properties that are insensitive to both BH 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 BH masses ≳10 ⁶ M _⊙ , 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 BHs go through a quasi-star/LRD phase during their formation and growth.