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1. Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks

   https://doi.org/10.3847/1538-4357/ad323d  

   Manikantan, Vikram; Kaaz, Nicholas; Jacquemin-Ide, Jonatan; Musoke, Gibwa; Chatterjee, Koushik; Liska, Matthew; Tchekhovskoy, Alexander (April 2024, The Astrophysical Journal)

   Abstract The conventional accretion disk lore is that magnetized turbulence is the principal angular momentum transport process that drives accretion. However, when dynamically important large-scale magnetic fields thread an accretion disk, they can produce mass and angular momentum outflows, known as winds,that also drive accretion. Yet, the relative importance of turbulent and wind-driven angular momentum transport is still poorly understood. To probe this question, we analyze a long-duration (1.2 × 10⁵r_g/c) simulation of a rapidly rotating (a= 0.9) black hole feeding from a thick (H/r∼ 0.3), adiabatic, magnetically arrested disk (MAD), whose dynamically important magnetic field regulates mass inflow and drives both uncollimated and collimated outflows (i.e., winds and jets, respectively). By carefully disentangling the various angular momentum transport processes within the system, we demonstrate the novel result that disk winds and disk turbulence both extract roughly equal amounts of angular momentum from the disk. We find cumulative angular momentum and mass accretion outflow rates of $\dot{L}\propto r^{0.9}$and $\dot{M}\propto r^{0.4}$, respectively. This result suggests that understanding both turbulent and laminar stresses is key to understanding the evolution of systems where geometrically thick MADs can occur, such as the hard state of X-ray binaries, low-luminosity active galactic nuclei, some tidal disruption events, and possibly gamma-ray bursts. 

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2. Bridging Scales in Black Hole Accretion and Feedback: Magnetized Bondi Accretion in 3D GRMHD

   https://doi.org/10.3847/2041-8213/ad1048  

   Cho 조, Hyerin 혜린; Prather, Ben S.; Narayan, Ramesh; Natarajan, Priyamvada; Su, Kung-Yi; Ricarte, Angelo; Chatterjee, Koushik (December 2023, The Astrophysical Journal Letters)

   Abstract Fueling and feedback couple supermassive black holes (SMBHs) to their host galaxies across many orders of magnitude in spatial and temporal scales, making this problem notoriously challenging to simulate. We use a multi-zone computational method based on the general relativistic magnetohydrodynamic (GRMHD) code KHARMA that allows us to span 7 orders of magnitude in spatial scale, to simulate accretion onto a non-spinning SMBH from an external medium with a Bondi radius ofR_B≈ 2 × 10⁵GM_•/c², whereM_•is the SMBH mass. For the classic idealized Bondi problem, spherical gas accretion without magnetic fields, our simulation results agree very well with the general relativistic analytic solution. Meanwhile, when the accreting gas is magnetized, the SMBH magnetosphere becomes saturated with a strong magnetic field. The density profile varies as ∼r⁻¹rather thanr^−3/2and the accretion rate $\dot{M}$is consequently suppressed by over 2 orders of magnitude below the Bondi rate ${\dot{M}}_{\mathrm{B}}$. We find continuous energy feedback from the accretion flow to the external medium at a level of $\sim {10}^{-2}\dot{M}{c}^2\sim 5\times {10}^{-5}{\dot{M}}_{\mathrm{B}}{c}^2$. Energy transport across these widely disparate scales occurs via turbulent convection triggered by magnetic field reconnection near the SMBH. Thus, strong magnetic fields that accumulate on horizon scales transform the flow dynamics far from the SMBH and naturally explain observed extremely low accretion rates compared to the Bondi rate, as well as at least part of the energy feedback. 

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3. Analytical Model of Disk Evaporation and State Transitions in Accreting Black Holes

   https://doi.org/10.3847/1538-4357/ac6d5c  

   Cho 조, Hyerin 혜린; Narayan, Ramesh (June 2022, The Astrophysical Journal)

   Abstract State transitions in black hole X-ray binaries are likely caused by gas evaporation from a thin accretion disk into a hot corona. We present a height-integrated version of this process, which is suitable for analytical and numerical studies. With radiusrscaled to Schwarzschild units and coronal mass accretion rate ${\dot{m}}_c$to Eddington units, the results of the model are independent of black hole mass. State transitions should thus be similar in X-ray binaries and an active galactic nucleus. The corona solution consists of two power-law segments separated at a break radiusr_b∼ 10³(α/0.3)⁻², whereαis the viscosity parameter. Gas evaporates from the disk to the corona forr>r_b, and condenses back forr<r_b. Atr_b, ${\dot{m}}_c$reaches its maximum, ${\dot{m}}_{c,\max }\approx 0.02{\left(\alpha /0.3\right)}^3$. If atr≫r_bthe thin disk accretes with ${\dot{m}}_d<{\dot{m}}_{c,\max }$, then the disk evaporates fully before reachingr_b, giving the hard state. Otherwise, the disk survives at all radii, giving the thermal state. While the basic model considers only bremsstrahlung cooling and viscous heating, we also discuss a more realistic model that includes Compton cooling and direct coronal heating by energy transport from the disk. Solutions are again independent of black hole mass, andr_bremains unchanged. This model predicts strong coronal winds forr>r_b, and aT∼ 5 × 10⁸K Compton-cooled corona forr<r_b. Two-temperature effects are ignored, but may be important at small radii. 

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4. The Comprehensive Archive of Substellar and Planetary Accretion Rates

   https://doi.org/10.3847/1538-3881/ad06b8  

   Betti, S K; Follette, K B; Ward-Duong, K; Peck, A E; Aoyama, Y; Bary, J; Dacus, B; Edwards, S; Marleau, G-D; Mohamed, K; et al (November 2023, The Astronomical Journal)

   Abstract Accretion rates ( $\dot{M}$) of young stars show a strong correlation with object mass (M); however, extension of the $\dot{M}–M$relation into the substellar regime is less certain. Here, we present the Comprehensive Archive of Substellar and Planetary Accretion Rates (CASPAR), the largest compilation to date of substellar accretion diagnostics. CASPAR includes: 658 stars, 130 brown dwarfs, and 10 bound planetary mass companions. In this work, we investigate the contribution of methodological systematics to scatter in the $\dot{M}–M$relation and compare brown dwarfs to stars. In our analysis, we rederive all quantities using self-consistent models, distances, and empirical line flux to accretion luminosity scaling relations to reduce methodological systematics. This treatment decreases the original 1σscatter in the $\log \dot{M}–\log M$relation by ∼17%, suggesting that it makes only a small contribution to the dispersion. The CASPAR rederived values are best fit by $\dot{M}\propto M^{2.02\pm 0.06}$from 10M_Jto 2M_⊙, confirming previous results. However, we argue that the brown-dwarf and stellar populations are better described separately and by accounting for both mass and age. Therefore, we derive separate age-dependent $\dot{M}–M$relations for these regions and find a steepening in the brown-dwarf $\dot{M}–M$slope with age. Within this mass regime, the scatter decreases from 1.36 dex to 0.94 dex, a change of ∼44%. This result highlights the significant role that evolution plays in the overall spread of accretion rates, and suggests that brown dwarfs evolve faster than stars, potentially as a result of different accretion mechanisms. 

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5. Strong Variability in AzV 493, an Extreme Oe-type Star in the SMC

   https://doi.org/10.3847/1538-4357/acb690  

   Oey, M. S.; Castro, N.; Renzo, M.; Vargas-Salazar, I.; Suffak, M. W.; Ratajczak, M.; Monnier, J. D.; Szymanski, M. K.; Phillips, G. D.; Calvet, N.; et al (April 2023, The Astrophysical Journal)

   Abstract We present 18 yr of OGLE photometry together with spectra obtained over 12 yr revealing that the early Oe star AzV 493 shows strong photometric (ΔI< 1.2 mag) and spectroscopic variability with a dominant, 14.6 yr pattern and ∼40 day oscillations. We estimate the stellar parametersT_eff= 42,000 K, $\log L/L_{\odot }=5.83\pm 0.15$,M/M_⊙= 50 ± 9, andvsini= 370 ± 40 km s⁻¹. Direct spectroscopic evidence shows episodes of both gas ejection and infall. There is no X-ray detection, and it is likely a runaway star. The star AzV 493 may have an unseen companion on a highly eccentric (e> 0.93) orbit. We propose that close interaction at periastron excites ejection of the decretion disk, whose variable emission-line spectrum suggests separate inner and outer components, with an optically thick outer component obscuring both the stellar photosphere and the emission-line spectrum of the inner disk at early phases in the photometric cycle. It is plausible that AzV 493’s mass and rotation have been enhanced by binary interaction followed by the core-collapse supernova explosion of the companion, which now could be either a black hole or a neutron star. This system in the Small Magellanic Cloud can potentially shed light on OBe decretion disk formation and evolution, massive binary evolution, and compact binary progenitors. 

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