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1. Simultaneous JWST, NuSTAR, and VLA Monitoring of Sgr A*: A Unified Picture of the Variable IR, X-Ray, and Radio Emission

   https://doi.org/10.3847/2041-8213/ae4bd2  

   Yusef-Zadeh, F; Wardle, M; Arendt, R G; Heinke, C O; Chandler, C J; Bushouse, H; Moellenbrock, G A; Michail, Joseph M (March 2026, The Astrophysical Journal Letters)

   Abstract Flux variability is a fundamental channel of information from Sgr A* because of its direct probe of processes occurring within an accretion disk under strong gravity. We present simultaneous JWST, NuSTAR, and Very Large Array observations of Sgr A* on 2024 April 5. We report the detection of a strong X-ray flare with a duration of about 40 minutes and a luminosity of 5.2 × 10³⁵erg s⁻¹coincident with a bright near-IR (NIR) flare, and a brightening in radio about an hour later. We investigate the candidate physical mechanisms for the X-ray flare emission and conclude that this can best be explained by inverse Compton scattering of NIR flare radiation. We propose a dynamic scenario analogous to a coronal mass ejection in which a magnetic flux rope is ejected from Sgr A*’s inner accretion flow with a current sheet extending down from the rope to the bulk of the accretion flow. The accelerated electrons are continually ejected from the reconnection X-point with a bulk flow at the Alfvén speed of 0.7c. IR radiation from the approaching energetic electrons is enhanced by beaming and upscattered by thermal electrons in the accretion flow to produce the strong X-ray flare. Meanwhile, the relativistic electrons moving in the opposite direction away from the disk experience weaker magnetic fields and so radiate at longer wavelengths. They feed into the magnetic flux tube explaining the detected delayed radio emission. This physical picture attempts to unify the origin of the variable emission from Sgr A* at IR, X-ray, and radio/submillimeter wavelengths. 

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2. First Mid-infrared Detection and Modeling of a Flare from Sgr A*. II. Mid-infrared Spectral Energy Distribution and Millimeter Polarimetry

   https://doi.org/10.3847/1538-4357/ae25ef  

   Michail, Joseph M; von_Fellenberg, Sebastiano D; Keating, Garrett K; Rao, Ramprasad; Roychowdhury, Tamojeet; Willner, S P; Ford, Nicole M; Haggard, Daryl; Markoff, Sera; Philippov, Alexander; et al (January 2026, The Astrophysical Journal)

   Abstract S. D. von Fellenberg et al. reported the first mid-infrared detection of a flare from Sgr A*. The JWST/MIRI/Medium Resolution Spectrometer observations were consistent with an orbiting hotspot undergoing electron injection with a spectrum that subsequently breaks from synchrotron cooling. However, mid-infrared extinction measurements appropriate for these data were not yet determined, and, therefore, the temporal evolution of the absolute spectral index remained unknown. This work applies new Sgr A* extinction measurements to the flare observations. The evolution of the spectral index after the peak is fully consistent with that reported in Paper I with a maximum absolute mid-infrared spectral indexα_MIR= 0.45 ± 0.01_stat± 0.08_sysduring the second mid-infrared flare peak, matching the known near-infrared spectral index during bright states (α_NIR≈ 0.5). There was a near-instantaneous change in the mid-infrared spectral index of Δα_MIR= 0.33 ± 0.06_stat± 0.11_sysat the flare onset. We propose this as a quantitative definition for this infrared flare’s beginning, physically interpreted as the underlying electron distribution’s transition into a hard power-law distribution. This paper also reports the Submillimeter Array millimeter polarization during the flare, which shows a small, distorted, but overall CW-oriented StokesQ–Uloop during the third mid-infrared peak. Extrapolating the mid-infrared flux power law to the millimeter yields a variable flux consistent with the observed 220 GHz emission. These results, together with the Paper I modeling, plausibly suggest a single hotspot produced both the mid-infrared and millimeter variability during this event. However, additional flares are required to make a general statement about the millimeter and mid-infrared connection. 

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3. Multiwavelength Variability of Sagittarius A* in 2019 July

   https://doi.org/10.3847/1538-4357/ac6104  

   Boyce, H.; Haggard, D.; Witzel, G.; Fellenberg, S. von; Willner, S. P.; Becklin, E. E.; Do, T.; Eckart, A.; Fazio, G. G.; Gurwell, M. A.; et al (May 2022, The Astrophysical Journal)

   Abstract We report a timing analysis of near-infrared (NIR), X-ray, and submillimeter data during a 3 day coordinated campaign observing Sagittarius A*. Data were collected at 4.5 μ m with the Spitzer Space Telescope, 2–8 keV with the Chandra X-ray Observatory, 3–70 keV with NuSTAR, 340 GHz with ALMA, and 2.2 μ m with the GRAVITY instrument on the Very Large Telescope Interferometer. Two dates show moderate variability with no significant lags between the submillimeter and the infrared at 99% confidence. A moderately bright NIR flare ( F K ∼ 15 mJy) was captured on July 18 simultaneous with an X-ray flare ( F 2−10 keV ∼ 0.1 counts s −1 ) that most likely preceded bright submillimeter flux ( F 340 GHz ∼ 5.5 Jy) by about + 34 − 33 + 14 minutes at 99% confidence. The uncertainty in this lag is dominated by the fact that we did not observe the peak of the submillimeter emission. A synchrotron source cooled through adiabatic expansion can describe a rise in the submillimeter once the synchrotron self-Compton NIR and X-ray peaks have faded. This model predicts high GHz and THz fluxes at the time of the NIR/X-ray peak and electron densities well above those implied from average accretion rates for Sgr A*. However, the higher electron density postulated in this scenario would be in agreement with the idea that 2019 was an extraordinary epoch with a heightened accretion rate. Since the NIR and X-ray peaks can also be fit by a nonthermal synchrotron source with lower electron densities, we cannot rule out an unrelated chance coincidence of this bright submillimeter flare with the NIR/X-ray emission. 

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4. General relativistic MHD simulations of non-thermal flaring in Sagittarius A*

   https://doi.org/10.1093/mnras/stab2466  

   Chatterjee, K; Markoff, S; Neilsen, J; Younsi, Z; Witzel, G; Tchekhovskoy, A; Yoon, D; Ingram, A; van der Klis, M; Boyce, H; et al (September 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Sgr A* exhibits regular variability in its multiwavelength emission, including daily X-ray flares and roughly continuous near-infrared (NIR) flickering. The origin of this variability is still ambiguous since both inverse Compton and synchrotron emission are possible radiative mechanisms. The underlying particle distributions are also not well constrained, particularly the non-thermal contribution. In this work, we employ the GPU-accelerated general relativistic magnetohydrodynamics code H-AMR to perform a study of flare flux distributions, including the effect of particle acceleration for the first time in high-resolution 3D simulations of Sgr A*. For the particle acceleration, we use the general relativistic ray-tracing code bhoss to perform the radiative transfer, assuming a hybrid thermal+non-thermal electron energy distribution. We extract ∼60 h light curves in the sub-millimetre, NIR and X-ray wavebands, and compare the power spectra and the cumulative flux distributions of the light curves to statistical descriptions for Sgr A* flares. Our results indicate that non-thermal populations of electrons arising from turbulence-driven reconnection in weakly magnetized accretion flows lead to moderate NIR and X-ray flares and reasonably describe the X-ray flux distribution while fulfilling multiwavelength flux constraints. These models exhibit high rms per cent amplitudes, $$\gtrsim 150{{\ \rm per\ cent}}$$ both in the NIR and the X-rays, with changes in the accretion rate driving the 230 GHz flux variability, in agreement with Sgr A* observations. 

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5. Gone with the Wind: JWST-MIRI Unveils a Strong Outflow from the Quiescent Stellar-mass Black Hole A0620-00

   https://doi.org/10.3847/1538-4357/adf6b9  

   Zuo, Zihao; Cugno, Gabriele; Michail, Joseph; Gallo, Elena; Russell, David M; Plotkin, Richard M; Zou, Fan; Baglio, M Cristina; Casella, Piergiorgio; Cowie, Fraser J; et al (September 2025, The Astrophysical Journal)

   Abstract We present new observations of the black hole X-ray binary A0620-00 using the Mid-Infrared (MIR) Instrument on the James Webb Space Telescope, during a state where the X-ray luminosity is 9 orders of magnitude below Eddington, and coordinated with radio, near-infrared, and optical observations. The goal is to understand the nature of the excess MIR emission originally detected by Spitzer redward of 8μm. The stellar-subtracted MIR spectrum is well modeled by a power law with a spectral index ofα = 0.72 ± 0.01, where the flux density scales with frequency asF_ν ∝ ν^α. The spectral characteristics, along with rapid variability—a 40% flux flare at 15μm and 25% achromatic variability in the 5–12μm range—rule out a circumbinary disk as the source of the MIR excess. The Low Resolution Spectrometer reveals a prominent emission feature at 7.5μm, resulting from the blend of three hydrogen recombination lines. While the contribution from partially self-absorbed synchrotron radiation cannot be ruled out, we argue that thermal bremsstrahlung from a warm (a few tens of thousands of Kelvin) wind accounts for the MIR excess; the same outflow is responsible for the emission lines. The inferred mass outflow rate indicates that the system’s low luminosity is due to a substantial fraction of the mass supplied by the donor star being expelled through a wind rather than accreted onto the black hole. 

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