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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. 

Abstract We present a thermal observation of Callisto's leading hemisphere obtained using the Atacama Large Millimeter/submillimeter Array at 0.87 mm (343 GHz). The angular resolution achieved for this observation was ∼0.″16, which for Callisto at the time of this observation (D∼ 1.″05) was equivalent to ∼six elements across the surface. Our disk-integrated brightness temperature of 116 ± 5 K (8.03 ± 0.40 Jy) is consistent with prior disk-integrated observations. Global surface properties were derived from the observation using a thermophysical model constrained by spacecraft data. We find that models parameterized by two thermal inertia components more accurately fit the data than single thermal inertia models. Our best-fit global parameters adopt a lower thermal inertia of 15–50 J m⁻²K⁻¹s^−1/2and a higher thermal inertia component of 1200–2000 J m⁻²K⁻¹s^−1/2, with retrieved millimeter emissivities of 0.89–0.91. We identify several thermally anomalous regions, including spots ∼3 K colder than model predictions colocated with the Valhalla impact basin and a complex of craters in the southern hemisphere; this indicates the presence of materials possessing either a higher thermal inertia or a lower emissivity. A warm region confined to the midlatitudes in these leading hemisphere data may be indicative of regolith property changes due to exogenic sculpting. 

Abstract The time-variable emission from the accretion flow of Sgr A*, the supermassive black hole at the Galactic center, has long been examined in the radio-to-millimeter, near-infrared (NIR), and X-ray regimes of the electromagnetic spectrum. However, until now, sensitivity and angular resolution have been insufficient in the crucial mid-infrared (MIR) regime. The MIRI instrument on JWST has changed that, and we report the first MIR detection of Sgr A*. The detection was during a flare that lasted about 40 minutes, a duration similar to NIR and X-ray flares, and the source's spectral index steepened as the flare ended. The steepening suggests that synchrotron cooling is an important process for Sgr A*'s variability and implies magnetic fields strengths ~ 40–70 G in the emission zone. Observations at 1.3 mm with the Submillimeter Array revealed a counterpart flare lagging the MIR flare by ≈10 minutes. The observations can be self-consistently explained as synchrotron radiation from a single population of gradually cooling high-energy electrons accelerated through (a combination of) magnetic reconnection and/or magnetized turbulence. 

Abstract We present multifrequency (5–345 GHz) and multiresolution radio observations of 1ES 1927+654, widely considered one of the most unusual and extreme changing-look active galactic nuclei (CL-AGNs). The source was first designated a CL-AGN after an optical outburst in late 2017 and has since displayed considerable changes in X-ray emission, including the destruction and rebuilding of the X-ray corona in 2019–2020. Radio observations prior to 2023 show a faint and compact radio source typical of a radio-quiet AGN. Starting in 2023 February, 1ES 1927+654 began exhibiting a radio flare with a steep exponential rise, reaching a peak 60 times previous flux levels, and has maintained this higher level of radio emission for over a year to date. The 5–23 GHz spectrum is broadly similar to gigahertz-peaked radio sources, which are understood to be young radio jets less than ∼1000 yr old. Recent high-resolution Very Long Baseline Array observations at 23.5 GHz now show resolved extensions on either side of the core, with a separation of ∼0.15 pc, consistent with a new and mildly relativistic bipolar outflow. A steady increase in the soft X-ray band (0.3–2 keV) concurrent with the radio may be consistent with jet-driven shocked gas, though further observations are needed to test alternate scenarios. This source joins a growing number of CL-AGNs and tidal disruption events that show late-time radio activity, years after the initial outburst.