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Abstract MAXI J1744−294 (MAXI J1477), likely a low-mass X-ray binary system, is a Galactic center transient source, detected at radio and X-ray wavelengths, located approximately 19″ southeast of Sgr A*. We report the first detection of its variable linear polarization in four epochs spanning 2025 April 4–9. The normalized 33 and 43 GHz Stokes parametersqanduover the four epochs imply a common Faraday rotation screen with a rotation measure (RM) $=-63606_{-861}^{+844}$radians m⁻², the third largest RM detected within the Galaxy. The RM is consistent with that of the Galactic center magnetar PSR J1745−2900, giving the first direct evidence that MAXI J1744 lies within the Galactic center region, is bound to Sgr A*, and therefore is part of the nuclear star cluster. The uniformity in the Galactic center Faraday screen suggests that Sgr A*’s ≈−10⁵radians m⁻²RM is intrinsic rather than originating from an unrelated line-of-sight source. On 2025 April 6, we detected a secondary polarized component with an additional RM ≈ −6000 radians m⁻², which was not seen at any other epoch. Assuming this secondary component primarily cools by synchrotron radiation, the implied local magnetic field strength is ∼15–30 G. In the context of a jetted X-ray binary progenitor, the additional RM screen and magnetic field strength are explainable with a short-lived knot in a putative jet. 

Abstract We determine the mid-infrared (MIR, ∼5–22μm) extinction toward the Galactic center using MIRI/Medium-Resolution Spectrometer (MRS) integral field unit observations of the central 3″ × 3″ region (near 5μm) to 7″ × 7″ region (near 22μm). To measure the MIR extinction, we employ two approaches: modeling the intrinsic-to-observed dust thermal spectrum and assessing the differential extinction between hydrogen recombination lines. Expanding on prior work, we directly model the dust-opacity distribution along the line of sight, and we make available a Python code that provides a flexible tool for deriving intrinsic dust emission spectra. We confirm the spatial variability of extinction across the field, demonstrating that dusty sources—such as IRS 29N—exhibit higher local extinction. Furthermore, we verify the absence of emission features from polycyclic aromatic hydrocarbons in the MIR spectra of the Galactic center. Using the two complementary methods, we derive a refined “best guess” MIR extinction law for Sgr A* and the surrounding Galactic-center region. By applying the extinction law to an MIR flare measurement discussed in a companion paper, we estimate a residual relative extinction uncertainty for the short MIRI/MRS grating of the order of 0.2 mag from ∼5 to ∼18μm and ∼0.3 mag from ∼18 to ∼22μm, consistent with our uncertainty estimate. 

Abstract JWST/MIRI observations can place photometric limits on the presence of an intermediate-mass black hole (IMBH) near the Galactic Centre. The stellar complex IRS 13E, a co-moving conglomerate of young and massive stars, is a prime location to study because it has been speculated to be bound by an IMBH. Assuming a standard radiatively inefficient accretion flow (RIAF) and a minimum fractional variability of 10% of the intrinsic luminosity, the wavelength of peak emission in the spectral energy distribution for an IMBH would lie in the mid-infrared (∼5–25μm), and the variability would be detectable in MIRI time-series observations. Monitoring fails to detect such variable emission (other than from Sgr A*) in and around the IRS 13E complex, and upper limits on a putative IMBH’s intrinsic variability on timescales of minutes to ∼1 hr are ≲ 1 mJy at 12μm and ≲2 mJy at 19μm. These translate to luminosities ≲ 25 × 10³²erg s⁻¹. The resulting limits on the IMBH mass and accretion rate rule out any IMBH with mass ≳10³M_⊙accreting at ≳10⁻⁶times Eddington rate at the location of IRS 13E. Further, the observations rule out an IMBH anywhere in the central 6″ × 6″ region that is more massive than ≈ 2 × 10³M_⊙and accreting at ≥10⁻⁶of the Eddington rate. Assuming Bondi accretion scaled to typical RIAF-accretion efficiencies, albeit somewhat uncertain, also allows us to rule out IMBHs moving with typical velocities ∼200 km s⁻¹and masses ≳ 2 × 10³M_⊙. These methods showcase the effectiveness of photometric variability measurements in constraining the presence of accreting black holes in Galactic centre-like environments. 

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 The integral-field unit mode of the Near-Infrared Spectrograph (NIRSpec+IFU) mounted on the James Webb Space Telescope has now enabled kinematic studies of smaller and less massive compact stellar systems in which to search for central massive black holes (BHs) than ever before. We present here the first such detection using NIRSpec+IFU in its highest resolution (R ∼ 2700) mode. We report the detection of a central BH with mass ${\mathcal{M}}_{\mathrm{BH}}$= 2.1 ± 1.1 × 10⁶M_⊙(1σuncertainties) in UCD736 orbiting within the Virgo galaxy cluster. Schwarzschild modeling of the 1D kinematic profile rules out a zero-mass central BH at the 3σlevel; however, two other independent modeling approaches fail to rule out a zero-mass BH at >1σsignificance. The presence of such a massive BH strongly argues against a globular cluster origin of this ultracompact dwarf galaxy (UCD), and rather suggests a tidally stripped formation route from a former ≳10⁹M_⊙dwarf galaxy host. This represents the detection of a BH in the most compact (r_h ≈ 15 pc) stellar system to date, with a ${\mathcal{M}}_{\mathrm{BH}}$corresponding to ∼8% of the system’s stellar mass, roughly in line with previously reported UCD BH detections and comparable to the BH detected in the compact elliptical galaxy NGC4486B. 

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.