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Black hole X-ray binaries are capable of launching compact radio jets, even in their quiescent state where they spend the majority of their time accreting at low Eddington ratios l_edd<1e-5. There are still several open questions regarding the physical properties of quiescent black hole jets and accretion flows. Here, we apply a steady-state multizone jet model BHJet to the broadband spectrum of the highly quiescent black hole X-ray binary XTE J1118+480 (l_Edd~1e-8.5), and we compare our results to previous work that applied a predecessor model to the same broadband spectrum. We find two statistically comparable model fits that explain the X-ray emission as either synchrotron self-Compton (from a thermal distribution of particles in the jet base) or as synchrotron-cooled radiation (from an accelerated tail of nonthermal particles). However, we argue that the synchrotron self-Compton fit is more realistic. Finally, we confirm previous suggestions that the jet base becomes cooler and more compact in quiescence, compared to hard state black hole X-ray binaries 

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 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 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. 

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 Infrared interferometry is a new frontier for precision ground-based observing, with new instrumentation achieving milliarcsecond (mas) spatial resolutions for faint sources, along with astrometry on the order of 10 microarcseconds (μas). This technique has already led to breakthroughs in the observations of the supermassive black hole at the Galactic centre and its orbiting stars, active galactic nucleus, and exo-planets, and can be employed for studying X-ray binaries (XRBs), microquasars in particular. Beyond constraining the orbital parameters of the system using the centroid wobble and spatially resolving jet discrete ejections on mas scales, we also propose a novel method to discern between the various components contributing to the infrared bands: accretion disc, jets, and companion star. We demonstrate that the GRAVITY instrument on the Very Large Telescope Interferometer should be able to detect a centroid shift in a number of sources, opening a new avenue of exploration for the myriad of transients expected to be discovered in the coming decade of radio all-sky surveys. We also present the first proof-of-concept GRAVITY observation of a low-mass XRB transient, MAXI J1820+070, to search for extended jets on mas scales. We place the tightest constraints yet via direct imaging on the size of the infrared emitting region of the compact jet in a hard state XRB. 

The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide.