Uwe Bach

Context . Modern digital radio astronomy receivers produce increasingly wide-bandwidth, high bit-rate data streams that necessitate the development of flexible, scalable, and maintainable back-end processing and recording systems. Historically, such back-end instrumentation has been tightly coupled to telescope observing modes, limiting reuse between observatories and science cases. Aims . We present the Effelsberg Direct Digitisation (EDD) back-end framework, a software-defined architecture for constructing real-time radio astronomy back ends on commodity off-the-shelf computing infrastructure. We describe its design, implementation, supported observing modes, and operational deployments. Methods . EDD separates a common core framework from plug-in-provided observing capabilities. The core provides orchestration, telescope interfaces, pipeline lifecycle management, monitoring, and deployment tooling, while plug-ins implement processing pipelines for specific observing modes. The framework is designed to support both single-dish and interferometric instruments through site-specific configuration and plug-in selection. Results . EDD currently supports spectroscopy and spectropolarimetry, pulsar timing and searching, baseband recording, very long baseline interferometry, correlation, and beam forming. Operational deployments include the Effelsberg 100-m telescope, the SKA-MPI prototype dish, the Thai National Radio Telescope, and the ARGOS interferometric prototype array. We show that when coupled with in-receiver digitisation, the EDD back end at Effelsberg provides ∼30% improvement in pulsar time-of-arrival estimation compared with previous instruments, given the same observing bandwidth and integration time. Conclusions . EDD provides a reusable open-source framework for developing and operating real-time radio astronomy back ends. By separating common services, observing-mode plug-ins, and site-specific configuration, it allows back-end capabilities to be deployed across heterogeneous telescope environments and provides a community resource for broadband radio astronomy instrumentation.

Relativistic jets launched by active galactic nuclei are fundamental for understanding the physics of accreting supermassive black holes and their immediate environment, but the mechanisms that drive the jet launching remain uncertain. We investigate the sub-parsec jet of 3C 84 using multi-epoch multi-frequency very long baseline interferometry (VLBI) observations with the European VLBI Network and the Very Long Baseline Array at 22 and 43 GHz. We analysed the evolution of the spectral index gradient in the core region to relate the observed structure to physical interpretations and to distinguish between competing jet-launching models. Furthermore, we examined the effect of the ambient medium and magnetic field configuration on the jet morphology and dynamics over time and explored their connection to a coinciding γ -ray flare. Our spectral analysis reveals significant changes across three epochs, indicating dynamic activity between filamentary structures on sub-parsec scales, evolving magnetic fields, and a complex interaction with the surrounding medium, all of which shape the innermost jet and might affect its high-energy emission.

Context. Sagittarius A* (Sgr A ^* ), the supermassive black hole at the center of the Milky Way, provides a unique laboratory to study accretion dynamics and plasma processes near the event horizon. Aims. We investigated the variability and polarization properties of Sgr A ^* using ALMA observations during the 2018 Event Horizon Telescope campaign. Methods. We analyzed high-cadence full-polarization light curves from ALMA at millimeter wavelengths, performed time-series analysis, and investigated the temporal behavior during an X-ray flare observed by Chandra on 2018 April 24. The variability characteristics are compared with expectations from standard accretion flow models. Results. We find low variability in total intensity ( σ / μ < 10%), but significantly higher variability in linear and circular polarization (∼30% and ∼50%, respectively). A time-series analysis reveals red-noise variability, with power spectral densities between −2 and −3 across all Stokes parameters. Polarized intensity shows stable intra-day timescales, while total intensity exhibits more variable timescales, suggesting distinct emission regions, with polarization likely arising from a coherent structure. On April 24, a statistically significant inter-band delay in polarized intensity coincides with a near-simultaneous X-ray and millimeter peak that deviates from the typical delayed flare scenario. This event also features enhanced millimeter variability and coherent polarization loop evolution. The observed simultaneity challenges standard models of transient synchrotron emission with cooling delays, favoring instead a scenario of continuous energy injection in an optically thin region. Conclusions. Our results offer new constraints on the physical mechanisms driving variability in Sgr A ^* , and provide key observational input for refining theoretical models of accretion and plasma behavior in the vicinity of supermassive black holes.

We investigate the presence and spatial characteristics of the jet base emission in M87* at 230 GHz, enabled by the significantly enhanced ( u,v ) coverage in the 2021 Event Horizon Telescope (EHT) observations. The integration of the 12−m Kitt Peak Telescope (USA) and NOEMA (France) stations into the array introduces two critical intermediate-length baselines to SMT (USA) and IRAM 30−m (Spain), providing sensitivity to emission structures at spatial scales of ∼250 μas and ∼2500 μas (∼ 0.02 pc and ∼ 0.02 pc). Without these new baselines, previous EHT observations of the source in 2017 and 2018 lacked the capability to constrain emission on large scales, where a “missing flux” of order ∼1 Jy is expected to reside. To probe these scales, we analyzed closure phases–robust against station-based gain calibration errors–and model the jet base emission using a simple Gaussian component offset from the compact ring emission at spatial separations > 100 μas. Our analysis revealed a Gaussian feature centered at (ΔRA ≈ 320 μas, ΔDec. ≈ 60 μ as), projected separation of ≈ 5500 AU, with an estimated flux density of only ∼60 mJy, implying that most of the missing flux identified in previous EHT studies had to originate from different, larger scales. Brighter emission at the relevant spatial scales is firmly ruled out, and the data do not favor more complex models. This component aligns with the inferred position of the large-scale jet and is therefore physically consistent with the emission of the jet base. While our findings point to detectable jet base emission at 230 GHz, the limited coverage provided by only two intermediate baselines limits our ability to robustly reconstruct its morphology. Consequently, we treated the recovered Gaussian as an upper limit on the jet base flux density. Future EHT observations with expanded intermediate baseline coverage will be essential to constrain the structure and nature of this component with higher precision.

In Very-Long Baseline Interferometric arrays, nearly co-located stations probe the largest scales and typically cannot resolve the observed source. In the absence of large-scale structure, closure phases constructed with these stations are zero and, since they are independent of station-based errors, they can be used to probe data issues. Here, we show with an expansion about co-located stations, how these trivial closure phases become non-zero with brightness distribution on smaller scales than their short baseline would suggest. When applied to sources that are made up of a bright compact and large-scale diffuse component, the trivial closure phases directly measure the centroid relative to the compact source and higher-order image moments. We present a technique to measure these image moments with minimal model assumptions and validate it on synthetic Event Horizon Telescope (EHT) data. We then apply this technique to 2017 and 2018 EHT observations of M87* and find a weak preference for extended emission in the direction of the large-scale jet. We also apply it to 2021 EHT data and measure the source centroid about 1 mas northwest of the compact ring, consistent with the jet observed at lower frequencies.

The γ -ray–loud blazar TXS 2013+370, a powerful multiwavelength emitter at z = 0.859, underwent an exceptional gigaelectron volt outburst in late 2020 to early 2021. In this work, we present full polarization VLBI imaging at 22, 43, and 86 GHz (11 February 2021) together with contemporaneous single-dish monitoring (OVRO 15 GHz; SMA 226 GHz) and Fermi –LAT light curves to localize the high-energy dissipation site and probe the magnetic field of the inner jet. The images enabled us to study the jet structure and field topology on sub-parsec scales, revealing a compact near-core knot at r ≃ 40 − 60 μas along with the gigaelectron volt (GeV) flare and a flat core-dominated spectrum ( α ≳ −0.5). The core has strong linear polarization and exhibits a ∼50° electric vector polarization angle rotation at 86 GHz. The pixel-based and integrated fits we employed yielded a high, uniform rotation measure, RM = (7.8 ± 0.2)×10 ⁴ rad m ⁻² , consistent with an external Faraday screen. Performing a cross-correlation of Fermi –LAT and 15 GHz light curves revealed a highly significant peak, with the γ rays leading by Δ t = (102 ± 12) d. Adopting β _app = 4.2 ± 0.5 and θ = 4.1° ±0.2° implies a de-projected separation of Δ r _{γ  − 15} = (2.71 ± 0.47) pc and locates the GeV emission between the jet apex and ∼0.42 pc (in the 1 σ range) downstream. Our results do not pinpoint the emission site; rather, they support two valid scenarios. The γ -ray production occurs within the broad-line region (∼0.07 pc), where external-Compton scatters optical/UV photons to γ -rays, and beyond the broad-line region, reaching ∼0.42 pc (1 σ ) within the inner parsecs, where external-Compton scattering of dusty-torus infrared photons dominates. Both scenarios are compatible in the allowed range of emission distances, while opacity-driven core shifts modulate the observed radio– γ delay without requiring large relocations of the dissipation zone.

We report the milliarcsecond localization of a high (∼1379 pc cm ⁻³ ) dispersion measure (DM) repeating fast radio burst (FRB), FRB 20190417A. Combining European VLBI Network detections of five repeat bursts, we confirm the FRB’s host to be a low-metallicity, star-forming dwarf galaxy at z = 0.12817, similar to the hosts of FRB 20121102A, FRB 20190520B, and FRB 20240114A. We also confirm that it is associated with a previously reported persistent radio source (PRS), which is compact on milliarcsecond scales. Visibility-domain model fitting constrains the transverse physical size of the PRS to μ Jy at 1.4 GHz. Though we do not find significant evidence for DM evolution, FRB 20190417A exhibits a time-variable rotation measure (RM) ranging between +3958 ± 11 rad m ⁻² and +5061 ± 24 rad m ⁻² over a 50-day period. We find no evidence for intervening galaxy clusters in the FRB’s line of sight and place a conservative lower limit on the rest-frame host DM contribution of 1228 pc cm ⁻³ (90% confidence)—the largest known for any FRB so far. This system strengthens the emerging picture of a rare subclass of repeating FRBs with large and variable RMs, above-average host DMs, and luminous PRS counterparts in metal-poor dwarf galaxies. Our results suggest that these systems are the result of environmental selection, or a distinct engine for FRB emission.

We report three epochs of polarized images of M87* at 230 GHz using data from the Event Horizon Telescope (EHT) taken in 2017, 2018, and 2021. The baseline coverage of the 2021 observations is significantly improved through the addition of two new EHT stations: the 12 m Kitt Peak Telescope and the Northern Extended Millimetre Array (NOEMA). All observations result in images dominated by a bright, asymmetric ring with a persistent diameter of 43.9 ± 0.6 μas, consistent with expectations for lensed synchrotron emission encircling the apparent shadow of a supermassive black hole. We find that the total intensity and linear polarization of M87* vary significantly across the three epochs. Specifically, the azimuthal brightness distribution of the total intensity images varies from year to year, as expected for a stochastic accretion flow. However, despite a gamma-ray flare erupting in M87 quasi-contemporaneously to the 2018 observations, the 2018 and 2021 images look remarkably similar. The resolved linear polarization fractions in 2018 and 2021 peak at ∼5%, compared to ∼15% in 2017. The spiral polarization pattern on the ring also varies from year to year, including a change in the electric vector position angle helicity in 2021 that could reflect changes in the magnetized accretion flow or an external Faraday screen. The improved 2021 coverage also provides the first EHT constraints on jet emission outside the ring, on scales of ≲1 mas. Overall, these observations provide strong proof of the reliability of the EHT images and probe the dynamic properties of the horizon-scale accretion flow surrounding M87*.