Master Colloquium
Lennart Böhm	SCHEDULED

MPIfR

With the origin of chlorine presumed to be in core-collapse supernovae, stellar models predict the ratio of its two stable isotopes 35Cl/37Cl, between 1 and 4, but observations of Cl-bearing molecules have been limited due to their high-lying transitions. I will present observations of the HCl (1–0) line at 625 GHz, carried out using the SEPIA660 receiver on the APEX telescope. We detected both isotopes of HCl toward 27 Galactic sources, spanning a range of galactocentric radii, doubling the number of sources toward which it has previously been detected. Toward 11 sources we see pure emission with hints of an outflow wing while the remaining sources display complex profiles with both emission and absorption. In addition, the HCl detected in absorption toward NGC 4945, the first detection of this species in a nearby galaxy. For the Galactic sources, we obtained an isotopic ratio between 2.0 and 2.6 with an average value of 2.2+/-0.2. Further, we performed a radiative transfer analysis using RADEX with recently computed collisional rate coefficients between HCl and H2, which constrained HCl-bearing gas to trace warm, dense gas in the core and hot, translucent gas in the outflow.

	Special Colloquium
Dr. Felix Thiel	SCHEDULED

Queen University Canada

Very Long Baseline Interferometry (VLBI) is a technique in radio astronomy commonly used to image radio galaxies and their central black hole shadows at extremely high resolution. The resolution of ground-based VLBI however is limited both in baseline and observing frequency by the size of the Earth and molecular absorption of the atmosphere resepectively. While space missions are extremely costly, balloon-borne VLBI would give access to frequencies beyond those of the Event Horizon Telescope (EHT) at a fraction of the cost. As a first proof of concept for balloon-borne VLBI, the Balloon-borne VLBI Experiment (BVEX) will launch as part of the Canadian Space Agency (CSA) Stratos program from Timmins, Ontario, Canada in August 2025 and will operate above 99% of the Earth's atmosphere. This experimet consists of a K-band 22 GHz radio telescope and receiver, where the singals will be correlated with a large ground-based telescope. In this talk I will give an overview of the telescope, receiver and backend as well as technical challenges such as position tracking and timing reference stability that arise when attempting VLBI from the stratosphere. To wrap up, I will talk about how lessons learned from BVEX will help in designing a mm/sub-mm VLBI mission in the future, which could improve the uv-coverage of VLBI networks such as the EHT or next-generation EHT (ngEHT).

	Special Colloquium
Alejandro Mus	SCHEDULED

Universitat de Valencia

In this talk, I present new frameworks for mm-VLBI data analysis, which include 1) A scattering mitigation framework for dynamic imaging of time-variable or static objects and 2) A machine learning pipeline for transient detection in VLBI data. We expect the combination of the two frameworks would have a strong impact on a variety of science topics like the long period repeating transient searches. First, I introduce the advanced optimization techniques MOEA/D (Müller&Mus+2023, Mus&Müller+2024) and Particle Swarm Optimization (Mus+2024) whose flexibility allows the modeling for scattering mitigation based on Stochastic Optics (Johnson16).Then, I present results on synthetic movies at horizon scales, exploring different intrinsic structures, source evolution and scatteringparameters (like the screen distance, and speed). In a second step, I will show the performance of this framework at 86 GHz, by using synthetic Sagittarius A* (SgrA*) Global mm-VLBI Array(GMVA) data with various corruptions. We show that our algorithm can recover the intrinsic SgrA* ring and the scattering screen, thanks to the advantages of MOEA/D and PSO. Next, I will introduce a new machine learning pipeline for detecting transients in VLBI data. Very recently, pulsars with unexpected low period have been found (for instance Caleb+2022,2024,Hurley-Walker+2023). We aim to revisit old data using our fast and unsupervised algorithm to try to detect hidden slow transient. Finally, I will talk about ongoing work using full bandwidth capabilities aim to mitigate scattering at lower frequencies, helping the transient search with novel telescopes like next-generation VLA.