Special Colloquium
Dr. Willem Baan	CANCELED

ASTRON, The Netherlands

TBD

	Informal Colloquium
Dr. Sheperd S. Doeleman	ORATED

Center for Astrophysics | Harvard & Smithsonian

The next-generation Event Horizon Telescope (ngEHT) is a transformative instrument that will be capable of making real-time and time-lapse movies of supermassive black holes on event horizon scales. These movies will resolve complex structure and dynamics on Schwarzschild radius dimensions, bringing into focus not just the persistent strong-field gravity features predicted by General Relativity (GR), but details of active accretion and relativistic jet launching that drive large scale structure in the Universe. This effort builds upon recent results by the Event Horizon Telescope (EHT): the first image of M87’s supermassive black hole and its magnetic field structure, as well as resolved images of SgrA*, the central black hole at the heart of the Milky Way. These images are scientifically rich, and show that evolution of the EHT to a more capable array can address even deeper questions across physics and astronomy. The central concept behind the ngEHT is that the addition of many modest-diameter dishes at new geographic locations will enable high-dynamic-range imaging and time-resolved dynamical studies of the black hole boundary that extend beyond planned upgrades to the existing global mm/submm VLBI array.

	Main Colloquium
Prof. Lukasz Wyrzykowski	ORATED

Warsaw University (Poland)

Most of about 60 known stellar-mass black holes were found in binaries (X-ray binaries and GW mergers). Gravitational microlensing is the only tool capable of detecting single black holes, which are not interacting with anything. I will present our long-term project aiming at discovering and studying the microlensing black holes with the OGLE, Gaia and forthcoming Rubin/LSST surveys. Microlensing non-detections towards the Magellanic Clouds have put strong limits on the compact dark matter content in the Galaxy Halo, while the statistical studies of microlensing events towards the Galactic Centre, revealed a continuum of masses of dark lenses and hint at a lack of the mass gap between neutron stars and black holes. However, in order to study individual events and obtain the masses of individual lenses it is crucial to measure the size of the Einstein Radius, which defines a separation between the lensed images and is a physical parameter degenerated in the classical microlensing light curve model. This is possible en masse only with the Gaia space mission, which scans the entire sky and is providing not only brightness and colour temporal evolution for nearly 2 billion stars but also positional time series with sub-milliarcsecond precision. I will describe how we search for on-going microlensing events within daily Gaia data using Gaia Science Alerts system and how their photometric, spectroscopic and interferometric follow-up is conducted. I will present the first candidates for dark lenses from Gaia and then describe how these lessons learnt can be applied to the forthcoming Rubin/LSST survey and GRAVITY+ instrument.

	Main Colloquium
Dr. Dominic Pesce	ORATED

Center for Astrophysics - CfA Harvard

Water megamasers residing in the accretion disks around supermassive black holes in active galactic nuclei (AGN) provide unique tools for bypassing the distance ladder and making one-step, geometric distance measurements to their host galaxies. The Megamaser Cosmology Project (MCP) is a multiyear campaign to find, monitor, and map such AGN accretion disk megamaser systems, with the goal of constraining the Hubble constant to a precision of several percent. In this talk I will cover the latest results from the MCP. We have systematically applied an updated disk modeling technique to measure the distances to six megamaser-hosting galaxies, which together constrain the Hubble constant to 73.9 +/- 3.0 km/s/Mpc. This value relies solely on maser-based distance and velocity measurements, and it does not use any peculiar velocity corrections. We have explored a variety of different approaches for correcting peculiar velocities, none of which modify this constraint by more than 1-sigma. Our measurement is independent of distance ladders, the cosmic microwave background, and gravitational lenses, and it corroborates prior indications that the local Hubble constant exceeds the early-Universe prediction.

	Master Colloquium
Kathrin Grunthal	ORATED

MPIfR

We will explore the method of precision pulsar timing as a tracer of space-time deformations both in the vicinity of the pulsar, caused by ist host system, as well as a probe of gravitational waves (GWs) crossing the lines-of-sight between multiple pulsars and the Earth. Subsequently, this work is set up in a two-fold manner: In the first part, we present an analysis of the millisecond pulsar (MSP) J1618-3921 using radio observations from MeerKAT, Parkes and Nancay. We investigate its emission behavior, comprising a change of its mean stable profile, as well as a Rotating-Vector-Model fit of the linear polarisation position angle swing. The main focus lies on the following timing analysis using the 23-year long timing baseline stretching back to the discovery observations in 1999. We present the first timing solution of this pulsar containing a binary model and a mass estimate for both the pulsar and its companion. We also discuss the measurement of the change of the orbital period, which strongly diverges from its expected value, indicating the presence of a third massive object in the pulsar's vicinity. With J1618-3921 belonging to the class of eccentric MSPs, all these results shed new light on this enigmatic pulsar population, raising new questions to current stellar evolution models. The second part holds a theoretical approach to the detection of continuous GWs with current pulsar timing arrays (PTAs). We discuss the mathematical framework of PTA sensitivity curves and its impementation. We apply it to the European Pulsar Timing Array (EPTA) using its latest 25 pulsar data set, followed by a comparison with the Northern American Nanohertz Observatory for Gravitational Waves (NANOGrav), as well as the full GW detector network available in the mid 2030s.

	Main Colloquium
Prof. Nanase Harada	ORATED

National Astronomical Observatory of Japan

ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) is a spectra scan towards the nearby starburst galaxy NGC 253. It covers a wide frequency range of ALMA Bands 3-7, and was conducted as an ALMA Large Program in Cycle 5. The interstellar medium (ISM) in starburst galaxies is expected to have different physical properties from the ISM in the Milky Way due to the high star formation activity. This difference should also appear in chemical composition. The ALCHEMI survey aims to analyze this ISM property in one of the nearest starburst galaxies. The survey detected about 1500 transitions and more than 100 species. They include complex organic molecules that originated from the hot and dense starburst region. Comparing chemical models and observed chemical abundances, our results show 1-2 orders of magnitude higher cosmic-ray ionization rates than the Galactic Center. They also demonstrate multiple signs of shocks. A statistical study to extract physical features from the survey images is also on the way.

	Main Colloquium
Prof. Stephen Taylor	ORATED

Vanderbilt University, Nashville, USA

The nanohertz-frequency band of gravitational waves should be awash with signals from supermassive black-hole binaries, as well as cosmological signatures of phase transitions, cosmic strings, and other relics of the early Universe. Pulsar-timing arrays (PTAs) are poised to chart this new frontier of gravitational-wave discovery within the next several years using precision timing data recorded by flagship radio telescopes from dozens of pulsars in the Milky Way. I will present exciting new results from recent cutting-edge searches, discuss some milestones on the road to the next decade of PTA discovery, and how these results will influence and shape other fields.

	Master Colloquium
Nipesh Dulal	ORATED

MPIfR

In this work, we examined the properties of the molecular gas in the Central Molecular Zone (CMZ) of our Galaxy from the APEX CMZ survey data, which imaged the 13CO(2-1) emission from CMZ at an unprecedented sensitivity. We employed the automatic algorithms dendrograms and SCIMES to segment clouds and construct a catalog of the integrated properties of almost 4200 clouds in the CMZ. Our catalog contains clouds ranging from small (1 pc) scale clouds to large cloud complexes, with a distribution of sizes similar to that of the inner Galaxy. However, the clouds in the CMZ appear more turbulent than those of the inner Galaxy. The average molecular cloud surface mass densities estimated were lower than in previous catalogs, and we found a lower CO-to-H2 conversion factor (X_CO) in the CMZ, which was calculated using local thermodynamic equilibrium assumptions. Additionally, the star formation efficiency and the dense gas mass fraction are significantly lower for the clouds in the CMZ compared to those in the inner part of the Galaxy. Overall, the extreme ambient environment of the CMZ prevents its surrounding molecular gas from forming stars.

	Special Colloquium
Alejandro Mus	ORATED

Universitat de Valencia

The Galactic Center (GC) presents one of the highest stellar densities in our Galaxy, making its surroundings an environment potentially rich in radio transients, such as pulsars and different kinds of flaring activity. In this talk, I will present the first study of transient activity in the region of the GC based on Atacama Large Millimeter/submillimeter (mm/submm) Array (ALMA) continuum observations at 230 GHz. This search is based on a new self-calibration algorithm, especially designed for variability detection in the GC field. Using this method, we have performed a search of radio transients in the effective field of view of ∼30arcseconds of the GC central supermassive black hole Sagittarius A* (SgrA*) using ALMA 230 GHz observations taken during the 2017 Event Horizon Telescope (EHT) campaign, which span several observing hours (5-10) on 2017 April 6, 7, and 11. This calibration method allows one to disentangle the variability of unresolved SgrA* from any potential transient emission in the wider field of view and residual effects of the imperfect data calibration. Hence, a robust statistical criterion to identify real transients can be established: the event should survive at least three times the correlation time and it must have a peak excursion of at least seven times the instantaneous root-mean-square between consecutive images. Our algorithms are successfully tested against realistic synthetic simulations of transient sources in the GC field. Having checked the validity of the statistical criterion, we provide upper limits for transient activity in the effective field of view of the GC at 230 GHz.

	Special Colloquium
Dr. Nicolas Aimar	ORATED

LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université

For the past two decades, flares (i.e. outbursts of radiation) have been observed from the centre of the Milky Way where a massive compact object of 4.3 millions solar masses resides at only 8.3 kpc. This makes this object called Sgr A* the closest supermassive black hole candidate to Earth and an unique laboratory for relativistic astrophysics. Recent observations have shown that the source of these outbursts is close to the event horizon and has an orbital motion around the black hole. Many scenarios are envisaged to explain this phenomenon without reaching a consensus. Among these scenarios, magnetic reconnection is one of the most promising, supported by many GRMHD and PIC studies. During this presentation I will present a realistic semi-analytical magnetic reconnection model based on kinetic simulations. I will examine the diversity of observables associated to these models and discuss them in the light of the recent VLTI/GRAVITY observations of Sgr A* flares

	Main Colloquium
Prof. Ralf Klessen	ORATED

Universität Heidelberg

Stars and star clusters form by gravitational collapse in regions of high density in the dynamically evolving magnetised multi-phase interstellar medium. The process of stellar birth is controlled by the intricate interplay between the self-gravity of the star-forming gas and various opposing agents, such as supersonic turbulence, magnetic fields, radiative feedback, cosmic rays, and gas pressure. I will discuss examples of recent progress and controversy, specifically, I will focus on current attempts to bring numerical simulations and theoretical models closer to the observational domain. In this context, I would like to report on efforts to model magnetic field evolution and generate synthetic polarisation and Faraday rotation maps for comparison with the observational data.