Pulsar Timing Array (PTA) collaborations around the globe recently
announced compelling evidence for a gravitational-wave background (GWB)
at nanohertz frequencies. This breakthrough achievement has important
implications for astrophysics, as the GWB signal, if genuine, is likely
to originate from a cosmic population of supermassive black holes
orbiting each other at the centers of galaxies. As I will illustrate in
this talk, the new PTA data is, however, also of great interest to the
high-energy physics community, as it allows to probe a broad range of
particle physics models of the early Universe that predict the
generation of a cosmological GWB in the Big Bang. In this sense, the PTA
data opens a new window onto the very early Universe and enables
particle physicists to constrain scenarios of new physics beyond the
Standard Model at extremely high energies. In my talk, I will give an
overview of these searches for new physics at the PTA frontier and
highlight several cosmological scenarios that underline the relevance of
PTA observations for fundamental problems such as dark matter, neutrino
masses, and the matter-antimatter asymmetry of the Universe. Finally, I
will conclude with a brief outlook on future measurements that may help
in discriminating between a GWB signal of astrophysical origin and a GWB
signal from the Big Bang.
A journey of inference around black holes: From ALMA to VLBI (EHT) data ; from semi-analytic to GRMHD models.
Special Colloquium
Aristomenis Yfantis
ORATED
U. Radboud
Since the turn of the millennia, there has been a substantial growth in the field of black hole physics, both in observational methods and theoretical predictions. This has created a great need for methods of inference, in order to extract the most of our data and inform our models as best possible. In this talk I’ll be presenting my work on that topic exploring two different datasets and model types from SgrA*.
Jocelyn Bell-Burnell’s Role in the Discovery of Pulsars, and An Update on the Parkes Pulsar Timing Array Project
Main Colloquium
Dr. Dick Manchester
ORATED
ATNF, CSIRO, Sydney, Australia
Pulsars are among the most fascinating and important objects in the
Universe. Not only is their formation process and how they function of
great interest, but they also serve as sensitive probes of, for
example,
gravitational theories and the Galactic and extragalactic interstellar
medium. This talk is in two parts. In the first part I will describe
how
pulsars were discovered and, in particular, Jocelyn Bell-Burnell’s
role in the 1968 discovery. With her thesis supervisor, Antony Hewish,
she first helped construct and test the “4-acre” array near
Cambridge, England. She then searched several km of chart recordings
looking for compact radio sources that scintillated in the
inter-planetary medium, serendipitously discovering pulsars in the
process. In the second part of the talk, I will describe the Parkes
Pulsar Timing Array (PPTA) and its role in the international search for
nanoHertz gravitational waves using PTAs. Since mid-2018, the PPTA has
made use of the “Ultra-Wideband Low” (UWL) receiver. I will
describe
this receiver and some recent results obtained using it.
Pulsar Scintillation and Interstellar Lenses
Promotionskolloquium
Tim Sprenger
ORATED
MPIfR
The study of scintillation arcs is a quickly growing field aiming at
understanding the small-scale structure of the interstellar medium
(ISM). Scintillation is caused by interference of scattered paths
through the ISM. The resulting intensity variations contain information
on the scattering structures, which manifests in scintillation arcs. I
worked on a novel technique to extract this information with high
resolution. The main object of study in my thesis is PSR B1508+55, which
showed peculiar scintillation phenomena in the past. I will present
observations made at the Effelsberg 100-m telescope spanning from 2020
to 2023, during which the pulsar goes through various phases of
scintillation arcs with rich substructure. Many observables could be
monitored over four years, one of which was newly discovered in this
study. The result is one of the most constraining data sets of pulsar
scintillation, which clearly points to two scattering screens instead of
one. I will show how such an extended model can explain the
scintillation properties of PSR B1508+55.
Properties of Outer Galaxy Large-Scale Filaments
Master Colloquium
Eleonore Dann
ORATED
MPIfR
Recent results of the Outer Galaxy High Resolution Survey (OGHReS,
PI's: C. König, J. Urquhart) by Colombo et al. (2021) have shown
that
a significant number of large-scale filaments exist in the outer Milky
Way as well. As the physical and chemical conditions in the outer
Milky
Way differ significantly compared to the inner Galaxy, these highly
resolved large-scale filaments serve as a unique laboratory on how
star
formation changes with environment and Galactocentric distance. To
study the star formation activity of these outer Galaxy large-scale
filaments, we used APEX 12CO(2-1) data from the OGHReS Survey, their
dendrograms (Colombo et al. 2021, Rosolowsky et al. 2008) and dust
emission of the Herschel Infrared GALactic Plane Survey (Hi-GAL,
Molinari et al. 2010). We investigated the 13CO(2-1) emission and
determined the 13CO(2-1) to 12CO(2-1) line ratios in the filaments.
Many
Galactic parameters, such as the gas-to-dust ratio (Giannetti et al.
2017), are expected to change with Galactocentric distance in line
with
the physical and chemical conditions. The gas-to-dust ratio in the
star-forming regions of the filaments has been derived and compared
with the expected gas-to-dust ratio from Giannetti et al. (2017).
Furthermore, we have searched for warm and cold HI gas towards the
filaments using the HI4PI (Bailin et al. 2016) and SGPS survey
(Griffiths et al. 2005, Haverkorn et al. 2006). While almost all
filaments appear to be quiescent based on the absence of protostellar
clumps, we found that the longest filament (~140 pc) in the sample
stands out as a prominent candidate to host ongoing high-mass star
formation. The filament is located in an inter-arm region at a
Galactocentric distance of 9.6 kpc (Colombo et al. 2021) and appears
to
be associated with HI shells. The gas-to-dust ratios in the filaments
differ significantly from each other, whereby our average logarithmic
gas-to-dust ratio is in agreement with the expected value of
log(ɣ)=2.36 at the average Galactocentric Distance of the filaments
of
10.6 kpc by Giannetti et al. (2017). Furthermore, we observed that
most
of the outer Galaxy large-scale filaments are associated with warm and
dense HI gas.
Numerical Simulations of accreting black holes and relativistic jets
Main Colloquium
Dr. Christian Fromm
ORATED
Uni. Wuerzburg
Relativistic jets are among the most powerful objects in the universe.
They are launched from rotating supermassive black holes and accelerate
particles to highest energies. However, the detailed mechanism behind
their formation and acceleration is still unknown. In order to study
relativistic jets we perform numerical simulation of jet launching as
well as propagation simulation on horizon scales and beyond. In order
to
compare our simulations with observations we perform radiative transfer
calculations and generate realistic synthetic data.
In the talk we will present recent results from our numerical
simulations including the exploration of particle heating mechanisms on
horizon scales and jet asymmetries on parsec scales. In addition, we
present possible radiative signatures which could be detected with
future Very-Long-Baseline Interferometric (VLBI) observations.
Astronomical Analysis of Next-Generation Global Geodetic Observations
Informal Colloquium
Mr. Ezequiel Albentosa-Ruiz
ORATED
Universitat de València, Spain
The VLBI Global Observing System (VGOS) is a next-generation VLBI
system
developed by the International VLBI Service for Geodesy and Astrometry
(IVS) to provide geodetic observations with ultra-wideband receivers
observing from 3 GHz to 11 GHz. These receivers observe in linear
polarisation to achieve optimal polarisation purity.
In this talk we will explore the intricacies of calibrating VGOS data,
including the conversion from linear to circular polarisation using
PolConvert and the implementation of the Wideband Global Fringe Fitting
(GFF) algorithm. Departing from the conventional geodetic approach of
performing fringe-fitting separately for each baseline, we present the
first results obtained by performing global fringe-fitting over a
global
IVS array of eight antennas with intercontinental baselines operating
over the full VGOS bandwidth.
This approach yields promising results and allows us to preserve
critical closure information, crucial for reconstructing the full
polarisation brightness distribution structure of the observed sources,
which we present as preliminary images for a selection of the observed
sources.
Role of the Thermal and Non-thermal Processes in Structure Formation and Evolution of Galaxies
Main Colloquium
Prof. Dr. Fatemeh Tabatabaei
ORATED
MPIA
Investigating the physics and pressure balance of the medium where
galactic structures, on various scales, are formed is the most
fundamental step to understanding the formation and evolution of
galaxies. Galaxy evolution models suggest gas accretion from the
intergalactic medium (IGM) or from cosmic filaments as a mechanism to
maintain star formation and an active galactic nucleus (AGN). These
models also propose supernova/ AGN feedback as mechanisms to quench
massive star formation. Observational studies, however, show that
feedback can, in some cases, trigger star formation, leaving the issue
as an open challenge. It seems that our understanding of the formation
and evolution of structures in the interstellar medium (ISM) and IGM
still needs to be fundamentally improved: What are the physical
parameters/factors governing structure formation on various scales? How
does the ISM/IGM energy balance change over cosmic time? There is
increasing evidence showing the important role of the non-thermal
pressures inserted by cosmic rays, magnetic fields, and turbulence. We
study the impact of the thermal and non-thermal processes in the
evolution of the ISM/IGM structures over cosmic time using both
observations and simulations from the radio to optical domains. I will
review our findings in nearby galaxies as well as those at high
redshifts.
Updates on PolConvert for non-ALMA use
Informal Colloquium
Prof. Dr. Iván Martí-Vidal
ORATED
Dep. Astronomia & Observatori Astronòmic, Universitat de València, Spain
The design of broadband and/or multiband receivers for VLBI may involve
the use of linear polarisation feeds to minimise the effects of
instrumental polarisation. I will discuss the software-based conversion
of VLBI observations taken with linear polarisation feeds.
In particular, I will present new results of PolConvert successfully
applied to a global IVS VGOS experiment (8 linear stations; 74 sources;
24 hours). I will discuss the reliability of the PolConvert conversion
and its residual instrumental polarisation as estimated from a complete
and consistent analysis of the entire dataset.
The talk could be followed by a discussion of the strategy and
applicability of PolConvert to future multi-band receivers to be
implemented at Effelsberg.
Magnetic Fields in the Milky Way and the Small Magellanic Cloud
Special Colloquium
Dr. Yik Ki Ma
ORATED
The Australian National University, Canberra, ACT, Australia
The magnetic field is a crucial constituent of the interstellar medium,
and
accurately understanding its strength and structure in galaxies has
significant implications for various areas of galactic astrophysics.
Recent
advancements in sensitivity and frequency coverage of radio telescopes
have
provided clearer insights into the magnetic fields of our Milky Way
Galaxy
and its neighbouring Small Magellanic Cloud (SMC). In this talk, I will
share
the new magnetic knowledge gained from these advanced radio
instruments,
using both linear polarisation and high-resolution HI measurements. In
the
case of the Milky Way, I utilised the Faraday rotation experienced by
linearly polarised emission from extragalactic radio sources to find
that (1)
contrary to theoretical expectations, the galactic-scale magnetic field
in
the Galactic disk can reverse its direction across the mid-plane
towards
the
Sagittarius spiral arm, and (2) the Galactic magnetic structures can
undergo
significant changes at <~ 10" scales, most likely due to the
anisotropic
turbulent magnetic fields shaped by compression and shearing. Meanwhile
in
the SMC, I find that the filamentary HI structures near its centre are
preferentially aligned with the magnetic fields traced by starlight
polarisation, suggesting that these HI structures can have the
potential
of
being a new 3D tracer of the SMC's plane-of-sky magnetic field. I will
conclude by highlighting the exciting prospects in magnetism studies
offered
by on-going southern sky radio survey, such as the POSSUM survey in
polarisation and the GASKAP survey in HI.
Advanced VLBI Imaging
Promotionskolloquium
Hendrik Mueller
ORATED
MPIfR
Very Long Baseline Interferometry (VLBI) is an observational technique developed in astronomy for combining multiple radio telescopes into a single virtual instrument with an effective aperture reaching up to many thousand kilometers and enabling measurements at highest angular resolutions. Despite the proven successes of VLBI, analysis and imaging of VLBI data still remain difficult, owing in part to the fact that VLBI imaging inherently constitutes an ill-posed inverse problem. Historically, this problem has been addressed in radio interferometry by the CLEAN algorithm, a matchingpursuit inverse modeling method developed in the early 1970-s and since then established as a de-facto standard approach for imaging VLBI data. In recent years, the constantly increasing demand for improving quality and fidelity of interferometric image reconstruction has resulted in several attempts to employ new approaches, such as forward modeling and Bayesian estimation, for application to VLBI imaging. While the current state-of-the-art forward modeling and Bayesian techniques may outperform CLEAN in terms of accuracy, resolution, robustness, and adaptability, they also tend to require more complex structure and longer computation times, and rely on extensive finetuning of a larger number of non-trivial hyperparameters. This leaves an ample room for further searches for potentially more effective imaging approaches and provides the main motivation for this dissertation and its particular focusing on the need to unify algorithmic frameworks and to study VLBI imaging from the perspective of inverse problems in general. In pursuit of this goal, and based on an extensive qualitative comparison of the existing methods, this dissertation comprises the development, testing, and first implementations of two novel concepts for improved interferometric image reconstruction. The concepts combine the known benefits of current forward modeling techniques, develop more automatic and less supervised algorithms for image reconstruction, and realize them within two different frameworks.
Radio-loud AGN in the discovery era of X-ray polarimetry
Special Colloquium
Dr. Yannis Liodakis
ORATED
University of Turku (Gruber Fellow)
X-ray polarization is a crucial probe of the magnetic field structure
and emission processes in astrophysical systems. This is particularly
true for active galactic nuclei (AGN). In radio-loud AGN, X-ray
polarimetry allows us to investigate particle acceleration and
composition in jets. Until now, polarization observations have been
limited to the radio-to-optical range, thereby leaving a gap in our
knowledge of the processes and physical conditions in the most
energetic
objects. The recently launched Imaging X-ray Polarimetry Explorer --
IXPE, the first X-ray polarization mission to target AGN, offers a
radically new way of studying high-energy processes in relativistic
jets. I will discuss the multiwavelength polarization observations and
results from the first two years of IXPE observations of radio galaxies
and blazars that clearly demonstrate the importance of X-ray
polarization in our understanding of the Universe.
Searching for the Ashes of Dead Stars
Promotionskolloquium
Rohit Dokara
ORATED
MPIfR
A supernova event marks the death of either a high-mass (>8 Msol) star,
or a white dwarf in a binary system. Such an event expels most of the
stellar interiors into the surrounding interstellar medium and forms a
supernova remnant (SNR). The global effects of SNRs on their native
galaxies (e.g., creating/destroying dust, suppressing/accelerating
star-formation activity) are poorly understood, and a complete sample of
the SNRs in our own Galaxy is necessary to obtain key observational
constraints in this regard. However, only about 300 SNRs are catalogued
in the Milky Way so far, while the expected number is of the order of
1000 or more. In this talk, I will describe our attempts to rectify
this apparent deficiency by finding new SNRs.
We identified 157 SNR candidates in the images of the 4-8 GHz GLObal
view on STAR formation (GLOSTAR) survey, which utilizes the Jansky Very
Large Array and the 100-meter Effelsberg radio telescopes. By measuring
polarization and spectral indices, we established the nonthermal nature
of the radio emission from at least seven SNR candidates, providing
evidence that they are indeed SNRs. In addition, we also found spurious
entries in the catalog of Milky Way's SNRs. We performed a Monte-Carlo
simulation of supernova events and the subsequent evolution of their
remnants, which suggested that the biggest gains in the quests to find
new SNRs can be made by searching for small angular-size SNRs that are
only a few arcminutes wide.
X-ray quasi-periodic eruptions from the nuclei of low-mass galaxies
Special Colloquium
Dr. Riccardo Arcodia
ORATED
MIT
Quasi-Periodic Eruptions (QPEs) are high-amplitude bursts of X-ray
radiation recurring every few hours and originating near the central
black holes in galactic nuclei of low-mass galaxies. So far, only a
handful of such events have been found. I will give an overview of their
multi-wavelength observational properties and of some of the models
proposed to explain them. The latest models suggest that these systems
are extreme mass ratio inspirals (EMRI), which would make QPEs their
electromagnetic counterparts.
Search for kinematic signatures of feedback in the molecular gas of the LMC
Master Colloquium
Niklas Czubkowski
ORATED
MPIfR
With its short distance and nearly face-on inclination, the LMC is an
ideal object to study star formation in molecular clouds and its
feedback onto the environment on a variety of spatial scales.
Our goal is to trace and analyse kinematic signatures of stellar
feedback mechanisms and colliding gas flows in the molecular gas phase
of the LMC. This work is based on sensitive, high-spectral resolution
observations of the 12CO(3-2) transition line with the APEX telescope at
5 pc spatial resolution in the course of the APEX LMC Legacy Survey.
Thereby we will focus on identifying kinematically disturbed regions
that may trace protostellar molecular outflows or other signatures of
feedback over large parts of the molecular disk of the LMC in the
currently covered $17.0$~deg$^2$ of the survey.\\
In order to find potentially interesting regions we searched for
line-splitting in the spectra as a first step and performed a line
fitting with multiple Gaussians on a selected sub-sample afterwards.
In total we identified 2762 regions with disturbed kinematics, taking up
6.6 % of molecular complexes identified in the LMC. Through statistical
comparisons using a multi-wavelength approach we found line-splitting
regions to be mainly associated with signatures of on-going star
formation, to have on average higher energy input, higher column
densities and to be less virialized than single-peak regions.
On the selected sub-sample we found $\sim300$ areas with multiple
velocity components, including several candidates of protostellar
outflows or colliding flows. For outflow candidates we also calculated
molecular gas mass, momentum and energy of the individual components.
Our study suggests that feedback from star formation (e.g. in form of
protostellar outflows) might impact the environment of molecular gas on
larger scales than typically observed in the Milky Way.
TBD
Main Colloquium
Prof. Tomasz Kamiński
CANCELED
Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences
TBD
Past, present and future of Hungarian radio astronomy
Main Colloquium
Dr. Viktor Toth
ORATED
Eötvös University, Budapest
Drawing the history of Hungarian radio astronomy, one may in principal
start with the successful Moon radar measurements in 1946, but at least
with the HI 21cm measurements of the 1960s. Hungarian astronomers,
lately even our MSc students, became users of world class facilities,
carried out single-dish and interferometric measurements with ground
based and space born observatories. I will sample those results in the
prelude of my talk with special focus on a few topics where also the
Effelsberg-100m radio telescope was used.
Last decades saw a few attempts to build Hungarian radio astronomical
facilities again after the mid 20s century. We conducted site tests,
considered possible scientific goals and appropriate specifications to
work out a concept in the last 5 years. I will give a short review of
our recent activities towards establishing the first Hungarian radio
astronomical observatory.
How do the most luminous black holes accrete and expel gas?
Special Colloquium
Dr. Matthew Liska
CANCELED
Center for Astrophysics - CfA Harvard
The gravitational pull of a black hole attracts gas and forms a
physical
laboratory whose extreme conditions cannot be replicated on Earth. The
infalling gas forms an accretion disk where the interplay between
hydromagnetic processes and the warping of space-time releases
gravitational energy in the form of radiation, relativistic jets, and
winds. It is likely that most gas falls into supermassive black holes
when the accretion rate approaches the Eddington limit (L=Ledd), at
which point radiation pressure overcomes gravity. To date, our
knowledge
of such 'luminous’ black hole accretion disks mostly relies on
semi-analytical models, supplemented by a very limited set of numerical
simulations. In my talk I will discuss new insights gained from the
first radiative general relativistic magnetohydrodynamics (GRMHD)
simulations of luminous accretion disks. I will demonstrate that
magnetic fields lead to the formation of a hot corona and that
misalignment between the disk and black hole spin axis can explain
quasi-periodic oscillations, which have remained a mystery for over 30
years. I will finish my talk by discussing the opportunities the
next-generation of GRMHD simulations bring in addressing physical
processes driving spectral variability in accreting black holes.
Investigating Protostellar Accretion and Outflow with IR Surveys and JWST
Special Colloquium
Prof. Tom Megeath
ORATED
University of Toledo, Ohio
The assembly of stellar masses via accretion primarily occurs in the
early, deeply embedded phase of protostellar evolution, and is shaped by
powerful outflows. Infrared studies of this early phase requires
sensitive observations at wavelengths > 3 microns from space-based
platforms. I will first overview a program to measure the role of
outbursts in protostellar accretion using space telescope data spanning
25 years, from ISO, to Spitzer and Herschel, to NEOWISE. These
observations indicate low mass, Class 0 protostars undergo multi-decade,
accretion driven outbursts every ~400 years.
I will then present results from a Cycle I JWST, medium sized program
targeting five protostars with luminosities of 0.2 to 10,000 solar
luminosities and central, keplerian masses of 0.12 to 10 solar masses. I
will show NIRSpec and MIRI 2.9-28 micron IFU data which map the
accretion driven outflows and their interactions with the inner
envelopes with spatial resolutions down to 30 au. While the shapes of
the outflow cavities are traced in scattered light, the interiors of the
cavities show structures in molecular hydrogen. Collimated jets are
primarily visible in ionic, atomic and occasionally molecular lines. Yet
even in predominately ionic jets, we find knots of molecular emission. I
discuss our ongoing efforts to measure the properties of the outflows
and their dependence on the masses and accretion luminosities of the
protostars. Finally, I will discuss the use of mid-IR OH lines to
measure UV from accretion onto the very low mass IRAS 16253-2429
protostar.
Astrophysics and fundamental physics with LISA observations
Main Colloquium
Dr. Stanislav Babak
ORATED
APC, Paris
The sky is bright in gravitational waves in the milli-Hz band. We expect
emission from millions
of inspiralling white dwarf binaries in our Galaxy, merging massive
black holes, stellar-mass black holes plunging
into massive holes, stochastic gravitational wave signal and more. LISA
is L3 ESA-led mission aiming at detecting
all these sources. These observations will allow us to infer the
channel of formation and evolution of massive
(and not very massive) black holes, estimate the Universe's expansion
rate and constrain (or observe) the deviations
from General Relativity. I will review the rich science achievable with
LISA.
TBD
Main Colloquium
Dr. Maria Rioja
CANCELED
ICRAR-UWA/CSIRO, OAN, Australia
TBD
Exploring the Impact of Intrinsic Properties on Shocks and Variabilities in Relativistic Jets
Main Colloquium
Prof. Zakaria Meliani
ORATED
Observatoire de Paris
Relativistic jets in radio-loud active galactic nuclei (AGN) are
fascinating astrophysical phenomena that emit non-thermal radiation
across the electromagnetic spectrum and display diverse characteristics
at different scales. With their frequent high flux variability, they are
ideal objects to study high-energy physical processes. In this
presentation, we conduct a comprehensive investigation of the intrinsic
properties of relativistic jets and their impact on dynamics,
interaction with the external medium, and resulting observed light
curve.To explore the properties of relativistic jets, we employ a
combination of relativistic magneto-hydrodynamic (MHD) simulations using
the MPI-AMRVAC code and radiative transfer simulations with the
post-processing code REPTIDE. We focus on the impact of various
intrinsic properties of the jet, such as magnetization, rotation
profile, and transverse structure and variability, on the formation,
evolution, and behavior of standing knots and their interaction with
moving knots. We also examine the observables associated with these
features and their connection to observed variabilities at different
wavelengths.
VLBI astrometry in the epoch of Gaia
Special Colloquium
Prof. Laurent Loinard
ORATED
National University of Mexico
Gaia and phase-referenced VLBI observations deliver similar levels of
astrometric accuracy - of order 20 micro-arcseconds on trigonometric
parallax measurements. The two types of measurements, however, are
affected by different sources of error and have different limitations.
Gaia, of course, provides results for more than a billion objects while
VLBI astrometric observations exist only for a few hundred sources. On
the other hand, VLBI observations are largely immune to dust
extinction,
enabling observations deep into the Galactic disk. Also, the angular
resolution of VLBI observations is more than a hundred times better
than
Gaia's, enabling the resolution of tight binary systems that cannot be
resolved with Gaia. In this talk, I will present a comparison between
Gaia and VLBI emphasizing the complementarity between the two
techniques. I will also briefly touch on expected VLBI astrometry
improvements in the coming few decades.
The nuclear stellar disc of the Milky Way
Main Colloquium
Dr. Rainer Schödel
ORATED
Instituto de Astrofísica de Andalucía - CSIC, Granada, Spain
The Galactic Centre region is dominated by the nuclear stellar disc
(NSD), a rotating structure with a radius of ~200 pc that is
kinematically and chemically different from the surrounding bulge. The
NSD overlaps with the central molecular zone (CMZ), where ~10% of the
Galaxy’s molecular gas is concentrated. The extreme conditions in the
NSD and CMZ closely match those in high-redshift star forming regions.
I
will summarise our current knowledge about the NSD, obtained mostly
through high angular resolution near-infrared studies, and how the NSD
relates to the larger structure of the Milky Way. I will describe how
my
group are currently working on obtaining proper motions for millions of
stars in the NSD, a region inaccessible to Gaia, and how we use the new
data to better understand structure, formation history and recent star
formation in the NSD.
Fast Radio Burst Radiation Physics & Cosmology
Special Colloquium
Prof. Pawan Kumar
ORATED
University of Texas
The detection of a Fast radio burst (FRB) in 2007 was a major unexpected
discovery in astronomy in decades. Hunting for FRBs and measuring their
physical properties have become major scientific goals in astronomy. It
is well established that most FRBs are located at cosmological
distances, and therefore they are the brightest known transients in the
universe in the radio band. An FRB was discovered in our galaxy in April
2020, and it confirmed that at least some FRBs are associated with
neutron stars with very strong magnetic fields (magnetars). I will
describe recent works regarding how the FRB radiation is produced and
provide a unified picture for the weak Galactic FRB as well as the
bright bursts seen at cosmological distances. I will discuss how FRBs
can be used as a probe of the baryon distribution in the universe and
for investigating the epoch of reionization.
Energy extraction and particle acceleration in black hole magnetosphere
Special Colloquium
Dr. Arman Tursunov
ORATED
MPIfR (Humboldt Fellow)
One of the most fascinating aspects of Einstein’s general theory of
relativity is the uncanny correspondence of the black hole mechanics
and
thermodynamics. It implies that up to 29% of the total energy of a
black
hole is rotational extractable energy. It is therefore most pertinent
to
tap these
enormous sources most effectively and ultra efficiently. In this talk,
I
will present two new energy extraction mechanisms, which are the
ultra-efficient regime
of the magnetic Penrose process (MPP) of acceleration of charged
particles to ultra-high-energies and the radiative Penrose process
(RPP)
of energy gain
by radiating charged particles inside the black hole’s ergosphere.
The
former one predicts supermassive black holes as the source of the
highest-energy
cosmic rays, while the later one -- a powerful non-thermal
(synchrotron)
emission peak at the edge of the ergosphere, potentially observable by
future event
horizon telescope observations. I will also discuss possible
observational implications of given predictions in the case of the
Galactic centre supermassive
black hole Sgr A*.
FS CMa stars: overlooked channel of mergers
Main Colloquium
Dr. Daniela Korcakova
ORATED
Charles University
Our new N-body simulations show that more than half of merger products
are of B-type stars. Unfortunately, the focus of the community is either
to massive binaries, that later explode as supernovae, or Ap stars. This
largest channel of mergers is overlooked. Recently, we identified a
post-merger in a peculiar B-type group called FS CMa stars; the first
post-merger found in the phase where the envelope started to be
transparent. FS CMa stars are a subgroup of the B[e] stars, i.e., the
forbidden emission lines and infrared excess are present in their
spectra. This are signs of very extended circumstellar gaseous and dusty
region. The properties of several other FS CMa stars indicate that there
may be other post-mergers hidden in this group. These intermediate-mass
mergers contribute to the enrichment of the interstellar, even
intergalactic, material by heavier elements. Nowadays, more effective
"dust factories" are AGB stars, supernovae, and red giants, however,
B-type mergers may play an important role in the early universe.
The long-term modulation of the binary star LS I +61°303 across the electromagnetic spectrum
Special Colloquium
Dr. Frederic Jaron
ORATED
TU Vienna
The stellar binary system LS I +61°303 is composed of a Be type star
and a compact object in an eccentric orbit. Detected across the
electromagnetic spectrum, from the radio until the very high-energy
gamma-rays, its emission is not only variable but also periodic on
different time-scales. Focus of my talk is a super-orbital modulation
with a period of 4.6 years. Decades-spanning radio monitoring reveals
that this long-term modulation has remained stable for eight cycles.
Ongoing monitoring shows that this behavior continues. I will present
how the long-term modulation pattern shows a systematic trend as a
function of energy at all observed wavelengths. This systematic trend
finds a straightforward explanation in a scenario including a
precessing
jet. But I will also mention alternative scenarios.
Observations of interstellar hydrides and what they tell us
Special Colloquium
Dr. Arshia Maria Jacob
ORATED
Johns Hopkins University (Baltimore, Maryland)
Formed at an early stage of gas-phase ion-molecule chemistry,
hydrides– molecules containing a heavy element covalently bonded to
one or more hydrogen atoms – play an important role in interstellar
chemistry as they are the progenitors of larger and more complex
species
in the interstellar medium. In recent years, the careful analysis of
the
spectral signatures of hydrides have led to their use as tracers of
different constituents, and phases of the interstellar medium and in
particular the more diffuse environments. Diffuse clouds form an
essential link in the stellar gas life-cycle as they connect both the
late and early stages of stellar evolution. As a result, diffuse clouds
are continuously replenished by material which makes them reservoirs
for
heavy elements and hence ideal laboratories for the study of
astrochemistry. In this talk we will journey through a renaissance of
hydride observations detailing the past, present and future of hydride
studies and in particular the legacy of the SOFIA HyGAL program.
Violent cosmic explosions in young and massive clusters: Remains of a proto-stellar merge?
Special Colloquium
Dr. Luis Alberto Zapata
ORATED
National University of Mexico
The phenomenon of molecular outflow was first discovered in the
1980’s. Very high velocity motions were detected in the line wings of
the carbon monoxide molecule, seen towards Orion-KL. With the subsequent
detection of the bipolar outflows in many sources, they became
recognized as a ubiquitous stage in star formation. Excess angular
momentum was carried outwards by the molecular outflow, which allowed
the remaining material to fall onto the protostar. However, around 10
years ago, our paradigm changed radically, the eponymous outflow in
Orion-KL turned out to be an explosive outflow. These explosive flows
are possibly powered by the release of gravitational energy related with
the formation of a massive (proto)stellar merger or a close stellar
binary, a totally different process from the classical bipolar outflows.
In this talk, I will review this new phenomenon, and show the latest
efforts to reveal more cases where possible mergers events could led the
violent flows.
The Laser Interferometer Space Antenna (LISA)
Main Colloquium
Prof. Guido Müller
ORATED
Max Planck Institute for Gravitational Physics (Hannover)
Since their first detection, gravitational waves (GW) have become a
major part of current and future plans to study the universe. The
current ground based observatories opened an observational channel which
allows us to listen to GW from mergers between neutron stars and black
holes up to about 100 solar masses. The first space-based observatory,
the Laser Interferometer Space Antenna (LISA), will enable us to listen
to the mHz gravitational wave band which is the natural frequency of GW
sources involving a million solar mass black holes. LISA is currently
passing through the first project phases and is expected to be adopted
by ESA as its next L-class mission later this year with a launch
anticipated in the mid 2030s. I will discuss the science case for LISA
as well as its technology and also initial ideas about ‘beyond LISA’
missions.
TBD
Main Colloquium
Dr. Roberto Neri
CANCELED
IRAM Grenoble (France)
TBD
Interstellar extinction and the distances to supernova remnants and molecular clouds
Special Colloquium
Prof. Biwei Jiang
ORATED
Beijing University
Interstellar extinction increases with distance and presents a
significant jump at the position of dense medium. This property makes
it
an important indicator of distance to the extended objects such as
supernova remnants and molecular clouds. By taking the bluest color as
the intrinsic color for a given set of stellar parameters, the
interstellar extinction is calculated with relatively high accuracy for
millions of stars from the stellar parameters derived in large-scale
spectroscopic surveys including LAMOST and APOGEE. With the Gaia
distance, the distances to several tens of SNRs and molecular clouds.
Accordingly, the dust properties implied in the extinction law are
discussed.
Multi-messenger observations of cosmic collisions: progenitors, relativistic ejecta, and remnants
Main Colloquium
Prof. Alessandra Corsi
ORATED
Texas Tech University, Lubbock, USA
The births and mergers of neutron stars and black holes, the most
exotic
objects in the universe, can launch the fastest cosmic jets and shake
the very fabric of space-time with gravitational waves. GW170817, the
merger of two neutron stars witnessed through both its gravitational
wave siren and its glow at all wavelengths of light, marked the
beginning of a golden age in multi-messenger astrophysics. Starting
from
the example of GW170817, I will show how observations at radio
wavelengths can probe the ejecta and environments of compact binary
mergers and help unveil their progenitors and remnants. I will then
discuss opportunities and challenges ahead, as new observational
facilities will transform a trickle of multi-messenger discoveries into
a flood. I will conclude by highlighting prospects for extending the
reach of radio and gravitational wave studies to the heaviest black
holes in merging galaxies.
BLR radius-luminosity relation and its applications in cosmology
Special Colloquium
Dr. Michal Zajaček
ORATED
Department of Theoretical physics and Astrophysics, Faculty of Science, Masaryk University, Brno, Czech Republic
Over the last decades, tight correlations between the time delays of prominent broad lines - Hbeta, MgII, CIV - and corresponding underlying monochromatic luminosities at 510, 300, 135 nm were established thanks to reverberation mapping (RM). For optical Hbeta broad line, RM has been performed for more than 120 sources, for UV MgII line, we currently have time delays for 94 sources, and for the UV CIV broad line, 38 sources were reverberation mapped. The radius-luminosity relation implies that RM quasars can in principle be standardized, i.e. by measuring the time delay, we can determine monochromatic luminosity, and from the measured flux density, we can infer the luminosity distance. I will demonstrate the method how to constrain both radius-luminosity relation parameters and cosmological parameters from the measured time delay, the measured flux density, and the source redshift, thus avoiding the circularity problem. So far uur analysis shows that the RM quasars are standardizable, i.e. the radius-luminosity parameters do not change significantly for different cosmologies. Specifically, MgII and CIV quasars (on average more distant) give results that are weaker but fully consistent with better established cosmological probes (BAO+H(z)). In contrast, closer Hbeta quasars do show a small, 2sigma tension with the better established probes.
The low radio frequency foreground: thrive on what cosmologist want to avoid
Special Colloquium
Prof. Nirupam Roy
ORATED
Indian Institute of Science, Bangalore
The redshifted 21 cm signal from neutral hydrogen is a very useful probe
of cosmic evolution. A particularly interesting phase of this evolution
is the Epoch of Reionization (EoR), when the first ionizing sources were
born in the early universe. The current and the upcoming low frequency
radio telescopes, like the upgraded GMRT (Giant Metrewave Radio
Telescope) or the Square Kilometre Array (SKA), are expected to play a
crucial role in studying cosmic evolution during the EoR and the
post-EoR era, and thus addressing many important but unresolved issues.
One of the challenging aspects in this regard is the fast and efficient
data processing necessary for weak EoR/post-EoR signal detection in
presence of the strong, confusing foreground. However, the foreground is
interesting in its own right. In this talk, I plan to share some of our
recent efforts to study the low radio frequency foreground using the
uGMRT data. This work may help us to understand the near-future pathways
of studying the atomic hydrogen power spectrum from the cosmological
EoR/post-EoR, but it also allows us to study, in details, the properties
of a large population of extragalactic sources as well as of the
interstellar medium of the Milky Way.
Algorithms for pulsar detection.
Special Colloquium
Dr. Barak Zackay
ORATED
Weizmann Institute of Science, Israel
Since its very beginning, discoveries in pulsar astronomy had required algorithmic sophistication and ingenuity. The basic problem, detecting a pulsar by its periodicity modulated by intrinsic, orbital or barycentric modulation is computationally prohibitive as the number of combinations of all of these parameters can be as large as 10^40. While there had been substantial developments in algorithms for pulsar detection in the last 50 years, the state of the art algorithms trade sensitivity (in some cases, orders of magnitude) for computational tractability. In this talk, I will show a road map for solving the pulsar detection problem in observationally relevant regimes using algorithms that are prevalent in the field of cryptography. I will show in detail how to take the discrete timing problem, relevant for gamma-ray pulsar astronomy, and transform it to the problem of finding a short vector in a lattice, that is then solved in seconds using standard implementations. I will then outline in rough sketches three different algorithms for detecting pulsars, creating a roadmap for unlocking the full potential of pulsar surveys to detect "holy grail" systems - a tight pulsar black hole system and a pulsar in the vicinity of the galactic center.
GLOSTAR: Investigating Tracers of High-Mass Star Formation
Promotionskolloquium
Hans Nguyen
ORATED
MPIfR
High-mass stars (>8 msol) are not as numerous as their less massive
counterparts but have a much more profound impact on their environment
within their native galaxies. Despite their importance, our
understanding of their formation process is not as developed as that of
low-mass stars. As such, the GLObal view on STAR formation (GLOSTAR)
Galactic Plane survey aims to address this question of how high-mass
stars form by finding all sites of high-mass star formation at
different evolutionary stages in the Milky Way and characterising their
physical properties in order to develop a complete view of massive star
formation.
In the first half of this talk, I present our findings of 5.8 GHz radio
continuum observations of the region known as the Central Molecular Zone
(CMZ) using the Karl G. Jansky Very Large Array (VLA) to detect HII
regions, which are known tracers of high-mass star formation. We
investigated the presence of young stellar object (YSO) candidates with
HII regions as it is unknown how many of the near-infrared (NIR)
identified YSOs trace HII regions. Our results show that the majority of
these NIR-identified YSOs lack VLA radio counterparts. We found that
most of these sources also lack dust emission (using data from
ATLASGAL), and it is likely that they are older and have dispersed their
natal clouds.
The second half of this talk focuses on 6.7 GHz methanol masers as they
have been shown to be an exclusive tracer of the earliest stages of
high-mass star formation. Many surveys have already been conducted,
however, with the upgraded VLA, GLOSTAR can provide the most sensitive
unbiased survey of 6.7 GHz methanol masers to date. I present the
results of this catalogue where we detect many new masers. To study the
environments of early high-mass star formation we used ancillary dust
continuum and radio continuum data. We find that the newly detected
masers are more likely to be associated with less massive stars.
Furthermore, by comparing the radio source and the maser flux
distributions, we find no correlation with respect to angular offset,
suggesting that the maser and the radio source are powered by unrelated
mechanisms.
Gravity tests with pulsars using new-generation radio telescopes
Promotionskolloquium
Huanchen Hu
ORATED
MPIfR
General relativity has so far passed all experimental tests, with some of the most stringent tests in strong fields coming from observations of pulsars — magnetised rotating neutron stars that form from the collapsed cores of massive stars during supernovae. Such compact objects contain some of the densest forms of matter in the Universe and therefore produce a strong gravitational field in their vicinity. Thanks to their excellent rotational stability, these cosmic clocks are powerful tools for studying a wide range of topics in fundamental physics. These include the equation of state of matter at supranuclear densities, tests of general relativity and its alternatives, and the search for nanohertz gravitational waves with pulsar timing arrays, all of which are investigated in this dissertation using the new-generation of radio telescopes. In this talk, I will focus on two aspects based on studies of the unique Double Pulsar system, PSR J0737-3039A/B. Firstly, I will highlight the timing results of the Double Pulsar using data from the MeerKAT telescope, in particular the (so far) best test of higher-order signal propagation effects predicted by general relativity. Secondly, I will introduce a new approach to constrain the equation of state by measuring the neutron star moment of inertia through pulsar timing and show predictions for future measurements.
Supermassive Black Holes and Their Host Galaxies
Special Colloquium
Dr. Vardha N. Bennert
ORATED
CalPoly, San Luis Obispo
Supermassive black holes (BHs) are ubiquitous in the center of massive
galaxies. When actively growing through accretion, Active Galactic
Nuclei (AGNs) become some of the brightest objects in the universe. It
has been two decades that relations between BH mass and the properties
of their host galaxies (such as luminosity, stellar mass and stellar
velocity dispersion) were first discovered. Interpreted as evidence for
a co-evolution between BHs and galaxies, these scaling relations remain
a hot topic for contemporary
studies with many open questions remaining, including the role of AGN
feedback or hierarchical merging, and the nature of the host galaxies
(elliptical versus spiral galaxies,
bars, pseudo-bulges and mergers). While reverberation mapping has been
the gold standard for BH mass measurements in AGNs, traditional methods
yield only sample-average recipes for BH mass estimates. Recent advances
include dynamical modeling of reverberation-mapped data to obtain BH
mass for individual objects and directly resolving the broad emission
around AGNs spatially. Studying the co-evolution as a function of cosmic
history can shed light onto origin and fundamental drivers, but relies
on AGNs for which host-galaxy properties are intrinsically difficult to
measure. Thus, a robust local baseline for AGN MBH-scaling relations is
the key. I will review our understanding of the nature and origin of the
scaling relations between supermassive black holes and their host
galaxies.
TBD
Main Colloquium
Prof. Thorsten Kleine
CANCELED
Max Planck Institute for Solar System Research
TBD
What does gender have to do with physics?
Special Colloquium
Professor Tomas Brage
ORATED
Lund University
Physics is often seen, by Physicists not the least, as objective and we
believe we are surrounded by a "culture with no culture". At the same
time our history, class and board rooms are dominated by men. This is a
paradox that should awaken the curiosity of anyone. In this talk I will
give some examples on how you can approach the question in the title.
There have been several studies of Scientists and I will combine a
discussion of these with some general theory and personal experiences,
to paint a picture on how gender transgresses Physics, like all other
fields. By using the levels of change introduced by Schiebinger, I refer
to studies of e.g. Anthropologists, Sociologists and Psychologists. The
bias against women, since Science is stereotypically male, combined with
the "myth of meritocracy" could be key to understand the lack of women
in the field. The talk is intended as a translation of results from
recent progress in Gender Science to an audience of non-experts in the
field, especially people within STEM-fields. The aim is to give some
answers to the question in the title, but also to show that this is an
extremely interesting and active field of research.
Investigating the star-forming quenching mechanisms in nearby EDGE-CALIFA galaxies
Master Colloquium
Evdokia Makroleivaditi
ORATED
MPIfR
The main mechanisms that suppress the star-formation in nearby galaxies,
known as “star-formation quenching” processes, are considered to
play a key role in galaxy evolution. Nearby galaxies across the Hubble
sequence exhibit various quenching patterns based on the distribution
of the ionised gas and the resolved maps of the equivalent width of
Hα, WHα. In Kalinova et al., 2021 they found six repetitive patterns
for their sample of 238 Calar Alto Legacy Integral Field Area (CALIFA)
galaxies and defined the following “stages”: star-forming (SF) –
systems, dominated by star formation; quiescent-nuclear-ring (QnR) –
galaxies that have a quiescent ring within 0.5 effective radius (Re);
centrally quiescent (cQ) – objects that are fully quiescent within
0.5Re and star-forming in the outskirts; mixed (MX) – galaxies with
no clear pattern of the star-forming; nearly retired (nR) – retired
systems in which small star-formation regions can be found within 2Re,
and fully retired (fR) – objects that are completely retired within
2Re with no sign of star-formation. A further categorisation based on
their nuclear activity, lead to two groups: active systems that host a
weak or strong active galactic nucleus (wAGN or sAGN) at their centre,
and non-active objects. In our work, we analyze the molecular gas
properties by using resolved data from the CARMA Extragalactic Database
for Galaxy Evolution (EDGE)- CAL- IFA survey, (see Bollato et al. 2017)
for 115 galaxies across quenching stage and nuclear activity. Our
sample includes 104 non-Active galaxies and 11 AGN- hosting galaxies (2
wAGN and 9 sAGN). We explore the radial variation of the CO gas, star
formation rate (SFR), star formation efficiency (SFE) and stellar-mass
surface density as well as the age, metallicity, stellar velocity, stel-
lar velocity dispersion, specific star formation rate (sSFR) and gas
mass fraction of our sample. Additionally, we build-up resolved
(pixel-by-pixel) relations be- tween quenching indexes of the galaxies
(defined as thresholds in the equivalent width of the Hα line) and
molecular gas properties.
Constraining scalarization in Gauss-Bonnet gravity through binary pulsars
Gravity models admitting scalarization are attracting considerable attention due to their natural ability to evade constraints from weak field observations. In the present talk, I will discuss one such modified theory of gravity, namely the scalar-Gauss-Bonnet gravity. It offers the possibility for spontaneous growth of scalar hair both for neutron stars and black holes with a trigger of the process being the curvature of the spacetime itself. I will discuss the sectors of the theory that have already been tested with binary pulsar observations and what is the implication on black hole scalarization. Special attention will be paid to future prospects of testing further Gauss-Bonnet gravity in terms of theory development and pulsar observations.
A challenge to the standard cosmological model
Special Colloquium
Prof. Subir Sarkar
ORATED
University of Oxford
In the ΛCDM cosmological model the Universe is assumed to be isotropic
& homogeneous when averaged on large scales. That the CMB has a dipole
anisotropy is interpreted as due to our peculiar motion because of local
inhomogeneity. There should then be a similar dipole in the sky
distribution of high redshift sources. Using catalogues of radio sources
and quasars we find that this expectation is rejected at >5σ, i.e. the
distribution of distant matter is not isotropic in the 'CMB frame’.
This calls into question the standard practice of boosting to this frame
to analyse cosmological data, in particular to infer isotropic
acceleration of the Hubble expansion rate from Type Ia supernovae, which
is interpreted as due to Λ.
Mapping the Milky Way: Galactic cartography in the age of high precision parallaxes
Main Colloquium
Dr. Ronald Drimmel
ORATED
Osservatorio astrofisico di Torino, INAF (Italy)
Galactic studies is undergoing a renaissance in the current decade,
thanks not only to Gaia, but to ground-breaking astrometry in the NIR
and radio. I will give an update on the latest Gaia results following
on
the it's last data release about the structure of our Galaxy, with
particular focus on the disk. Thanks to a recent Lorentz Center
workshop
with the same title as this talk, I will attempt to put Gaia's
contribution in the context of other recent results from the NIR and
radio. Integrating all these together I hope to share what we are
learning and hope to soon learn about the Milky Way.
High mass star formation towards G358.69+0.03
Master Colloquium
Anahat Cheema
ORATED
MPIfR
In our work, we investigate the high mass star formation activity in the
region centered at (l,b)=(358.69°, 0.03°) near the Galactic center. In
order to identify and characterise the HII regions, we use data from the
GLOSTAR survey, which is a wideband radio (4-8 GHz) survey of the Milky
Way combining data from the Karl G. Jansky Very Large Array (VLA) and
the Effelsberg 100m telescope. Using BLOBCAT source extraction software
which uses flood-fill algorithm to detect sources in 2 dimensional
astronomical images, we identified 46 radio sources in the region.
Candidate HII regions are cataloged based on association with at least
one multiwavelength counterpart. For this, we used mid-infrared
(GLIMPSE, MIPSGAL and WISE), far-infrared (Hi-GAL) and submillimeter
(ATLASGAL) archival data. In total, we have identified 43 HII region
candidates. We compute the properties of these HII regions, such as the
Lyman continuum photon rate, ZAMS spectral type, dynamical age etc. The
HII region candidates are distributed in a ring-like formation. We also
observe enhancement of ATLASGAL clumps and infrared sources towards
south-east of the ring, suggesting triggered star formation in the
region.
Extragalactic Megamaser
Special Colloquium
Dr. Willem Baan
CANCELED
ASTRON, The Netherlands
TBD
The next-generation Event Horizon Telescope
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.
Studying the hidden population of black holes in the Milky Way with microlensing.
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.
A geometric measurement of the Hubble constant by the Megamaser Cosmology Project
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.
Pulsars as probes of gravity
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.
A wide-frequency spectra scan in the starburst galaxy NGC 253: The ALCHEMI survey
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.
Exploring the nanohertz-frequency gravitational-wave landscape with pulsar timing arrays
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.
A fully resolved molecular cloud catalog of the Central Molecular Zone
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.
A first search of transients in the Galactic Center from 230 GHz ALMA observations
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.
Semi-analitycal model for the flares of Sagittarius A* observed by GRAVITY
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
Star Formation and ISM Dynamics
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.