Wednesday Colloquium



10.01.2024

"WD binaries in MOCCA - comparisons to close and wide observed binaries"

Lukas Hellström (CAMK, Warsaw)


31.01.2024

"CAMK Annual Conference (till Feb. 2nd)"

(CAMK, PAN)


07.02.2024

"News from the M87* black hole"

Maciej Wielgus (Max Planck Institute for Radio Astronomy (Bonn, Germany))

In 2017 Event Horizon Telescope (EHT), a global array of radio telescopes, observed the giant supermassive black hole M87* in the center of the M87 galaxy. These observations resulted in the first image of a black hole resolved at the event horizon scale. We have now completed the analysis of the corresponding data, with results on total intensity (2019), linear polarization (2021), and circular polarization (2023). It is a good time to summarize and systematize what we have learned about black holes, accretion disks, and magnetic fields from the EHT observations. Moreover, I will discuss the most recent first EHT results from 2018 observations of M87*, allowing us to study the persistence and variability of the black hole shadow image.


14.02.2024

"Quantum computers — tiny universes at our command"

Piotr Gawron (CAMK/Astronet, Warsaw)

Quantum computing is at the same time a fascinating model of computation that uses the fundamental principles of Nature; great engineering challenge; and a promise of calculating the impossible and acquiring unimaginable riches. During the talk we will try to present what quantum computing really means, what it is not, what are hopes related to and what are the challenges in implementation of quantum computing. We will show simultaneously how little is possible to achieve with quantum computers today and how unimaginably marvellous the quantum computers — those tiny universes at our command — are.


21.02.2024

"Multiple Stellar Populations - Speculations on Cluster Migration and Gas Re-Accretion"

Mirosław Giersz (CAMK, Warsaw)

I will very briefly discuss the formation and evolution of multiple populations in globular clusters from the point of view of observation and theory. Then I will present the main scenarios of their formation and discuss their weaknesses. Finally, I will focus on describing the results of simulations carried out with the MOCCA Monte Carlo code and summarize the conclusion resulting from these simulations in the form of a speculative scenario.


28.02.2024

"Propagation and lensing of gravitational waves in modified theories"

Sreekanth Harikumar (National Center for Nuclear Research, Warsaw)

The first direct detection of gravitational waves by LIGO collaboration has opened a new era of Gravitational Wave astronomy. The bending of light by massive objects is a prediction of General Relativity (GR) and this phenomenon known as gravitational lensing has now become an indispensable tool in astrophysics. Therefore, in this era of astronomy, the next most anticipated event is the detection of gravitational waves lensed by massive sources along the line of sight. The lensed gravitational waves has many applications such as detection of Intermediate Mass BlackHoles (IMBH), Primordial Black Holes, precision cosmology etc. Another emerging avenue is the test of GR, in this talk I will discuss the predictions of f(R) and Palatini f(R) in the context of lensing of gravitational waves.


06.03.2024

"https://events.camk.edu.pl/event/96/overview"

Young astronomers meeting at CAMK (CAMK, Warsaw)


13.03.2024

"TESS photometry of hot subdwarf stars"

Sumanta Kumar Sahoo (CAMK, Warsaw)

Hot subdwarf stars are extreme horizontal branch stars, consisting of a convective helium-burning core, helium shell, and a thin (in mass) hydrogen envelope. A few of these subdwarfs pulsate, which opens the window to study these stars using asteroseismology. The majority of such pulsating stars detected are B-type stars (sdBV). From the TESS mission, we have detected a few tens of rich gravity mode sdBVs by analyzing their short cadence (SC) light curves and identified their pulsation mode geometries. Apart from the SC data, we also extracted light curves from TESS full frame images for the targets not observed in SC mode and analyzed them to detect pulsations. I will be talking about these pulsating subdwarf stars and our study to understand their pulsation properties.


20.03.2024

"Binary coalescences as sources of ultrahigh-energy cosmic rays"

Jonas Pereira (Núcleo de Astrofísica e Cosmologia & Departamento de Física, Universidade Federal do Espírito Santo, Vitória, Brasil and CAMK, W)

Binary coalescences are known sources of gravitational waves (GWs) and they encompass combinations of black holes (BHs) and neutron stars (NSs). I’ll show that when BHs are embedded in magnetic fields (B’s) larger than approximately e10 G, charged particles colliding around their event horizons can easily have ultrahigh energies (≳ e18 eV) and become more likely to escape. Such B-embedding and high-energy particles can take place in BH-NS binaries, or even in BH-BH binaries with one of the BHs being charged (with charge-to-mass ratios as small as e-5, which do not change GW waveforms) and having a residual accretion disk. The number of collisions leading to ultrahigh energy particles is estimated to range from a few up to millions before the merger of binary compact systems. Thus, binary coalescences may also be efficient sources of ultrahigh energy cosmic rays (UHECRs) and constraints to NS/BH parameters would be possible if UHECRs are detected along with GWs.

The presentation will be based on recent PRL work: Binary Coalescences as Sources of Ultrahigh-Energy Cosmic Rays Jonas P. Pereira, Carlos H. Coimbra-Araújo, Rita C. dos Anjos, and Jaziel G. Coelho Phys. Rev. Lett. 132, 091401 – Published 27 February 2024


27.03.2024

"Thermal effects in nuclear matter and neutron stars"

Lucas Tonetto (Dipartimento di Fisica, “Sapienza” University of Rome, Italy)

In different astrophysical systems involving neutron stars, such as mergers or newly born stars, a reliable model of a finite-temperature equation of state is needed. Temperature has implications in equilibrium and dynamical phenomena, therefore a fully consistent framework should be able to take into account thermal effects in single-nucleon properties alongside yielding accurate results for average thermodynamic quantities. In this talk, I present the results of employing a recently developed effective interaction based on the Correlated Basis Functions theory, being able to account for nuclear correlations and two- and three-nucleon potentials. After discussing the properties of its generalisation to nonzero temperature, we apply it in the calculation of the neutrino mean free path and emissivity. In the latter, we study how in-medium effects alter the results by using effective weak transition operators.


03.04.2024

"Magnetic flux saturation mechanism at accreting black holes"

Krzysztof Nalewajko (CAMK, Warsaw)

Black holes (BH) acquire relativistic magnetospheres by accreting magnetized gas. Once they collect significant magnetic flux across the horizon, aided by the spin they can drive powerful relativistic jets by the Blandford-Znajek mechanism. Large enough BH magnetic flux backreacts on the accretion flow, which has been described in terms of arresting or choking. Magnetic flux eruptions have been identified as the mechanism of BH magnetic flux saturation. These eruptions can potentially dissipate a large fraction of magnetic energy in the BH magnetosphere by means of relativistic magnetic reconnection, accelerating particles and producing flares of non-thermal radiation. We analyze the results of 3D general-relativistic ideal magnetohydrodynamic (GRMHD) numerical simulations of accretion flows onto magnetically saturated Kerr BHs, focusing on the initiation of magnetic flux eruptions.


10.04.2024

"Population of exoplanets found using microlensing technique"

Radek Poleski (Astronomical Observatory Warsaw University)

There are four main methods of finding exoplanets: transits, radial velocity, microlensing, and direct imaging. Each of these methods comes with its own biases and limitations. The microlensing technique allows finding planets on orbits similar to the ones in the Solar System as well as free-floating planets (i.e., not bound to any star) of low mass. I'll present how the microlensing method works and what are the current constraints on the population of bound and free-floating planets. I'll also present prospects for a microlensing survey that will use the Nancy Grace Roman Space Telescope - a NASA flagship mission currently built and scheduled to be launched in 3 years.


17.04.2024

"The impact of magnetic field on beamed emission of neutron star ULXs in Radiative GRMHD simulations"

Fatemeh Kayanikhoo (CAMK, PAN, Warsaw)

In our simulations accreting magnetized neutron stars are considered to be potential pulsating ultraluminous X-ray sources. Due to the neutron star's magnetic field, the emission is beamed, resulting in apparent luminosity that exceeds the Eddington luminosity expected from stellar mass objects. In my research, I investigate the impact of the surface magnetic field of a neutron star on the photosphere and luminosity by using GRRMHD simulations. Our study shows that strong magnetic fields truncate the disk and quench the outflows, which lowers the luminosity. Moreover, our simulations demonstrate that the beaming emission decreases as the magnetic field increases.


24.04.2024

"A unified accretion ejection paradigm for X-ray binaries: the JED-SAD paradigm"

Grégoire Marcel (CAMK, Warsaw)

The hysteresis behavior of X-ray binaries during their outbursts remains a mystery. In this work, we developed a paradigm where the disk material accretes in two possible, mutually exclusive, ways (Ferreira et al. 2006). In the usual alpha-disk mode (SAD, Shakura & Sunayev 1973), the dominant local torque is due to a radial transport of the disk angular momentum. In the jet-emitting disk mode (JED), magnetically-driven jets carry away mass, energy, and angular momentum vertically. Within this framework, the transition from one mode to another is related to the magnetic field distribution, an unknown. We have shown that typical hard states of X-ray binaries can be reproduced up to unprecedented X-ray luminosities in this paradigm (Marcel et al. 2018a,b,2019). Direct spectral fits have since been performed on an AGN (Ursini et al,. 2020), as well as two X-ray binaries (Marino et al. 2021, Barnier et al. 2022), showing striking dynamical similarities between the two accretion flow structures despite the factor > 10^6 in mass. Moreover, we have addressed the production of low frequency quasi-periodic oscillations during the outbursts (Marcel et al. 2020, Marcel & Neilsen 2021), the radiative efficiency of the accretion flow and the associated radio--X-ray correlation (Marcel et al. 2022), as well as the production of winds (Petrucci et al. 2021), the latest results from IXPE (Zhang et al., in prep.), and the timing properties of X-ray binaries (Malzac & Marcel, to be submitted).


15.05.2024

"Using the Alcock-Paczyński test to constrain the amplitude of the cosmological constant"

Andrzej Sołtan (CAMK, Warsaw)

The galaxy autocorrelation function (ACF) is constructed using the galaxies selected from the DR12 of the SDSS. The ACF amplitude increase at separa- tions expected to those generated by the Baryon Acoustic Oscillations (BAO) is clearly detected. We study the dependence of this correlation signal on the angle between the line of sight and vectors defined by galaxy pairs. Using the Alcock-Paczyński test we estimate acceptable amplitudes of the matter and cosmological constant density parameters, Ωm and ΩΛ. Only flat cosmological models are considered, e.g. Ωm + ΩΛ = 1. We found that in the local Universe 0.25 . Ωm . 0.45, what is in agreement with the results based the CMB me- asurements. Strong stoochastic variations of local galaxy concentrations generate substantial scatter of the ACF signal unrelated to the BAO, what significantly degrades the accuracy of the present estimates.


22.05.2024

"On the hot flows in AWDs: broadband noise, spectral characteristics and disk structure"

Solen Balman (Department of Astronomy and Space Sciences, Istanbul University)

Cataclysmic Variables (CVs) and related systems (e.g., AM CVn, Symbiotics) are compact systems with white dwarf (WD) primaries referred as accreting white dwarfs (AWDs). They are excellent laboratories to study astrophysical plasmas, accretion flows and disks, gas dynamics, outflows, and transient outbursts. Broadband noise and its variations in accretion flows have been a diagnostic tool for understanding the structure of accretion disks together with accretion history and state changes. CVs demonstrate band limited noise (mainly 1-6 mHz) in the optical, UV and X-ray energy bands, which can be adequately explained in the framework of the model of propagating fluctuations yielding break frequencies. I will discuss broadband noise structure in CVs with a broader sense including some different nonmagnetic AWDs elaborating on similarities along with spectral characteristics. The spectral and/or broadband noise studies show that advective hot flow structure (ADAF-like) resides inside nonmagnetic CV disks mainly detected in the X-ray regime (Balman 2020, Balman et al. 2022 and references therein) indicating other characteristics like outflows in the X-rays and warm absorber effects.


29.05.2024

"Tidally induced bars in galaxies"

Ewa L. Łokas (CAMK, Warsaw)

Galactic bars are generally believed to form via disk instability. There is however another channel for the formation of these structures: by tidal effects. I will review the process of formation and evolution as well as describe the properties of bars induced in galaxies as a result of tidal interactions between them and a larger or equal-mass companion. I will start by describing the results of controlled simulations of dwarf galaxies orbiting the Milky Way and then move to normal-size galaxies interacting during flybys or orbiting galaxy clusters. Then I will place this scenario in the cosmological context and discuss tidally induced bars in galaxies formed in the simulations of the IllustrisTNG project, including a recently identified analog of the Sagittarius dwarf.


05.06.2024

"How useful at ML Deep NN for reconstruction of the large scale structure?"

Adi Nusser (Technion, Haifa)

The equivalence principle ensures that galaxy motions are unbiased with respect to the velocity field of the underlying mass density field, making galaxy peculiar motions our only direct probe of the three-dimensional distribution of dark matter. However, analyzing peculiar velocity data is challenging due to large errors that increase with distance and the sparse spatial coverage of the data. The talk will discuss methods for constraining cosmology using observed galaxy motions, including traditional approaches and new AI/ML-based methods.


12.06.2024

"The time is out of joint: Shakespeare’s astronomical imagination"

Anna Cetera-Włodarczyk and Jarosław Włodarczyk (Institute of English Studies, Warsaw University (ACW), Institute for the History of Science PAN (AW))

London at the turn of the 17th century is one of the most fascinating places to investigate the reception of the new astronomy propelled by the Copernican revolution and astronomical observations. William Shakespeare’s imagery has been typically placed within the geocentric model of the universe, firmly anchored in the medieval paradigm, denying any impact of the new models of cosmos. But was he really so ignorant of the ongoing revolution? The much overused dichotomy between geo- vs. heliocentrism does not give justice to the complex intellectual climate of the age. Drawing on the history of astronomy and literary linguistic analysis, we argue that Shakespeare’s references to the unsphered and disorbed planets and stars are better understood when viewed in the light of the disputes of the times, the decline of the concept of the celestial orbs in particular. Hence we shall rehearse the 16 th century astronomy to read Shakespeare’s metaphors.