Wednesday Colloquium


"R Aquarii: direct insights into complex binary interactions with ALMA"

Joanna Mikołajewska (NCAC, Warsaw)

R Aqr is one of the closest symbiotic binaries composed of a white dwarf and Mira-type AGB star orbiting each other with ~44 yr period. It is one of the best studied symbiotic systems with the two binary components independently resolved by VLT and ALMA imaging. The binary is surrounded by very complex bipolar nebula with a spectacular jet outflow. I will review the properties of this system and present our first results on ALMA imaging of the molecular CO lines and the continuum distributions. In particupar, we have for the first time directly imaged the strong gravitational effects of the white dwarf secondary on the circumstellar wind leaving the AGB primary as well as the stream of material flowing from the Mira to the accretion disk around the white dwarf.


"Emergent de Sitter epoch of the quantum Cosmos"

Tomasz Pawłowski (Center for Theoretical Physics, Warsaw)

By using the regularization techniques of Loop Quantum Gravity we reexamine the quantum nature of the Big Bang in the framework of Loop Quantum Cosmology. The studies of the dynamics of a simple isotropic quantum Universe lead to a qualitative modification to the 'traditional' LQC bounce paradigm. Quantum gravity effects still lead to a quantum bounce connecting deterministically large classical Universes. However, the evolution features a large epoch of de Sitter Universe, with emergent cosmological constant of Planckian order, smoothly transiting into a flat expanding Universe.


"What was the last Noble Prizein physics given for?"

Czesław Radzewicz (Physics Department, Warsaw University)


"Modeling the X-ray Emissions from Accreting Black Holes"

Javier Garcia (CalTech, Pasadena)

In the region close to compact object such as black holes or neutron stars, the extreme conditions created by the strong gravitational field produces copious amounts of energetic radiation (ultra-violet, X-rays, and Gamma-rays). The interaction of this radiation with the surrounding material results in observables that carry important physical information. X-ray spectral and timing techniques provide direct access to the accretion physics on these systems, such as the black hole spin, the location of the inner-edge of the accretion disk, its ionization stage and composition, among others. In this talk, I will discuss the current state of modern relativistic reflection models computed specifically for modeling the X-ray spectrum from supermassive black holes in AGN and black hole binary systems. I will show examples of the implementation of our new models to observational data from several X-ray observatories (e.g., RXTE, Swift, XMM-Newton, Suzaku, and NuSTAR), and discuss current outstanding issues, such as the large iron abundances frequently required to fit the reflection spectra, controversies on the disk truncation, the origin of the soft excess in AGN, and the effects of high density in the observed spectra.


"Net-baryon fluctuations in Magnetized QCD matter"

Marcio Ferreira (University of Coimbra)

The kurtosis and skewness of net baryon-number fluctuations are studied for the magnetized phase diagram of three-flavor quark matter within the Polyakov extended Nambu–Jona-Lasinio model. The effect of magnetic catalysis and inverse magnetic catalysis is considered using two distinct scalar interactions. Special attention is given to the fluctuations dependence near the critical end points (CEPs). Regions with large fluctuations but no CEP in non-magnetized matter develop a CEP under the action of a strong magnetic field.


"Interstellar nanoparticles and millimeter measurement of high redshifts"

Alain Omont (IAP, Paris)

1. Interstellar carbon nanoparticles. Carbonaceous nanoparticles are one of the main reservoirs of interstellar carbon together with dust grains and gaseous C+, C and CO. I shall briefly review their different known and possible forms and the likelihood that they could bring a solution to various problems of the interstellar medium, especially the nature of the carriers of the diffuse interstellar bands (DIBs). I shall consider in turn polycyclic aromatic hydrocarbons (PAHs), fullerenes, carbon chains, nanodiamonds and nanotubes. In particular, I shall briefly present our recent work about the interest of polyacene PAHs (linear chains of N fused hexagons) as possible DIB carriers. 2. Measurement of high redshifts of submillimeter galaxies from CO lines. Redshift determination is the main difficulty for exploiting the few 105 high-redshift submillimeter galaxies (SMGs) discovered, e.g., by the Herschel Space Observatory, at z~2-4. CO line blind search is the most efficient way for such redshift determination. The broad band (twice that of ALMA) of the new receptor system of the IRAM NOEMA interferometer is very efficient for that, as we have just proved in a pilot project. It is now extended in a NOEMA large program for determining 120 redshifts of strongly lensed or hyper-luminous (LFIR > 1013 Lsun) Herschel SMGs, which should bring new light on these exceptional objects.


"The repercussions of pulsar glitches"

Cristobal Manuel Espinoza Romo (Pontificia Universidad Católica de Chile)

Glitches are discrete positive steps in spin frequency that occasionally interrupt the smooth rotation of pulsars. Our current understanding points to an internal origin, in which a neutron superfluid component that rotates at a higher rate than the rest of the star sometimes shares its angular momentum, thereby causing the observed glitches. Observations show a broad range of glitch sizes, covering more than 7 decades, and exhibiting a clear bimodal distribution. The largest events distribute narrowly in size, show different occurrence rates to the small glitches and more regular times between glitches. Furthermore, large glitches are followed by significant negative changes in spin-down rate. Enhanced spin-down rate regimes, known as glitch recoveries, are seen to last for months and slowly evolve towards pre-glitch values over years, or even decades. We have characterised the long-term (>20 yr) evolution of a number of pulsars with three or more large glitches and found that all of them evolve with particularly low frequency second derivatives. I will show that such findings suggest that large glitches and their transient recoveries are perturbations that alter the rotational behaviour of pulsars over long periods of time. Moreover, their cumulative effect can be substantial and should be considered when assessing the spin evolution of the young population and their ages.


"On the origin of LIGO/VIRGO gravitational wave sources"

Tomasz Bulik (OA UW and CAMK)


"A new way to measure the neutron star mass and radius from atmospheric oscillations"

Ananda Deepika Bollimpalli (CAMK, Warsaw)

Neutron stars are excellent astrophysical laboratories for studying matter at very high nuclear densities. To understand the equation of state (EoS) of such dense material, it is very important to know the mass and radius of the neutron star. There is plenty of observational evidence that some neutron star systems reach super-Eddington luminosities, either through the accretion of matter onto the stellar surface or by thermonuclear burning. Stars with such high luminosities are shown to harbor levitating atmospheres, supported by the radiation pressure from the star at a certain height above the stellar surface. We study the oscillations of these radiation-supported atmospheres to find a family of relativistic eigenmodes and eigenfrequencies of the radial oscillations. The frequency of these oscillations depends on the stellar parameters and varies with the stellar luminosity/ flux. We also find that damping due to radiation drag limits the frequency of these oscillations introducing a characteristic maximum in the frequencies. The main focus of this talk will be to discuss how this maximum frequency and the frequency variation with the flux can be used to determine the mass and radius of the neutron star. In addition to the stellar parameters, observation of the variation of the oscillation with flux would allow us to estimate the stellar luminosity and therefore the distance to the source with an accuracy of a few percents.


"Equation of state of neutron stars: confronting modern bounds from nuclear physics with GW170817"

Paweł Haensel (NCAC, Warsaw)


"Stellar Dynamics and Tidal Disruptions in Galactic Nuclei"

Rainer Spurzem (Frankfurt Institute for Advanced Study)

Black Holes are difficult to detect directly; indirect evidence for them comes e.g. from stellar motions or from tidal disruption events; stars getting on their orbit too close to the black hole are destroyed due to strong tidal forces, and the debris forms a transient disk with electromagnetic signature. Another interesting topic is the possible presence of a central gaseous disk in active galactic nuclei (AGN); stars interact with such disks and tidal disruption processes are dynamically distinct from the standard case. Tidal disruption events also may provide clues to the dynamical state of galaxies, for example whether there has been a recent merger. We study with computer simulations based on direct N-body simulations with Post-Newtonian general relativity (if needed) how nuclear star clusters, surrounding supermassive black holes, evolve, and look for evidence of galaxy mergers and central disks in the dynamical characteristics of tidal disruption events. We will also briefly touch the issue of gravitational wave emission in case of compact objects, which are too compact to be tidally disrupted, but will be just accreted to the central black hole under emission of gravitational waves. The computer simulations are quite challenging and can only be done due to the use of high performance GPU accelerated supercomputers. Some fresh benchmarks using the newest Volta V100 GPU hardware are shown.


"M1-92 revisited: the chemistry of an ejected common envelope ?"

Javier Alcolea (Observatorio Astronomico Nacional, Madrid)

The formation of axisymmetric planetary nebulae from the evolution of isotropic AGB circumstellar envelopes has been a matter of debate since more than two decades. A good deal of information on this process has been obtained from the observations of in between nebulae, which have been termed pre-planetary nebulae (pPNe). Going beyond the usual CO observation, which provide the main parameters of these post-AGB envelopes, we report on observations of other species in the bipolar pPN M1–92. The new IRAM 30m-MRT and NOEMA data show the presence of shock induced chemistry in the nebula. In addition, from the derived [17O]/[18O] ratio, we suggest that the sudden mass loss event responsible for the formation of the nebula 1200 yr ago may also resulted in the premature end of the AGB phase of the central star. Depending on the prevalence of such sudden mass losses, this may have and impact of the yields of AGB and the chemical evolution of the galaxy.


"Extremes of Power in the Universe"

Andrew Fabian (Institute of Astronomy, Cambridge, UK)


"Stochastic blazar variability"

Arti Goyal (Astronomical Observatory, Jagiellonian University)

Blazars are a rare class of active galactic nuclei (AGN) whose total radiative energy output is dominated by non-thermal processes; synchrotron (radio–to–optical frequencies) and inverse-Compton (IC; ∼X-ray–to–γ−ray frequencies) in a relativistic, non-stationary jet. Besides showing extreme luminosities up to ∼ 1047−48 ergs s−1 , blazars are also extremely variable on timescales ranging from decades to hours and even down to minutes. The power-law form of variability power spectral densities, P(νk) = A ν −β k , where νk is the temporal frequency, A is the normalization and β is the slope, indicate that the variability is generated by the underlying stochastic processes which is of colored noise type (i.e., β ' 1−3). In this talk, I will summarize the results of our analysis using multiwavelength data sets at TeV (HESS and VERITAS), GeV (Fermi-LAT), X-ray (Swift-XRT and RXTE– PCA), multiband optical/infrared and radio (GHz band from MRO, UMRAO and OVRO programmes) frequencies covering a few decades to minutes timescales. The novelty of this study is that at optical frequency, by combining long-term (historical optical light curves) and densely sampled intra-night lightcurves, the PSD characteristics are investigated for temporal frequencies ranging over 7 orders of magnitude. Our main results are: (1) nature of processes generating flux variability at synchrotron frequencies is different from those at IC frequencies (β ∼ 2 and 1, respectively); this could imply, that γ−ray variability, unlike the Synchrotron (radio-to-optical) one, is generated by the superposition of two stochastic processes with different relaxation timescales, (2) the main driver behind the optical variability is the same on years, months, days, and hours timescales (β ∼ 2), which argues against the scenario where different drivers behind the long-term flux changes and intra-night flux changes are considered, such as internal shocks due to the jet bulk velocity fluctuation (long-term flux changes) versus small-scale magnetic reconnection events taking place at the jet base (intra-night flux changes). Implications of these results are discussed in the context of commonly employed blazar emission models.


"First results from the Event Horizon Telescope"

Maciej Wielgus (Harvard-Smithsonian Center for Astrophysics, Cambridge, MA)

Event Horizon Telescope (EHT) is a global very long baseline interferometry array, capable of performing observations in 1 millimeter wavelength. After over a decade of tests and technical developments, in April 2017 EHT has performed first observations as a mature instrument, with sensitivity, resolution and coverage unparalleled in the history of the millimeter wavelength radioastronomy, for the first time expected to allow for the imaging of the observed sources. Observations of black hole candidates in the centers of Milky Way and M87 were conducted with a nominal resolution better than the diameter of a 'shadow of a black hole' expected from general relativity. The data set was processed and analyzed since then, finally leading to the announcement of the results on April 10th 2019.


"Star-disc magnetospheric interaction and launching of outflows and jets"

Miljenko Cemeljic (NCAC, Warsaw)

I present results in numerical simulations of star-disc magnetospheric interaction. I performed resistive and viscous MHD simulations, in which a quasi-stationary state is reached. Sweeping through the parameter space, I computed the torque exerted on the star, and the mass and angular momentum flux expelled from the system with the stellar wind, outflows and jets launched from the magnetosphere. I show the trends found in such solutions and derive some hints on the analytical solutions for the magnetic thin accretion discs.


"Gravitational lensing of gravitational waves and precision cosmology"

朱宗宏 Zong-Hong Zhu (Wuhan University)

The standard siren approach of gravitational wave cosmology appeals to the direct luminosity distance estimation through the waveform signals from inspiralling double compact binaries, especially those with electromagnetic counterparts providing redshifts. It is limited by the calibration uncertainties in strain amplitude and relies on the fine details of the waveform. We will show the next generation detector, e.g., the Einstein Telescope, is expected to produce 10^4 −10^5 gravitational wave detections per year, 50−100 of which will be lensed. Then we report a waveform-independent strategy to achieve precise cosmography by combining the accurately measured time delays from strongly lensed gravitational wave signals with the images and redshifts observed in the electromagnetic domain. We demonstrate that just 10 such lensing systems can provide a Hubble constant uncertainty of 0.68% for a flat Lambda Cold Dark Matter universe in the era of third generation ground-based detectors.


"Multi-messenger Astronomy"

Nayantara Gupta (Raman Research Institute, Bangalore, India)

Our group at Raman Research Institute, Bangalore works on cosmic rays (Galactic and extragalactic), multi-wavelength modelling of AGN and neutrino astronomy. In this talk I select two topics from our current research activities. In the first part of my talk I will discuss about physics of extended jets of quasars. The radio, optical and X-ray emission from extended jets is difficult to explain with the popular inverse Compton emission model. I will discuss about the other possible scenarios. In the second part I will discuss about injection, propagation and composition of ultrahigh energy cosmic rays with Pierre Auger data. We have used the publicly available code CRPropa3 to infer some favourable scenarios of source distribution, spectral index of injected cosmic rays and their composition for the EBL model by Dominguez et al.


"Unexpected presence of hot gamma Doradus and A-F type hybrid pulsators"

Filiz Kahraman (NCAC, Warsaw)

Gamma Doradus and A-F type hybrid pulsators are supposed to have an effective temperature range of about 6500 – 7500 K. However, the high-quality Kepler data has revealed some gamma Doradus and A-F hybrid pulsators hotter than 7500 K. To drive the pulsation mechanism in these oscillating stars, a sufficient convective envelope is necessary. However, according to theory in these hot objects, the convective envelope should not be sufficient enough to excite the gamma Doradus type pulsations. To explain the unexpected situation of these objects, some opinions were proposed such as possible incorrect atmospheric parameters, rapid rotation, chemical peculiarity, and binarity. A detailed spectroscopic and photometric investigation is needed to clarify these opinions. Therefore, we present the conclusions of our comprehensive spectroscopic and photometric study of twenty-four hot gamma Doradus and A-F hybrid candidate stars. Consequently, we confirm that the hot gamma Doradus and A-F hybrid stars exist. The surface chemical peculiarity and the binarity suggestions don’t appear to be a possible answer for the existence of these objects. It has been suggested that hot gamma Doradus and hybrid stars might be fast rotating slowly pulsating B stars (Balona et al. 2016). The stars in our sample have high projected rotational velocity (on average 130 and 190 km/s for gamma Doradus and hybrid stars, respectively) but they don’t show B type spectral lines. The result of this study provides new information to improve the understanding of the pulsation mechanism that occurs in the gamma Doradus stars.


"Particle acceleration in relativistic magnetospheres"

Benoît Cerutti (Université Grenoble Alpes)

Rapidly rotating neutrons stars and black holes are the central engines of some of the most extreme astrophysical phenomena such as gamma-ray bursts, pulsars, X-ray binaries, binary mergers or active galactic nuclei. The activity of these compact objects is often associated with the creation and the launching of a relativistic magnetized plasmas accompanied by efficient particle acceleration and non-thermal radiation, but the underlying physical mechanisms are still poorly understood. The particle-in-cell method is well-suited to model these processes from first principles. Recent numerical simulations have clearly established that relativistic magnetic reconnection within the magnetosphere of pulsars and black holes plays a crucial role in dissipating magnetic energy which is then efficiently channeled into energetic particles and high-energy radiation. Results will be discussed in the context of gamma-ray pulsars, merging binary neutron stars and weakly accreting Kerr black holes.


"The Milky Way as seen by the OGLE survey"

Igor Soszyński (Astronomical Observatory, Warsaw University)

The Optical Gravitational Lensing Experiment (OGLE) is currently the world's largest survey aimed at searching for variability in the sky. Currently, the project monitors brightness of about two billion objects in the densest stellar regions of the sky: central regions of the Galaxy, the Galactic disk, and the Magellanic Clouds. The OGLE Collection of Variable Stars currently contains nearly one million objects of various types and this is the largest set of variable stars ever obtained by any astronomical project. Recently, OGLE has greatly extended the list of known Cepheids in the Milky Way disk and used them to explore the structure, dynamics, and history of our Galaxy. I will present these results as well as other most spectacular latest OGLE discoveries in the field of variable stars.


"Radio sources in the SDSS spectroscopic catalogues"

Dorota Kozieł-Wierzbowska (Astronomical Observatory, Jagiellonian University)

I will present the catalogue of radio sources with galactic counterparts and unresolved or extended morphologies I (ROGUE I), which is the largest, handmade catalogue of visually classified radio objects. It was created by cross-matching galaxies from the SDSS with the FIRST and NVSS catalogues. The catalogue contains more than 32,000 galaxies with a FIRST core within 3arcsec of the optical position. However, in the radio catalogues with a low flux limit like ROGUE I, the separation of objects in which the radio emission is associated with jets from the radio emission arising in the star-forming regions is a challenging task. In recent papers (Best & Heckman 2012, Sabater et al. 2019) this population separation was made based on several different diagrams. However, this procedure is complicated and requires the knowledge of many different parameters, like optical emission line luminosities or ratios, galaxy mass or Dn(4000). Therefore, during my talk I will also discuss a single diagram that can be used to distinguish radio emission produced by an active galactic nucleus from the one emitted in star-forming regions.


"Watchmaking - craft or art ?"

Eugeniusz Szwed (Polish Watch and Clock Collectors Club)


"Flames on Neutron Stars: Type I Bursts and Instabilities"

Yuri Cavecchi (University of Southampton)

The Type I Bursts are powerful X-ray flashes from the surface of accreting neutron stars. They develop from the ignition of thermonuclear reactions and the spreading of the flame in the freshly accreted material. Outshining the accretion powered emission for tens of seconds, the Type I Bursts are one of the best observable phenomena of accreting neutron stars, and yet many questions remain unanswered. The light curves of the bursts encode information about the mass and radius of the star which can be linked to the unknown equation of state core of the neutron stars. However, in order to unambiguously extract such information we need to know the details of the emission on the surface and that crucially depends on fuel composition, its distribution and how the burning front propagates. I will discuss the puzzling phenomenology of the bursts, focusing on the burning conditions and flame propagation.


"Testing general relativity using X-ray reflection spectroscopy"

Cosimo Bambi (Fudan University, Shanghai, PRC)

Einstein's theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in weak gravitational fields. However, the strong field regime is still largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein's gravity for weak fields and present deviations only when gravity becomes strong. X-ray reflection spectroscopy is potentially a powerful tool for testing the strong gravity region around astrophysical black holes with electromagnetic radiation. In this talk, I will present the reflection model RELXILL_NK designed for testing the metric around black holes and the current constraints on possible new physics from the analysis of a few sources.


"Stable sub-Eddington black hole accretion disks"

Włodzimierz Kluźniak (CAMK, Warsaw)

I will present a new class of solutions of black hole accretion disks that we found through 3D, global, radiative magnetohydrodynamic simulations in general relativity (Debora Lan čová, David Abarca et al. 2019). It combines features of the canonical thin, slim, and thick disk models, but differs from all of them. The inner disk luminosity in the presented solution is about 0.3 of the Eddington value. The disk is optically thick and radiation-pressure dominated, but stable thanks to the presence of magnetic fields, with the beta parameter on the order of unity. The appearance of the disk to a distant observer strongly varies with the inclination angle.


"Particle acceleration and magnetic field amplification in astrophysical jets"

Anabella Araudo (Czech Academy of Sciences)

We study diffusive shock acceleration and magnetic field amplification in AGN and protostellar jets. By combining observational data, numerical simulations, plasma physics, and semi analytical theory of shock acceleration we go beyond the state of the art in modeling non-thermal jets. I will discuss our recent results on particle acceleration in shocks in extragalactic and galactic jets. In the former case, we conclude that acceleration in the mildly relativistic shocks in the jet lobes in radiogalaxies can accelerate Ultra High Energy Cosmic Rays. In the later case, we show that protostellar jets can accelerate TeV electrons and protons and emit gamma rays.


"Compressible Small-Scale Kinematic Dynamos"

Michael Mond (Ben Gurion University Negev, Israel)

The origin of the magnetic fields in the universe is one of the fundamental issues in astrophysics. In the current-day paradigm, seed magnetic fields are amplified over many orders of magnitude by turbulent magnetic dynamo action. In particular, the small-scale dynamo gives rise to the amplification of small-scale fluctuations of the magnetic field due to the random stretching of the field by the turbulent flow. In this talk I will concentrate on the role of compressibility of the turbulent flow. Its effect on small-scale field amplification are investigate by employing the Kazantsev-Kraichnan theory for kinematic dynamos. It is shown that the sonic scale, defined as the scale where typical turbulent eddy velocity equals the speed of sound, imposes a robust structure on the growth process. Three as well as two dimensional cases will be discussed.


"Accretion disks with outflows and their implications on massive black hole growth in the early universe"

Xinwu Cao (Zhejiang University, Hangzhou, China (浙江大学))

We derive a global solution to a slim disk with radiation-driven outflows, and find that the outflows are driven from the disk surface if the dimensionless mass accretion rate is around 1.8 (in units of Eddington rate: mdot=Mdot_Edd = L_Edd/0.1c^2). The rate of the gas swallowed by the black hole (BH) is always limited to mdot∼1.8−1.9 even if the mass accretion rate at the outer edge of the disk is very high. This implies it rather difficult to grow a massive BH with billions of solar masses at z>7 via accretion from a stellar mass BH. We develop a self-consistent accretion disk model with magnetically driven outflows, in which most angular momentum of the disk is removed by the outflows. It is found that the SMBH with several billion solar masses discovered at z>7 may probably be grown through chaotic accretion predominantly driven by magnetic outflows from a stellar mass BH, when the disks are radiating at moderate luminosity (∼ 0:5 Eddington luminosity) with mild outflows. Most SMBHs are found to be spinning at moderate values of spin parameter a at high redshifts, which may imply only a small fraction of quasars having radio jets.


"Numerical studies of white dwarf explosions: type Ia supernovae and tidal disruption events"

Ataru Tanikawa (University of Tokyo, Graduate School of Arts and Sciences)

White dwarfs (WDs) are the final states of intermediate- and low-mass stars. They will just cool over a long time if alone. However, they can experience explosion due to nuclear burning triggered by mass accretion, tidal disruption, and so on. First, I will talk about three dimensional simulation of double-detonation explosion in double WD system, so-called the D6 model, for modeling type Ia supernovae (SNe Ia). The D6 model predicts that the heavier WD explodes, while the lighter WD does not. Our simulation has shown that the explosion of the heavier WD strips materials from the lighter WD. This stripped materials consist of carbon and oxygen, and may contribute to low-velocity ejecta components as observationally interred for several sub-luminous SNe Ia. Second, I will introduce our numerical simulations of tidal disruption events of WDs, and discuss about nuclear ignition mechanism of such WDs.


"Single-sided pulsators - a new type of oscillating stars in close binaries"

Gerald Handler (CAMK, Warsaw)

Oscillating stars in binary systems usually have their pulsation axis located perpendicular to the orbital plane. It has long been hypothesized, but never observationally confirmed, that the gravitational force of the companion can alter the orientation of the pulsation axis of an oscillating star. Here we report the discovery of "single-sided pulsators" (working title), stars that have pulsation amplitudes strongly enhanced in one hemisphere. We explain this by the presence of a close companion, that deforms the geometry of the pulsator and tilts its pulsation axis into the orbital plane. So far, we have discovered three such stars in data from NASA's TESS mission. Each one of them behaves differently. We present the (short) history of this new type of pulsating stars and summarize the knowledge gathered on them so far.


"Hydrogen Burning White Dwarfs and their Mysterious Short Pulsations in X-Rays"

Marina Orio (Department of Astronomy, University of Wisconsin/INAF - Astronomical Observatory Padova)

Luminous supersoft X-ray sources are usually white dwarfs (WDs) undergoing shell burning in a thin layer at the bottom of an envelope accreted from an interacting binary companion (either a main sequence or a red giant star). The vast majority are in post-outburst novae that continue thermonuclear burning for a period varying from days to years, but also a few steady or recurrent non-nova burning WDs have been discovered. Several of them are in symbiotics and a few are in massive binaries, with a Be companion. In a number of novae after the outburst, and in the non-nova binaries CAL 83 of the LMC and "r3-8" of M31, short pulsations of the X-ray flux with periods around one minute have been discovered. These were initially proposed to be non-radial oscillations caused by the shell burning, as observed in some AGB stars (so called "epsilon mechanism"), however recent work shows that effective temperature and other WD parameters are not consistent with the observed times for such a mechanism. In recent work on Nova LMC 2009 and on CAL 83, we discovered that the period of the oscillations drifts by a few percent during time scales of hours, and even the pulsation amplitude varies. I will show the data and discuss possible interpretations for this puzzling phenomenon of time variability, and what it may imply for the evolution of white dwarf binaries.


"Unified equation of state consistent with astrophysical, gravitational, high- and low- energy nuclear physics data and its applications"

Violetta Sagun (Coimbra University)

I will present a novel equation of state, which includes the surface tension contribution induced by the interparticle interaction and the asymmetry between neutrons and protons, to the study of neutron star properties. This equation of state is obtained from the virial expansion for the multicomponent particle mixtures that takes into account the hard-core repulsion between them. The considered model is in full concordance with all the known properties of normal nuclear matter, provides a high quality description of the proton flow constraints, hadron multiplicities created during the nuclear-nuclear collision experiments and equally is consistent with astrophysical data coming from neutron star observations and GW170817 merger. I will show how the induced surface tension (IST) equation of state opens an elegant way to describe the properties of matter across a very wide range of densities and temperatures. In the second part of the presentation I am going to present our recent results of an impact of asymmetric dark matter on properties of the neutron stars and their ability to reach the two solar masses limit. It allows us to present a new upper constraint on the mass of dark matter particle. Our analysis is based on the observational fact of existence of three pulsars reaching this limit and on the theoretically predicted reduction of the neutron star maximal mass caused by accumulation of dark matter in its interior.


"What was the last Nobel Prize in Physics given for?"

Ewa Łokas, Maciej Konacki (NCAC, Warsaw)