The seminar of the Astro & Theory Section takes place normally Tuesdays in D5-106, starting at 14.15. If you would like to suggest a seminar speaker or want to be added to the email list, please contact the organizer (Michael.Kachelriess@ntnu.no).
The (planned) seminars in 2023 are
- 22.02., Alexander van der Horst (George Washington U.): Radio View on Gamma-Ray Burst Extremes
Abstract: Many high-energy astrophysical sources accelerate electrons to extreme velocities, resulting in multi-wavelength emission from synchrotron and other radiation processes. Gamma-ray bursts represent some of the most extreme electron accelerators, in the collimated outflows, or jets, of massive stellar explosions and the mergers of compact objects. Observations of gamma-ray bursts across the electromagnetic spectrum, from GHz radio frequencies to TeV gamma-ray energies, provide a unique probe of electron acceleration due to the relativistic nature of their jets. I will discuss recent developments in this area, both observationally and in modeling the observations, focusing on gamma-ray burst extremes from a radio perspective.
- 15.03., Egor Podlesnyi (IFY, NTNU): Modelling the persistent low-state γ-ray emission of the PKS 1510-089 blazar
Abstract: Blazars may accelerate protons and/or nuclei as well as electrons. The hadronic component of accelerated particles in blazars may constitute the bulk of their high-energy budget; nevertheless, this component is elusive due to a high value of the energy threshold of proton interaction with photon fields inside the source. However, broad line regions (BLRs) of some flat spectrum radio quasars (FSRQs) may contain a sufficient amount of matter to render primary protons "visible" in γ rays via hadronuclear interactions. In the present work, we study the persistent γ-ray emission of the FSRQ PKS 1510-089 in its low state utilizing the publicly-available Fermi-LAT data and the spectrum measured with the MAGIC imaging atmospheric Cherenkov telescopes. We find an indication for an excess of γ rays at the energy range ≳20 GeV with respect to a simple baseline log-parabolic intrinsic spectral model. This excess could be explained in a scenario invoking hadronuclear interactions of primary protons on the BLR material with the subsequent development of electromagnetic cascades in photon fields. We present a Monte Carlo calculation of the spectrum of this cascade component, taking as input the BLR photon field spectrum calculated with the Cloudy code.
27.03., Tor Nordam (SINTEF and IFY NTNU): Transport modelling with stochastic differential equations
Abstract: Stochastic transport models have existed at least the early 1900s, when Einstein (1905) and Langevin (1908) published two papers with mathematical models for Brownian motion. Einstein described this as a random displacement process, where a particle moves a distance at during each time interval. Langevin, on the other hand, described a random acceleration process, where a particle changes its velocity by a random amount during each time interval. In this talk, I will give a short historical introduction, and then discuss differences and similarities between the two models.
- 19.04. Enrico Peretti (NBI): Particle acceleration and multi-messenger radiation from astrophysical outflows
Abstract: Winds and outflows are ubiquitous at several scales throughout the Cosmos. They often develop a bubble structure characterized by strong shocks and turbulence where high-energy particles can be efficiently produced. I will present a model in which diffusive shock acceleration is a key process to energize particles in such astrophysical winds.I will show some model applications in the context of young massive stellar clusters, starburst galaxies and active galactic nuclei andI will discuss the associated multi-messenger implications in terms of high-energy photons, neutrinos and escaping cosmic rays.
- 26.04., Mark Kennedy (U Cork): Invisible Monsters and Spider Webs
Abstract: Where is the missing Galactic population of black holes? How do we obtain precise and accurate masses for black holes and neutron stars in our galaxy? These are two of the most pressing questions in astrophysics, and ones which the enormous databases produced by telescopes such as GAIA, the Zwicky Transient Factory (ZTF), and the upcoming Vera Rubin Observatory can help us answer. During this talk, I will discuss where I believe an answer to the missing black hole population may be found. This involves combining data taken by GAIA and ZTF and applying machine learning techniques to find light curves which challenge our understanding of binary classification. I will also highlight the many challenges, pitfalls, and rabbit holes that can be encountered when dealing with these large data sets. I will also give a summary of how the masses of invisible companions can be estimated in these types of binaries, with a focus on some interesting recent results and advances in how the companions in these systems are being modelled.
- 03.05. Jordan Simpson (IFY, NTNU): Cataclysmic variables with HiPERCAM
Abstract: Cataclysmic variables (CVs) are close interacting binary systems consisting of a white dwarf star (primary) accreting matter from a low-mass companion (secondary). As the end-state of many main sequence binaries, and potential progenitors to Type Ia supernovae, CVs form a crucial stage in the evolution of a wide variety of systems. The classical theory of CV evolution has persisted for over 40 years, despite its continuing failures to explain observations. Recently, a new empirical model has emerged that potentially explains many issues in CV evolution - however, a physical basis for this model is still needed. Using the ultra-fast quintuple-band imager HiPERCAM, along with advanced modelling techniques, the team at Sheffield (and collaborators) measures CVs with unprecedented precision to test this new model and ultimately resolve the long-standing problems in CV evolution once and for all.
- 10.05. Alice Harding (LANL): High-energy emission from spider binaries
Abstract: Compact binary systems containing rotation-powered millisecond pulsars with powerful winds that interact with their companions produce shocks that can accelerated particles to very high energies. The pulsar winds also heat and ablate the companion stars, and are appropriately named Black Widow and Redback systems. The Fermi Gamma-Ray Space Telescope has discovered most of the present population of 32 Black Widows and 13 Redbacks. I will discuss the acceleration and radiation from the intrabinary shocks in such systems and present a model for orbitally modulated emission from X-rays to Very-High-Energy gamma-rays.
- 7.6., Mathias Pavely Noedtvedt (IFY, NTNU): Detecting gravitational waves with the Moon
Abstract: It has been suggested to measure the eigenmodes of the Moon excited by gravitational waves. Such a resonant bar detector could become a valuable partner observatory for gravitational waves working alongside LISA and other future detectors. We discuss in this talk the excitation of resonant eigenmodes by gravitational waves within Einstein and Brans-Dicke gravity. In the latter case, gravitational waves obtain an additional polarisation. In spherical detectors, it is possible to extract information on the polarisation of the gravitational wave. Such detectors are therefore a way to search for theories of gravity beyond general relativity. I will discuss the calculations of the displacement by gravitational wave resonant with the eigenmodes of the Earth and Moon, and comment on the frequency range for the potential Lunar gravitational wave antenna.
- 14.6., Kari Koljonen (IFY, NTNU): The origin of optical emission lines in the soft state of X-ray binary outbursts
Abstract: The optical emission line spectra of X-ray binaries are thought to be produced in an irradiated atmosphere, potentially the base of a wind, positioned above the outer accretion disc. However, the physical nature and conditions of the region that forms these lines are not well understood. In this presentation, I will discuss our study of X-ray binary MAXI J1820+070 in the soft state associated with its 2018 outburst. We modeled a line-rich, soft state, broadband spectrum using the X-Shooter instrument at the Very Large Telescope, with a Monte Carlo radiative transfer code that employed a simple biconical disc wind model. This model was inspired by radiation-hydrodynamic simulations of irradiation-driven outflows from X-ray binary discs. I will demonstrate that such a model can qualitatively replicate the observed features and that almost all of the optical emission in the model arises from the transonic "transition region" between the disc atmosphere and a thermally-driven wind. Our research shows that this transition region can be the source of the abundant optical lines seen in the soft state spectra of X-ray binaries during outburst events. These "wind bases" can also have a significant effect on the infrared-to-ultraviolet continua, and can even dominate the emission in the red part of the spectrum. Overall, the strong irradiation of the outer disc by the inner accretion flow can naturally create both the optical line-forming layer and an overlying outflow/atmosphere that generates X-ray absorption lines that are typically observed during the bright soft states of X-ray binary outbursts.
- summer break
- 29.08. Benedikt Schroer (U Chicago): Transport of Galactic Cosmic-Ray Nuclei
Abstract: Understanding the cosmic-ray transport properties in our Galaxy is of utmost importance for many different observations. Using a well-motivated, semi-analytical model, we prove that a single population of cosmic-ray sources accelerating all elements with the same slope is able to explain the available data. We further test the self-consistency of our model by investigating particle transport on intermediate scales with a special focus on the escape of accelerated particles into the interstellar medium. These escaping particles are able to excite the non-resonant streaming instability inside a flux tube. As a result, one expects efficient self-confinement of high-energy particles, which we confirm by performing 2D and 3D hybrid particle-in-cell simulations with the code dHybridR. These bubbles of self-confinement might introduce some modifications to the previously described model of transport on Galactic scales.
- 12.09. Anlaug Amanda Djupvik (NOT): Star formation studies with the Nordic Optical Telescope
Abstract: I will present the Nordic Optical Telescope and two ongoing star formation studies using the NOT. One is based on infrared observations of embedded protostellar jets and another on high-resolution optical spectroscopy to study the UXOR phenomenon. 1) Protostellar jets serve as footprints of the ejection history of a protostar, believed to reflect also the accretion history. A kinematic study of jet knots with infrared imaging and spectroscopy in a dense cloud in Serpes reveals space velocities and locate their driving sources. We find time-variable ejection/accretion and deduce a higher accretion rate, by orders of magnitude, for the youngest protostar. 2) The UXOR phenomenon, a specific and irregular variability in intermediate-mass young stars, is believed to be caused by variable circumstellar extinction. We monitor the variable emission line profiles of hydrogen and other lines formed in the inner part of the circumstellar disk as the star is going through a fading event. Simulations based on hybrid models of magnetospheric accretion and magnetocentrifugal disk winds are used with variable obscuration scenarios. Modelling finds that the disk wind plays a dominant role in the emission line profiles. An obscuration scenario where dust moves vertically up from the disk fits the data best for one of our targets, RR Tau, and it is suggested that the dust is lifted by the disk wind.
- 19.09.. Vittoria Vecchiotti (IFY, NTNU): The high-energy Galactic gamma-ray emission: the diffuse component and the role of unresolved pulsar wind nebulae
Abstract: The study of the Galactic gamma-ray and neutrino diffuse emission represents an indirect way to investigate cosmic-ray propagation in the Galaxy. However, the signal produced by unresolved sources contaminates diffuse emission measurements, making their interpretation challenging. In order to quantify the relevance of the unresolved source component, we performed a population study of the H.E.S.S. Galactic Plane Survey under the assumption of sources powered by pulsar activity. We estimate the total TeV flux produced by all the sources in our Galaxy. In particular, our results show that a non-negligible fraction of this flux is produced by faint sources below the H.E.S.S. detection threshold. In the GeV energy range, a significant fraction of the TeV source population cannot be resolved by Fermi-LAT providing a relevant contribution to the large-scale diffuse emission, ranging within $\sim 4\%-40\%$ of the total diffuse $\gamma$-ray emission in the inner Galaxy. Hence, this unresolved component may account for a large part of the spectral index variation observed by Fermi-LAT as a function of the Galactocentric distance. Lastly, we calculate the hadronic diffuse emission in the sub-PeV energy range under different model assumptions and compare it to the large-scale diffuse emission observed by Tibet AS$\gamma$. We show that a pure hadronic emission underpredicts the measurements requiring the presence of an unresolved source component.
- 10.10. Lluis Mas-Ribas (IFY, NTNU): Oes
- 17.10. Roger Blandford (U Stanford): On the Acceleration of Atomic Nuclei to a Hundred Joules
Abstract: Cosmic rays are observed to have energies measured to be as high as 200 EeV. A major puzzle is to explain how they are accelerated. In this lecture, I will summarize what we have learned about the composition, spectrum and isotropy from non-relativistic to these “Ultra High" energies. I will then briefly outline the most promising of the mechanisms that have been proposed. Finally I will sketch a scheme in which the entire spectrum is produced, hierarchically, by a “bootstrap” process operating at astrophysical shock fronts of successively larger scale.
- 14.11. Raphaël Mignon-Risse (IFY, NTNU): Oes
- tbd. Jens Oluf Andersen (IFY, NTNU): Pion condensation and pion stars
Abstract: In this talk I will discuss pion condensation in the context of two and three-flavor chiral perturbation theory. I will present results for quark and pion condensates as functions of the isospin chemical potential. The results compare favorably to those of lattice QCD. As an application, I will discuss pion condensation in a dense neutrino cloud and the possibility of pion stars. These are compact objects with a mass up to 20 solar masses and radii of up to 140km.
- tbd: . Michael Unger (KIT and IFY, NTNU): ??