Contact
University of Wroclaw
Faculty of Physics and Astronomy
Institute of Theoretical Physics
Division of Elementary Particle Theory
Plac Maksa Borna 9
50-204 Wroclaw
Poland



Tobias Fischer
office 432
Tel. +48 71 375 9251
Fax. +48 71 321-4454
tobias.fischer (at) uwr.edu.pl

Research page

Our reseach activities center around the "SUPERNOVA PROBLEM". It concerns the death of massive stars (i.e. stars that are more than 10 times as massive as the sun) and, in particular, the understanding of the physical processes involved. Such events are the most energetic outbursts in the modern universe with an energy output in excess of an entire galaxy - containing billions of stars - for the period of several months. The understanding of such events requires a profound knowledge about all four fundamental forces of nature - gravity, electromagnetism, strong, and weak interactions - as well as large scale numerical modeling. Supernovae are also an ideal site to probe the yet incopmpletely known state of matter, in particular at extreme conditions that are inaccessible otherwise, e.g., in terrestrial experiments.



Failed core-collapse supernovae and subsequent black-hole formation


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In the absence of an earlier supernova explosion, continuous mass accretion onto the central protoneutron star will eventually lead to the formation of a solar mass black hole, on a timescale of several 100 milliseconds up to few seconds, for stellar progenitors in the zero-age main sequence mass range of 40 to 50 solar masses. Details of this scenario depend on the equation of state (EOS) and can be probed via the neutrino luminosities and spectra. Astron. Astrophys. 499, 26 (2009)




Equation of state for core-collapse supernova studies


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The large variety of conditions which are covered by the supernova equation of state range from low temperatures ≤ 0.5 MeV, where time-dependent nuclear reactions determine the composition, towards complete chemical and thermal equilibrium known as NSE (nuclear statistical equilibrium) with increasing temperature. In NSE, the nuclear composition is determined by the following three independent variables: temperature, rest-mass density (or baryon number density), and charge density. With increasing rest-mass and charge density, nuclei become heavier and more neutron rich, as a consequence of electron captures on protons bound in nuclei. At normal nuclear density as well as above temperatures of about 10 MeV, nuclei dissolve at the liquid–gas phase transition into homogeneous nuclear matter. Publ. Astron. Soc. Austr. 34, e064 (2017)



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An interesting consideration is the possibility of a phase transition, from normal nuclear (in general hadronic matter) to the quark-gluon plasma. If such transition is of first order, i.e. featuring an unstable hadron-quark matter co-excistence region, then it might yield an observable signature in the neutrino signal. Nat. Astron. 2, 980 (2018)



Long-term proto-neutron star evolution: deleptonization and cooling


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Neutrinos of all flavors will be emitted from core-collapse supernovae for several tens of seconds. Current neutrino detectors, e.g., Super-Kamiokande in Japan are setup to yield several tens of tousand events, where only the first few hundrets of milliseconds provide details about the yet incompletely known central engine that drives the supernopva explosion. Once the supenrova explosion proceeds, the nascant central compact object -- proto-neutron star -- delptonizes and cools on a timescale on the order of tens of seconds, developing thereby into the final supernova remnant neutron star. Phys. Rev. D. 94, 085012 (2016)



Development of neutrino transport methods and weak interactions


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Boltzmann transport equation for ultra-relativistic particles: Evolution of the neutrino phase-space distribution functions within the general-relativistic framework -- left-hand side: phase space deriavtive (transport) -- right-hand side: collision integral Phys. Rev. D. 94, 085012 (2016)


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In astrophysical applications, e.g., in multi-dimensional supernova simulations, it is indispensable to develop approximate neutrino-transport schemes due to the present computational limitations. Astrophys. J. 698, 1174 (2009)


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Set of standard neutrino-matter interactions that are considered in simulations of core-collapse supernovae. Weak processes under supernova conditions are modified by the nuclear medium. In particular reactions which involve neutrons and protons have a strong medium dependence. Beyond the mean-field treatment, the dominant modification is due to the dressing of the vertex function. Astron. Astrophys. 593, A103 (2016)



Axions: constraining the couplings and excess cooling in simulations


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The existence of axion-like particles (ALP) is a prediction by several extensions of the standard model of particle physics. In a core-collapse supernova, ultralight ALP would be emitted via the Primakoff process, and eventually convert into gamma rays in the galacvtic magnetic field. The lack of a gamma-ray signal in the observing instruments coinciding with SN1987A, therefore provides a strong bound on the ALP coupling to photons. Revisiting this constraint was possible with modern supernova models. J. Cosm. Astropart. Phys. 02, 006 (2015)


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There is also the prediction of strongly interacting axions, as a possible solution to the strong CP problem of QCD. The relevant process of axion emission in a supernova is the nucleon-nucleon axion bremsstrahlung, which involves the axion-nucleon coupling. The associated additional source of energy loss could potentially enhance the cooling, which in turn may reduce the associated neutrino flux. The reliable prediction of the impact from excess axion cooling requires accurate models of in-medium nucleon-nucleon bemsstrahlung processes. J. Cosm. Astropart. Phys. 10, 016 (2019)

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