Victor Oknyansky (University of Haifa)
Dragana Ilic (University of Belgrade)
Tomasz Krajewski (NCAC, Warsaw)
Terminal velocity reached by bubble walls in cosmological first order phase transitions is an important parameter determining both primordial gravitational-wave spectrum and production of baryon asymmetry in models of electroweak baryogenesis. We developed a numerical code to study the real-time evolution of expanding bubbles and investigate how their walls reach stationary states. Our results agree with profiles obtained within the so-called bag model with very good accuracy, however, not all such solutions are stable and realised in dynamical systems. Depending on the exact shape of the potential there is always a range of wall velocities where no steady state solutions exist. This behaviour in deflagrations was explained by hydrodynamical obstruction where solutions that would heat the plasma outside the wall above the critical temperature and cause local symmetry restoration are forbidden. For even more affected hybrid solutions causes are less straight forward, however, we provide a simple numerical fit allowing one to verify if a solution with a given velocity is allowed simply by computing the ratio of the nucleation temperature to the critical one for the potential in question.
Biswaraj Palit (NCAC, Warsaw)
How black holes consume and eject matter has been the subject of intense studies for more than 60 years. The luminosity of these systems are often compared to the Eddington limit, the border at which the spherical accretion is inhibited by the radiation pressure of photons it produces. The discovery of ultraluminous X-ray sources (ULXs) showed that accretion can proceed even when the apparent luminosity exceeds the Eddington limit (Kaaret et al. 2017). High apparent luminosity might be produced by the beaming of the incident radiation by a thick collimated outflow or by a truly super-Eddington accretion flow. However, possibilities to study these outflows in detail are limited, as ULXs are typically found in distant galaxies. Using the Imaging X-ray Polarimetry Explorer (IXPE, Weisskopf et al. 2022), we made the first measurement of X-ray polarization in Galactic X-ray binary Cyg X-3. The detection of high, ≈25\%, nearly energy-independent linear polarization, orthogonal to the direction of the radio ejections, unambiguously indicates the primary source is obscured and the observer on Earth only sees reflected and scattered light. Modelling shows there is an optically thick envelope with a narrow funnel around the primary X-ray source in the system. We derive an upper limit on the opening angle of the funnel that implies a lower limit on the beamed luminosity exceeding the Eddington value. We show that Cyg X-3 is viewed as a ULX to an extragalactic observer located along the axis of the funnel. Our findings reveal this unique persistent source as an ideal laboratory for the study of the inner workings of ULX central engines.
