Wednesday Colloquium


"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.