Talks

What powers the hard, non-thermal X-rays from accreting compact objects has been a longstanding mystery. In my talk, I will address the underlying question of what energizes the particles of the Comptonizing “corona” against the strong inverse Compton (IC) cooling losses with first-principle particle-in-cell simulations of magnetic reconnection subject to IC cooling in magnetically dominated electron-positron plasmas, and in mildly-magnetized electron-ion plasmas. I will also show---using results of global resistive GRMHD simulations of accreting black holes---that the black hole jet sheath is a site of efficient electromagnetic dissipation through processes such as magnetic reconnection and turbulence. The distribution of bulk motions of the radially outflowing plasma along the jet sheath also resembles a Maxwellian distribution with an effective bulk temperature of a few 100 keV, and this could be a candidate for the Comptonizing corona.

A new version of the Athena++ AMR MHD code implemented using the Kokkos library for performance-portability will be described. The code is being used for a variety of problems related to accretion onto (and merger of) compact objects. This talk will focus on results from general relativistic radiation MHD models of accretion in luminous systems such as X-ray binaries and AGN. The models allow observationally important quantities such as the radiative efficiency, variability, and kinetic feedback in winds to be measured over a range of luminosities, from sub-Eddington to highly super-Eddington. A new “cyclic-zoom” approach for modeling such flows over very long time scales will also be briefly introduced.

An alternative interpretation of the EHT observations of M87, involving a strongly magnetized ergomagnetosphere, that expels essentially of the gas supplied at large radius as a jet-collimating MHD wind is described. In this case, the emitting regions are approximated as rapidly evolving flares or sparks. Six generic consequences, that can be sought in existing data, are described.

Primordial Magnetic Fields (PMFs), long studied as potential relics of the early Universe, can accelerate recombination and have been proposed as a possible solution to the Hubble tension. In this seminar, I will present the latest observational evidence supporting this idea, based on comprehensive magnetohydrodynamic (MHD) simulations of recombination in the presence of PMFs. Current Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillation (BAO) data exhibit a mild preference for PMFs while predicting a higher Hubble constant. Notably, the inferred field strengths align with those required to explain galaxy cluster magnetic fields purely from primordial origins, eliminating the need for additional dynamo amplification or stellar contributions. Future high-resolution CMB temperature and polarization measurements will be crucial in confirming or further constraining the role of PMFs during recombination.

In this session, we'll discuss the nuts and bolts of science writing for the general public. You'll learn how produce engaging science content for blogs and articles, as well as how to pitch ideas for publication in media outlets.