1. Dmitri Uzdensky (University of Colorado Boulder)
Relativistic magnetic flares and cosmic particle accelerators at the frontier of plasma astrophysics
Dmitri Uzdensky
(University of Colorado Boulder)
In collaboration with G.Werner, V.Zhdankin, M.Begelman (Colorado)
RosnerFest, Univ. Chicago, September 8, 2017
D. Uzdensky
6/12/2017

2. Coronae of Black-Hole (BH) Accretion Disks
An important part of Bob’s scientific legacy: GRV-1979
BH accretion disks have hard X-ray coronae.
Analogy with solar corona:
accretion disk is turbulent (MRI) --> dynamo;
magnetic loops rise above the disk;
reconnection accelerates coronal electrons;
relativistic electrons produce X-rays by up-scattering soft disk photons (inverse-Compton emission);
D. Uzdensky
6/12/2017

3. Main Questions (the “Why?”):
energy partitioning between electrons and ions
nonthermal particle (electron) acceleration
observable radiative signatures
effects of radiation reaction
efficiency of pair production
Differences with traditional (e.g., magnetosphere, solar) reconnection physics:
Relativistic /semi-relativistic plasmas,
powerful radiation cooling of electrons
e+e- pair production
D. Uzdensky
6/12/2017

4. Kinetic Numerical (PIC) Simulations
These questions are now subject of intense investigation via first-principles kinetic (PIC) numerical simulations.
This astrophysical problem motivated us to develop of new computational capabilities, adding new physics (e.g., radiation and pair creation) to astrophys. PIC codes.
We use our own new radiative-PIC code Zeltron:
includes synchrotron and inverse-Compton radiation reaction;
self-consistently computes radiation spectra, light-curves, etc..
Our biggest simulations (3D) are on done on Mira at Argonne/ALCF/INCITE.
D. Uzdensky
6/12/2017

5. Semi-Relativistic* and Relativistic Reconnection in Electron-Ion Plasmas
(G. Werner et al. 2017)
[Black hole accretion flows, accretion disk coronae, jets]
* Semi-relativistic: ultra-relativistic electrons but non-relativistic ions.
D. Uzdensky
6/12/2017

6. Electron-Ion reconnection in 2D semirelativistic and ultra-relativistic regimes
Evolution of magnetic field and current density in 2D electron-ion reconnection with σi=0.1.
D. Uzdensky
6/12/2017

7. Relativistic e-i reconnection: Key PIC Sims Results I
Reconnection rate: vin/c = E/B0
Energy partitioning between electrons and ions
ions σi
electrons σi σi
In semi-relativistic case ions gain 3 times more energy than electrons.
Werner et al. 2017
D. Uzdensky
6/12/2017

8. Relativistic e-i reconnection: Key PIC Sims Results II
Particle Acceleration:
- electrons: ✔
- ions ?
Electron power-law index p and cutoff γc:
γc/σi pe σi σi
Werner et al. 2017
D. Uzdensky
6/12/2017

9. Cautionary Tale: Chaotic Behavior and Natural System Noise
Particle energy spectra in multiple realizations of ei reconnection in simulations with identical setup and initial parameters:
Long reconnection current layers are unstable;
Reconnection is not laminar but highly dynamic/stochastic;
But it is not fully turbulent, so statistical properties vary from one realization to another;
This leads to uncertainty of, e.g., nonthermal particle spectral indices (δp~0.3)
and cutoffs γc (factor of 2).
D. Uzdensky
6/12/2017

10. Relativistic reconnection with external inverse Compton Cooling
Effects of radiative IC cooling on nonthermal particle acceleration:
D. Uzdensky
6/12/2017