1. Variability and Flares From Accretion onto Sgr A* Eliot Quataert (UC Berkeley) Collaborators: Josh Goldston, Ramesh Narayan, Feng Yuan, Igor Igumenshchev

2. Two Sources of Variability Dynamical: , T, & B in accretion flow change with time (it’s turbulent!) John Hawley

3. Two Sources of Variability Transient Heating & Electron Acceleration Soho’s View of the Sun Note: dynamics and heating coupled: e.g., fluctuations in magnetic field probably correlated with electron acceleration

4. Newtonian Simulations & Radiative Transfer (next step is GR …) Thermal electrons + power-law tail (5% of e - energy) Encouraging: at ~ THz, emission is strongly peaked near black hole where GR effects important (e.g, Falcke et al. 2000)  10 R S Synchrotron Emission in MHD Simulations of RIAFs Goldston, Quataert, & Igumenshchev 2004 ~ THz

5. Synchrotron Lightcurves (optically thin) Radio (thermal) IR (Power law e - ) At high frequencies, factors of ~ few-10 variability on ~ hour timescales (~ orbital period near BH) Difft. freq. well correlated with < hr time delay

6. Variability more rapid & larger amplitude at higher frequencies, in accord with observations Fractional Variability 1 hour timescale 1 day timescale Photon Frequency

7. variability decreases at optically thick frequencies Flux & RMS Variability Fractional Variability Photon Frequency

8. Linear Polarization Linear Polarization Fraction Photon Frequency 32 random time-slices optically thin; no Faraday rotation Polarization vector predicted to be in the plane of the accretion flow (due to coherent B  )

9. encouraging that variability from turbulent accretion flow is broadly consistent with observations Significant fluctuations on ~ hour time-scales But... 1. probably insufficient changes on 10s min (IR) - particle acceleration or rotating hole? 2. large-amplitude X-ray flares - particle acceleration? A Day in the Life of Sgr A*

10. Flaring from Electron Acceleration Yuan, Quataert, & Narayan 2004 well motivated by strong dynamical changes near BH ( + hot magnetized plasma) assume ~ 10% of electron thermal energy transiently dumped into a ‘hard’ power law tail IR: synchrotron from  ~ 10 3 e - X-rays: synch. from  ~ 10 5 e - (x-rays could also be SSC)

11. Why our Galactic Center? Key is L <<<<< L EDD : analogous ‘flares’ harder to detect in more luminous systems because they are swamped by thermal SSC emission (next best bet is probably M32) Yuan, Quataert, & Narayan 2004

12. Summary SgrA* variability broadly consistent w/ turbulent RIAF Synchrotron radiation in MHD simulations shows –~ order of mag. variability on ~ hour timescales at optically thin freq. –increasing variability with increasing photon frequency –strong linear polarization in the plane of the accretion flow at all optically thin freq. (neglecting Faraday effects) Largest amplitude, shortest timescale X-ray & IR flaring probably traces transient electron acceleration

13. Two Sources of Variability Transient Heating & Electron Acceleration Soho’s View of the Sun Note: dynamics and heating coupled: e.g., fluctuations in magnetic field probably correlated with electron acceleration