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Variability and Flares From Accretion onto Sgr A*

Variability and Flares From Accretion onto Sgr A*. Eliot Quataert (UC Berkeley). Collaborators: Josh Goldston, Ramesh Narayan, Feng Yuan, Igor Igumenshchev. Two Sources of Variability. Dynamical : , T, & B in accretion flow change with time (it’s turbulent!). John Hawley.

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Variability and Flares From Accretion onto Sgr A*

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  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 Note: dynamics and heating coupled: e.g., fluctuations in magnetic field probably correlated with electron acceleration Soho’s View of the Sun

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

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

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

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

  8. Linear Polarization 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) Linear Polarization Fraction Photon Frequency

  9. A Day in the Life of Sgr A* 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?

  10. Flaring from Electron Acceleration 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  ~ 103 e- X-rays: synch. from  ~ 105 e- (x-rays could also be SSC) Yuan, Quataert, & Narayan 2004

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

  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 Note: dynamics and heating coupled: e.g., fluctuations in magnetic field probably correlated with electron acceleration Soho’s View of the Sun

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