Accretion onto Black Holes

1
Variability and Flares From Accretion onto Sgr A
Eliot Quataert (UC Berkeley)
Collaborators Josh Goldston, Ramesh Narayan,
Feng Yuan, Igor Igumenshchev
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Two Sources of Variability
Dynamical ?, T, B in accretion flow
change with time (its 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
Sohos 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 lt 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
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variability decreases at optically thick
frequencies
Flux RMS Variability
Fractional Variability
Photon Frequency
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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
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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
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Why our Galactic Center?
Yuan, Quataert, Narayan 2004
Key is L ltltltltlt LEDD analogous flares harder to
detect in more luminous systems because they are
swamped by thermal SSC emission (next best bet
is probably M32)
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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

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