Post on 19-Jan-2018
description
Variability and Flares From Variability and Flares From Accretion onto Sgr A*Accretion onto Sgr A*
Eliot Quataert(UC Berkeley)
Collaborators: Josh Goldston, Ramesh Narayan, Feng Yuan, Igor Igumenshchev
Two Sources of VariabilityTwo Sources of Variability
Dynamical: , T, & B in accretion flow change
with time (it’s turbulent!)
John Hawley
Two Sources of VariabilityTwo Sources of Variability
Transient Heating &Electron Acceleration
Soho’s View of the Sun
Note: dynamics and heatingcoupled: e.g., fluctuations in
magnetic field probably correlated with
electron acceleration
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 RS
Synchrotron Emission in MHD Simulations of RIAFsSynchrotron Emission in MHD Simulations of RIAFs
Goldston, Quataert, & Igumenshchev 2004
~ THz
Synchrotron LightcurvesSynchrotron Lightcurves(optically thin)(optically thin)
Radio(thermal)
IR (Powerlaw e-)
At high frequencies, factors of ~ few-10 variability on ~ hour timescales (~ orbital period near BH)
Difft. freq. well correlated with < hr time delay
Variability more rapid & larger amplitude at Variability more rapid & larger amplitude at higher frequencies, in accord with observationshigher frequencies, in accord with observations
Frac
tiona
l Var
iabi
lity
1 hourtimescale
1 daytimescale
Photon Frequency
variability decreases at optically variability decreases at optically thick frequenciesthick frequencies
Flux & RMSVariability
FractionalVariability
Photon Frequency
Linear PolarizationLinear PolarizationLi
near
Pol
ariz
atio
n Fr
actio
n
Photon Frequency
32 random time-slices
optically thin; no Faraday rotation
Polarization vectorpredicted to be
in the plane of the accretion flow
(due to coherent B)
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*
Flaring from Electron AccelerationFlaring from Electron Acceleration
Yuan, Quataert, & Narayan 2004
well motivated by strongdynamical 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)
Why our Galactic Center?Why our Galactic Center?
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)
Yuan, Quataert, &
Narayan 2004
SummarySummary
SgrA* variability broadly consistent w/ turbulent RIAFSgrA* variability broadly consistent w/ turbulent RIAF
Synchrotron radiation in MHD simulations showsSynchrotron radiation in MHD simulations shows
– ~ order of mag. variability on ~ hour timescales at optically thin freq.~ order of mag. variability on ~ hour timescales at optically thin freq.– increasing variability with increasing photon frequencyincreasing variability with increasing photon frequency– strong linear polarization in the plane of the accretion flow at all strong linear polarization in the plane of the accretion flow at all
optically thin freq. (neglecting Faraday effects)optically thin freq. (neglecting Faraday effects)
Largest amplitude, shortest timescale X-ray & IR flaring Largest amplitude, shortest timescale X-ray & IR flaring probably traces transient electron accelerationprobably traces transient electron acceleration
Two Sources of VariabilityTwo Sources of Variability
Transient Heating &Electron Acceleration
Soho’s View of the Sun
Note: dynamics and heatingcoupled: e.g., fluctuations in
magnetic field probably correlated with
electron acceleration