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