Variability: from X-ray binaries to AGN

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Variability from X-ray binaries to AGN

• Phil Uttley
• University of Southampton
• With thanks to Ian McHardy, Patricia Arévalo

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Overview

• The remarkable connection between BHXRB and AGN
  X-ray variability
• Caveats and surprises (AGN are different in at
  least one respect)
• Interpreting AGN continuum spectral
  variability evidence for multiple variability
  time-scales,

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X-ray variability in accreting black hole systems
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Black hole XRB spectral states
(Done Gierlinski 2007)
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Quantifying variability thepower spectrum
(PSD) of Cyg X-1
Soft state Hard state


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A gallery of black hole XRB PSDs
Done Gierlinski 2005
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The origin of the variability

• Bottom line its fluctuations in the accretion
  flow (disk or ADAF/accreting corona) - please
  drop the flaring solar corona analogy!
• Very broad range of variability time-scales
• Neutron-star/BH timing similarities (e.g.
  Klein-Wolt van der Klis 2007)
• Rms-flux relations/non-linearity (e.g. Uttley et
  al. 2005)
• Optical/X-ray variability in AGN
• Flickering is standard outcome of MHD
  simulations
• Spectral-timing properties

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Building the PSD one disk-annulus at a time
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Characteristic time-scales scale with black hole
mass
gt108 M?
106 M?
4?107 M?
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Characteristic time-scales scale inversely with
accretion rate
Markowitz Uttley 2005
For AGN with the same BH mass, PSD break
time-scale appears to depend on luminosity
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The BH variability fundamental plane
mass-luminosity-timescale
Now with more detailed fitting Does break
time-scale tB scale with bolometric luminosity
of AGN? Try tB? MBHA/LbolB
McHardy et al., Nature, 2006
AGN
BH XRBs
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Variability vs. Luminosity
Black squares NLS1, Crosses BL Seyferts (Turner
et al. 1999)
?2rms
Log L2-10 keV
Can use excess-variance to estimate mass e.g.
ONeill et al., Gierlinski et al. 2007, Nikolajuk
(this conf.) but should take accretion rate into
account!
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Power covers a broad bandwidth soft-state like
fake intermediate
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Problems caveats
GRO J1655-40 X-ray Flux

• Is Cyg X-1 unusual, is it really soft state?
  Other soft state BHXRBs show much lower rms (few
  per cent or less), c.f. AGN 20-50 rms
• Are Seyferts really soft state? Maybe normal
  Seyferts hard state, narrow line Seyfert 1s soft?
  (e.g. Gierlinski et al. 2007)

Hardness
Fractional rms
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Are soft states variable enough?
Cyg X-1 diskPL energy spectra
mean
Cyg X-1 PSDs 6-13 keV 2-6 keV
rms
The variability is produced primarily by the
power-law, not the disk, which is constant
dilutes variability - dont expect in AGN (disk
in optical/UV, but see later)
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Are soft states variable enough?
Cyg X-1
GRO J1655-40 (Done et al. 2007)
Variability even more heavily diluted by very
large constant disk in other soft-state sources
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How typical is the Cyg X-1 soft-state PSD?
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Are Seyferts in soft states?
NGC 5548 (Gaskell et al. 2008)
Yes, most/all have soft SEDs (mdotgt
1) Low/hard states are LINERs/FR Is?
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Surprise AGN disks are variable!
Not X-ray reprocessing! Simple energetics
argument Long-term optical fractional
variability amplitude gt X-ray, absolute
luminosity variations even larger! But 1 year
few seconds in BHXRBsnot observed. AGN soft
state disks unstable, BHXRB stable?
(NGC 5548, From Uttley et al. 2003)
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What about reprocessing?
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Optical still requires some reprocessing (phew!)
MR 2251-178 (Arévalo et al., submitted) good
X-ray/opt correlation but small lags of
short-term variations require small (lt30)
reprocessed variability component
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Understanding complex spectral variability in AGN
Example Ark 564 shows complex spectral
variability and time lags (Dewangan 2007)
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Cyg X-1 hard state spectral-timing behaviour
Basic observations Multiple-Lorentzian Power
spectrum (PSD) Harder X-rays ? more power at
high frequencies (short time-scales) Hard lags
soft, lag is frequency dependent Spectral-timing
shifts between Lorentzians
PSD
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PSD energy dependence in Ark 564
(McHardy et al. 2007)
PSD is band-limited (unlike soft state). Can be
fitted by two Lorentzians, higher frequency
component stronger at higher energies
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Complex X-ray lags in Ark 564
(McHardy et al. 2007)
In Ark 564, lags seem to show similar stepped
behaviour to Cyg X-1, this time may be very
high state (also multi-Lorentzians)
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Mapping the emission region
The lag between two bands is the difference
between emissivity-weighted propagation times
(Arévalo Uttley 2006), which is a function of
radius of origin of the signal and hence the
signal time-scale.
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Conclusions

• AGN/BHXRB variability is consistent with being
  the same type of process, scaled by mass and mdot
• Seyfert variability and SEDs are consistent with
  soft states
• AGN disks are intrinsically unstable on short
  time-scales, unlike in soft state BHXRBs
• Many properties of AGN variability may already
  be known from BHXRBs, which will help to decode
  the variability