Long-term X-ray and optical variability of radio-quiet AGN

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Long-term X-ray and optical variability of
radio-quiet AGN
Phil Uttley, University of Southampton with Ian
McHardy, University of Southampton Iossif
Papadakis, University of Crete Rick Edelson,
UCLA Alex Markowitz, UCLA Brad Peterson, Ohio
State
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Before RXTE short term variability and the PSD

• Suggests analogy with Galactic black hole
  candidates, but sparse sampling of long-term data
  leaves many uncertainties

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Short time-scale X-ray variability
luminosity/variability-amplitude correlations

• Long-look observations of day duration show an
  inverse correlation between variability amplitude
  and X-ray luminosity (e.g. Barr Mushotzky 1986,
  Green, McHardy Lehto 1993, König, Staubert
  Wilms 1997, Nandra et al. 1997)

Normalisedexcess variance vs. Log L2-10keV
Excess variance versus log(2-10 keV luminosity)
Nandra et al. 1997
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The RXTE Era Long-term X-ray light curves of
Seyfert Galaxies
6-8 year 2-10 keV light curves of Seyfert
galaxies (see also Markowitz et al. 2003 for
other objects)
NGC 4051 NLS1, least luminous (few 1041 erg/s),
low mass (few 105 M?)
MCG-6-30-15 borderline NLS1, moderately
luminous (1043 erg/s), mass prob few 106 M?
NGC 5548 Broad line Seyfert, high
luminosity (1044 erg/s) mass (108 M?)

• All 3 AGN show significant long-term variability

• Low and high Lx AGN show similar amplitudes on
  long time-scales (e.g. see also Markowitz
  Edelson 2001)

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Measuring the broadband PSD of SeyfertsCase
Study NGC 4051 (McHardy et al. 2003)

• Preliminary results on 4 other Seyferts showed 3
  with PSD breaks, but low-frequency PSD shapes not
  well-constrained (Uttley et al. 2002)

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Measuring the broadband PSD of SeyfertsCase
Study NGC 4051 (McHardy et al. 2003)
Power Spectral Density (PSD)
(see also Hayashida et al. (1998) for BH mass
estimates using high-freq data only)
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Luminosity BH mass dependence of PSD breaks
(Black hole masses estimated from reverberation
mapping or bulge stellar velocity dispersion)
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NGC 3227 Broad Line AGN, 1LEdd

• Strong long and short term variability!

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NGC 3227 Broad Line AGN, low accretion rate
Broadband PSD of NGC 3227 (Uttley et al. in prep)

• NGC 3227 appears to be similar to high state but
  is low accretion rate (1 Eddington) Broad Line
  AGN with hard X-ray spectrum

• Perhaps not surprising, since in BHXRBs
  transition can be as low as 1 Eddington (e.g.
  Maccarone 2003), but X-ray spectrum of NGC 3227
  is hard (?1.6 in 2-10 keV) unlike high/very-high
  state in BHXRBs

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X-ray variability summary

• Long-term variability is a continuation of the
  red-noise process seen at short time-scales
• Dependence of X-ray variability on luminosity is
  mainly due to an apparently universal dependence
  of PSD break time-scale on BH mass
• AGN with good PSDs appear similar to high state,
  but we may be limited by a selection effect
  against low accretion rate AGN

• Can look to X-ray binary research for clues to
  origin of variability (e.g. Uttley McHardy
  2001, Gleissner et al. 2003)

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Long-term optical variability

• Red-noise/flickering variability on long
  time-scales
• What is the relation to X-ray variability?
• Do X-rays drive the optical variations through
  reprocessing?
• Or does the same underlying variability process
  drive both X-ray and optical variations (e.g.
  accretion rate fluctuations in the disk,
  Kawaguchi et al. 1998)?

NGC 5548 13 year 5100Åcontinuum light
curve (Peterson et al. 2003)
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Optical/X-ray relation 1 NGC 7469

• UV and X-ray continua seem to be uncorrelated,
  ruling out simple reprocessing

month-long X-ray and UV light curve (Nandra et
al. 1998)
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Optical/X-ray relation 2 NGC 3516 and NGC 4051

• NGC 3516 (Maoz et al. 2002) no apparent
  correlation

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Optical/X-ray relation 3 NGC 5548 (Uttley et
al. 2003)
5100Å optical

• X-rays more variable than optical on short
  time-scales but long-term variability amplitude
  is similar

2-10 keV
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Comparing variability amplitudes

• After correcting for host galaxy starlight,
  optical variability of NGC 4051 is still very
  weak (few rms), especially compared to X-rays
  (e.g. see Done et al. 1990)

• Is different optical/X-ray behaviour a function
  of black hole mass?

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A possible model for the optical/X-ray relations

• Combine with reprocessing effects and Compton
  cooling, can we explain the variety of
  optical/X-ray behavior?

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Summary

• Long time-scale X-ray monitoring of AGN has
  revealed a strong connection to black holes in
  X-ray binaries
• Combined X-ray and optical monitoring suggests
  that fluctuating accretion flow is responsible
  for optical and X-ray continuum variability in
  AGN
• Future monitoring of a larger sample of AGN over
  a long period (10 years or more) will test this
  model and map the multiwavelength emission in AGN
  
• Sensitive X-ray All Sky Monitors (e.g. MAXI,
  Argos-X, Lobster-ISS) and robotic ground-based
  telescopes will be essential for this task!