The origin of UV / Optical Variability of AGN

48 Pages·2016·4.22 MB·English

The origin of UV / Optical
Variability of AGN:

Relationship to X-ray Variability

c
Ian M Hardy

University of
Southampton
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Main Questions
•  What drives UV/optical variability in AGN?
•  How is the X-ray band related to UV/optical?

•  What do X-ray/UV/optical variations tell us
about AGN inner structure?
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Possible drivers of UV/optical
Variability
- Reprocessing of higher energy photons

- which “high” energy? X-ray? Far-UV?
- reprocessing off what? Disc? BLR?

- Intrinsic disc variations

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Observational Diagnostics

•  Reprocessing - High energies lead uv/optical by short
(hour-days) light travel time.
Allows ‘reverberation’ mapping of reprocessing structures.
Measure lag from different temperature regions (different λ ).
•  Intrinsic disc variability – High energies lag: two possibilities
–  Long lag (months), viscous propagation timescale for perturbations to
reach X-ray region from optical in disc
–  Short lag (hour-day), light travel time of UV seed photons to corona
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REPROCESSING
Wavelength dependence of lags
4 −1 −3
For standard Shakura-Sunyaev DISC, L(R)=σT ∝M m! R
!! BH . E
dissipating gravitational potential energy
( R in gravitational radii)
T ∝ M −1/4m!1/4R−3/4
i.e.
BH E
Disc illumination from point source, height H above disc,
-3
also falls off as H R
So for reprocessing 4/3
Lag ∝Wavelength (eg Cackett et al 2007)
from disc, we expect
and Lag ∝M2/3m! 1/3
!! E
For illumination of a shell-type structure, eg the BLR or torus,
-2
Illumination falls of as R giving
2
Lag ∝Wavelength
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Optical interband lags

NGC4051
Consistent with reprocessing
from a disc but no link to
high energies
(Cackett et al, 2007;
Sergeev et al 2005,6)
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The X-ray / Optical lag
Many RXTE + ground based optical programmes; eg Breedt et al 2010

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NGC 4051
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NGC4051
99% confidence
95% confidence
Optical lags by 1.5 +/- 0.5 d

Breedt et al 2010
(Possible secondary longer (40d) lag – torus?)
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MKN 79
(Breedt et al, 2009, MNRAS)
CCFs often asymmetric
-more than one variability component
Long timescales (years)
– poorly correlated behaviour. Intrinsic disc variations in optical?

Short timescales (days-weeks)
- well correlated. Usually a hint of optical lagging by ~day, but large uncertainty

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