1 reverberation mapping of active galactic nuclei planets to cosmology: essential science in...
TRANSCRIPT
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Reverberation Mappingof Active Galactic Nuclei
Planets To Cosmology: Essential Science In Hubble's Final Years 4 May 2004
Bradley M. PetersonThe Ohio State University
Keith HorneUniversity of St. Andrews
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~10 17 cm
The Inner Structure of AGNs• Black-hole/accretion-disk
produces a time-variable high-energy continuum that ionizes and heats nuclear gas, which produces broad emission lines.
• Angular size of nuclear regions is of order microarcseconds.
• Broad-line region (BLR): role in accretion process?
• How do we determine the geometry, kinematics, and role of the BLR?
The Nature of the BLR
• Double-peaked Balmer-line profiles– Characteristic of rotating
disks– Small subset of AGNs
NGC 1097Storchi-Bergmann et al. (2003)
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The Nature of the BLR
• Evidence for outflows– Clear blueward
asymmetries in some cases
– Blueshifted absorption features are common
Leighly (2001)
Chandra: Kaspi et al. (2002)HST: Crenshaw et al. (2002)FUSE: Gabel et al. (2002)
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The Nature of the BLR• Gravity is important
– Broad-lines show virial relationship between size of line-emitting region and line width, r V2
– Yields measurement of black-hole mass
Tremaine slope
Ferrarese slope
AGN Black Holes on the MBH – * Relationship
The AGN Mass–Luminosity Relationship
Strong
evid
ence
for o
utflo
ws
Evidence for disk
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Reverberation Mapping• Kinematics and
geometry of the BLR can be tightly constrained by measuring the emission-line response to continuum variations.
NGC 5548, the most closely monitored Seyfert 1 galaxy
Continuum
Emission line
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Reverberation Mapping Assumptions1 Continuum originates in a single central
source.– Continuum source (1013–14 cm) is much smaller
than BLR (~1016 cm)– Continuum source not necessarily isotropic
2 Light-travel time is most important time scale.• Cloud response instantaneous
rec = ( ne B)1 0.1 n101 hr
• BLR structure stable dyn = (R/VFWHM) 3 – 5 yrs
3 There is a simple, though not necessarily linear, relationship between the observed continuum and the ionizing continuum.
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The Transfer EquationUnder these assumptions, the relationship between the
continuum and emission lines is:
Emission-linelight curve
“Velocity-delay map”
ContinuumLight Curve
d t C V t V L) ( ) , ( ) , (
Simple velocity-delay map
Velocity-delay map is line response to a -function outburst.
Goal of reverberation mapping is torecover velocity-delay map
from observables
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= r/c
“Isodelay Surfaces”
All pointson an “isodelaysurface” have the same extralight-travel timeto the observer,relative to photonsfrom the continuumsource.
= r/c
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• Consider simple case of clouds in circular orbits at inclination i = 90°, orbital speed Vorb.
• Clouds at intersection of isodelay surface and orbit have line-of-sight velocities V = –Vorb sin.
• Circular orbit projects to an ellipse in the (V, ) plane.
Velocity-Delay Map for an Edge-On Ring
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Thick Geometries
• Generalization to a disk or thick shell is trivial.
• General result is illustrated with multiple-ring system.
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Two Simple Velocity-Delay Maps
Inclined Kepleriandisk
Randomly inclinedcircular Keplerian orbits
The profiles and velocity-delay maps are superficially similar,but can be distinguished from one other and from other forms.
Broad-line regionas a disk,
2–20 light daysBlack hole/accretion disk
Time after continuum outburst
Timedelay
Line profile atcurrent time delay
“Isodelay surface”
20 light days
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Recovering Velocity-Delay Maps from Real Data
• Existing velocity-delay maps are noisy and ambiguous
• In no case has recovery of the velocity-delay map been a design goal for an experiment!
C IV and He II in NGC 4151(Ulrich & Horne 1996)
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What Will It Take to Map the Broad-Line Region?
• Extensive simulations have been carried out, based on what has been learned over the last decade.
• Accurate mapping requires a number of characteristics (nominal values follow):– High time resolution ( 0.2 day)– Long duration (several months)– Moderate spectral resolution ( 600 km s-1)– High homogeneity and signal-to-noise (~100)
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10 Simulations Based on HST/STIS Performance
Each step increases the experiment duration by 25 days
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Some Important Points• For NGC 5548, all experiments succeed within
200 days– If you’re lucky, success can be achieved in as little as
~60 days (rare) or ~150 days (common)
• Results are robust against occasional random data losses– Nominal S/C and STIS safings have been built into the
simulations with no adverse effect
• What if the velocity-delay map is a “mess”?– You’ve still learned something important about the BLR
structure.– It probably won’t be a mess since long-term monitoring
shows persistent features that imply there is some order or symmetry.
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Key Points
1) Reverberation mapping provides a unique probe of the inner structure of AGNs.
2) Broad-line region size has been measured directly in 35 AGNs, leading to determination of their black-hole masses. Uncertain by factor ~3 Accuracy limited by unknown geometry/kinematics
of BLR
3) Recovery of complete velocity-delay maps is possible with Hubble Space Telescope.
