1 Reverberation Mapping of Active Galactic Nuclei Planets To Cosmology: Essential Science In Hubble's Final Years 4 May 2004 Bradley M. Peterson The Ohio State University Keith Horne University of St. Andrews

2 ~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 1097 Storchi-Bergmann et al. (2003)

4 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)

5 The Nature of the BLR Gravity is important –Broad-lines show virial relationship between size of line- emitting region and line width, r   V  2 –Yields measurement of black-hole mass

Tremaine slope Ferrarese slope AGN Black Holes on the M BH –  * Relationship

The AGN Mass–Luminosity Relationship

Strong evidence for outflows Evidence for disk

9 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

10 Reverberation Mapping Assumptions 1Continuum originates in a single central source. –Continuum source (10 13–14 cm) is much smaller than BLR (~10 16 cm) –Continuum source not necessarily isotropic 2Light-travel time is most important time scale. Cloud response instantaneous  rec = ( n e  B )  1  0.1 n 10  1 hr BLR structure stable  dyn = (R/V FWHM )  3 – 5 yrs 3There is a simple, though not necessarily linear, relationship between the observed continuum and the ionizing continuum.

11 The Transfer Equation Under these assumptions, the relationship between the continuum and emission lines is: Emission-line light curve “Velocity- delay map” Continuum Light Curve Simple velocity-delay map Velocity-delay map is line response to a  -function outburst. Goal of reverberation mapping is to recover velocity-delay map from observables

12  = r/c “Isodelay Surfaces ” All points on an “isodelay surface” have the same extra light-travel time to the observer, relative to photons from the continuum source.  = r/c

13 Consider simple case of clouds in circular orbits at inclination i = 90°, orbital speed V orb. Clouds at intersection of isodelay surface and orbit have line-of-sight velocities V = –V orb sin . Circular orbit projects to an ellipse in the (V,  ) plane. Velocity-Delay Map for an Edge-On Ring

14 Thick Geometries Generalization to a disk or thick shell is trivial. General result is illustrated with multiple- ring system.

15 Two Simple Velocity-Delay Maps Inclined Keplerian disk Randomly inclined circular Keplerian orbits The profiles and velocity-delay maps are superficially similar, but can be distinguished from one other and from other forms.

Broad-line region as a disk, 2–20 light days Black hole/accretion disk Time after continuum outburst Time delay Line profile at current time delay “Isodelay surface” 20 light days

18 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)

19 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

22 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.

23 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.