What Variability of (nonblazar) AGNs is Telling Us Martin Gaskell Dept. Physics & Astronomy Univ. Nebraska
[email protected]
Huatulco, April 18, 2007
Another area Deborah has worked in – variability of nonblazar AGNs!
1. 2. 3. 4. 5.
OUTLINE Review of simple accretion disk theory and what it means for variability. How similar is the variability of different types of AGNs? The nature of variability Explaining wavelength-dependent variability The X-ray / optical connection
1. DISKS AND VARIABILITY
Basic theory of quasar energy generation Schwarzschild radius Rg = 2GM/c2. ⇒ τvar ∝ Mbh ~ 3 km per solar mass. For NGC 5548 MBH ~ 108 solar masses (Koratkar & Gaskell 1991 etc.)
⇒Rg ~ 3×108 km ~ 2 a.u. radius ~ ½ light hour diameter
Black-Hole Accretion • Accretion onto a supermassive black hole (Zel’dovich & Novikov 1964, Salpeter 1964). • Gravitational PE = -GM/R • For bound system in equilibrium, virial theorem ⇒ KE = -½ PE
∴½ PE lost • Energy lost – mechanically (mass outflow) – radiatively
Matter settles into an accretion disk (Lynden-Bell 1969)
Structure of an accretion disk Pringle & Rees (1972), Shakura & Sunyaev (1973) Luminosity structure L (ergs s-1) per cm3 at any point depends on • PE (= GM/R) released per gram, and • the number of grams entering per sec per cm3. which in turn depends on • total accretion rate (dM/dt) • local density (ρ). Conservation of mass flux Area of a shell is πR2, so volume of a shell ∝ R2 ⇒ ρ ∝ 1/R2. [note: no assumptions about thickness] Putting it all together L ∝ (GM/R) (dM/dt)(1/R2) or L ∝ R-3.
Temperature Structure Assume black-body spectrum L ∝ T4. so T ∝ (L)1/4 ∝ (R-3)1/4 or
T ∝ R-3/4 Integrated spectrum Integrate flux from rings (add up black-body curves) If T ∝ R-p Fν ∝ ν(3-2/p) If p = 3/4,
Fν ∝ ν+1/3
Shakura & Sunyaev (1973)
Pringle & Rees (1972)
Don’t get obsessed with Fν ∝ ν+1/3 ! • Standard thin disk: T ∝ R-0.75 ⇒ Fν ∝ ν+0.33 • Non-local processes – additional energy transport (e.g., irradiation from inner regions) flattens T(R) • Observed spectrum is very sensitive to T(R). • E.g., “slim disk” (Abramowicz et al. 1988): T ∝ R-0.5 ⇒ Fν ∝ ν-1
Average spectral energy distributions of real AGNs: Elvis et al. (1994)
Observed distribution of UV-optical spectral indices: Gaskell, Goosmann, Antonucci & Whysong (2004)
Reddening
(See also talks by Luc Binette and Sinhue Haro-Corzo)
Bechtold et al. (1997) Filled - optically selected
R
ng i n e d ed
Open – radio-loud from Netzer et al. (1995)
Deduced temperature structure
• α = - 0.5 ⇒ p = 0.57 • Reasonable (cf. 0.75 for standard thin disk and 0.50 for “slim” disk)
Relative Sizes of Regions
Real AGN Variability • All AGNs vary (gives a technique for finding them – see Vicki Sarejedini’s talk) • So AGN variability is normal.
Lyuti (2006)
First two things we get from variability: 1. Amplitude 2. Timescale Amplitude Tells if variability is important or unimportant. E.g.: • Variable stars – amplitude small (bolometrically) ⇒ variability unimportant (main energy mechanism not varying) • Supernovae – amplitude huge ⇒ variability fundamental (main energy generation mechanism is varying) • Quasars? – amplitude large (see later) assert: main energy generation mechanism is varying ⇒ variability fundamental.
2. HOW SIMILAR IS THE VARIABILITY OF DIFFERENT CLASSES OF AGNs?
Questions: 1. Do bright radio-quiet AGNs vary as much in the optical as radio-loud AGNs of comparable brightness? 2. Do high-accretion-rate AGNs vary as much in the optical as low-accretion-rate AGNs of comparable luminosity?
Conventional wisdoms: • Radio-loud quasars show higher amplitude optical variability than radio-quiet ones because there is contribution of a jet-related, non-thermal component in the optical (a “blazar” component). • Some indications that NLS1s (high-accretionrate) AGNs vary less in the optical. (but NLS1s known to vary more in soft X-rays).
Compare the three brightest nearby quasars • 3C 273 (z = 0.158, MV= -26.6) brightest and nearest high-luminosity radio-loud quasar – well-known variable; light-curve well studied for decades. Two recently-discovered, comparable-luminosity, nearby, radio-quiet quasars: • PDS 456 (z = 0.184, MV = -26.9) Broad lines like 3C 273 (Torres et al. 1997; Simpson et al. 1999) • PHL 1811 (z = 0.192, MV = -25.9) Narrow-Line Seyfert 1 quasar (Leighly et al. 2001)
PHL 1811 V-band light curve (2003) 14.80 14.90
V Magnitude
15.00 15.10 15.20 15.30 PHL 1811 15.40 15.50 0
20
40 60 80 100 2003 (relative day number)
120
PDS 456 V-band light curve (2000) 14.00
V Magnitude
14.05
PDS 456
14.10 14.15 14.20 14.25 14.30 -10
0
10
20
30
40
Day Number
50
60
70
80
Comparison of Variability – Seasonal RMS Variability • PDS 456 (quiet; broad lines) ±0.042m (2000) ±0.036m (2003) • PHL 1811 (quiet; narrow lines) ±0.104m (2003) • 3C 273 (radio-loud) ±0.042m mean for 11 seasons with comparable coverage. rms seasonal variability has exceeded ±0.10m only once over the past 30 years.
Comparing high- and lowaccretion rate quasars. Klimek, Gaskell, & Hedrick (2004), ApJ, 609
No clear differences once you allow for selection effects (we observe NGC 5548 because it varies!)
SHORT TIMESCALES 1. Radio-louds and radio-quiets show similar occurrences of optical sub-diurnal variability (de Diego, Dultzin-Hacyan, Ramirez, & Benitez 1998; Wiita, Stalin, Gopal-Krishna, & Sagar 2004)
2. High-accretion-rate AGNs show a similar occurrence of optical sub-diurnal variability (Klimek, Gaskell, & Hedrick, 2004)
“Conventional wisdoms” seem to be wrong on both long and short timescales. ⇒ OPTICAL VARIABILITY MECHANISMS FUNDAMENTALLY THE SAME FOR DIFFERENT CLASSES OF AGNs
3. The Nature of the Variability
ARE THERE FLARES? Gaskell (2004)
(Soft X-rays)
Variability depends on flux level. (Lyutyi & Oknyanskij,1987; Gaskell 2004)
Gaskell (2004)
Variability has a log-normal distribution
Gaskell (2004)
(a) Explains light curves without flares (b) No “high” and “low” states Gaskell (2004)
(One of these is IRAS 132243809!)
4. Wavelength-Dependent Variability (Gaskell, 2007, astro-ph/0612474)
Wavelength-dependent delays • Expected delays on sound-crossing or dynamical (orbital) timescales (long – see table above) FIRST SURPRISE: • Not seen at first (e.g., NGC 5548, Korista et al. 1995; NGC 4151, Edelson et al. 1996). Upper limits ruled out long (dynamic) timescales ⇒ light-crossing timescales.
• NGC 7469 – Wanders et al. (1997), Collier et al. (1998), Kriss et al. (2000)
UV
Delays found on light-crossing timescales
Important discovery (Sergeev et al. 2005): Delay ∝ Luminosity
Current model: - “Lamp post” model (E.g., Goosmann et al. 2006)
Expect τ = R/c ∝ T-4/3 ∝ λ4/3
Collier et al. (1998)
Wavelength
Mrk 279
(Gaskell et al., in prep – see Arav et al. 2007)
PROBLEM 1: Lopt can vary by an order of magnitude. ⇒ Irradiance would dominate over viscous energy production in the disk!! ⇒ Main energy source would not be the disk!! (i.e., our old model is totally inconsistent!)
2. What is this amazing light bulb?! 3. Even if it does exist, WHY DON’T WE SEE IT?! Have to have “fullcutoff” fixtures! (International Dark Sky Association approved!)
AVERAGE NORMALIZED DELAYS FOR 14 AGNS
τ ∝ λ4/3 looks good, but …
. . . IT PREDICTS WRONG UV-OPTICAL DELAY BY ALMOST AN ORDER OF MAGNITUDE.
Upper limits for NGC 4151 and NGC 5548
A NEW MODEL (astro-ph/0612464) 1. Intrinsic continuum variability has essentially no wavelengthdependent lag. 2. OBSERVED LAGS PRODUCED BY CONTAMINATION BY A SMALL AMOUNT OF LIGHT WITH A LARGE DELAY FROM THE DUSTY TORUS. IR emission comes from hottest dust = dust at sublimation temperature
Example: NGC 4151 – 2.2 μm lags 0.55 μm by ~ 50 days Minezaki et al. (2006)
NGC 4151
WHAT DOES HOT DUST LOOK LIKE? A well-known device operating at the dust sublimation temperature: A candle shines in the optical! So hot AGN dust shines in the optical too!
3. Delay depends linearly on the relative strengths of the simultaneous component and the delayed one. I, R
Wien tail of torus emission
Hot dust
(Hα )
Model also automatically explains why Delay ∝ Luminosity …
(Sergeev et al. 2005)
… because 2.2 μm delay ( = inner radius of torus = dust sublimation radius) ∝ L1/2.
Suganuma et al. (2006)
Hence I and R band lags ∝ L1/2.
Model also predicts: hysteresis in colour-magnitude (or colourcolour) diagrams.
Similar V fluxes; different K fluxes because of history.
NGC 4151 Based on Minezaki et al. (2006)
Can quantitatively predict (V-I) vs. V just from observed V light curve: • Predict IR delay from IR luminosity – radius relationship. • Create a “psuedo I” from (simultaneous) V and delayed K. • Add non-varying host galaxy starlight contributions. I, R
Torus
RESULTS: Observed
Model
Bachev & Strigachev (2003)
(Not “high” and “low” states.)
5. The X-ray / Optical Connection (Gaskell, 2006 - astro-ph/0701008)
The short-term relationship between X-ray and optical flux
Gaskell & Klimek (2003)
~ 1 day lag (V – X)
Gaskell & Klimek (2003)
On short timescales the optical can ignore the X-rays.
Gaskell (2006)
NGC 3516 - Maoz et al. (2002)
KEY FACTS: • X-ray timescale is short • Optical timescale can be short too (but only sometimes). ⇒ ORIGIN OF VARIABILITY IS ELECTROMAGNETIC
Why? • Needs to move near the speed of light. • Gas dynamical timescales much too long. • If we believe in energy equipartition: Emagnetic ~ Ekinetic.
• The large amplitudes in short times require relativistic beaming (e.g., Boller et al. 1998, Reeves et al. 2002).
The relationship between optical and X-ray flux is not simple. • Sometimes correlated with small lags • Sometimes X-rays vary without optical • Sometimes optical varies without the Xrays
A QUALITATIVE MODEL
X-rays, no optical
Both Optical, no X-rays
Gaskell (2006)
Long-term X-ray/optical relationship
Uttley et al. (2003)
Gaskell (2006)
• Long-term X-ray and optical have close to zero phase lag (within a day or two)! • ⇒ OPTICAL AND X-RAY HAVE SAME UNDERLYING LONG-TERM DRIVING MECHANISM.
UV X-rays
Optical
Gaskell (2006)
UV
X-rays
Optical
Gaskell (2006)
Few of the results of this talk make sense in the standard accretion disk picture!!
CONCLUSIONS • Variability is very similar for all AGNs. • Most variability timescales are too short to be explained by standard (or modified) accretion disks. • Variability amplitudes are much too large to be explained by standard accretion disks. • Variability timescales correlate with BH mass. • Simple reprocessing models don’t work. • Optical/X-ray – long-term correlations with zero lag, but short-term correlations erratic. • Long-term variability drives short-term variability • Electromagnetic and relativistic effects very important • PLENTY STILL TO EXPLAIN!
