Characterizing activity in AGN

with X-ray variability

Rick Edelson

Snippets of history• Optical discovery & study came first

– Seyfert classification based on emission lines– First observations only possible in optical– Still most accessible, well-studied waveband

• Flat IR thru X-ray SEDs, e.g. Elvis (1987)

• Mushotzky (2004, astro/ph0405144) review– Concl: most effective AGN surveys in X-rays– Essentially all “Radiating Supermassive Black

Holes” (AGN) show detectable hard X-ray activity

X-rays are best activity indicator:

1) Reach deepest into heart of the AGN– Rapid var → emission from inner lt-hrs– Natural probe of central engine

2) No confusing emission components– Other local components and external sources

generally don’t emit strongly in X-rays

• Other s provide info on orientation, etc. – Produced lt-days to lt-years out

Principal Component Analysis

• “PCA” first applied to AGN by Boroson & Green (1992, ApJS, 80,109)– Optical data on 92 opt/UV-selected quasars– “Principal Eigenvector”: strong correlation of

H width and Fe II strength, other line params– Secondary strongly correlated with luminosity

• Principal eigenvector linked to X-ray slope– Boller, Brandt & Fink (1996, A&A, 305, 53)– X-ray softness correlated with H width

Boller et al. (1996) correlation of H FWHM and X-ray

X-ray variability in Radiating Supermassive Black Holes

• Non-statistical indications of “extreme” variability in X-ray soft sources– IRS 13224: Boller et al. (1997, MN, 289, 393)– Akn 564: Edelson et al. (2002, ApJ, 568, 610)

• Statistical link w/X-ray var. amplitude (xs)– Turner et al. (1999, ApJ, 524, 667) and

O’Neill et al. (2005, MNRAS, 358, 1405)• Correlated “excess variance” w/ various

properties for ~day-long ASCA light curves• Found corr. w/ luminosity, optical params.

35 days of X-ray coverage of Akn 564. Note strong X-ray variability; UV/optical varied 15% peak-peak in this period.

Sixteen single-orbit light curves (1 point on previous graph) in which Akn 564 varies by factor of 2 within 3000 sec.

Why X-ray Varibility Classification?• AGN “stick out” the most in the X-rays

• X-rays give best access to nuclear region– Bulk of lower-energy from lt-weeks–years out– Optical emission lines formed lt-days out– X-rays come from inner lt-hours

• Variability indicates activity time/size scale

• Test this by correlating X-ray variability with traditional eigenvectors of activity

XMM and X-ray variability

• Rapid X-ray variability is a powerful tracer of activity in Radiating SMBHs

• XMM provides best opportunity to exploit it– LEO light curves (ASCA, Swift) are interrupted

this destroys key info on 3-10 ks timescale– XMM can detect var. on <100 sec timescales– Chandra also uninterrupted, but lower sens.

• Sensitive, uninterrupted XMM light curves ideal probes of critical short timescales

XMM Variability Study• w/ Simon Vaughan, Ken Pounds

• XMM Variability Sample– 29 Sy1s w/ >30 ks obs, good bkgd, opt. data

• Measured Excess Variance (xs)

– Measured 4 ks time scale: shortest ever– Errors on individual estimate of order unity– Averaged multiple (10-100) estimates to

beat down errors

– Confirmed that xs stable in different periods

XMM light curves of sources w/ a range of variability levels. Note the tabulated quantity is Fvar = sqrt(xs

2).

Fvar = 41%

Fvar = 19% Fvar = 11%

Fvar < 1.7%

Fvar < 1.7%

Fvar = 22%

Variability Study Results• Used ASURV to correlate 4 parameters:

1) X-ray excess variance (xs)

2) X-ray slope ()

3) H FWHM

4) Luminosity (0.2-10 keV)

• Strongest correlations involved H– xs vs. H FWHM (p < 0.01%)

– vs. H FWHM (p = 0.26%)

– xs vs. Lx weaker than expected (p = 1.6%)

Multi-parameter correlations. The strongest correlations are shown on the left.

p < 0.01%

p = 22%p = 6.7%

p = 0.26%

p = 1.6%p = 0.52%

Implications• Short time scale X-ray variability better

correlated w/ H FWHM than luminosity

• X-ray variability most likely linked to mass of supermassive black hole

→ H FWHM is a better mass indicator than luminosity

→ Efficiency is not constant

• Improved X-ray, optical data; censored PCA methods key to further progress

State of X-ray Astronomy• Right now lots of X-ray satellites: XMM,

Chandra, RXTE, Suzuki & Swift

• Con-X, XEUS mega-missions planned for the 2020s

– Doubtful they will proceed fully as hoped

• No missions are planned for the interim

• We will lose the ability to see in the X-rays starting in about 10 years

• This would be a disaster for AGN studies

Conclusions• Rapid X-ray variability most strongly

correlated with H FWHM (an indicator of SMBH mass)

• X-rays are allowing the deepest probes of the central environment

• Access to the X-rays will be lost in next ~10 years unless we act quickly