Long-term X-ray and optical variability of radio-quiet AGN Phil Uttley, University of Southampton with: Ian McHardy, University of Southampton Iossif Papadakis, University of Crete Rick Edelson, UCLA Alex Markowitz, UCLA Brad Peterson, Ohio State

Before RXTE: short term variability and the PSD ‘Long-look’ observations showed ‘red-noise’/‘flickering’ type variability on time-scales up to a few days: no periodicities or characteristic time- scales (e.g. Lawrence et al. 1987, McHardy & Czerny 1987) Exosat ME: ~2.5 day observation of NGC 5506 monthsdaysyearshours NGC 5506 PSD (McHardy 1988) Power-law power spectral density (PSD) over time-scales up to days Combining with very sparsely sampled long-term archival data from different missions suggested possible flattening or ‘break’ in PSD. monthsdaysyearshours NGC 5506 PSD (McHardy 1988) Suggests analogy with Galactic black hole candidates, but sparse sampling of long-term data leaves many uncertainties…

Short time-scale X-ray variability: luminosity/variability-amplitude correlations Long-look observations of ~day duration show an inverse correlation between variability amplitude and X-ray luminosity (e.g. Barr & Mushotzky 1986, Green, M c Hardy & Lehto 1993, König, Staubert & Wilms 1997, Nandra et al. 1997) ‘Excess variance’ versus log(2-10 keV luminosity) Nandra et al But Narrow Line Seyfert 1 do not appear to follow this correlation (e.g. Turner et al. 1999, Leighly 1999) Black squares: NLS1 Crosses: Broad Line Seyferts Turner et al ‘Normalised excess variance’ vs. Log L 2-10keV

The RXTE Era: Long-term X-ray light curves of Seyfert Galaxies All 3 AGN show significant long-term variability NGC 4051: NLS1, least luminous (few erg/s), low mass (few 10 5 M  ) MCG : borderline NLS1, moderately luminous (10 43 erg/s), mass prob few 10 6 M  NGC 5548: Broad line Seyfert, high luminosity (10 44 erg/s) & mass (10 8 M  ) 6-8 year 2-10 keV light curves of Seyfert galaxies (see also Markowitz et al for other objects) Low and high L x AGN show similar amplitudes on long time-scales (e.g. see also Markowitz & Edelson 2001)

Measuring the broadband PSD of Seyferts: Case Study: NGC 4051 (M c Hardy et al. 2003) Preliminary results on 4 other Seyferts showed 3 with PSD breaks, but low-frequency PSD shapes not well-constrained (Uttley et al. 2002) NGC 4051 has best-sampled long-term X-ray light curve of any AGN NGC keV light curve Can probe shortest timescales (minutes or even seconds) with ~day-long XMM-Newton observation NGC 4051 XMM-Newton keV

NGC 4051 shows 1/f PSD shape at low frequencies (flat top in frequency*power) Measuring the broadband PSD of Seyferts: Case Study: NGC 4051 (M c Hardy et al. 2003) Power Spectral Density (PSD) Incorporating XMM-Newton data shows high-frequency (HF) break Like HF break in Cyg X-1 low/hard state, but no low-frequency (LF) break seen in NGC 4051 NGC 4051 PSD looks like high state of Cyg X-1  M BH ~3  10 5 M  What about other AGN? Not enough LF data to tell ‘state’… but can compare HF break time- scale with independent mass estimates (see also Hayashida et al. (1998) for BH mass estimates using high-freq data only)

(Black hole masses estimated from reverberation mapping or bulge stellar velocity dispersion) Luminosity & BH mass dependence of PSD breaks Markowitz et al. 2003M c Hardy et al Broad line, low state?  - Narrow line, high state?

NGC 3227: Broad Line AGN, ~1%L Edd Strong long and short term variability!

NGC 3227: Broad Line AGN, low accretion rate Broadband PSD of NGC 3227 (Uttley et al. in prep) NGC 3227 appears to be similar to high state but is low accretion rate (1% Eddington) Broad Line AGN with hard X-ray spectrum Perhaps not surprising, since in BHXRBs transition can be as low as ~1% Eddington (e.g. Maccarone 2003), but X-ray spectrum of NGC 3227 is hard (  ~1.6 in 2-10 keV) unlike high/very-high state in BHXRBs

Can look to X-ray binary research for clues to origin of variability (e.g. Uttley & McHardy 2001, Gleissner et al. 2003) X-ray variability: summary Long-term variability is a continuation of the red-noise process seen at short time-scales Dependence of X-ray variability on luminosity is mainly due to an apparently universal dependence of PSD break time-scale on BH mass AGN with good PSDs appear similar to high state, but we may be limited by a selection effect against low accretion rate AGN Propagation of fluctuations in accretion flow? (e.g. Mineshige et al. 1994, Lyubarskii 1997, Misra 2000)

Long-term optical variability Red-noise/flickering variability on long time- scales What is the relation to X- ray variability? Do X-rays drive the optical variations through reprocessing? Or does the same underlying variability process drive both X-ray and optical variations (e.g. accretion rate fluctuations in the disk, Kawaguchi et al. 1998)? NGC year 5100Åcontinuum light curve (Peterson et al. 2003)

Optical/X-ray relation 1: NGC 7469 UV and X-ray continua seem to be uncorrelated, ruling out simple reprocessing But UV flux appears to correlated with X-ray continuum slope (Nandra 2000): suggests Compton cooling of corona by UV seed photons? (see also poster by Petrucci et al.) ~month-long X-ray and UV light curve (Nandra et al. 1998)

Optical/X-ray relation 2: NGC 3516 and NGC 4051 NGC 3516 (Maoz et al. 2002): no apparent correlation NGC 4051 (Peterson et al. 2000): optical/X-ray variability is correlated but optical variability is weak (see also Mason et al. 2002, Shemmer et al. 2003)

Optical/X-ray relation 3: NGC 5548 (Uttley et al. 2003) X-rays more variable than optical on short time- scales but long-term variability amplitude is similar Variability very well correlated, but large optical variability amplitude rules out reprocessing! 5100Å optical 2-10 keV

Comparing variability amplitudes After correcting for host galaxy starlight, optical variability of NGC 4051 is still very weak (few % rms), especially compared to X- rays (e.g. see Done et al. 1990) Difference is not primarily an accretion rate effect, since PG quasars show >10% rms on time-scales of years (e.g. Giveon et al. 1999, Kaspi et al. 2000) Optical light curve of PG quasar Is different optical/X-ray behaviour a function of black hole mass?

Lower mass AGN have higher temperature disks and optical emission from outer disk, further from X- ray emitting region Radial profile of 5100 A emission NGC4051 NGC5548 Radial profile of 5100 A emission Higher mass AGN have cooler disks and optical emission mainly from the inner disk, co-spatial with X-ray emitting region A possible model for the optical/X-ray relations Combine with reprocessing effects and Compton cooling, can we explain the variety of optical/X-ray behavior?

Summary Long time-scale X-ray monitoring of AGN has revealed a strong connection to black holes in X- ray binaries Combined X-ray and optical monitoring suggests that fluctuating accretion flow is responsible for optical and X-ray continuum variability in AGN Future monitoring of a larger sample of AGN over a long period (10 years or more) will test this model and map the multiwavelength emission in AGN Sensitive X-ray All Sky Monitors (e.g. MAXI, Argos-X, Lobster-ISS) and robotic ground-based telescopes will be essential for this task!