Variability: from X-ray binaries to AGN

1. Variability: from X-ray binaries to AGN Phil Uttley University of Southampton With thanks to: Ian McHardy, Patricia Arévalo

2. Overview • The remarkable connection between BHXRB and AGN X-ray variability • Caveats and surprises (AGN are different in at least one respect…) • Interpreting AGN continuum spectral variability: evidence for multiple variability time-scales,

3. X-ray light curve of NGC 5506 X-ray light curve of Cyg X-1 X-ray variability in accreting black hole systems

4. Black hole XRB spectral states (Done & Gierlinski 2007)

5. Quantifying variability: the‘power spectrum’ (PSD) of Cyg X-1 Soft state Hard state

6. A gallery of black hole XRB PSDs Done & Gierlinski 2005

7. The origin of the variability • Bottom line: it’s fluctuations in the accretion flow (disk or ADAF/accreting corona) - please drop the ‘flaring solar corona’ analogy! • Very broad range of variability time-scales • Neutron-star/BH timing similarities (e.g. Klein-Wolt & van der Klis 2007) • Rms-flux relations/non-linearity (e.g. Uttley et al. 2005) • Optical/X-ray variability in AGN • ‘Flickering’ is standard outcome of MHD simulations • Spectral-timing properties…

8. Building the PSD one disk-annulus at a time

9. Ark 564

10. RXTE 1995-2009?

11. Characteristic time-scales scale with black hole mass >108 M 106 M 4107 M

12. Uttley & McHardy 2005 Time-scale normalised by BH mass scales inversely with Lbol/LEdd Characteristic time-scales scale inversely with accretion rate Markowitz & Uttley 2005 For AGN with the same BH mass, PSD break time-scale appears to depend on luminosity

13. A Yes! tB MBH2/Lbol B The BH variability fundamental plane: mass-luminosity-timescale McHardy et al., Nature, 2006 Now with more detailed fitting: Does break time-scale tB scale with bolometric luminosity’ of AGN? Try: tB MBHA/LbolB AGN BH XRBs

14. observed band observed band power  frequency power  frequency frequency frequency Variability vs. Luminosity Black squares: NLS1, Crosses: BL Seyferts (Turner et al. 1999) 2rms Log L2-10 keV Can use ‘excess-variance’ to estimate mass [e.g. O’Neill et al., Gierlinski et al. 2007, Nikolajuk (this conf.)] but should take accretion rate into account!

15. Power covers a broad bandwidth: soft-state like fake intermediate

16. Problems & caveats • Is Cyg X-1 unusual, is it really ‘soft state’? Other soft state BHXRBs show much lower rms (few per cent or less), c.f. AGN 20-50% rms • Are Seyferts really soft state? Maybe normal Seyferts hard state, narrow line Seyfert 1s soft? (e.g. Gierlinski et al. 2007) GRO J1655-40 X-ray Flux Hardness Fractional rms

17. Are soft states variable enough? Cyg X-1 disk+PL energy spectra mean Cyg X-1 PSDs 6-13 keV 2-6 keV rms The variability is produced primarily by the power-law, not the disk, which is ~constant & dilutes variability - don’t expect in AGN (disk in optical/UV, but see later…)

18. Are soft states variable enough? Cyg X-1 GRO J1655-40 (Done et al. 2007) Variability even more heavily diluted by very large constant disk in other soft-state sources

19. How typical is the Cyg X-1 soft-state PSD?

20. Are Seyferts in soft states? NGC 5548 (Gaskell et al. 2008) Yes, most/all have ‘soft’ SEDs (mdot> 1%) Low/hard states are LINERs/FR Is?

21. Surprise: AGN disks are variable! Not X-ray reprocessing! Simple energetics argument: Long-term optical fractional variability amplitude > X-ray, absolute luminosity variations even larger! But 1 year = few seconds in BHXRBs…not observed. AGN soft state disks unstable, BHXRB stable? (NGC 5548, From Uttley et al. 2003)

22. 80% of incident flux 0.5% of incident flux 20% of incident flux Disk reflection implies a thermal reprocessed component What about reprocessing?

23. Optical still requires some reprocessing… (phew!) MR 2251-178 (Arévalo et al., submitted): good X-ray/opt correlation but small lags of short-term variations require small (<30%) reprocessed variability component

24. Understanding complex spectral variability in AGN Example: Ark 564 shows complex spectral variability and time lags (Dewangan 2007)

25. Ratio of soft/hard PSDs 8-13 keV vs 2-4 keV lags Cyg X-1 hard state spectral-timing behaviour Basic observations: Multiple-Lorentzian Power spectrum (PSD) Harder X-rays  more power at high frequencies (short time-scales) Hard lags soft, lag is frequency dependent Spectral-timing shifts between Lorentzians PSD

26. Soft PSD (0.6-2 keV) Hard PSD (2-10 keV) PSD energy dependence in Ark 564 (McHardy et al. 2007) PSD is ‘band-limited’ (unlike soft state). Can be fitted by two Lorentzians, higher frequency component stronger at higher energies

27. Lag (s) Frequency (Hz) Complex X-ray lags in Ark 564 (McHardy et al. 2007) In Ark 564, lags seem to show similar ‘stepped’ behaviour to Cyg X-1, this time may be ‘very high’ state (also multi-Lorentzians)

28. = ‘Response Function’ Flux emitted per unit travel time * mdot flux Travel time delay from origin of signal ? Time Time Mapping the emission region The lag between two bands is the difference between emissivity-weighted propagation times (Arévalo & Uttley 2006), which is a function of radius of origin of the signal and hence the signal time-scale.

29. Conclusions • AGN/BHXRB variability is consistent with being the same type of process, scaled by mass and mdot • Seyfert variability and SEDs are consistent with soft states • AGN disks are intrinsically unstable on short time-scales, unlike in soft state BHXRBs • Many properties of AGN variability may already be known from BHXRBs, which will help to decode the variability