1. Disentangling disc variability in the hard state
Phil Uttley
T. Wilkinson, P. Cassatella (Southampton)
J. Wilms, M. Hanke, M. Böck (FAU)
K. Pottschmidt (NASA-GSFC)

2. Introduction
BHXRB hard state spectra show clear evidence for both disc and hot coronal components, as well as interaction between them (reflection).
Hard states also show strong X-ray variability: an ideal laboratory to study the disc- corona interaction.
What drives the variability – disc instabilities, or an intrinsically unstable/flaring corona?
What role does X-ray reverberation play in driving variability?
We need the high throughput and fast timing capability of the XMM-Newton EPIC-pn to extend spectral-timing measurements down to soft X-rays

3. Intrinsic disc variability?
GX hard state keV (XMM-Newton EPIC-pn)
Light curve
PSD
Strong X-ray variability on a broad range of time-scales could have a natural origin in the accretion disc
(e.g. Lightman & Eardley 1974, Shakura & Sunyaev !976, Lyubarskii 1996, King et al. 2002, Reynolds & Miller 2009)

4. Characteristic time-scales in the disc?GX GX BH
NS (Atoll)
ms pulsar
(Wijnands & van der Klis 1999)
Characteristic frequencies (QPOs, ‘breaks’, broad Lorentzians) evolve together.
Frequency increases linked to spectral softening, stronger reflection components (e.g. Gilfanov et al. 1999).
Changing inner disc radius? (see also Emrah Kalemcı’s talk!)

5. The ‘quiet disc’ problem
However:
The most variable spectral states have power-law- dominated SEDs
Soft disc-dominated states show very weak variability
Cyg X-1 soft state shows that variable component has power-law spectral shape (Churazov et al. 2001)
Cyg X-1
‘mean’ (time-averaged) spectrum
rms (time-varying) spectrum
Is the disc intrinsically stable – variability generated in the corona?

6. Disc X-ray reverberation
X-ray heating of the disc (thermal reprocessing) is the counterpart to reflection.
Can produce significant fraction of total luminosity, emitted at temperature of reflection site: can place strong constraints on reflection geometry.
Expect strong, variable contribution in hard states.
~70% of incident flux
~30% of incident flux
~1% of incident flux

7. GX 339-4 hard state rms spectra
(Wilkinson & Uttley, 2009)
Use XMM-Newton 2004 long- look (Miller et al. 2006, Done & Díaz Trigo 2009)
Measure rms variability amplitude in count/s units as function of energy to make an rms spectrum
We use new ‘covariance’ technique: measures rms of component correlated with a broad bandpass (e.g keV) – better S/N more suited to picking up correlated disc/power-aw variations.
rms spectra:short time-scale variations (0.1-4s) long time-scale ( s) 7

8. Fourier-resolved spectra
Ratio to Γ=1.65 absorbed power-law (NH=6×1021 cm-2)
0.5-1 keV
3-10 keV
The amplitude of variable disc emission is strongly frequency-dependent

9. Intrinsic disc variability or disc heating with variable geometry?
Increase in variable disc component could be due to extra variability in reprocessor geometry (coronal scale-height or disc inner radius)
However reflection features do not show corresponding increase in strength (so bb variability could be intrinsic to disc?)
BUT this argument assumes reflection and disc soft X-ray emission are from the same region – may not be true

10. Interband time lags in the hard state (> 3 keV)
Nowak et al. 1999
Kotov et al. 2001
Variations at harder energies lag those at softer energies
~log-linear energy dependence (but lags probably not due to Comptonisation,e.g. Nowak et al. 1999)

11. Extending lag measurements to soft X-rays
Lags for individual energy bands are measured from phase of cross-spectrum averaged over Hz frequency range, with respect to the full keV band.
=0.07 s
or ~1400 RG/c

12. Fourier-resolved lag-energy spectra
The lag behaviour is also strongly frequency-dependent, with a sign-reversal corresponding to drop in variable disc emission

13. Interpretation
At low frequencies, variations in mdot are produced at larger radius in disc, modulating disc emission before propagating in to the corona on the disc viscous time-scale
At high frequencies, variations in mdot are produced at small radius in disc or in corona itself. Only a fraction of disc emission can respond, but all of corona does, and coronal heating dominates variability  disc reverberation

14. Conclusions
The disc is responsible for hard state variability, at least up to ~1 Hz (over to you, theorists…)
Ergo, soft-hard state transition is linked to the onset of disc instabilities
We see viscous-time-scale hard lags at low frequencies, light-travel time soft lags at high frequencies (similar behaviour seen in AGN, Fabian et al & see talk by Abdu Zoghbi)
Soft X-ray coverage+timing is a very powerful combination (pathfinder for IXO-HTRS)

15. Fourier-resolved spectra
Ratio to Γ=1.65 absorbed power-law (NH=6×1021 cm-2)
0.5-1 keV
3-10 keV

16. Fourier-resolved lag-energy spectra