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Truncated disc and X-ray spectral states of black holes

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1 Truncated disc and X-ray spectral states of black holes
Marek Gierliński University of Durham Truncated disc and X-ray spectral states of black holes I would like to tell you about accretion onto compact objects

2 TRUNCATED DISC STRIKES BACK THE DISC WARS

3 Outline X-ray spectral states Geometry of the accretion flow
Hard state and Comptonisation Geometry of the accretion flow Variability Truncated disc model Potential problem: strong disc in the hard state Solution: irradiated disc model Conclusions

4 Black hole binaries: transients
Truncated disc and X-ray spectral states 26 March 2008 Black hole binaries: transients 1 year ASM ( keV) Done, Gierliński & Kubota 2007

5 GX 339-4, 2002 outburst

6 X-ray spectral states GRO J1655–40
Truncated disc and X-ray spectral states 26 March 2008 X-ray spectral states GRO J1655–40 Spectral appearance changes as a function of luminosity Low luminosity (<0.01 LEdd): low/hard state High luminosity (>0.1 LEdd): high/soft and very high states Ultrasoft – extreme high/soft I will generally distinguish between hard and soft states Done, Gierliński & Kubota 2007

7 Emission mechanisms Spectral states of Cyg X-1 disc Comptonisation
Truncated disc and X-ray spectral states 26 March 2008 Emission mechanisms Spectral states of Cyg X-1 disc Comptonisation Soft state Hard state The observed spectra consist generally of two components. One of them is the standard geometrically thin, optically thick disc, which locally emits as a blackbody. This is thermal radiation. The temperature of such a disc scales simply with black hole mass, as M^(-1/4) There is another component, at higher energies, which means that a fraction of accretion power is dissipated in hot, optically thin medium. The dominating emission mechanism is Comptonisation – inverse Compton scattering of the seed disc photons off hot electrons Standard accretion disc: blackbody-like spectrum of temperature less then ~1 keV Spectrum at higher energies: fraction of accretion power is dissipated in a hot, optically thin medium Emission mechanism: inverse Compton scattering of disc photons off hot electrons

8 Outline X-ray spectral states Geometry of the accretion flow
Hard state and Comptonisation Geometry of the accretion flow Variability Truncated disc model Potential problem: strong disc in the hard state Solution: irradiated disc model Conclusions

9 Optically thin plasma – Comptonisation
Truncated disc and X-ray spectral states 26 March 2008 Optically thin plasma – Comptonisation Cyg X-1 Emission mechanism: repeated inverse Compton scattering of soft disc photons off hot electrons (Comptonisation) Comptonisation on Maxwellian electrons can explain the hard state Typical electron temperature ~ 100 keV Soft state requires non-thermal electrons Hard state thermal Soft state non-thermal

10 Seed photon input shapes the spectrum
Truncated disc and X-ray spectral states 26 March 2008 Seed photon input shapes the spectrum Less seed photons: harder Heating (accretion) Comptonisation More seed photons: softer The spectra in different states are shaped by the balance between heating and cooling. Here it is how it works. We have a cloud of hot plasma. Heating is the power supplied by accretion. Cooling is due Comptonisation of the disc photons. If you change the balance between heating and cooling you can get spectra of different shape. With less cooling the Comptonised spectrum is hard. When we increase the disc emission – that is the amount of cooling – the spectrum becomes softer. Cooling (disc photons)

11 Varying hard-to-soft ratio, Lh/Ls
Truncated disc and X-ray spectral states 26 March 2008 Varying hard-to-soft ratio, Lh/Ls Theoretical model Data from Cyg X-1 And it really works! On the left you can see a model of Comptonisation, with varying seed photon input, which affects balance between cooling and heating. This can explain the observed X-ray spectral states. Varying ratio betwen seed photon input (Ls) and heating of electrons (Lh) changes the shape of the Comptonised spectrum. This can explain observed X-ray spectral states!

12 Outline X-ray spectral states Geometry of the accretion flow
Hard state and Comptonisation Geometry of the accretion flow Variability Truncated disc model Potential problem: strong disc in the hard state Solution: irradiated disc model Conclusions

13 What geometry? We need hot, optically thin plasma; but where?
Truncated disc and X-ray spectral states 26 March 2008 What geometry? We need hot, optically thin plasma; but where? The disc extends down to the last stable orbit and there is a hot corona above the disc (Beloborodov 1999) The outer disc can be truncated at some radius and replaced be a hot inner flow (Esin, McClintock & Narayan 1997) Transition from a standard disc to the hot flow can be achieved by evaporation (Różańska & Czerny 2000) The outer disc can be truncated at some radius and replaced be a hot inner flow (Esin, McClintock & Narayan 1997)

14 Geometry of the truncated disc
Truncated disc and X-ray spectral states 26 March 2008 Geometry of the truncated disc jet active region accretion disc hot inner flow black hole So, what is the geometry? Where is this hot plasma and how can you change the amount of disc photons? One of the models that can explain both spectra and variability of accretion discs is the so-called truncated disc model. According to this model the standard disc is truncated at some radius and evaporates in the hot, optically thin flow in the middle. The seed photons for Comptonisation come from the disc itself. outflow

15 Spectral states – moving truncation radius
Truncated disc and X-ray spectral states 26 March 2008 Spectral states – moving truncation radius Lh/Ls hard state soft state hard state soft state Moving the truncation radius would change the balance between heating and cooling and, in effect, would change the observed spectrum. When the disc is truncated far away, there is few disc photons cooling the hot plasma and the spectrum is hard. When the disc extends to the last stable orbit, the plasma is cooled and the spectrum is soft. The Comptonisation now takes place in active regions above the disc, similar to our Sun.

16 Outline X-ray spectral states Geometry of the accretion flow
Hard state and Comptonisation Geometry of the accretion flow Variability Truncated disc model Potential problem: strong disc in the hard state Solution: irradiated disc model Conclusions

17 Light curves – different timescales
Truncated disc and X-ray spectral states 26 March 2008 Light curves – different timescales Available instruments allow us to observe X-ray sources with sub-millisecond resolution Black holes show variability on all timescales down to milliseconds A useful tool to study variability is Fourier transforms Create power spectra We can also study X-ray variability. We see that black holes show variability on all timescales down to milliseconds. And we can study variability using Fourier transforms and creating power spectra.

18 Truncated disc and X-ray spectral states
26 March 2008 Power spectra Here are a few examples of power density spectra from X-ray binaries. One interesting thing you can see on top of the broad-band noise is quasi-periodic oscillations, which have very specific frequency, quite easy to measure. Quasi-periodic oscillations (QPOs) – unlike, e.g., pulsations, they are not coherent

19 Do we understand power spectra?
Truncated disc and X-ray spectral states 26 March 2008 Do we understand power spectra? Not really We have some idea how to obtain certain frequencies (e.g. QPOs) These might come from disc oscillations, depend on size Moving the truncation radius will change frequency Hard state – disc is truncated lower frequencies So, do we understand power spectra? Unfortunately, the honest answer is no. We don’t how the broad-band noise and QPOs are created. We have, however, some idea about how to obtain specific frequencies. These might come from oscillations in the disc and should depend on its size. So, moving the truncation radius should change the frequencies. Soft state – disc extending down higher frequencies

20 Music of the truncated disc
Truncated disc and X-ray spectral states 26 March 2008 Music of the truncated disc We can link the observed QPO frequency with the truncation radius in the disc There are frequencies in the disc: orbital, periastron and nodal precession Change in the truncation radius = change in the QPO frequency Soft Hard Di Matteo & Psaltis (1999)

21 Propagation of fluctuations – broad band PDS
Truncated disc and X-ray spectral states 26 March 2008 Propagation of fluctuations – broad band PDS Rtrunc RLSO Frequency P How can we explain this? As I mentioned before, we don’t really understand how the power spectra are formed. So, this is only a toy model. Assume we have fluctuations propagating in the disc and the power spectrum is created in the inner hot flow. Far from the centre we see only low frequencies. When we get closer to the centre, all the timescales are faster and frequencies higher. So, this part of the power spectrum would be near the truncation radius, and this part would be formed near the last stable orbit.

22 Propagation of fluctuations – broad band PDS
Truncated disc and X-ray spectral states 26 March 2008 Propagation of fluctuations – broad band PDS RLSO Rtrunc How can we explain this? As I mentioned before, we don’t really understand how the power spectra are formed. So, this is only a toy model. Assume we have fluctuations propagating in the disc and the power spectrum is created in the inner hot flow. Far from the centre we see only low frequencies. When we get closer to the centre, all the timescales are faster and frequencies higher. So, this part of the power spectrum would be near the truncation radius, and this part would be formed near the last stable orbit. P Frequency

23 Propagation of fluctuations – broad band PDS
Truncated disc and X-ray spectral states 26 March 2008 Propagation of fluctuations – broad band PDS RLSO Rtrunc How can we explain this? As I mentioned before, we don’t really understand how the power spectra are formed. So, this is only a toy model. Assume we have fluctuations propagating in the disc and the power spectrum is created in the inner hot flow. Far from the centre we see only low frequencies. When we get closer to the centre, all the timescales are faster and frequencies higher. So, this part of the power spectrum would be near the truncation radius, and this part would be formed near the last stable orbit. P Frequency

24 Behold the last stable orbit
Truncated disc and X-ray spectral states 26 March 2008 Behold the last stable orbit XTE J1550–564 Done, Gierliński & Kubota 2007 Rtr RLSO Now, you do the spectral transition and move the inner disc radius towards the centre, The characteristic frequencies would increase. On the high-frequency end we will have a low-pass filter related to the last stable orbit, which fill cut off the power spectrum in the same place. This seems to reproduce the data! The universal shape of the high-frequency power spectrum is a signature of the last stable orbit around a black hole. Hard-to-soft transition: decrease truncation radius, increase frequencies The disc: high-pass filter Last stable orbit: low-pass filter

25 Hard-to-soft transition
Truncated disc and X-ray spectral states 26 March 2008 Hard-to-soft transition XTE J1650–500 Let’s come back to our nice animation. This panel shows power spectra of this source. Then the energy spectrum softens and the sources goes from hard to the soft state, the QPO frequencies increase. You can see them moving!

26 Outline X-ray spectral states Geometry of the accretion flow
Hard state and Comptonisation Geometry of the accretion flow Variability Truncated disc model Potential problem: strong disc in the hard state Solution: irradiated disc model Conclusions

27 Accretion disc at low luminosities (< 0.03 LEdd)
Truncated disc and X-ray spectral states 26 March 2008 Accretion disc at low luminosities (< 0.03 LEdd) Accretion rate  Truncation radius  Seed photon input  Lh /Ls  Spectrum softens Luminosity Ibragimov et al. 2005

28 Accretion disc at low luminosities (< 0.03 LEdd)
Truncated disc and X-ray spectral states 26 March 2008 Accretion disc at low luminosities (< 0.03 LEdd) Accretion rate  Truncation radius  Disc irradiation  Reflection from the disc  Luminosity Spectral index hard soft Reflection amplitude Ibragimov et al. 2005

29 Accretion disc at low luminosities (< 0.03 LEdd)
Truncated disc and X-ray spectral states 26 March 2008 Accretion disc at low luminosities (< 0.03 LEdd) Accretion rate  Truncation radius  Timescales  QPO frequency  Luminosity hard Spectral hardness soft QPO frequency Axelsson et al. 2005

30 Accretion disc at low luminosities (< 0.03 LEdd)
Truncated disc and X-ray spectral states 26 March 2008 Accretion disc at low luminosities (< 0.03 LEdd) Accretion rate  Truncation radius  High-pass filter  Power spectrum narrows Luminosity hard soft Gierliński, Done & Kubota 2007

31 Jet in the hard state Jets are strongly suppressed in the soft state
Truncated disc and X-ray spectral states 26 March 2008 Jet in the hard state Meier & Nakamura 2006 Jets are strongly suppressed in the soft state Meier (2005): vertical magnetic field in the hot inner flow is required to launch a jet (magnetically-dominated accretion flow) Needs truncated disc!

32 Outline X-ray spectral states Geometry of the accretion flow
Hard state and Comptonisation Geometry of the accretion flow Variability Truncated disc model Potential problem: strong disc in the hard state Solution: irradiated disc model Conclusions

33 Prominent disc in the hard state?...
Truncated disc and X-ray spectral states 26 March 2008 Prominent disc in the hard state?... Miller et al. (2006) analysed XMM-Newton and RXTE data of GX 339–4 Hard-state X-ray spectra They found very small inner radius of the disc, comparable to the last stable orbit Does this rule out the truncated disc model? GX 339-4 Prominent disc after Miller et al. 2006

34 Prominent disc in the hard state?... maybe
Truncated disc and X-ray spectral states 26 March 2008 Prominent disc in the hard state?... maybe Miller et al. (2006) analysed XMM-Newton and RXTE data of GX 339–4 Hard-state X-ray spectra They found very small inner radius of the disc, comparable to the last stable orbit Does this rule out the truncated disc model? X-ray spectral fitting is complicated and non-unique Some hard-state spectra show a soft excess above the disc (Ebisawa et al. 1996; Wilms et al. 1999; Di Salvo et al. 2001; Ibragimov et a. 2005) GX 339-4 Prominent disc after Miller et al. 2006

35 Soft excess above the disc
Truncated disc and X-ray spectral states 26 March 2008 Soft excess above the disc GX 339-4 Cyg X-1 Cyg X-1 Disc Soft excess Soft excess Broken power law Wilms et al (ASCA) Broken power law to account for the soft excess Di Salvo et al (SAX) Additional thermal Comptonisation hotter than the disc Ibragimov et al. (2005) (RXTE) Non-thermal Comptonisation; disc so cool and outside RXTE band The soft excess, when added to the model, pushes the disc away: decreasing its temperature and increasing its inner radius

36 More problems with the truncated disc model
Truncated disc and X-ray spectral states 26 March 2008 More problems with the truncated disc model Rykoff et al. (2007) analysed Swift observations of a transient XTE J They traced the transition from the soft to the hard state Picture shows luminosity-temperature diagram The disc seems to keep constant inner radius during transition This is a killer for the truncated disc model! Or is it? Rykoff et al. 2007

37 Accretion disc in the hard state
Truncated disc and X-ray spectral states 26 March 2008 Accretion disc in the hard state RXTE (green) and Swift (black) data during the outburst Results from multicolour blackbody disc model Soft state: inner radius remarkably constant Transition: disc recedes! Hard state: disc comes back? soft hard Disc radius (arbitrary units) Gierliński, Done & Page 2008

38 Outline X-ray spectral states Geometry of the accretion flow
Hard state and Comptonisation Geometry of the accretion flow Variability Truncated disc model Potential problem: strong disc in the hard state Solution: irradiated disc model Conclusions

39 Irradiated disc Hot plasma produces hard X-ray spectrum
Truncated disc and X-ray spectral states 26 March 2008 Irradiated disc Hot plasma produces hard X-ray spectrum Regardless of the geometry the hot flow illuminates the disc We see this: hard X-rays are reflected, typically /2 ~ 0.3 If there is reflection there must be reprocessing: Lrep = /2 (1 – a) LComp Effectively, an inner portion of the disc is heated up by irradiation Lrep LComp Gierliński, Done & Page 2008

40 Intrinsic disc emission
Truncated disc and X-ray spectral states 26 March 2008 Irradiated disc Hot plasma produces hard X-ray spectrum Regardless of the geometry the hot flow illuminates the disc We see this: hard X-rays are reflected, typically /2 ~ 0.3 If there is reflection there must be reprocessing: Lrep = /2 (1 – a) LComp Effectively, an inner portion of the disc is heated up by irradiation Explanation for the soft excess! If fitted by a standard disc model, temperature is overestimated and the inner radius underestimated Lrep LComp 0.1 Intrinsic disc emission Effect of irradiation Gierliński, Done & Page 2008

41 Effect of irradiation on disc radius
Truncated disc and X-ray spectral states 26 March 2008 Effect of irradiation on disc radius The graph shows RXTE (green) and Swift observations interpreted with standard and irradiated disc models There is no difference in the soft state (no irradiation) During the transition and in the hard state the inner disc radius is larger when irradiation is taken into account More effects that can lead to underestimated disc radius: No stress-free boundary condition, appropriate for LSO (2.7) Varying colour correction (1.5) Scattering in the corona (2–3) After corrections the disc recedes continuously with decreasing flux XTE J soft Disc radius hard standard disc irradiated disc irradiated disc with no stress-free boundary condition soft hard standard disc irradiated disc irradiated disc with no stress-free boundary condition Gierliński, Done & Page 2008

42 Truncated disc and X-ray spectral states
26 March 2008 Irradiated outer disc Let us assume that a (small) fraction of the central X-ray flux illuminated the outer disc Could be scattered back to the disc by outflowing material Could be warped disc This changes the shape of the standard disc blackbody Gierliński, Done & Page 2008

43 Fit the UV data (Swift UVOT+XRT)
Truncated disc and X-ray spectral states 26 March 2008 Fit the UV data (Swift UVOT+XRT) XTE J Gierliński, Done & Page 2008

44 Hard state is different
Truncated disc and X-ray spectral states 26 March 2008 Hard state is different Irradiation fraction fout: what fraction of central X-ray luminosity is intercepted by the outer disc Plot versus spectral state Soft state: fout ~ 10-3 Hard state: fout ~ 710-3 Increased vertical size of corona? Contribution from jet? hard Irradiation fraction soft transition Comptonisation-to-disc ratio Gierliński, Done & Page 2008

45 Truncated disc and X-ray spectral states
26 March 2008 Conclusions X-ray binaries make an excellent laboratory for accretion physics We can easily study various spectral states at various luminosities Truncated disc model: in the hard state the inner disc is replaced by hot optically thin flow Hugely supported by spectral and timing data; as disc recedes: Spectrum hardens Reflection amplitude drops QPO frequencies decrease Power spectrum broadens Some recent observations interpreted with simple models contradicted truncated disc The truncated disc strikes back: irradiated disc model is consistent with new data

46 TRUNCATED DISC STRIKES BACK DISC WARS
THE DISC WARS DISCLAIMER: this talk is presented ‘as-is’ and without warranty of any kind. In no event shall the authors be liable for any special, incidental, indirect or consequential damages, whatsoever, including, without limitation, heart attack, cerebral haemorrhage, stupor, cataract, epilepsy, remorse, gastric ulcer, Nobel prize, appendicitis, gout, death or resurrection, whether or not advised of the possibility of damage, and on any theory of liability, arising out of or in connection with the use or inability to use this talk. WRITTEN BY MAREK GIERLIŃSKI AND CHRIS DONE All sources depicted in this presentation are not fictional. Any resemblance to actual black holes, accretion discs and X-ray binaries, living or dead, is purely intentional. SPONSORED BY THE MINISTRY OF EDUCATION OF POLAND X-RAY SATELLITES PROVIDED BY NASA & ESA PICTURES BY MAREK GIERLIŃSKI PERFOMED BY MAREK GIERLIŃSKI COMMISSIONED BY JURI POUTANEN MUSIC THE DEAFS DURHAM UNIVERSITY PRODUCTION

47 Truncated disc and X-ray spectral states
26 March 2008 Broad line in GX 339-4? Diskbb + power law + Laor line fit gives line with Rin= 40.4 Rg (Miller et al 2006) Inconsistent with truncated disc But Comptonisation gives continuum with high and low energy cutoff – but now depends how model reflection Rin=10  3Rg with good models or 4 0.4 Rg with pexriv! Irradiated truncated disc can have yet more complex continuum! Crucially determines the extent of the red wing of the line….. Done, Gierliński, Díaz Trigo 2008

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