Slim Accretion Disks: Theory and Observational Consequences
Round 1
Reviewer 1 Report
Report on manuscript titled: “ Slim accretion disks: Theory and observational consequences”
This is good review on Slim accretion disks written in a simple and understandable manner. The review will be very useful to young scientists who plan to start research in this field and for others who would like to understand the salient features of the topic.
I recommend the manuscript for publications. However, I have a few suggestions listed below, which the author may wish to consider while revising the manuscript.
1) It would be good to have more discussion of the earlier “Thick disk model”(Pacynski & Witta) and to show that the slim disk model is different.
2) There should be some discussion on ADAF, otherwise a reader not well acquainted with the field may get confused if he/she has heard of ADAF and may mistake the slim disk as being ADAF.
3) It is not clear whether the author is claiming that the zero torque boundary condition is invalid for the standard disk in general or whether it is invalid only for slim disks and if so, when exactly does it become important.
4) Perhaps it could be mentioned that the luminosity of the slim disk scales approximately with the logarithm of the accretion rate.
5) Line 233, I guess that author means that the “radiation pressure instability does not develop”, instead of “radiation pressure does not develop..”, also one could refer also Janiuk and Misra (2012).
6) There are several spelling mistakes in the manuscript which should be corrected.
Comments for author File: 
Comments.pdf
Author Response
REPORT 1
This is good review on Slim accretion disks written in a simple and understandable manner. The review will be very useful to young scientists who plan to start research in this field and for others who would like to understand the salient features of the topic.
I recommend the manuscript for publications. However, I have a few suggestions listed below, which the author may wish to consider while revising the manuscript.
1) It would be good to have more discussion of the earlier “Thick disk model”(Pacynski & Witta) and to show that the slim disk model is different.
- We now added that to Sect. 2.
2) There should be some discussion on ADAF, otherwise a reader not well acquainted with the field may get confused if he/she has heard of ADAF and may mistake the slim disk as being ADAF.
- Indeed. We added a general list of models with short description. However, my understanding was that this article is just one of the series of many articles on the accretion processes, including other flow types etc.
3) It is not clear whether the author is claiming that the zero torque boundary condition is invalid for the standard disk in general or whether it is invalid only for slim disks and if so, when exactly does it become important.
- Yes, the zero torque is used both in standard disk and in slim disk, only in slim disk this is moved to the black hole horizon. Clarification of that issue is now at the beginning of Sect. ???.
4) Perhaps it could be mentioned that the luminosity of the slim disk scales approximately with the logarithm of the accretion rate.
- Yes, it was mentioned but indeed we now stress that more strongly. Now it is given as Eq. (3).
5) Line 233, I guess that author means that the “radiation pressure instability does not develop”, instead of “radiation pressure does not develop..”, also one could refer also Janiuk and Misra (2012).
- Indeed. Corrected. Also Janiuk & Mistra made an important point. Now this suggestion is also added to Section 8.
6) There are several spelling mistakes in the manuscript which should be corrected.
- Indeed. Hopefully all were corrected.
Reviewer 2 Report
This paper is a potentially useful summary of slim accretion disk models and if it is revised to take into account the comments below it would be a good addition to the literature on accretion disks.
Comments:
1. The author indicates three features of slim accretion disks, which are not covered by the standard Shakura-Sunyaev model (i) The inner boundary condition and associated flow to the black hole (ii) Geometrically thick accretion disks (iii) Disk instability. What readers would appreciate here is a physical description of how the SS model is deficient or why it needs to be generalised. This is done for (i) but for (ii) and (iii). What is it about the SS model that forces us towards a geometrically thick configuration and why stop at `slim’? Furthermore, on the question of stability, is the SS model unstable or do the descriptions of unstable regions only apply to slim disks?
2. Figure 1 is a schematic representation of the stability curves (accretion rate vs surface density) of slim accretion disk models. Since this is important part of the entire review, it would be good if it the stability curves were to be based on calculations derived from actual slim disk models rather than indicative sketches.
3. At various parts of the text, the author mentions the connection of slim desks with the time variability and/or duty cycle of radio galaxies. This means that the accretion jet is launching a jet. What is the mechanism for doing this in the context of slim disks?
4. Again, with respect to young radio galaxies, Baldi et al. (2016, A&A, 576, A38) have discovered a class of weak, apparently stifled compact radio galaxies which may be related to the short ) ~ 103-4 yr timescale associated with slim accretion disks. On the same topic, is there a possible association of slim disks and larger radio galaxies with a 107-8 yr timescale?
5. The discussion of turbulent `viscosity’ in section 9 seems to me to be somewhat oversimplified with the discussion of hydrodynamic viscosity apparently based on Prandtl’s idea of eddy viscosity in which the mean free path is related to the distance of propagation of a turbulent element of plasma. The mean free path of ions is not relevant so that the notion of accretion from the corona needs to await the analysis of stability in that region – just as the magnetic stress in accretion disks is related to the MRI.
6. Finally, I need to remark that the standard of language in this review is very poor, with numerous spelling and grammatical errors. These require rigorous attention.
Author Response
REPORT 2
1. The author indicates three features of slim accretion disks, which are not covered by the standard Shakura-Sunyaev model (i) The inner boundary condition and associated flow to the black hole (ii) Geometrically thick accretion disks (iii) Disk instability. What readers would appreciate here is a physical description of how the SS model is deficient or why it needs to be generalised. This is done for (i) but for (ii) and (iii). What is it about the SS model that forces us towards a geometrically thick configuration and why stop at `slim’? Furthermore, on the question of stability, is the SS model unstable or do the descriptions of unstable regions only apply to slim disks?
- This is discussed at the end of Sec. 5. I stressed there before that slim disks are never geometrically very thick. I reformulated the preceeding sentence to show more clearly why the disk thickness rises at all with the rise of accretion rate. So the geometrical thickness is not a real issue for slim disks. As for the instability, of course the standard disks are unstable, as was shown immediately after Shakura-Sunyaev paper apperad (their paper - 1973, thermal instability paper - 1974, viscous instability - 1974, the two combined - 1976), and this is discussed, and the references to all these papers are given. The point is that BEFORE slim disks were introduced, this instability lead to a catastrophic event, while new branch of solutions - slim disk branch - closed the system and the evolution became a limit cycle. I added an additional sentence on that point to Sect. 5.
2. Figure 1 is a schematic representation of the stability curves (accretion rate vs surface density) of slim accretion disk models. Since this is important part of the entire review, it would be good if it the stability curves were to be based on calculations derived from actual slim disk models rather than indicative sketches.
- I do not have a working code at the moment. My 40 years old fortran code has some problems now, and it was never full GR anyway. The code of Sadowski is not available (I asked him for that some time ago, before he left astronomy). Anyway, the plot would depend on a specific value of the black hole mass and viscosity coefficient.
3. At various parts of the text, the author mentions the connection of slim desks with the time variability and/or duty cycle of radio galaxies. This means that the accretion jet is launching a jet. What is the mechanism for doing this in the context of slim disks?
- It has no relation whatsoever. This is why I did not discuss that in more detail. The jet is launched by the geometrically thick optically thin strongly magnetized plasma, either in the form of an ADAF (and this is the case of a steady jet in low states), or in the form of episodic coronal flows (intermittent jet activity at very high states).
Since the formation of this corona is not well predicted (slim disk does not do that), we still have a long way to go to really model jet formation.
4. Again, with respect to young radio galaxies, Baldi et al. (2016, A&A, 576, A38) have discovered a class of weak, apparently stifled compact radio galaxies which may be related to the short ) ~ 103-4 yr timescale associated with slim accretion disks. On the same topic, is there a possible association of slim disks and larger radio galaxies with a 107-8 yr timescale?
- The timescales for the limit cycle based on the radiation pressure instability ranges from 100 yr to 1e5 years (see Fig. 4 of Czerny et al. 2009). So these short timescales are of Baldi are fine, I have added the citation to this paper to Sect. 6. Timescales 1.e7 - 1.e8 years, are too long, and (in my opinion) rather suggest a stable disk powered by the mass reservoire lasting that long.
5. The discussion of turbulent `viscosity’ in section 9 seems to me to be somewhat oversimplified with the discussion of hydrodynamic viscosity apparently based on Prandtl’s idea of eddy viscosity in which the mean free path is related to the distance of propagation of a turbulent element of plasma. The mean free path of ions is not relevant so that the notion of accretion from the corona needs to await the analysis of stability in that region – just as the magnetic stress in accretion disks is related to the MRI.
- Yes, it is oversimplified. But I think we have an issue there.
6. Finally, I need to remark that the standard of language in this review is very poor, with numerous spelling and grammatical errors. These require rigorous attention.
- I have corrected spelling. I am not a native English speaker, so for the grammar I cannot do better. I do not have a co-author on that paper who would help.
Reviewer 3 Report
The author presents a reviewonthe slim accretion disks studies, withfocuson unsolved issues as the disk stability, radiation pressure, the role of magnetic fields, etc. This reviewer thinks that the paper describes well the state of the art in this field, but the paper can be improved to gain soundness and impact.
There is a lack of physics along the text. Only very basic equations are provided for the Eddington mass and luminosity, and the viscosity. For example, the viscosity might be discussed in the frame of the Navier-Strokes equations, more details about the Shakura-Sunyaev (SS) model would be helpful, as well as a discussion about the applications of the upgrade of SS model using general relativity (Novikov & Thorne 1973), or the role of the black hole spin. Other important aspects of the accretion disk theory, such as the disk temperature and emitted spectrum, g-factor (gravitational and Doppler energy shift combined effect), angle dependence, the vertical structure and radiative transfer, the importance of the angular momentum conservation and energy balance, are not even introduced.
Other minor issues are:
Line 50: 'This line of research was vigorously studied in Warsaw..." This sentence is appropriate for an institutional review, not for an international publication.
Lines 210-211: '...the use of the $\alpha \sqrt{P_{gas} P_{tot}}$ or even more general models...' To understand this sentence, the reader should be aware of the alpha-p prescription in SS (1973), otherwise it is meaningless.
Lines 261-262: '...the phenomenon of Changing Look (CL) AGN...'is not been defined.
Conclusions: They look like a 'to do' list more than truly conclusions.
Finally, several typos andgramaticalerrors should be revised.
Author Response
REPORT 3
The author presents a reviewonthe slim accretion disks studies, withfocuson unsolved issues as the disk stability, radiation pressure, the role of magnetic fields, etc. This reviewer thinks that the paper describes well the state of the art in this field, but the paper can be improved to gain soundness and impact.
There is a lack of physics along the text. Only very basic equations are provided for the Eddington mass and luminosity, and the viscosity. For example, the viscosity might be discussed in the frame of the Navier-Strokes equations, more details about the Shakura-Sunyaev (SS) model would be helpful, as well as a discussion about the applications of the upgrade of SS model using general relativity (Novikov & Thorne 1973), or the role of the black hole spin. Other important aspects of the accretion disk theory, such as the disk temperature and emitted spectrum, g-factor (gravitational and Doppler energy shift combined effect), angle dependence, the vertical structure and radiative transfer, the importance of the angular momentum conservation and energy balance, are not even introduced.
- The review would then have to be much longer. Anyway, the basic things like derivation of the disk structure equations from Navier-Strokes are well covered by books, for example Frank, King & Raine, Accretion Power in Astrophysics, Third Edition. Qualitatively, GR are of no key importance here. The detailed study of the disk atmosphere has no meaning if we cannot describe (and we cannot!) the formation of the coronal layer. This will change all predictions. The aim of my review was exactly to stress, how little we know, instead of summarizing what we were able to write on the topic of accretion disks. And in adition, I was asked to write about Slim disks, I assume that some other authors write about other disks etc.
Other minor issues are:
Line 50: 'This line of research was vigorously studied in Warsaw..." This sentence is appropriate for an institutional review, not for an international publication.
- I would think that in the section 'historical remarks' this is still apropriate
Lines 210-211: '...the use of the $\alpha \sqrt{P_{gas} P_{tot}}$ or even more general models...' To understand this sentence, the reader should be aware of the alpha-p prescription in SS (1973), otherwise it is meaningless.
- Yes, I assume that the reader is well familiar with the standard disk theory. The reader should not start from slim disks.
Lines 261-262: '...the phenomenon of Changing Look (CL) AGN...'is not been defined.
- At present, there is no good definition of the CL phenomenon. They just change the way how they look. Either line change in opt/UV, or X-ray appearance change. It might be obscuration, TDE, instabilities - many phenomena which now go under this catchy term.
Conclusions: They look like a 'to do' list more than truly conclusions.
- Yes, and it was the aim of the review. I think it is really important that the progress in accretion disk modlling was very limited, and we did not answer the basic questions, the same since years. I hope that with now huge data flow we will be able/we will have to do it.
Finally, several typos andgramaticalerrors should be revised.
- I removed a number of typos.
Round 2
Reviewer 2 Report
I think that the changes made to the paper and, for the most part, the author's responses are OK. I think it is a shame that real stability curves could not be presented in Fig. 1 but I accept the author's explanation.
I think that this paper will provide a useful introduction to the concept of slim accretion disks.
Author Response
Indeed I share the frustration of the referee. On the other hand, there are some approximate models (used for stationary fitting) which are ok for testing, but I did not want to used them in the present review since it would create only an apparent illusion of accuracy.
Reviewer 3 Report
The author has partially paid attention to the suggestion of this referee, but I feel that the new version has achieved a substantial improvement. Still, some language and style revision is needed.
Author Response
Without detail comments, I cannot see the places where improvements are apparently needed.
