ANU Astrophysical Theory Centre

THE ANU ASTROPHYSICAL THEORY CENTRE

1. STAFF

Head and Professor

D. T. Wickramasinghe, B. A.(Cantab), M. A. (Cantab), Ph. D. (Cantab), F.R.A.S.

Senior Fellow

G. Bicknell, M. Sc, Ph. D (Syd)

Senior Lecturer

L. Ferrario, Laurea in Astronomia (Bologna), Ph. D. (ANU)

Senior ARC Research Associate

S. Vennes, Ph. D. [Montreal]

Research Fellow/Lecturer

M. de Kool, Ph. D. (University of Amsterdam)

P. Francis, Ph. D. (Cantab)

J. Li, Ph. D. (Sussex)

R. Sutherland, Ph. D.(ANU)

ARC Research Associate

J. Murray, Ph. D. (Monash)

Postdoctoral Fellow

Z. Kuncic, M. Sc. (Syd), Ph. D.(Cantab)

ARC International Fellows

Professor R. Wehrse, Institute Theoretische Astrophysik, Heidelberg

Dr. Stephan Wagner, Heidelberg

Honorary Fellow

Emeritus Professor D. W. N. Stibbs, M.Sc. (Syd), D. Phil. (Oxon), F.R.A.S., F.R.S.E.

Associate Members

Professor M. Dopita, MSSSO

Dr. D. J. Faulkner, MSSSO

Professor K. C. Freeman, MSSSO

Dr. A. Kalnajs, MSSSO

Dr. D. McClelland, Department of Physics

Dr. P. Wood, MSSSO

MSSSO Annual Report 1997

Visiting Fellows

Dr. D. Buckley, SAAO

Professor G. Rudiger, Potsdam University

Prof. L. Titarchuck, NASA

Dr. C. Tout, Cambridge, UK.

Dr. W. van Breugel

Professor R. Webbink, Illinois, USA

Dr. R. Whitehurst, DSTO, Melbourne

Graduate Students

Michelle Buxton

Tavis Hamer

Roberto Soria

Mike Burgess

Walid El-Khoury

Lisa Kewley

Honours Students

Daniel Shaw

Holly Syms

Greg Charles

Greg Wilson

Chairs of ATC Committees

Visitor Program Committee: Lilia Ferrario

Colloquium Committee: Jianke Li

Outreach Committee: Geoff Bicknell

Curriculum Committee: Dayal Wickramasinghe

Library Committee: Walter Stibbs

Computing Committee: Ralph Sutherland

2. OPERATION OF THE CENTRE

2.1 GENERAL

The Centre reached its full complement consistent with its current level of funding in late 1997. Dr. Stephane Vennes from the Centre for EUV Astrophysics, Berkeley joined the Centre as a Senior ARC Research Associate early in the year. Dr. James Murray, who was a post doctoral fellow at the Canadian Centre for Theoretical Astrophysics, arrived late in the year to take up his position as an ARC Research Associate. Dr. Martijn de Kool from the Joint Institute for Laboratory Astrophysics

ANU Astrophysical Theory Centre

took up an appointment as Research Fellow. The Centre also welcomed Dr. Paul Francis from the University of Melbourne, the first joint appointment between the Department of Physics and MSSSO linked to the Centre. Dr. Zdenka Kuncic completed one year at the Centre and left on a Fellowship to the Dominion Astrophysical Observatory.

With the arrival of new staff, the research interests of the Centre broadened to encompass new areas such as numerical fluid dynamics, high energy astrophysics, radiation processes in supernovae, binary evolution and population synthesis. The Centre now has a strong group in compact stars, binaries, radiative transfer, interstellar physics, astrophysical magnetohydrodynamics and active galactic nuclei and one can fairly say that a critical mass has been achieved in these core areas of astrophysics which form the basis for many astrophysical investigations. I am pleased to record that there is regular interaction between staff with exchange and cross-fertilisation of ideas, and that the morale and research ethos in the Centre is high. I am confident that with time, the Centre will develop new directions building on the complementary research interests of its current staff.

My own contention is that a Theory Centre should have a centralised location, as is presently the case, and that this is essential for the exchange of ideas and for engendering intellectual intercourse. The strong collaborations that are occurring between members of the Centre, as is evidenced by the research output of the Centre, clearly demonstrate that the current organisational structure is working well and should be maintained. Normally this would not be an issue. Theory Centres should operate as "think tanks", and this is the principle under which other similar organisations operate. The circumstance that some staff allocated to the Centre have substantive appointments in other Departments is not unusual. These staff have offices in the School of Mathematical Sciences where the ATC is located, and have generally succeeded in maintaining a strong presence at the ATC.

Some of the organisational difficulties in running a multi-site centre which I alluded to in my last report have in a sense disappeared with the setting up of a Management Committee under the chairmanship of Professor Ken Freeman. The Committee has representatives from the different sites, meets regularly, and has acted as an important sounding board for new ideas on the management of the Centre. The responsibility for the day to day operation of the major activities of the Centre has been delegated to sub-committees, and this appears to be working quite well. I would like to record my thanks to Professor Freeman for undertaking the onerous task of Chairing the Management Committee, and to the Chairs of the various ATC subcommittees for assisting in the smooth running of the Centre.

The route of expansion of the Centre in the immediate future is through ARC Fellowships and Grants, and I am very pleased to record the success of one of our staff members, Dr. Stephane Vennes, in winning a prestigious Queen Elizabeth II

MSSSO Annual Report 1997

Research Fellowship in the 1997 competitive round of ARC Fellowships. It is important that all members of staff who have appointments to the Centre from Mathematics, Physics or MSSSO should have the opportunity to compete for ARC funds. It may be possible to achieve this by re-classifying the Centre as a University Centre. This is a matter that I am currently pursuing.

As already noted in last year's annual report, the Large ARC grant scheme is not geared towards promoting the development of new areas of research which are clearly outside the range of expertise of the applicants. I would like to reiterate that an area which falls into this category, and which requires development as a matter of urgency, is Theoretical or Physical Cosmology. The strength of MSSSO in observational cosmology is unquestionable, and it is clear that any plans for the future expansion of the Centre should involve the development of Theoretical Cosmology as a matter of high priority.

The Honours Program in Astronomy and Astrophysics has continued to attract students of high quality. It is my pleasure to record that one of our astrophysics students, Greg Wilson, was awarded a University Medal in 1997. Finally, it is my pleasure to record my thanks to Dr. Don Faulkner, who retired from MSSSO at the end of 1997, for his continued support of the ATC throughout its formative years. I am particularly indebted to him for the important role that he played in the teaching of courses in the Honours Program in Astronomy and Astrophysics which I was instrumental in setting up jointly with MSSSO and Physics three years ago.

2.3 Visitors to the ATC

The ATC Visitor Program was only partially funded by internal funds in 1997, and as a result there was a marked drop in the number of years compared to previous years. However, this was offset somewhat by long term visits by Professor Rainer Wehrse and Dr. Stephan Wagner, both on ARC International Visiting Fellowships.

Professor Wehrse brought to the Centre expertise in 3-D radiative transfer. During his visit he interacted strongly with a number of members of the Centre and was instrumental in originating several new research projects in diverse areas such as the 3-D structure of accretion funnels (with Dr. Ferrario), advective discs (with Wickramasinghe) and accretion disc structure (with Ph. D. student Walid El-Khoury).

Professor Ron Webbink from the University of Illinois visited the Centre partially funded by a grant from NASA. He worked with Ferrario, Burgess and Wickramasinghe on problems associated with binary evolution.

ANU Astrophysical Theory Centre

Dr. Chris Tout from the Institute of Astronomy Cambridge visited the Centre and completed a paper on Black Hole binary evolution with Regos and Wickramasinghe.

The visit of Professor Lev Titarchuk from NASA was funded jointly by the ATC and RCfTA, and resulted in the initiation of collaborative work with Wickramasinghe on the radiation properties of accreting black holes.

Professor Gunter Rudinger of the University of Potsdam visited the Centre on a grant obtained by Jianke Li and worked with him and Wickramasinghe on a project on turbulence in discs.

Dr. Wil Van Breugel visited the ATC in 1997 during a sabbatical which also involved visits to the Australia Telescope National Facility and the Anglo-Australian Observatory. His visit to the ATC stimulated theoretical work on the high redshift radio galaxy 4C 41.17 and further work (in progress) on the radio galaxy 3C338 Van Breugel's sabbatical was partially funded by the ATC and by the ATNF and AAO.

Dr. Stefan Wagner began a one year's visit in December, 1997 as an ARC International Fellow following a successful application coordinated by Dr. Geoff Bicknell. Wagner is an expert in extragalactic astronomy - Intraday Variability of quasars and high energy emission (X-Ray - TeV gamma rays) from blazars and is currently working with Bicknell on these topics. He also has other X-Ray data on the nearest active galaxy Centaurus A and optical emission line data on a sample of Seyfert galaxies and it is anticipated that later in his visit he will work with other ATC academics on these topics. His visit has been very popular with other Australian scientists and in a recent visit to institutions in Sydney gave three lectures on various aspects of his work.

2.4 THE HONOURS AND GRADUATE PROGRAMS

The BSc (Hons) Program in Astronomy and Astrophysics run jointly by MSSSO and the Departments of Mathematics and of Physics and Theoretical Physics, continued to attract students from both Departments. Lecturers from ATC and MSSSO participated in the Program with courses offered in " Black Holes, White Dwarfs and Neutron Stars", "Stellar Structure and Pulsations", "Magnetohydrodynamics", "Physics of the Interstellar Medium" and " Galaxies and Cosmology".

During June and July, Professor Stibbs conducted a series of Seminars on Data Analysis for First-Year Research Students in the ATC and the Mount Stromlo Observatory.

MSSSO Annual Report 1997

The Honours year Bok scholarship was awarded to Greg Wilson who went on to receive a First Class Honours degree in Mathematics in the Astrophysics Program, and a University Medal.

The following students are presently enrolled in a Ph. D. degree, and are supervised by staff members of the ATC, either as principal supervisor or as advisor.

Mike Burgess (Wickramasinghe, Wu [RCfTA] and Wood [MSSSO])

Michelle Buxton (Vennes, Ferrario and Wickramasinghe)

Walid El-Khoury (Wickramsinghe, Roberts [SMS] and Ferrario)

Tavis Hamer (Bicknell)

Lisa Kewley (Dopita [MSSSO], Heisler [MSSSO] and Sutherland )

Roberto Soria (Wickramasinghe, Li and Bessell [MSSSO])

Witting, M. (Francis jointly with Webster [Melbourne])

2.5 ANUATC WORKSHOPS

The ANUATC hosted two well attended workshops in 1997. The first was on 'Compact Stars and Accretion Discs' organised by Jianke Li, Stephane Vennes and Lilia Ferrario, and coincided with visits to the Centre by Professor Ron Webbink from the University of Illinois, Professor Rainer Wehrse from the University of Heidelberg and Dr. Chris Tout from the University of Cambridge. The workshop, which was held between August 11-12th 1997, attracted over 25 participants from around the country, and included members of the Special Research Centre for Theoretical Astrophysics, University of Sydney led by Professor Don Melrose, Professor Dick Manchester from the ATNF, Professor Ravi Sood from ADFA, Dr. John Greenhill from the University of Tasmania and Dr. Rosemary Mardling from Monash University. Dr. Helen Johnston from AAO and Dr. Kurt Liffman from AFDL also attended the workshop. The topics covered ranged from Binary Evolution and Pulsars to black holes. The decision by the LOC to have half of the sessions at Mount Stromlo Observatory and the other half on campus at the ATC turned out to be quite popular, and was in keeping with the spirit of the ATC. While at Stromlo, the participants had the opportunity to visit the newly built MSO Exploratory. The workshop was funded by an ARC grant.

The second workshop was on 'Active Galactic Nuclei' and was organised by Bicknell, Francis, Heisler (MSSSO) and Sutherland on Dec. 4 and 5. The venue was chosen by the LOC to be MSSSO. The workshop was timed to precede the ESO/Australia workshop, 'Looking Deep in the Southern Sky', held in Sydney on Dec. 10-12. With the organisational assistance of Dr. Rafaella Morganti (ATNF) we were able to attract a number of speakers from the ESO/Australia meeting to attend the AGN workshop. The meeting also attracted about 18 participants from other Australian institutions. 27 papers were presented including 10 from ANU scientists and visitors working in this

ANU Astrophysical Theory Centre

area. This workshop provides a low cost way for Australian scientists working in the area of AGN to meet and discuss their research, and it was unanimously agreed by the attendees that this meeting should be a regular annual event.

3. RESEARCH ACTIVITIES

Advective Discs around Black Holes in AGN and Soft-X-ray Transients

Shaviv, Wickramasinghe and Wehrse have investigated the self consistency of advective disc models in relation to recent proposals that such models may be applicable to low mass transfer rate accretion discs around black holes in AGN, our galactic centre, and Soft X-ray Transients. Advection dominated accretion flows have usually been discussed in the (1+1) D slim disc approximation, where vertically averaged equations are introduced for pressure, temperature and surface density, and it is explicitly assumed that the vertical energy transport decouples from the radial structure equations. Shaviv, Wickramasinghe and Wehrse have shown that this assumption, which can be justified in the Shakura - Sunyaev thin disc models, could lead to gross errors in the slim disc advective models when cooling is inefficient. They have shown that the coupling with the vertical structure is non- trivial, and may in fact be the determining factor in the hydrodynamics of the accretion flow in advective discs. Their results question the validity of the canonical model for advective discs proposed by Narayan, and may have far reaching consequences for current models of low mass transfer rate accretion discs around black holes in Soft X-ray transients and in galactic nuclei.

Measuring the mass of the Black Hole in GRO 1655-40

The single most important characteristic which distinguishes neutron star (NS) and black hole (BH) X-ray binaries is the mass of the compact object associated with the X-ray source. If the mass is greater than 3 solar masses, the collapsed star is likely to be a black hole. Soria and Wickramasinghe, in collaboration with Hunstead and Wu of the University of Sydney, have measured the motion of the compact object in GRO1655-40 which was discovered nearly three years ago as an enigmatic X-ray and gamma ray source exhibiting superluminal motion. The measurement was made using a unique set of observations obtained with the 2.3m ANU telescope during an outburst of GRO1655-40 in 1996 and has yielded a firm lower limit of 5.4 solar masses and a probable mass of 7 solar masses for the compact star in this system. GRO1655-40 thus becomes one of the best established candidates for a black hole in a Low Mass X-ray Binary in our galaxy.

The Evolutionary Status of GRO1655-40

Regos, Tout and Wickramasinghe have investigated the unusual evolutionary state of the Soft X-ray Black Hole transient GRO 1655-40 which seems to challenge the theory of stellar and binary evolution. A crucial problem is that both the observationally estimated mean mass transfer rate, and the upper limit to the mass transfer rate that

MSSSO Annual Report 1997

would allow accretion disc limit cycles is much smaller than the mass transfer rate expected for a 2-2.5 solar mass donor. One of the proposed models is that the donor star is crossing the Hertsprung gap, and filling its Roche lobe, but the calculations of Regos et al. have shown that the expected mass transfer rate is about 1000 times greater than the estimated upper limit based on observations. The proposal that the star is in the Pennington Hyatus is also rejected on the grounds that the star will shrink into its Roche lobe during this phase. The best model appears to be one in which the star is still on the main sequence in which convective overshooting or some equivalent process occurs at the core-envelope boundary.

Disc Instability in Black Hole Transients

El-Khoury and Wickramasinghe have investigated the thermal stability of X-ray irradiated accretion discs in the low and high mass transfer rate regimes by calculating the vertical structure using a bi-gray approximation. The effects of X-ray heating depends on the hardness of the incident X-ray spectrum, and the magnitude of the x-ray flux relative to the viscously generated flux. For heating by soft x-rays a temperature inversion occurs in the upper atmosphere of the disc when X-ray heating dominates, while for heating by hard X-rays, no significant inversion occurs. In general the S curves (mass transfer rate versus surface density relationship) become increasingly distorted as the irradiation flux increases affecting the stability of the disc. The black hole soft X-ray transients tend to hug the X-ray irradiated instability line, thus providing strong support to the idea that the outbursts in BH-SXTs are triggered by thermal instability in the disc.

Neutron Star Accretion Processes

Sutherland has been investigating the coupled radiation/dynamics problem of instability in highly sub-Eddington accretion in isolated old neutron stars. Together with collaborators, he has shown that if accretion occurs on old isolated neutron stars, which have spun down sufficiently to start accreting interstellar gas, we can use an iterative scheme involving the MAPPINGS III code to couple the accretion dynamics with non-equilibrium atomic heating and ionization. We have found, for both blackbody and more realistic ionizing spectra, that steady accretion is not possible, even for accretion luminosities as small a 10^-7 of the Eddington luminosity of the system. This new extremely low luminosity limit is found for the first time because we include a more complete set radiation cross-sections and atomic processes than has been hitherto possible.

The Thermal Structure of Accretion Funnels

Accretion funnels occur whenever a central magnetised star interacts with an accretion disc and channels matter along magnetic field lines onto the central object. Thus funnels occur in proto planetary systems such as the T Tauri stars, the AM Herculis type variables and the Intermediate polars. The infalling material forms stand