Relativistic Astrophysics

Paperpresented at the 13th Int. Conf. on General Relativity and Gravitation 387 Cordoba, Argentina, 1992: Part 2, Workshop Summaries Relativistic astrophysics Richard H. Price Department of Physics University of Utah Salt Lake City, UT 84112 USA Abstract. Work reported in the workshop on relativistic astrophysics spanned a wide varicy of topics. Two specific areas seemed of particular in— terest. Much attention was focussed on gravitational wave sources, especially on the waveforms they produce, and progress was reported in theoretical and observational aspects of accretion disks. The title “relativistic astrophysics” is rather broad, and has illedeiined boundaries separating it from the concerns of other branches of relativity, and of other workshops at the conference. As l interpret it, the subject should be driven (although sometimes indirectly) by observations. On this basis the following topics would seem to be some of the interesting devel» opnients in the past year or so: (i) The discovery of the isotropy of y—i‘ay burst sources suggests a cosmologicalorigin. (ii) The identification of blaclehole candidates in compact systems, systems with signatures rather different from Cygnus Xel, is a sign of matu— ration ol. the field. (iii) There is increasing evidence that our own galaxy has a mass concentration near its center, suggesting a central black hole, and/or other strongefield phenomena. (iv) Data from the llubble telscope confirm the longstanding belief that there is a mass cusp at the center of M87. (v) Recent work, especially the numerical simulations by Shapiro and Teukolsky [1], reported at this conference, presagc the down, {all of cosmic censorhip. What are the astrophysical consequences of Shapiro-'l‘eukolsky spindle singularities? Along with these new, or reactivated, problems there remain some venerable issues, problems on which progress continues, and on which further work is needed. We are still far from having an adequate understanding the physics of accretion disks around black holes. Perhaps related to this is one of the Classic question of relativistic astrophysics: the central structure of active galactic nuclei. The range of subjects covered was broad and somewhat scattered. I shall review here two subjects, which were well represented: sources of gravitational radiation, and accretion disks/toroids. Both, are questions the “long-standing problem” variety. The first of these, gravitational wave sources, was a subject of marked concentration in the workshop, and was the focus of five of the thirteen oral presentations. The focus, (a 1997i lOP PublishimT Ltd 388 General Relan'vily and Gravitation J 992 in fact, was rather narrow. The great interest in details of source waveforms leaves no doubt that the anticipated advent of laser interferometer gravitational wave detectors is having a considerable impact. One presentation holds out an interesting hope of tying together observations of gravitational waves and the recent discoveries about 7-ray bursts. Perhaps the reason these bursts have not created the same flurry among relativists as among astrophysicists is that in the 20 or so years since they were first discovered the bursts have been assumed to be dramatic, but not relativistic, events involving strongly magnetized neutron stars. All this has changed with the findings of BATSE (Burst and Transient Source Experi- ment) on the Compton GRO (Gamma Ray Observatory) [2]. There are two important features of bursts that it has discovered: (i) the bursts are distributed isotropically, and (ii) plots of number of bursts vs. burst strength show a fall off at low strength. These results almost certainly point to sources at z N 1. At this point the field is open for much speculation, and the data leave room for some differences of interpretation. For one thing, the PVC (Pioneer Venus Orbiter) which has higher threshold (and different energy range) than BATSE has been monitoring strong 7»ray bursts since 1978. A com~ parison of the strong burst data of PVC with the extensive statistics of weak bursts from BATSE suggests a deficiency of weak bursts that points to evolution of the source population and rules out early universe sources [3]. The way the data are to be interpreted is an open question. There might, for example, be two different classes of phenomena contributing to the observations. The nature of the source is a question that is open even wider. Exotic possibilities, such as stars made of strange matter, have been suggested [4], as has the tidal disruption of an ordinary star by a massive ( 1W ~ 108MB) black hole [5]. Perhaps the favored modeleat least the most conservative modeleis that of the coalescence of two compact objects, either a neutron star binary as suggested by Piran [6] and others, or a neutron star— black hole binary, as suggested by Paczynski [7]. The expected frequency of neutron—star coalescences is on the order of 10—6 year“1 per galaxy) which would be compatible with the rate at which bursts are being detected. The energy requirement for a 2 ~ 1 burst is around 1050 ergs, and would be compatible with the expected 1053 ergs for a binary coalescence. This all should, of course, means that 7—1‘ay bursts are of great potential interest to relativists, since coalescence of compact binaries is also the presently favored source of detectable gravitational waves. At the workshop David Nicholson reported on work with Schutz that points out how this can be exploited for gravitational wave (GW) observations. In particular, the 7—ray burst and the GW burst for the coalescence should coincide within 1 sec The pattern matching necessary for the search for GW bursts would be simplified by searches limited to the data within 1 sec of an observed 7-ray burst. This allows, for example, the use of a finer mesh of filters and improved sensitivity. The detection of a burst by BATSE would provide directional information on the burst source, and would therefore constrain the form of the signal arriving in GW detectors. Monte Carlo simulations were used by Nicholson and Schutz to explore the magnitude of the improvement to be gained from a search for coincident bursts. They find (for an optimally configured detectors) that 2 GVV detectors would be able to detect a binary neutron star coalescence out to z = 0.4. Other work reported at the workshop was also motivated by the problem of the pattern matching necessary to find GW signals from coalescing binaries. Very recent Relativistic astrophysics 389 results by the CalTech group [8] show that much improved detailed understanding of the final inspiral of compact binaries will be needed if astrophysical information is to be efficiently extracted from GW waveforms. Larry Kidder reported on work with Will and Wiseman [9] on one facet of the progress that will be needed. They study the existence of innermost stable orbits for compact binary systems. The approach is a hybrid of (post)2- Newtonian analysis [so that (post)5/2 radiation damping is omitted] and test body effects from the Schwarzschild geometry. The results show that, for a given mass of the binary pair, the radius of the innermost stable orbit is 20% larger for an equal mass binary than for the test body case. The need for more precise predictions of source waveforms was part of the motivation for the presentation, by Alan Wiseman, of a study with Will of “heredity effects” in GW phenomena. These are nonlinear effects that in general require integration over the past history of the system. Another motivation was the relation of this work to the “Christodoulou nonlinear memory” [10]. A gravitational wave train is said to have “memory’7 if the strain hU-(t) is different after the the passage of the waves than it was before the arrival of the waves. In principle, this would leave the free masses of a GW de- tector in positions different from their positions before the waves arrived, and the shifted positions would constitue a souvenir of the burst, the “memory.” In practice this DC offset of the burst would be swamped by noise, but it can be inferred by measurement of the low frequency content of the burst, the components of the burst at frequencies below any characteristic frequency of the source. A major reason for the interest in GW memory is the recent correction of a widespread and longstanding conceptual error concerning its calculation. Only 1/1" contributions to hi, are radiative; for the memory only the initial and filial values of h;, are needed; before and after the strong accelerations producing the fat middle of the burst, the stress energy is simply that of “particles” moving at uniform velocity; the 1/r fields of these particles is just the Coulomb fields due to their masses, and those contributions are completely within the scope of linear theory. It was therefore the common wisdom that the calculation of the memory produced by an astrophysical event required only linearized gravity theory, even for a strong field events like black hole collisions. As long as the waves were weak at the detectors, it was thought, linearized theory should suffice for calculations of the memory. Christodoulou [10], with a rigorous mathematical analysis, showed that this was wrong, that there are in fact nonlinear contributions to the memory that are of the same order as the linear contributions. It was quickly shown to be equivalent, for waves from astrophysical sources, to including the emerging GWs themselves as part of the source [11][12]. Though this nonlinear part of the memory is present in principle, it is a separate question whether it could be detected. Preliminary model calculations [12] suggest that detection with laser interferometers will be very difficult, but may be possible.

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