======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@mpifr-bonn.mpg.de http://www.mpifr-bonn.mpg.de/gcnews ======================================================================== Vol. 11, No. 10 Apr 18, 2000 Recently submitted papers: -------------------------- 1) The role of the outer boundary condition in accretion disk models: theory and application (Yuan et al., ApJ) 2) Possible evidence for the disk origin for the powering of jets in Sgr A* and nearby elliptical galaxies (Yuan et al., MNRAS) ------------------------------------------------------------------------ Email : fyuan@nju.edu.cn Title : The role of the outer boundary condition in accretion disk models: theory and application Author(s): Feng Yuan(1), Qiuhe Peng(2), Ju-fu Lu(3), Jianmin Wang(4) Institute: (1) Department of Astronomy, Nanjing University, Nanjing 210093,, and Beijing Astrophysics Center, Beijing 100080, China , Email: fyuan@nju.edu.cn (2) Department of Astronomy, Nanjing University, Nanjing 210093, China (3) Center for Astrophysics, University of Science \& Technology of China, Hefei, 230026, China,, and National Astronomical Observatories, Chinese Academy of Sciences (4) Laboratory of Cosmic Ray and High Energy Astrophysics,, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China Paper : ApJ, June 2000, in press EPrint : astro-ph/0002068 Abstract: In a previous paper (Yuan 1999, hereafter Paper I), we find that the outer boundary conditions(OBCs) of an optically thin accretion flow play an important role in determining the structure of the flow. Here in this paper, we further investigate the influence of OBC on the dynamics and radiation of the accretion flow on a more detailed level. Bremsstrahlung and synchrotron radiations amplified by Comptonization are taken into account and two-temperature plasma assumption is adopted. The three OBCs we adopted are the temperatures of the electrons and ions and the specific angular momentum of the accretion flow at a certain outer boundary. We investigate the individual role of each of the three OBCs on the dynamical structure and the emergent spectrum. We find that when the general parameters such as the mass accretion rate \dot M and the viscous parameter alpha are fixed, the peak flux at various bands such as radio, IR and X-ray, can differ by as large as several orders of magnitude under different OBCs in our example. Our results indicate that OBC is both dynamically and radiatively important therefore should be regarded as a new ``parameter'' in accretion disk models. As an illustrative example, we further apply the above results to the compact radio source Sgr A^* located at the center of our Galaxy. The advection-dominated accretion flow (ADAF) model has turned out to be a great success to explain its luminosity and spectrum. However, there exists a discrepancy between the mass accretion rate favored by ADAF models in the literature and that favored by the three dimensional hydrodynamical simulation, with the former being 10-20 times smaller than the latter. By seriously considering the outer boundary condition of the accretion flow, we find that due to the low specific angular momentum of the accreted gas, the accretion in Sgr A^* should belong to a new accretion pattern, which is characterized by possessing a very large sonic radius. This accretion pattern can significantly reduce the discrepancy between the mass accretion rate. We argue that the accretion occurred in some detached binary systems, the core of nearby elliptical galaxies and active galactic nuclei(AGNs) very possibly belongs to this accretion pattern. ------------------------------------------------------------------------ Email : fyuan@nju.edu.cn Title : Possible evidence for the disk origin for the powering of jets in Sgr A* and nearby elliptical galaxies Author(s): Feng Yuan^1,2,3 Institute: ^1 Department of Astronomy, Nanjing University, Nanjing 210093, China. Email: fyuan@nju.edu.cn, ^2 Beijing Astrophysical Center, Beijing, 10080, China, ^3 Korea Institute for Advanced Study, Seoul, 130-012, Korea Paper : MNRAS, submitted EPrint : astro-ph/0004197 Abstract: Recent VLBA observation indicates the existence of an elongated (jet) structure in the compact radio source Sgr A^*. This is hard to explain in the context of advection-dominated accretion flow (ADAF) model for this source. On the other hand, the mass accretion rate favored by ADAF is 10-20 times smaller than that favored by the hydrodynamical simulation based on Bondi capture. If the latter were adopted, the predicted radio flux would significantly exceed the observation by about two orders of magnitude. A similar situation exists in the case of nearby giant ellipticals, where the canonical ADAF model - the widely assumed standard model for these sources - also significantly overpredict the radio flux. Based on these facts, in this paper we propose a truncated ADAF model for Sgr A^* and three ellipticals M87, NGC 4649 and NGC 4636. We assume that the accretion disk is truncated at a certain radius R_tr within which the jet forms by extracting the energy of the disk. The radio flux is greatly suppressed due to the radiative truncation of the disk and the fits to the observational data are excellent. For example, for Sgr A^*, the model fits the observational spectrum very well from radio including the ``excess'' below the break frequency to hard X-ray under a high accretion rate near the simulation value, and the predicted size-frequency relationship is also in excellent agreement with the observation; for M87, the predicted upper limit of the jet location is 24 R_g, in excellent agreement with the observational result that the jet is formed on scales smaller than 30 R_g, and the 20 % variability at 1 Kev - which is hard to explain in another model trying to explain the low radio flux of M87 - is also marginally interpreted. The success of the model supplies a possible piece of evidence for the disk rather than the hole origin for the powering of jets. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================