======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 24, No. 3 Mar 30, 2006 Recently submitted papers: -------------------------- 1) ANNIHILATION EMISSION FROM THE GALACTIC BLACK HOLE (Cheng et al., accepted) 2) Testing RIAF model for Sgr A* using the size measurements (Yuan et al., ApJL) 3) Particle Re-Acceleration in Colliding Winds Systems? Radio, X-ray, and gamma -ray Emission Models of WR 140 (Pittard & Dougherty, MNRAS) ------------------------------------------------------------------------ Email : anisia@bohr.physics.hku.hk Title : ANNIHILATION EMISSION FROM THE GALACTIC BLACK HOLE Author(s): K. S. Cheng(1), D. O. Chernyshov(1,2), and V. A. Dogiel(1,3) Institute: (1) Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, China (2) Moscow Institute of Physics and Technology, Institutskii lane, 141700 Moscow Region, Dolgoprudnii, Russia (3) I.E.Tamm Theoretical Physics Division of P.N.Lebedev Institute, Leninskii pr, 53, 119991 Moscow, Russia. Paper : accepted by ApJ EPrint : astro-ph/0603659 Abstract: Both diffuse high energy gamma-rays and an extended electron-positron annihilation line emission have been observed in the Galactic Center (GC) region. Although X-ray observations indicate that the galactic black hole Sgr A^* is inactive now, we suggest that Sgr A^* can become active when a captured star is tidally disrupted and matter is accreted into the black hole. As a consequence the galactic black hole could be a powerful source of relativistic protons. We are able to explain the current observed diffuse gamma-rays and the very detailed 511 keV annihilation line of secondary positrons by p-p collisions of such protons, with appropriate injection times and energy. Relativistic protons could have been injected into the ambient material if the black hole captured a 50M_o star at several tens million years ago. An alternative possibility is that the black hole continues to capture stars with 1M_o every hundred thousand years. Secondary positrons produced by p-p collisions at energies > 30 MeV are cooled down to thermal energies by Coulomb collisions, and annihilate in the warm neutral and ionized phases of the interstellar medium with temperatures about several eV, because the annihilation cross-section reaches its maximum at these temperatures. It takes about ten million years for the positrons to cool down to thermal temperatures so they can diffuse into a very large extended region around the Galactic center. A much more recent star capture may be also able to account for recent TeV observations within 10 pc of the galactic center as well as for the unidentified GeV gamma-ray sources found by EGRET at GC. The spectral difference between the GeV flux and the TeV flux could be explained naturally in this model as well. ------------------------------------------------------------------------ Email : fyuan@shao.ac.cn Title : Testing RIAF model for Sgr A* using the size measurements Author(s): Feng Yuan(1,2), Zhi-Qiang Shen(1,2) and Lei Huang(1,3) Institute: (1) Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China;fyuan,zshen,muduri@shao.ac.cn (2) Joint Institute for Galaxy and Cosmology (JOINGC) of SHAO and USTC (3) Graduate School of Chinese Academy of Sciences, Beijing 100039, China Paper : ApJL, 2006 in press EPrint : astro-ph/0603683 Web : http://www.shao.ac.cn/fyuan Abstract: Recent radio observations by the VLBA at 7 and 3.5 mm produced the high-resolution images of the compact radio source located at the center of our Galaxy--Sgr A*, and detected its wavelength-dependent intrinsic sizes at the two wavelengths. This provides us with a good chance of testing previously-proposed theoretical models for Sgr A*. In this Letter, we calculate the size based on the radiatively inefficient accretion flow (RIAF) model proposed by Yuan, Quataert \& Narayan (2003). We find that the predicted sizes after taking into account the scattering of the interstellar electrons are consistent with the observations. We further predict an image of Sgr A* at 1.3 mm which can be tested by future observations. ------------------------------------------------------------------------ Email : jmp@ast.leeds.ac.uk Title : Particle Re-Acceleration in Colliding Winds Systems? Radio, X-ray, and gamma -ray Emission Models of WR 140 Author(s): J.M. Pittard(1) and S.M. Dougherty(2) Institute: (1) School of Physics and Astronomy, The University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK (2) National Research Council of Canada, Herzberg Institute for Astrophysics, Dominion Radio Astrophysical Observatory, P.O. Box 248, Penticton, British Columbia V2A 6J9, Canada Paper : MNRAS, submitted EPrint : astro-ph/0603787 Abstract: We present calculations of the spatial and spectral distribution of the radio, X-ray and gamma -ray emission from shock accelerated electrons in the wind-collision region (WCR) of WR 140. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission at this phase in conjunction with the radio emission to constrain the mass-loss rates, we find that the O-star mass-loss rate is consistent with the reduced estimates for O4-5 supergiants by (Fullerton:2005), and the wind momentum ratio, \eta = 0.02. The observed low frequency turnover at 3 GHz in the radio emission is due to free-free absorption, since models based on the Razin effect have an unacceptably large fraction of energy in non-thermal particles. A key result is the index of the non-thermal electron energy distribution is flatter than the canonical value for diffusive shock acceleration (DSA), namely p<2. It is argued that this requires re-acceleration of non-thermal particles in multiple wind-embedded shocks, which are then injected as seed particles for further acceleration at the global shocks bounding the WCR. There are some tantalizing hints that shock modification occurs in these systems. For example, the estimated amount of energy placed into non-thermal particles is high enough that non-linear effects should be important, and changes in the degree of shock modification with orbital phase may account for the asymmetry of the radio lightcurve and the smaller than expected variation in the X-ray lightcurve. Shock modification also results in softer X-ray emission from the post-shock plasma due to a reduction in the velocity jump across the subshock. While this is also consistent with observations, the long timescale for energy transfer between the post-shock ions and electrons may be the dominant cause. Tighter constraints on p and the nature of the shocks in WR 140 will be obtained from future observations at MeV and GeV energies, for which we generally predict lower fluxes than previous work. Since the high stellar photon fluxes prevent the acceleration of electrons beyond gamma > 10^5-10^6, TeV emission from CWB systems will provide unambiguous evidence of pion-decay emission from accelerated ions. We finish by commenting on the emission and physics of the multiple wind collisions in dense stellar clusters, paying particular attention to the Galactic Centre. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Sera Markoff, Loránt Sjouwerman, Joseph Lazio, Cornelia Lang, Rainer Schödel, Robin Herrnstein - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================