======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 29, No. 13 Aug 20, 2009 Recently submitted papers: -------------------------- 1) Interstellar magnetic fields in the Galactic center region (Ferriere, A&A) 2) A Magnetohydrodynamical Model for the Formation of Episodic Jets (Yuan et al., MNRAS) 3) On post-Newtonian orbits and the Galactic-center stars (Preto & Saha, ApJ) 4) IS THERE A SUPERMASSIVE BLACK HOLE AT THE CENTER OF THE MILKY WAY? (Reid, International Journal of Modern Physics D) 5) An X-Ray Face-on View of the Sgr B Molecular Clouds, Observed with Suzaku (Ryu et al., PASJ) 6) THE ANISOTROPIC SPATIAL DISTRIBUTION OF HYPERVELOCITY STARS (Brown et al., ApJ) 7) MMT Hypervelocity Star Survey (Brown et al., ApJ) ------------------------------------------------------------------------ Email : ferriere@ast.obs-mip.fr Title : Interstellar magnetic fields in the Galactic center region Author(s): K. Ferriere Institute: (1) LATT, Universite de Toulouse, CNRS, 14 avenue Edouard Belin, F-31400 Toulouse, France Paper : A&A, August 2009, in press Web : http://arxiv.org/abs/0908.2037 Abstract: We seek to obtain a picture of the interstellar magnetic field in the Galactic center region that is as clear and complete as possible. We review the observational knowledge that has built up over the past 25 years on interstellar magnetic fields within 200 pc of the Galactic center. We then critically discuss the various theoretical interpretations and scenarios proposed to explain the existing observations. We also study the possible connections with the general Galactic magnetic field and describe the observational situation in external galaxies. We propose a coherent picture of the magnetic field near the Galactic center, which reconciles some of the seemingly divergent views and which best accounts for the vast body of observations. Our main conclusions are the following. In the diffuse intercloud medium, the large-scale magnetic field is approximately poloidal and its value is generally close to equipartition with cosmic rays ( 10 mu G), except in localized filaments where the field strength can reach 1 mG. In dense interstellar clouds, the magnetic field is approximately horizontal and its value is typically 1 mG. ------------------------------------------------------------------------ Email : fyuan@shao.ac.cn Title : A Magnetohydrodynamical Model for the Formation of Episodic Jets Author(s): Feng Yuan(1,2), Jun Lin(3,4), Kinwah Wu(5), and Luis C. Ho(6) Institute: (1) Shanghai Astronomical Observatory, Shanghai 200030, China (2) Joint Institute for Galaxy and Cosmology (JOINGC) of SHAO and USTC (3) National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011, China (4) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA (5) Mullard Space Science Laboratory, University College London, Holmbury St Mary, Surrey RH5 6NT, UK (6) The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA Paper : MNRAS, 2009, 395, 2183 Web : http://www.shao.ac.cn/fyuan/yuan.html Abstract: Episodic ejection of plasma blobs have been observed in many black hole systems. While steady, continuous jets are believed to be associated with large-scale open magnetic fields, what causes the episodic ejection of blobs remains unclear. Here by analogy with the coronal mass ejection on the Sun, we propose a magnetohydrodynamical model for episodic ejections from black holes associated with the closed magnetic fields in an accretion flow. Shear and turbulence of the accretion flow deform the field and result in the formation of a flux rope in the disk corona. Energy and helicity are accumulated and stored until a threshold is reached. The system then loses its equilibrium and the flux rope is thrust outward by the magnetic compression force in a catastrophic way. Our calculations show that for parameters appropriate for the black hole in our Galactic center, the plasmoid can attain relativistic speeds in about 35 minutes. ------------------------------------------------------------------------ Email : miguelp@ari.uni-heidelberg.de Title : On post-Newtonian orbits and the Galactic-center stars Author(s): Miguel Preto(1), Prasenjit Saha(2) Institute: (1) Astronomisches Rechen-Institut, Zentrum fuer Astronomie, University of Heidelberg, D-69120 Heidelberg, Germany (2) Institute for Theoretical Physics, University of Zuerich, Winterthurerstrasse 190, CH-8057 Zuerich, Switzerland Paper : ApJ, Oct 2009, in press Abstract: Stars near the Galactic center reach a few percent of light speed during pericenter passage, which makes post-Newtonian effects potentially detectable. We formulate the orbit equations in Hamiltonian form such that the O(v^2/c^2) and O(v^3/c^3) post-Newtonian effects of the Kerr metric appear as a simple generalization of the Kepler problem. A related perturbative Hamiltonian applies to photon paths. We then derive a symplectic integrator with adaptive time-steps, for fast and accurate numerical calculation of post-Newtonian effects. Using this integrator, we explore relativistic effects. Taking the star S2 as an example, we find that general relativity would contribute tenths of mas in astrometry and tens of km s^-1 in kinematics. (For eventual comparison with observations, redshift and time-delay contributions from the gravitational field on light paths will need to be calculated, but we do attempt these in the present paper.) The contribution from stars, gas, and dark matter in the Galactic center region is still poorly constrained observationally, but current models suggest that the resulting Newtonian perturbation on the orbits could plausibly be of the same order as the relativistic effects for stars with semi-major axes > 0.01 pc (or 250 mas). Nevertheless, the known and distinctive time dependence of the relativistic perturbations may make it possible to disentangle and extract both effects from observations. ------------------------------------------------------------------------ Email : reid@cfa.harvard.edu Title : IS THERE A SUPERMASSIVE BLACK HOLE AT THE CENTER OF THE MILKY WAY? Author(s): Mark J. Reid Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA, Paper : International Journal of Modern Physics D, June EPrint : 0808.2624 Abstract: This review outlines the observations that now provide an overwhelming scientific case that the center of our Milky Way Galaxy harbors a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M) of 4*10^6 \Msun within a radius of 100 AU. Orbital speeds have been observed to exceed 5,000 km/sec . At the focal position there is an extremely compact radio source (\SgrA), whose apparent size is near the Schwarzschild radius (2GM/c^2). This radio source is motionless at the 1 km/sec level at the dynamical center of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon. ------------------------------------------------------------------------ Email : ryu@cr.scphys.kyoto-u.ac.jp Title : An X-Ray Face-on View of the Sgr B Molecular Clouds, Observed with Suzaku Author(s): Syukyo G. Ryu, Katsuji Koyama, Masayoshi Nobukawa, Ryosuke Fukuoka, and Takeshi Go Tsuru Institute: (1) Department of Physics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502 Paper : PASJ Aug 2009 61 EPrint : 0904.4550 Web : http://pasj.asj.or.jp/v61/n4/610419/610419.pdf Abstract: We present a new methodology to derive the positions of the Sgr B molecular clouds (MCs) along the line of sight, as an application study of the Galactic center diffuse X-rays (GCDX). The GCDX is composed of hot plasma emission of about 7 keV and 1 keV temperatures, and non-thermal continuum emission including the 6.4 keV line from neutral irons. The former, the Galactic center plasma emission (GCPE), is uniformly distributed over 1 degree in longitude, while the latter is clumpy emission produced by Thomson scattering and fluorescence from MCs irradiated by external X-rays (the X-ray reflection nebula emission: XRNE). We examined the Suzaku X-ray spectra of the GCPE and XRNE near to the Sgr B MC complex, and found that the spectra suffer from two different absorptions of N_H (Abs1) >= 10^23 H cm^-2 and N_H (Abs2) 6*10^22 H cm^-2. Abs1 is proportional to the 6.4 keV-line flux, and hence is due to the MCs, while Abs2 is typical of interstellar absorption toward the Galactic center. Assuming that the GCPE plasma is spherically-extended around Sgr A* with a uniform density and the same angular distribution of the two temperature components, we quantitatively estimated the line-of-sight positions of the MCs from the flux ratio the GCPE spectrum suffered by Abs1 and that with no Abs1. The results suggest that the Sgr B MCs are located at the near side of Sgr A* in the GCPE. ------------------------------------------------------------------------ Email : wbrown@cfa.harvard.edu Title : THE ANISOTROPIC SPATIAL DISTRIBUTION OF HYPERVELOCITY STARS Author(s): Warren R. Brown(1), Margaret J. Geller(1), Scott J. Kenyon(1) Benjamin C. Bromley(2) Institute: (1) Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 (2) Department of Physics, University of Utah, 115 S 1400 E, Rm 201, Salt Lake City, UT 84112 Paper : ApJ, 2009, 690, L69 Abstract: We study the distribution of angular positions and angular separations of unbound hypervelocity stars (HVSs). HVSs are spatially anisotropic at the 3- sigma level. The spatial anisotropy is significant in Galactic longitude, not in latitude, and the inclusion of lower velocity, possibly bound HVSs reduces the significance of the anisotropy. These observations are plausibly explained by an anisotropic central bulge potential. In the future, measuring the distribution of HVSs in the southern sky will provide additional constraints on the spatial anisotropy and the origin of HVSs. ------------------------------------------------------------------------ Email : wbrown@cfa.harvard.edu Title : MMT Hypervelocity Star Survey Author(s): Warren R. Brown, Margaret J. Geller, Scott J. Kenyon Institute: (1) Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 Paper : ApJ, 2009, 290, 1639 Abstract: We describe a new survey for unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole is their most likely origin. We investigate the possible contribution of unbound runaway stars, and show that the physical properties of binaries constrain low mass runaways to bound velocities. We measure radial velocities for HVS candidates with the colors of early A-type and late B-type stars. We report the discovery of 6 unbound HVSs with velocities and distances exceeding the conservative escape velocity estimate of Kenyon and collaborators. We additionally report 4 possibly unbound HVSs with velocities and distances exceeding the lower escape velocity estimate of Xue and collaborators. These discoveries increase the number of unbound HVSs by 60% - 100%. Other survey objects include 19 newly identified z 2.4 quasars. One of the HVSs may be a horizontal branch star, consistent with the number of evolved HVSs predicted by Galactic center ejection models. Finding more evolved HVSs will one day allow a probe of the low-mass regime of HVSs and will constrain the mass function of stars in the Galactic center. ------------------------------------------------------------------------ (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, Masaaki Sakano, Feng Yuan - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================