======================================================================== 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. 15 Nov 25, 2009 Recently submitted papers: -------------------------- 1) Localising the VHE gamma -ray source at the Galactic Centre (HESS, Van Eldik, MNRAS) 2) A Multiwavelength View of a Mass Outflow from the Galactic Center (Law, ApJ) 3) Hyper Velocity Stars and The Restricted Parabolic 3-Body Problem (Sari et al., ApJ) 4) Testing Properties of the Galactic Center Black Hole Using Stellar Orbits (Merritt et al.) 5) Radiative Models of Sgr A* from GRMHD Simulations (Moscibrodzka et al., ApJ) 6) Transition from radiatively inefficient to cooling dominated phase in two temperature accretion discs around black holes (Sinha, RAA accepted) 7) Stellar interactions in dense and sparse star clusters (Olczak et al., A&A) 8) Tidal effects in the vicinity of a black hole (Cadez et al., proceedings) 9) Enhanced tidal disruption rates from massive black hole binaries (Chen et al., ApJ) ------------------------------------------------------------------------ Email : Christopher.van.Eldik@mpi-hd.mpg.de Title : Localising the VHE gamma -ray source at the Galactic Centre Author(s): HESS Collaboration (through Chr. van Eldik) Paper : MNRAS, accepted EPrint : 0911.1912 Abstract: The inner 10 pc of our galaxy contains many counterpart candidates of the very high energy (VHE; > 100 GeV) gamma-ray point source HESS J1745-290. Within the point spread function of the H.E.S.S. measurement, at least three objects are capable of accelerating particles to very high energies and beyond, and of providing the observed gamma-ray flux. Previous attempts to address this source confusion were hampered by the fact that the projected distances between those objects were of the order of the error circle radius of the emission centroid (34'', dominated by the pointing uncertainty of the H.E.S.S. instrument). Here we present H.E.S.S. data of the Galactic Centre region, recorded with an improved control of the instrument pointing compared to H.E.S.S. standard pointing procedures. Stars observed during gamma-ray observations by optical guiding cameras mounted on each H.E.S.S. telescope are used for off-line pointing calibration, thereby decreasing the systematic pointing uncertainties from 20'' to 6'' per axis. The position of HESS J1745-290 is obtained by fitting a multi-Gaussian profile to the background-subtracted gamma-ray count map. A spatial comparison of the best-fit position of HESS J1745-290 with the position and morphology of candidate counterparts is performed. The position is, within a total error circle radius of 13'', coincident with the position of the supermassive black hole Sgr A* and the recently discovered pulsar wind nebula candidate G359.95-0.04. It is significantly displaced from the centroid of the supernova remnant Sgr A East, excluding this object with high probability as the dominant source of the VHE gamma-ray emission. ------------------------------------------------------------------------ Email : claw@astro.berkeley.edu Title : A Multiwavelength View of a Mass Outflow from the Galactic Center Author(s): C. J. Law Institute: (1) Radio Astronomy Lab, University of California, Berkeley, CA 94720 Paper : ApJ, 2009, accepted EPrint : 0911.2061 Abstract: The Galactic center (GC) lobe is a degree-tall shell of gas that spans the central degree of our Galaxy. It has been cited as evidence for a mass outflow from our GC region, which has inspired diverse models for its origin. However, most work has focused on the morphology of the GC lobe, which has made it difficult to draw strong conclusions about its nature. Here, I present a coherent, multiwavelength analysis of new and archival observations of the GC lobe. New radio continuum observations show that the entire structure has a similar spectral index, indicating that it has a common origin. The radio continuum emission shows that the GC lobe has a magnetized layer with a diameter of 110 pc and an equipartition field strength ranging from 40 to 100 mu G. I show that optical and radio recombination line emission are associated with the GC lobe and are consistent with being located in the GC region. The recombination line emission traces an ionized shell nested within the radio continuum with diameter of 80 pc and height 165 pc. Mid-infrared maps at 8 and 15 micron show that the GC lobe has a third layer of warm dust and PAH-emission that surrounds the radio continuum shell with a diameter of 130 pc. Assuming adibatic expansion of the gas in the GC lobe, its formation required an energy input of about 5*10^52 ergs. I compare the physical conditions of the GC lobe to several models and find best agreement with the canonical starburst outflow model. The formation of the GC lobe is consistent with the currently observed pressure and star formation rate in the central tens of parsecs of our Galaxy. Outflows of this scale are more typical of dwarf galaxies and would not be easily detected in nearby spiral galaxies. Thus, the existence of such an outflow in our own Galaxy may indicate that it is relatively common phenomenon in the nuclei of spiral galaxies. ------------------------------------------------------------------------ Email : issor.em@gmail.com Title : Hyper Velocity Stars and The Restricted Parabolic 3-Body Problem Author(s): Re'em Sari^1,2, Shiho Kobayashi^3,1, Elena M. Rossi^1 Institute: (1) Racah Institute of Physics, Hebrew University, Jerusalem, Israel, 91904 (2) Theoretical astrophysics 350-17, California Institute of Technology, Pasadena, CA, 91125 (3) Astrophysics Research Institute, Liverpool John Moores University, United Kingdom Paper : 2009, ApJ, accepted Abstract: Motivated by detections of hypervelocity stars that may originate from the Galactic Center, we revist the problem of a binary disruption by a passage near a much more massive point mass. The six order of magnitude mass ratio between the Galactic Center black hole and the binary stars allows us to formulate the problem in the restricted parabolic three-body approximation. In this framework, results can be simply rescaled in terms of binary masses, its initial separation and binary-to-black hole mass ratio. Consequently, an advantage over the full three-body calculation is that a much smaller set of simulations is needed to explore the relevant parameter space. Contrary to previous claims, we show that, upon binary disruption, the lighter star does not remain preferentially bound to the black hole. In fact, it is ejected exactly in 50% of the cases. Nonetheless, lighter objects have higher ejection velocities, since the energy distribution is independent of mass. Focusing on the planar case, we provide the probability distributions for disruption of circular binaries and for the ejection energy. We show that even binaries that penetrate deeply into the tidal sphere of the black hole are not doomed to disruption, but survive in 20% of the cases. Nor do these deep encounters produce the highest ejection energies, which are instead obtained for binaries arriving to 0.1-0.5 of the tidal radius in a prograde orbit. Interestingly, such deep-reaching binaries separate widely after penetrating the tidal radius, but always approach each other again on their way out from the black hole. Finally, our analytic method allows us to account for a finite size of the stars and recast the ejection energy in terms of a minimal possible separation. We find that, for a given minimal separation, the ejection energy is relatively insensitive to the initial binary separation. ------------------------------------------------------------------------ Email : merritt@astro.rit.edu Title : Testing Properties of the Galactic Center Black Hole Using Stellar Orbits Author(s): David Merritt Tal Alexander Seppo Mikkola Clifford M. Will Abstract: The spin and quadrupole moment of the supermassive black hole at the Galactic center can in principle be measured via astrometric monitoring of stars orbiting at milliparsec (mpc) distances, allowing tests of general relativistic ``no-hair''theorems \citeWill-08. One complicating factor is the presence of perturbations from other stars, which may induce orbital precession of the same order of magnitude as that due to general relativistic effects. The expected number of stars in this region is small enough that full N-body simulations can be carried out. We present the results of a comprehensive set of such simulations, which include a post-Newtonian treatment of spin-orbit effects. A number of possible models for the distribution of stars and stellar remnants are considered. We find that stellar perturbations are likely to obscure the signal due to frame-dragging for stars beyond 0.5 mpc from the black hole, while measurement of the quadrupole moment is likely to require observation of stars inside 0.2 mpc. A high fraction of stellar remnants, e.g. 10\msun black holes, in this region would make tests of GR problematic at all radii. We discuss the possibility of separating the effects of stellar perturbations from those due to GR. ------------------------------------------------------------------------ Email : mmosc@illinois.edu Title : Radiative Models of Sgr A* from GRMHD Simulations Author(s): Monika Moscibrodzka(1), Charles F. Gammie(1,2), Joshua C. Dolence(2), Hotaka Shiokawa(2), Po Kin Leung(2) Institute: (1) Department of Physics, University of Illinois, 1110 West Green Street, Urbana, IL 61801 (2) Astronomy Department, University of Illinois, 1002 West Green Street, Urbana, IL 61801 Paper : ApJ, Oct 2009, in press EPrint : 0909.5431 Abstract: Using flow models based on axisymmetric general relativistic magnetohydrodynamics (GRMHD) simulations, we construct radiative models for Sgr A*. Spectral energy distributions that include the effects of thermal synchrotron emission and absorption, and Compton scattering, are calculated using a Monte Carlo technique. Images are calculated using a ray-tracing scheme. All models are scaled so that the 230 GHz flux density is 3.4 Jy. The key model parameters are the dimensionless black hole spin a_*, the inclination i, and the ion-to-electron temperature ratio Trat. We find that: (1) models with Trat = 1 are inconsistent with the observed submillimeter spectral slope; (2) the X-ray flux is a strongly increasing function of a_*; (3) the X-ray flux is a strongly increasing function of i; (4) 230 GHz image size is a complicated function of i, a_*, and Trat, but the Trat = 10 models are generally large and at most marginally consistent with the 230 GHz VLBI data; (5) for models with Trat = 10 and i = 85 DEG the event horizon is cloaked behind a synchrotron photosphere at 230 GHz and will not be seen by VLBI, but these models overproduce NIR and X-ray flux; (6) in all models whose SEDs are consistent with observations the event horizon is uncloaked at 230 GHz; (7) the models that are most consistent with the observations have a_* 0.9. We finish with a discussion of the limitations of our model and prospects for future improvements. ------------------------------------------------------------------------ Email : msinha@physics.iisc.ernet.in Title : Transition from radiatively inefficient to cooling dominated phase in two temperature accretion discs around black holes Author(s): Monika Sinha and S. R. Rajesh and Banibrata Mukhopadhyay Institute: (1) Astronomy and Astrophysics Program, Department of Physics, Indian Institute of Science, Bangalore 560012, India Paper : accepted in RAA Abstract: We investigate the transition of a radiatively inefficient phase of a viscous two temperature accreting flow to a cooling dominated phase and vice versa around black holes. Based on a global sub-Keplerian accretion disc model in steady state, including explicit cooling processes self-consistently, we show that general advective accretion flow passes through various phases during its infall towards a black hole. Bremsstrahlung, synchrotron and inverse Comptonization of soft photons are considered as possible cooling mechanisms. Hence the flow governs a much lower electron temperature 10^8-10^9.5K compared to the hot protons of temperature 10^10.2-10^11.8K in the range of the accretion rate in Eddington units 0.01\lsim\mdot\lsim 100. Therefore, the solutions may potentially explain the hard X-rays and the gamma -rays emitted from AGNs and X-ray binaries. We finally compare the solutions for two different regimes of viscosity and conclude that a weakly viscous flow is expected to be cooling dominated compared to its highly viscous counterpart which is radiatively inefficient. The flow is successfully able to reproduce the observed luminosities of the under-fed AGNs and quasars (e.g. Sgr A^*), ultra-luminous X-ray sources (e.g. SS433), as well as the highly luminous AGNs and ultra-luminous quasars (e.g. PKS 0743-67) at different combinations of the mass accretion rate and ratio of specific heats. ------------------------------------------------------------------------ Email : olczak@ph1.uni-koeln.de Title : Stellar interactions in dense and sparse star clusters Author(s): C. Olczak (1), S. Pfalzner (1) and A. Eckart (1,2) Institute: (1) I. Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str.77, 50937 Koeln, Germany (2) Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany Paper : A&A, Nov 2009, accepted Abstract: Stellar encounters potentially affect the evolution of the protoplanetary discs in the Orion Nebula Cluster (ONC). However, the role of encounters in other cluster environments is less known. We investigate the effect of the encounter-induced disc-mass loss in different cluster environments. Starting from an ONC-like cluster we vary the cluster size and density to determine the correlation of collision time scale and disc-mass loss. We use the nbody6++ code to model the dynamics of these clusters and analyze the disc-mass loss due to encounters. We find that the encounter rate depends strongly on the cluster density but remains rather unaffected by the size of the stellar population. This dependency translates directly into the effect on the encounter-induced disc-mass loss. The essential outcome of the simulations are: i) Even in clusters four times sparser than the ONC the effect of encounters is still apparent. ii) The density of the ONC itself marks a threshold: in less dense and less massive clusters it is the massive stars that dominate the encounter-induced disc-mass loss whereas in denser and more massive clusters the low-mass stars play the major role for the disc mass removal. It seems that in the central regions of young dense star clusters - the common sites of star formation - stellar encounters do affect the evolution of the protoplanetary discs. With higher cluster density low-mass stars become more heavily involved in this process. This finding allows for the extrapolation towards extreme stellar systems: in case of the Arches cluster one would expect stellar encounters to destroy the discs of most of the low- and high-mass stars in several hundred thousand years, whereas intermediate mass stars are able to retain to some extant their discs even under these harsh environmental conditions. ------------------------------------------------------------------------ Email : uros.kostic@fmf.uni-lj.si Title : Tidal effects in the vicinity of a black hole Author(s): Andrej Cadez Uros Kostic Massimo Calvani Paper : proceedings of the Zeldovich Meeting 2009 EPrint : 0908.0117 Abstract: The discovery that the Galactic centre emits flares at various wavelengths represents a puzzle concerning their origin, but at the same time it is a relevant opportunity to investigate the environment of the nearest super-massive black hole. In this paper we shall review some of our recent results concerning the tidal evolution of the orbits of low mass satellites around black holes, and the tidal effect during their in-fall. We show that tidal interaction can offer an explanation for transient phenomena like near infra-red and X-ray flares from Sgr A*. ------------------------------------------------------------------------ Email : xchen3@ucsc.edu Title : Enhanced tidal disruption rates from massive black hole binaries Author(s): Xian Chen(1,2), Piero Madau(2), Alberto Sesana(3), \& F. K. Liu(1,4) Institute: (1) Department of Astronomy, Peking University, 100871 Beijing, China. (2) Department of Astronomy \& Astrophysics, University of California, Santa Cruz, CA 95064. (3) Center for Gravitational Wave Physics, The Pennsylvania State University, University Park, State College, PA 16802. (4) Kavli Institute for Astronomy and Astrophysics, Peking University, 100871 Beijing, China. Paper : ApJ, 2009, 697, L149 EPrint : 0904.4481 Abstract: ``Hard" massive black hole (MBH) binaries embedded in steep stellar cusps can shrink via three-body slingshot interactions. We show that this process will inevitably be accompanied by a burst of stellar tidal disruptions, at a rate that can be several orders of magnitude larger than that appropriate for a single MBH. Our numerical scattering experiments reveal that: 1) a significant fraction of stars initially bound to the primary hole are scattered into its tidal disruption loss cone by gravitational interactions with the secondary hole, an enhancement effect that is more pronounced for very unequal-mass binaries; 2) about 25% (40%) of all strongly interacting stars are tidally disrupted by a MBH binary of mass ratio q=1/81 (q=1/243) and eccentricity 0.1; and 3) two mechanisms dominate the fueling of the tidal disruption loss cone, a Kozai non-resonant interaction that causes the secular evolution of the stellar angular momentum in the field of the binary, and the effect of close encounters with the secondary hole that change the stellar orbital parameters in a chaotic way. For a hard MBH binary of 10^7 \msun and mass ratio 10^-2, embedded in an isothermal stellar cusp of velocity dispersion sigma _*=100 km/sec , the tidal disruption rate can be as large as \dot N_* 1 yr^-1. This is 4 orders of magnitude higher than estimated for a single MBH fed by two-body relaxation. When applied to the case of a putative intermediate-mass black hole inspiraling onto Sgr A^*, our results predict tidal disruption rates \dot N_* 0.05-0.1 yr^-1. ------------------------------------------------------------------------ (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 ========================================================================