======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 22, No. 5 Sep 16, 2005 Recently submitted papers: -------------------------- 1) Spectral analysis of the Galactic e^+e^- annihilation emission (Jean et al., A&A) 2) Frequency-Dependent Shift in the Image Centroid of the Black Hole at the Galactic Center as a Test of General Relativity (Broderick & Loeb, ApJL) 3) Imaging Optically-Thin Hot Spots Near the Black Hole Horizon of Sgr A* at Radio and Near Infrared Wavelengths (Broderick & Loeb, MNRAS) 4) Imaging Bright Spots in the Acceretion Flow Near the Black Hole Horizon of Sgr A* (Broderick & Loeb, MNRAS) 5) An Infrared Imaging Survey of the Faint Chandra Sources near the Galactic Centre (Bandyopadhyay et al., MNRAS) 6) A dual emission mechanism in Sgr A*/L' (Clenet et al., A&A) ------------------------------------------------------------------------ Email : Pierre.Jean@cesr.fr Title : Spectral analysis of the Galactic e^+e^- annihilation emission Author(s): P. Jean(1) and J. Knoedlseder(1) and W. Gillard(1) and N. Guessoum(2) and K, Ferriere(3) and A. Marcowith(1) and V. Lonjou(1) and J.P. Roques(1) Institute: (1) ^1 CESR, CNRS/UPS, B.P. 4346, 31028 Toulouse Cedex 4, France, ^2 American University of Sharjah, College of Arts \& Sciences, Physics Department, PO Box 26666, Sharjah, UAE, ^3 LATT, CNRS/OMP, 31000 Toulouse, France, Paper : A&A, sept 2005, in press EPrint : astro-ph/0509298 Web : http://arxiv.org/abs/astro-ph/0509298 Abstract: We present a spectral analysis of the e^+e^- annihilation emission from the Galactic Centre region based on the first year of measurements made with the spectrometer SPI of the INTEGRAL mission. We have found that the annihilation spectrum can be modelled by the sum of a narrow and a broad 511 keV line plus an ortho-positronium continuum. The broad line is detected (significance 3.2 sigma ) with a flux of (0.35 +/- 0.11) * 10^-3 photons s^-1 cm^-2. The measured width of 5.4+/-1.2 keV FWHM is in agreement with the expected broadening of 511 keV photons emitted in the annihilation of positroniums that are formed by the charge exchange process of slowing down positrons with hydrogen atoms. The flux of the narrow line is (0.72 +/- 0.12) * 10^-3 photons s^-1 cm^-2 and its width is 1.3+/-0.4 keV FWHM. The measured ortho-positronium continuum flux yields a fraction of positronium of (96.7+/-2.2)%. To derive in what phase of the interstellar medium positrons annihilate, we have fitted annihilation models calculated for each phase to the data. We have found that 49^+2_-23 % of the annihilation emission comes from the warm neutral phase and 51^+3_-2% from the warm ionized phase. While we may not exclude that less than 23% of the emission might come from cold gas, we have constrained the fraction of annihilation emission from molecular clouds and hot gas to be less than 8% and 0.5%, respectively. We have compared our knowledge of the interstellar medium in the bulge (size, density, and filling factor of each phase) and the propagation of positrons with our results and found that they are in good agreement if the sources are diffusively distributed and if the initial kinetic energy of positrons is lower than a few MeV. Despite its large filling factor, the lack of annihilation emission from the hot gas is due to its low density, which allows positrons to escape this phase. ------------------------------------------------------------------------ Email : abroderick@cfa.harvard.edu Title : Frequency-Dependent Shift in the Image Centroid of the Black Hole at the Galactic Center as a Test of General Relativity Author(s): Avery E. Broderick and Abraham Loeb Institute: (1) Institute for Theory and Computation, Harvard University, CfA, MS 51, 60 Garden Street, Cambridge, MA 02138, USA Paper : ApJL, submitted EPrint : astro-ph/0508386 Abstract: The inferred black hole in the Galactic center spans the largest angle on the sky among all known black holes. Forthcoming observational programs plan to localize or potentially resolve the image of Sgr A* to an exquisite precision, comparable to the scale of the black hole horizon. Here we show that the location of the image centroid of Sgr A* should depend on observing frequency because of relativistic and radiative transfer effects. The same effects introduce a generic dependence of the source polarization on frequency. Future detection of the predicted centroid shift and the polarization dependence on frequency can be used to determine the unknown black hole spin and verify the validity of General Relativity. ------------------------------------------------------------------------ Email : abroderick@cfa.harvard.edu Title : Imaging Optically-Thin Hot Spots Near the Black Hole Horizon of Sgr A* at Radio and Near Infrared Wavelengths Author(s): Avery E. Broderick \& Abraham Loeb Paper : MNRAS, submitted EPrint : astro-ph/0509237 Abstract: Sub milli-arcsecond astrometry and imaging of the black hole Sgr A* at the Galactic centre may become possible in the near future at infrared and sub-millimetre wavelengths. Motivated by observations of short-term infrared and X-ray variability of Sgr A*, in a previous paper we computed the expected images and light curves, including polarization, associated with an compact emission region orbiting the central black hole. We extend this work, using a more realistic hot-spot model and including the effects of opacity in the underlying accretion flow. We find that at infrared wavelengths the qualitative features identified by our earlier work are present, namely it is possible to extract the black hole mass and spin from spot images and light curves of the observed flux and polarization. At radio wavelengths, disk opacity produces significant departures from the infrared behaviour, but there are still generic signatures of the black hole properties. Detailed comparison of these results with future data can be used to test general relativity and to improve existing models for the accretion flow in the immediate vicinity of the black hole. ------------------------------------------------------------------------ Email : abroderick@cfa.harvard.edu Title : Imaging Bright Spots in the Acceretion Flow Near the Black Hole Horizon of Sgr A* Author(s): Avery E. Broderick \& Abraham Loeb Paper : MNRAS Sep 2005, in press EPrint : astro-ph/0506433 Abstract: Images from the vicinity of the black hole horizon at the Galactic centre (Sgr A*) could be obtained in the near future with a Very Large Baseline Array of sub-millimetre telescopes. The recently observed short-term infrared and X-ray variability of the emission from Sgr A* implies that the accretion flow near the black hole is clumpy or unsteady. We calculate the appearance of a compact emission region (bright spot) in a circular orbit around a spinning black hole as a function of orbital radius and orientation. We find that the mass and spin of the black hole can be extracted from their generic signatures on the spot image as well as on the light curves of its observed flux and polarization. The strong-field distortion remains noticeable even when the spot image is smoothed over the expected 20 mu as resolution of future sub-millimetre observations. ------------------------------------------------------------------------ Email : rmb@astro.ox.ac.uk Title : An Infrared Imaging Survey of the Faint Chandra Sources near the Galactic Centre Author(s): R.M. Bandyopadhyay^1, J.C.A. Miller-Jones^1,2, K.M. Blundell^1, F.E. Bauer^3, Ph. Podsiadlowski^1, A.J. Gosling^1, Q.D. Wang^4, E. Pfahl^5, S. Rappaport^6 Institute: ^1Department of Astrophysics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK, ^2Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, Amsterdam, The Netherlands, 1098 SJ, ^3Columbia Astrophysics Laboratory, Columbia University, 550 W. 120th St., New York, NY 10027, USA, ^4Dept. of Astronomy, University of Massachusetts, Amherst, MA 01003, USA, ^5Dept. of Astronomy, University of Virginia, Charlottesville, VA 22903, USA, ^6Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA, Paper : MNRAS, 2005, in press EPrint : astro-ph/0509346 Web : http://www-astro.physics.ox.ac.uk/~rmb/rmb_mn_f.ps Abstract: We present near-IR imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra ACIS-I survey towards the Galactic Centre (GC) (Wang etal 2002). These 800 discrete sources represent an important and previously undetected population within the Galaxy. From our VLT observations of 77 X-ray sources, we identify candidate K-band counterparts to 75% of the Chandra sources in our sample. The near-IR magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely to be accreting binaries at or near the GC. ------------------------------------------------------------------------ Email : yann.clenet@obspm.fr Title : A dual emission mechanism in Sgr A*/L' Author(s): Y. Clenet(1) and D. Rouan(1) and D. Gratadour(1) and O. Marco(2) and P. Lena(1) and N. Ageorges(2) and E. Gendron(1) Institute: (1) Observatoire de Paris, LESIA, 5 place Jules Janssen, F-92195 Meudon Cedex, France and European Southern Observatory (ESO), Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile Paper : A&A, 2005, 439, L9 EPrint : astro-ph/0507088 Abstract: We have collected in 2004 adaptive optics corrected L' images of the Galactic Center region with NAOS-CONICA at VLT. A strong variability was observed as well as a correlation between the photocenter positions and fluxes of the L'-band counterpart of Sgr A*. It is interpreted as the combined emission of a point-like flaring source at the position of Sgr A*/IR itself and an extended dust structure, 75 mas south west of Sgr A*/IR, which we name Sgr A*-f. We examine the different possible mechanisms to explain this dual Sgr A* L' emission and conclude it is likely a flaring emission associated to energetic events in the close environment of the black hole plus a quiescent emission resulting from the collision of Sgr A*-f by a jet from Sgr A*. ------------------------------------------------------------------------ (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 ========================================================================