======================================================================== 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. 7 Jun 16, 2006 Recently submitted papers: -------------------------- 1) Coupling the dynamics and the molecular chemistry in the Galactic center (Rodriguez-Fernandez et al., A&A) 2) Unveiling the nature of the highly absorbed X-ray source SAX J1748.2-2808 with XMM-Newton (Sidoli et al., A&A) 3) Flaring Activity of Sgr A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma (Yusef-Zadeh et al., ApJ) ------------------------------------------------------------------------ Email : nemesio.rodriguez@obs.u-bordeaux1.fr Title : Coupling the dynamics and the molecular chemistry in the Galactic center Author(s): N. J. Rodriguez-Fernandez (1,2) and F. Combes (3) and J. Martin-Pintado(4) and T. L. Wilson(5) and A. Apponi(6) Institute: (1) Observatoire de Bordeaux, L3AB (UMR 5804)/OASU, CNRS/Universite Bordeaux 1, BP 89, 2 rue de l'Observatoire, 33270 Floirac, France, (2) Universite Denis Diderot (Paris VII) \& Observatoire de Paris, 61 Av de l'Observatoire, 75014 Paris, France (3) LERMA, Observatoire de Paris, 61 Av de l'Observatoire, 75014 Paris, France, (4) DAMIR, IEM, CSIC, Serrano 121, Madrid, Spain, (5) ESO, Karl-Schwarzschild-Str. 2, D-85748 Garching bei Muenchen, Germany, (6) Steward Observatory, University of Arizona, Tucson, AZ 85721, USA Paper : A&A in press EPrint : astro-ph/0603851 Abstract: Most of the Galactic center (GC) gas moves in nearly circular orbits in a nuclear ring (hereafter the Galactic center ring, GCR). This is the case of cloud complexes such as Sgr A or Sgr B, where the gas is dense, warm and exhibits a rich molecular chemistry. The origin of these properties is thought to be shocks, in particular due to the large scale dynamics of the Galaxy. In addition, there are gas clouds moving in highly non-circular orbits known from observations of low density tracers such as CO(1-0). The physical conditions of the clouds moving with non-circular velocities are not well known. We have studied the physical conditions of the gas in non-circular orbits to better understand the origin of the unusual physical conditions of the GC molecular gas and the possible effect of the large scale dynamics on these physical conditions. Using published CO(1-0) data, we have selected a set of clouds belonging to all the kinematical components seen in the longitude-velocity diagram of the GC. We have carried out a survey of dense gas in all the components using the J=2-1 lines of CS and SiO as tracers of high density gas and shock chemistry. We have detected CS and SiO emission in all the kinematical components. The gas density and the SiO abundance of the clouds in non-circular orbits are similar to those in the GCR. Therefore, in all the kinematical components there are dense clouds that can withstand the tidal shear. However, there is no evidence of star formation outside the GCR. The high relative velocity and shear expected in the dust lanes along the bar major axis could inhibit the star formation process, as observed in other galaxies. The high SiO abundances derived in the non-circular velocity clouds are likely due to the large-scale shocks that created the dust lanes. ------------------------------------------------------------------------ Email : sidoli@iasf-milano.inaf.it Title : Unveiling the nature of the highly absorbed X-ray source SAX J1748.2-2808 with XMM-Newton Author(s): L. Sidoli^1, S. Mereghetti^1, F. Favata^2, T. Oosterbroek^3 and A.N. Parmar^2 Institute: (1) Istituto di Astrofisica Spaziale e Fisica Cosmica - Sezione di Milano - IASF/INAF, I-20133 Milano, Italy (2) Research and Scientific Support Department of ESA, ESTEC, Postbus 299, NL-2200 AG Noordwijk, The Netherlands (3) Science Payload and Advanced Concepts Office, ESA, ESTEC, Postbus 299, NL-2200 AG, Noordwijk, The Netherlands Paper : A&A, 2006, in press EPrint : astro-ph/0606307 Abstract: We report on the results of an EPIC XMM-Newton observation of the faint source SAX J1748.2-2808 and the surrounding field. This source was discovered during the BeppoSAX Galactic center survey performed in 1997-1998. A spatial analysis resulted in the detection of 31 sources within the EPIC field of view. SAX J1748.2-2808 is clearly resolved into 2 sources in EPIC images with the brighter contributing almost 80% of the 2-10 keV flux. Spectral fits to this main source are consistent with an absorbed power-law with a photon index of 1.4+/- 0.5 and absorption equivalent to 14 ^+6 _-4* 10^22 cm^-2 together with an iron line at 6.6 ^+0.2 _-0.2 keV with an equivalent width of 780 ^+620_-380 eV. The significantly better statistics of the \xmm observation, compared with \sax, allows to exclude a thermal nature for the X-ray emission. A comparison with other observations of SAXJ1748.2-2808 does not reveal any evidence for spectral or intensity long-term variability. Based on these properties we propose that the source is a low-luminosity high-mass X-ray binary located in the Galactic center region. \maketitle ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : Flaring Activity of Sgr A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma Author(s): F. Yusef-Zadeh, D. Roberts, M. Wardle, C. O. Heinke, G. C. Bower Paper : ApJ (in press) EPrint : astro-ph/0603685 Abstract: We have carried out Very Large Array (VLA) continuum observations to study the variability of Sgr A* at 43 GHz ( lambda =7mm) and 22 GHz ( lambda =13mm). A low level of flare activity has been detected with a duration of 2 hours at these frequencies, showing the peak flare emission at 43 GHz leading the 22 GHz peak flare by 20 to 40 minutes. The overall characteristics of the flare emission are interpreted in terms of the plasmon model of Van der Laan (1966) by considering the ejection and adiabatically expansion of a uniform, spherical plasma blob due to flare activity. The observed peak of the flare emission with a spectral index nu ^- alpha of alpha =1.6 is consistent with the prediction that the peak emission shifts toward lower frequencies in an adiabatically-expanding self-absorbed source. We present the expected synchrotron light curves for an expanding blob as well as the peak frequency emission as a function of the energy spectral index constrained by the available flaring measurements in near-IR, sub-millimeter, millimeter and radio wavelengths. We note that the blob model is consistent with the available measurements, however, we can not rule out the jet model of Sgr A*. If expanding material leaves the gravitational potential of Sgr A*, the total mass-loss rate of nonthermal and thermal particles is estimated to be <= 2*10^-8 M_o yr^-1. We discuss the implication of the mass-loss rate since this value matches closely with the estimated accretion rate based on polarization measurements. ------------------------------------------------------------------------ (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 ========================================================================