======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 26, No. 1 Dec 23, 2006 Recently submitted papers: -------------------------- 1) THE ARCHES CLUSTER MASS FUNCTION (Kim et al., ApJ) 2) The possibility of detecting Sagittarius A* at 8.6 micron from sensitive imaging of the Galactic center (Schoedel et al., A&AL) 3) Flaring Activity of Sgr A*: Expanding Hot Blobs (Yusef-Zadeh et al.) ------------------------------------------------------------------------ Email : figer@cis.rit.edu Title : THE ARCHES CLUSTER MASS FUNCTION Author(s): Sungsoo S. Kim(1), Donald F. Figer(2), Rolf P. Kudritzki(3), and F. Najarro(4) Institute: (1) Dept. of Astronomy \& Space Science, Kyung Hee University, Kyungki-do 446-701, Korea (2) Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623-5604 (3) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (4) Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientificas, Calle Serrano 121, 28006 Madrid, Spain Paper : ApJ, 2006, 653, L113 EPrint : astro-ph/0611377 Abstract: We have analyzed H and K_s-band images of the Arches cluster obtained using the NIRC2 instrument on Keck with the laser guide star adaptive optics (LGS AO) system. With the help of the LGS AO system, we were able to obtain the deepest ever photometry for this cluster and its neighborhood, and derive the background-subtracted present-day mass function (PDMF) down to 1.3 \msun for the 5 ''-9'' annulus of the cluster. We find that the previously reported turnover at 6 \msun is simply due to a local bump in the mass function (MF), and that the MF continues to increase down to our 50 % completeness limit (1.3 \msun) with a power-law exponent of \Gamma = -0.91 for the mass range of 1.3 < M/\msun < 50. Our numerical calculations for the evolution of the Arches cluster show that the \Gamma values for our annulus increase by 0.1-0.2 during the lifetime of the cluster, and thus suggest that the Arches cluster initially had \Gamma of -1.0 -1.1, which is only slightly shallower than the Salpeter value. ------------------------------------------------------------------------ Email : rainer@ph1.uni-koeln.de Title : The possibility of detecting Sagittarius A* at 8.6 micron from sensitive imaging of the Galactic center Author(s): R. Schoedel (1) and A. Eckart (1,2) and K. Mu\vzi\'c (1) and L. Meyer (1) and T. Viehmann (1) and G. C. Bower (3) Institute: (1) I.Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str.1, D-50937 Koeln, and Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, D-53121 Bonn and Astronomy Department and Radio Astronomy Laboratory, University of California, Berkeley. CA 94720 Paper : A&A Letter, in press EPrint : astro-ph/0611472 Web : http://www.arxiv.org/abs/astro-ph/0611472 Abstract: Sagittarius A* (Sgr A*) at the center of the Milky Way is a black hole accreting at extremely sub-Eddington rates. Measurements of its emission in the infrared and X-ray domains are difficult due to its faintness and high variability. The Galactic center was observed at 8.6 micron in order to detect a mid-infrared (MIR) counterpart to Sgr A*, parallel to NIR observations. The goal was to set constraints on possible emission mechanisms. Imaging data were acquired with the adaptive-optics assisted NIR instrument NACO and the MIR instrument VISIR at the ESO VLT. We present MIR imaging data of an unprecedented quality in terms of spatial resolution and sensitivity. An extended ridge of emission is found to be present in the immediate vicinity of Sgr A* thereby rendering any detection of a point source difficult. No MIR point source related to Sgr A* was detected during the observations. We derive a tight upper limit of 22+/-14 mJy (dereddened) on any possible point source present during the observations in the night of 4/5 June 2006. The absence of a flare in simultaneous observations at 2.2 micron and the low limits on any possible variability in the MIR strongly suggest that Sgr A* was in a quasi-quiescent state during this night. During the night from 5 to 6 June 2006, Sgr A* was found to be variable on a low level at 3.8 micron . No point source at 8.6 micron was detected during the simultaneous MIR observations. Due to the poorer atmospheric conditions, a higher upper limit of 60+/-30 mJy was found for Sgr A* at 8.6 micron during the second night. The observations are consistent with theoretical predictions. If the published models are correct, the observations demonstrate successfully that a 8.6 micron counterpart of Sgr A* can be easily detected in its flaring state. Spectral indices derived from simultaneous observations of flaring emission from Sgr A* at NIR and MIR wavelengths will enable us to distinguish between different kinds of flare models. ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : Flaring Activity of Sgr A*: Expanding Hot Blobs Author(s): F. Yusef-Zadeh, M. Wardle, D. A. Roberts, C. O. Heinke, C. D. Dowell, W. D. Cotton, G. C. Bower \& F. K. Baganoff Paper : IV Microquasar Workshop, astro-ph/0612156 EPrint : astro-ph/0612156 Abstract: Sgr A* is considered to be a massive black hole at the Galactic center and is known to be variable in radio, millimeter, near-IR and X-rays. Recent multi-wavelength observing campaigns show a simultaneous X-ray and near-IR flare, as well as sub-millimeter and near-IR flares from Sgr A*. The flare activity is thought to be arising from the innermost region of Sgr A*. We have recently argued that the duration of flares in near-IR and submillimeter wavelengths implies that the burst of emission expands and cools on a dynamical time scale before the flares leave Sgr A*. The detection of radio flares with a time delay in the range of 20 and 40 minutes between 7 and 12mm peak emission implies adiabatic expansion of a uniform, spherical hot blob due to flare activity. We suspect that this simple outflow picture shows some of the characteristics that are known to take place in microquasars, thus we may learn much from comparative study of Sgr A* and its environment vs. microquasars. ------------------------------------------------------------------------ (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 ========================================================================