======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 18, No. 4 Jul 14, 2004 Recently submitted papers: -------------------------- 1) High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz (Nord et al., AJ) 2) Intra-day Variation of Sagittarius A^* at Short Millimeter Wavelengths (Miyazaki et al., ApJL) 3) Diffuse X-ray Emission in a Deep Chandra Image of the Galactic Center (Muno et al., ApJ) 4) The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center (Muno et al., ApJ) 5) Metallicity in the Galactic Center:, The Arches cluster (Najarro et al., ApJL) ------------------------------------------------------------------------ Email : Michael.Nord@nrl.navy.mil Title : High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz Author(s): Michael E. Nord^1,6, T. Joseph W. Lazio^1, Namir E. Kassim^1, S. D. Hyman^2, T.N. LaRosa^3, C. Brogan^4, N. Duric^5 Institute: (1) Naval Research Laboratory, Code 7213, Naval Research Laboratory, Washington, DC 20375-5351 (2) Department of Physics and Engineering, Sweet Briar College, Sweet Briar, VA 24595 (3) Department of Biological and Physical Sciences, Kennesaw State University, 1000 Chastain Road, Kennesaw, GA 30144 (4) Institute for Astronomy, 640 North A'ohoku Place, Hilo, HI 96720 (5) Department of Physics and Astronomy, University of New Mexico, 800 Yale Boulevard NE, Albuquerque, NM 87131 (6) Doctoral Student, University of New Mexico Paper : AJ Accepted Web : http://lwa.nrl.navy.mil/nord/AAAB.pdf Abstract: We present a wide field, sub-arcminute resolution VLA image of the Galactic Center region at 330 MHz. With a resolution of 7'' * 12'' and an RMS noise of 1.6 mJy/beam, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency. The improved sensitivity has more than tripled the census of small diameter sources in the region, has resulted in the detection of two new Non Thermal Filaments (NTFs), 18 NTF candidates, 30 pulsar candidates, reveals previously known extended sources in greater detail, and has resulted in the first detection of Sagittarius A* in this frequency range. ------------------------------------------------------------------------ Email : amiya@nro.nao.ac.jp Title : Intra-day Variation of Sagittarius A^* at Short Millimeter Wavelengths Author(s): Atsushi Miyazaki^1, Takahiro Tsutsumi^2, Masato Tsuboi^1 Institute: (1) Nobeyama Radio Observatory National Astronomical Observatory of Japan,, Minamimaki, Minamisaku, Nagano 384-1305, Japan (2) National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Paper : ApJL 2004 accepted EPrint : astro-ph/0407252 Abstract: We have performed the monitoring observations of flux density of Sagittarius A^* at short millimeter wavelengths (100 and 140 GHz bands) on seven years in the period from 1996 to 2003 using the Nobeyama Millimeter Array (NMA). We found intra-day variation of Sgr A^* in March 2000 flare. The flux density at the peak of the flares increases 100-200% at 100 GHz and 200-400% at 140 GHz (Delta S/S), respectively. The two-fold increase timescale of the flare is estimated to be about 1.5 hours at 140 GHz. The intra-day variation at mm-wavelengths has similar increase timescale as those in the X-ray and infrared flares but has smaller amplitude. This short timescale variability suggests that the physical size of the emitting region is smaller than 12 AU (150 R_s). The decay timescale of the flare was at most 24 hours. Such a light curve with rapid increase and slow decay is similar to that often observed in outburst phenomena with ejections. ------------------------------------------------------------------------ Email : mmuno@astro.ucla.edu Title : Diffuse X-ray Emission in a Deep Chandra Image of the Galactic Center Author(s): M. P. Muno^1,2, F. K. Baganoff^3, M. W. Bautz^3, E. D. Feigelson^4, G. P. Garmire^4, M. R. Morris^1, S. Park^4, G. R. Ricker^3, L. K. Townsley^4 Institute: (1) Department of Physics and Astronomy, University of California, Los Angeles, CA 90095; mmuno@astro.ucla.edu (2) Hubble Fellow (3) Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 (4) Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802 Paper : ApJ, September 2004, in press EPrint : astro-ph/0402087 Abstract: We examine the spectrum of diffuse emission detected in the 17' by 17' field around \sgrastar during 625 ks of Chandra observations. The spectrum exhibits He-like and H-like lines from Si, S, Ar, Ca, and Fe, that are consistent with originating in a two-temperature plasma, as well as a prominent low-ionization Fe K- alpha line. The cooler, kT 0.8 keV plasma differs in surface brightness across the image between (0.2 - 1.8)*10^-13 \ergcmsarcmin (observed, 2-8 keV). This soft plasma is probably heated by supernovae, along with a small contribution from the winds of massive Wolf-Rayet and O stars. The radiative cooling rate of the soft plasma within the inner 20 pc of the Galaxy could be balanced by 1% of the kinetic energy of one supernova every 3*10^5 y. The hotter, kT 8 keV component is more spatially uniform, with a surface brightness of (1.5-2.6)*10^-13 \ergcmsarcmin (observed; 2-8) keV. The intensity of the hard plasma is correlated with that of the soft, but they are probably only indirectly related, because neither supernova remnants nor WR/O stars are observed to produce thermal plasma hotter than kT 3 keV. Moreover, a kT 8 keV plasma would be too hot to be bound to the Galactic center, and therefore would form a slow wind or fountain of plasma. The energy required to sustain such a freely-expanding plasma within the inner 20 pc of the Galaxy is 10^40 \ergsec. This corresponds to the entire kinetic energy of one supernova every 3000 y, which is unreasonably high. However, alternative explanations for the kT 8 keV diffuse emission are equally unsatisfying. The hard X-rays are unlikely to result from undetected point sources, because no known population of stellar object is numerous enough to the observed surface brightness. There is also no evidence that non-thermal mechanisms for producing the hard emission are operating, as the expected shifts in the line energies and ratios from their collisional equilibrium values are not observed. We are left to conclude that either there is a significant shortcoming in our understanding of the mechanisms that heat the interstellar medium, or that a population of faint (< 10^31 erg s^-1), hard X-ray sources that are a factor of 10 more numerous than CVs remains to be discovered. ------------------------------------------------------------------------ Email : mmuno@astro.ucla.edu Title : The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center Author(s): M. P. Muno^1,2, J. S. Arabadjis^3, F. K. Baganoff^3, M. W. Bautz^3, W. N. Brandt^4, P. S. Broos^4, E. D. Feigelson^4, G. P. Garmire^4, M. R. Morris^1, G. R. Ricker^3 Institute: (1) Department of Physics and Astronomy, University of California, Los Angeles, CA 90095; mmuno@astro.ucla.edu (2) Hubble Fellow (3) Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 (4) Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802 Paper : ApJ, October 2004, in press Abstract: We examine the X-ray spectra and variability of the sample of X-ray sources with L_X 10^31 - 10^33 \ergsec identified within the inner 9' of the Galaxy by Muno et al. (2003). Very few of the sources exhibit intra-day or inter-month variations. We find that the spectra of the point sources near the Galactic center are very hard between 2-8 keV, even after accounting for absorption. When modeled as power laws the median photon index is Gamma = 0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT > 8 keV. The combined spectra of the point sources is similarly hard, with a photon index of \Gamma = 0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly-ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source, or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact be to due local absorption that partially-covers the X-ray emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short time scales or changes in their mean X-ray flux on long time scales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy. ------------------------------------------------------------------------ Email : najarro@damir.iem.csic.es Title : Metallicity in the Galactic Center:, The Arches cluster Author(s): Francisco Najarro^1, Donald F. Figer^2, D. John Hillier^3, Rolf P. Kudritzki^4 Institute: (1) Instituto de Estructura de la Materia, CSIC, Serrano 121, 29006 Madrid, Spain (2) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (3) Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pittsburgh, PA 15260 (4) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 Paper : ApJL (2004), accepted EPrint : astro-ph/0407188 Abstract: We present a quantitative spectral analysis of five very massive stars in the Arches cluster, located near the Galactic center, to determine stellar parameters, stellar wind properties and, most importantly, metallicity content. The analysis uses a new technique, presented here for the first time, and uses line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared spectra of late-type nitrogen-rich Wolf-Rayet stars and OfI+ stars in the cluster. It relies on the fact that massive stars reach a maximum nitrogen abundance that is related to initial metallicity when they are in the WNL phase. We determine the present-day nitrogen abundance of the WNL stars in the Arches cluster to be 1.6% (mass fraction) and constrain the stellar metallicity in the cluster to be solar. This result is invariant to assumptions about the mass-luminosity relationship, the mass-loss rates, and rotation speeds. In addition, from this analysis, we find the age of the Arches cluster to be 2-2.5 Myr, assuming coeval formation. ------------------------------------------------------------------------ (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 ========================================================================