======================================================================== 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. 16 Dec 10, 2009 Recently submitted papers: -------------------------- 1) A time-dependent jet model for the emission from Sagittarius A* (Maitra et al., A&A) 2) A near-infrared survey of Miras and the distance to the Galactic Centre (Noriyuki et al., MNRAS) 3) Near-Infrared Counterparts to Chandra X-ray Sources Toward the Galactic Center. II. Discovery of Wolf-Rayet Stars and O Supergiants (Mauerhan et al., ApJ) 4) Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster (Schoedel et al., A&A) 5) Accurate photometry with adaptive optics in the presence of anisoplanatic effects with a sparsely sampled PSF (Schoedel, A&A) 6) Near infrared flares of Sagittarius A* - Importance of near infrared polarimetry (Zamaninasab et al., A&A) ------------------------------------------------------------------------ Email : dmaitra@umich.edu Title : A time-dependent jet model for the emission from Sagittarius A* Author(s): Dipankar Maitra(1), Sera Markoff(1) and Heino Falcke(2,3) Institute: (1) Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands (2) Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands (3) ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands Paper : A&A Lett., Dec 2009, in EPrint : 0911.0739 Abstract: The source of emission from Sgr A*, the supermassive black hole at the Galactic Center, is still unknown. Flares and data from multiwavelength campaigns provide important clues about the nature of Sgr A* itself. Here we attempt to constrain the physical origin of the broadband emission and the radio flares from Sgr A*. We developed a time-dependent jet model, which for the first time allows one to compare the model predictions with flare data from Sgr A*. Taking into account relevant cooling mechanisms, we calculate the frequency-dependent time lags and photosphere size expected in the jet model. The predicted lags and sizes are then compared with recent observations. Both the observed time lags and size-frequency relationships are reproduced well by the model. The combined timing and structural information strongly constrain the speed of the outflow to be mildly relativistic, and the radio flares are likely to be caused by a transient increase in the matter channelled into the jets. The model also predicts light curves and structural information at other wavelengths which could be tested by observations in the near future. We show that a time-dependent relativistic jet model can successfully reproduce: (1) the quiescent broadband spectral energy distribution of Sgr A*, (2) the observed 22 and 43 GHz light curve morphologies and time lags, and (3) the frequency-size relationship. The results suggest that the observed emission at radio frequencies from Sgr A* is most easily explained by a stratified, optically thick, mildly relativistic jet outflow. Frequency-dependent measurements of time-lags and intrinsic source size provide strong constraints on the bulk motion of the jet plasma. ------------------------------------------------------------------------ Email : matsu550107@yahoo.co.jp Title : A near-infrared survey of Miras and the distance to the Galactic Centre Author(s): Noriyuki Matsunaga,^1,2 Takahiro Kawadu,^2 Shogo Nishiyama,^2 Takahiro Nagayama,^2,3 Hirofumi Hatano,^3 Motohide Tamura,^4 Ian S. Glass^5 and Tetsuya Nagata^2 Institute: (1) Institute of Astronomy, the University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan, (2) Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan, (3) Department of Astrophysics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan, (4) National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan, (5) South African Astronomical Observatory, PO Box 9, 7935, Observatory, South Africa, Paper : MNRAS, Nov 2009, Volume 399, pp. 1709-1729 Abstract: We report the results of a near-infrared survey for long-period variables in a field of view of 20 * 30 arcmin^2 towards the Galactic Centre (GC). We have detected 1364 variables, of which 348 are identified with those reported in Glass et al. We present a catalogue and photometric measurements for the detected variables and discuss their nature. We also establish a method for the simultaneous estimation of distances and extinctions using the period-luminosity relations for the JHKs bands. Our method is applicable to Miras with periods in the range 100-350 d and mean magnitudes available in two or more filter bands. While J band means are often unavailable for our objects because of the large extinction, we estimated distances and extinctions for 143 Miras whose H- and K_s-band mean magnitudes are obtained. We find that most are located at the same distance to within our accuracy. Assuming that the barycentre of these Miras corresponds to the GC, we estimate its distance modulus to be 14.58 +/-0.02 (stat.) +/-0.11 (syst.) mag, corresponding to 8.24 +/-0.08 (stat.) +/-0.42 (syst.) kpc. We have assumed the distance modulus to the Large Magellanic Cloud to be 18.45 mag, and the uncertainty in this quantity is included in the above systematic error. We also discuss the large and highly variable extinction. Its value ranges from 1.5 mag to larger than 4 mag in except towards the thicker dark nebulae and it varies in a complicated way with the line of sight. We have identified mid-infrared counterparts in the Spitzer/IRAC catalogue of Ramirez et al. for most of our variable and find that they follow rather narrow period-luminosity relations in the 3.6-8.0 micron wavelength range. ------------------------------------------------------------------------ Email : mauerhan@ipac.caltech.edu Title : Near-Infrared Counterparts to Chandra X-ray Sources Toward the Galactic Center. II. Discovery of Wolf-Rayet Stars and O Supergiants Author(s): J. C. Mauerhan(1), M. P. Muno(2), M. R. Morris(3), S. R. Stolovy(1), A. Cotera(4) Institute: (1) Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125 (2) Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 (3) Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547 (4) SETI Institute, 515 N. Whisman Rd., Mountain View, CA Paper : ApJ, December 2009, accepted EPrint : 0912.1055 Abstract: We present new identifications of infrared counterparts to the population of hard X-ray sources near the Galactic center detected by the Chandra X-ray Observatory. We have spectroscopically confirmed 16 new massive stellar counterparts to the X-ray population, including nitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars, and O supergiants. These discoveries increase the total sample of massive stellar X-ray sources in the Galactic center region to 30 (possibly 31). For the majority of these sources, the X-ray photometry is consistent with thermal emission from plasma having temperatures in the range of kT=1-8 keV or non-thermal emission having power-law indices in the range of -1<=sssim\Gamma<=sssim3, and X-ray luminosities in the range of L_X 10^32- 10^34 erg s^-1 (0.5-8.0 keV). Several sources have exhibited X-ray variability of several factors between observations. These X-ray properties are not a ubiquitous feature of single massive stars but are typical of massive binaries, in which the high-energy emission is generated by the collision of supersonic winds, or by accretion onto a compact companion. However, without direct evidence for companions, the possibility of intrinsic hard X- ray generation from single stars cannot be completely ruled out. The spectral energy distributions of these sources exhibit significant infrared excess, attributable to free-free emission from ionized stellar winds, supplemented by hot dust emission in the case of the WC stars. With the exception of one object located near the outer regions of the Quintuplet cluster, most of the new stars appear isolated or in loose associations. Seven hydrogen-rich WN and O stars are concentrated near the Sagittarius B HII region, while other similar stars and more highly evolved hydrogen-poor WN and WC stars lie scattered within 50 pc, in projection, of Sagitarrius A West. We discuss various mechanisms capable of generating the observed X-rays and the implications these stars have for massive star formation in the Galaxy's Central Molecular Zone. ------------------------------------------------------------------------ Email : rainer@iaa.es Title : Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster Author(s): R. Schoedel(1) F. Najarro(2) K. Muzic(3) A. Eckart(3) Institute: (1) Instituto de Astrofisica de Andalucia - CSIC, Glorieta de la Astronomia S/N, 18008 Granada, Spain, (2) Centro de Astrobiologia (CSIC/INTA), Instituto Nacional de Tecnica Aeroespacial, Ctra de Torrejon a Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid, Spain, (3) I. Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Strasse 77, 50937 Koeln, Germany, Paper : A&A, accepted for publication EPrint : 0912.1273 Web : http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/. Abstract: The nuclear star cluster of the Galaxy is an important template for understanding its extragalactic counterparts, which can currently not be resolved into individual stars. Important drawbacks of observations of the Galactic center are, however, the presence of strong and spatially highly variable interstellar extinction and extreme crowding of the sources, which makes the use of adaptive optics techniques necessary. Both points pose serious obstacles to precise photometry that is needed for analyzing the stellar population. The aims of this work are to provide accurate photometry in multiple near-infrared broadband filters, to determine the power-law index of the extinction-law toward the central parsec of the Galaxy, to provide measurements of the absolute extinction toward the Galactic center, and finally to measure the spatial variability of extinction on arcsecond scales. We use observations of the central parsec of the Milky Way that were obtained with the near-infrared camera and adaptive optics system NAOS/CONICA at the ESO VLT unit telescope 4. The photometric method takes into account anisoplanatic effects and limits the corresponding systematic uncertainties to <=2%. Absolute values for the extinction in the H, Ks, and L'-bands as well as of the power-law indices of the H to Ks and Ks to L' extinction-laws are measured based on the well-known properties of red clump stars. Extinction maps are derived based on H-Ks and Ks-L' colors. We present Ks-band photometry for 7700 stars, and additionally photometry for stars detected in the H and/or L'-bands. From a number of recently published values we compute a mean distance of the Galactic center of R_0=8.03+/-0.15 kpc, which has an uncertainty of just 2%. Based on this R_0 and on the RC method, we derive absolute mean extinction values toward the central parsec of the Galaxy of A_H=4.48+/-0.13 mag, A_Ks=2.54+/-0.12 mag, and A_L'=1.27+/-0.18 mag. We estimate values of the power-law indices of the extinction-law of alpha _H-Ks=2.21+/-0.24 and alpha _Ks-L'=1.34+/-0.29. A Ks-band extinction map for the Galactic center is computed based on this extinction law and on stellar H-Ks colors. Both its statistical and systematic uncertainties are estimated to be <10%. Extinction in this map derived from stellar color excesses is found to vary on arcsecond scales, with a mean value of A_Ks=2.74+/-0.30 mag. Mean extinction values in a circular region with 0.5" radius centered on Sagittarius A* are A_H, SgrA*=4.35+/-0.12, A_Ks, SgrA*=2.46+/-0.03, and A_L', SgrA*=1.23+/-0.08. ------------------------------------------------------------------------ Email : rainer@iaa.es Title : Accurate photometry with adaptive optics in the presence of anisoplanatic effects with a sparsely sampled PSF Author(s): Rainer Schoedel(1) Institute: (1) Instituto de Astrofisica de Andalucia (CSIC), C/ Camino Bajo de Huetor 50, 18008 Granada, Spain Paper : A&A, accepted for publication EPrint : 0912.1219 Abstract: Anisoplanatic effects can cause significant systematic photometric uncertainty in the analysis of dense stellar fields observed with adaptive optics. Program packages have been developed for a spatially variable PSF, but they require that a sufficient number of bright, isolated stars in the image are present to adequately sample the PSF. Imaging the Galactic center is particularly challenging. We present two ways of dealing with spatially variable PSFs when only one or very few suitable PSF reference stars are present in the field. Local PSF fitting with the StarFinder algorithm is applied to the data. Satisfying results can be found in two ways: (a) creating local PSFs by merging locally extracted PSF cores with the PSF wings estimated from the brightest star in the field; (b) Wiener deconvolution of the image with the PSF estimated from the brightest star in the field and subsequent estimation of local PSFs on the deconvolved image. The methodology is tested on real, and on artificial images. The method involving Wiener deconvolution of the image prior to local PSF extraction and fitting gives excellent results. It limits systematic effects to 2-5% in point source photometry and 10% in diffuse emission on fields-of-view as large as 28"*28" and observed through the H-band filter. Particular attention is given to how deconvolution changes the noise properties of the image. It is shown that mean positions and fluxes of the stars are conserved by the deconvolution. However, the estimated uncertainties of the PSF fitting algorithm are too small if the presence of covariances is ignored in the PSF fitting as has been done here. An appropriate scaling factor can, however, be determined from simulated images or by comparing measurements on independent data sets. We present ways of obtaining reliable photometry and astrometry from images with a spatially variable, but poorly sampled PSF, where standard techniques may not work. ------------------------------------------------------------------------ Email : zamani@ph1.uni-koeln.de Title : Near infrared flares of Sagittarius A* - Importance of near infrared polarimetry Author(s): M. Zamaninasab(1,2), A. Eckart(1,2), G. Witzel(1), M. Dovciak(3), V. Karas(3), R. Schoedel(4), R. Giessuebel(1,2), M. Bremer(1), M. Garcia-Marin(1), D. Kunneriath(1,2), K. Muvzic(1), S. Nishiyama(5), N. Sabha(1), C. Straubmeier(1) and A. Zensus(2,1) 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 (3) Astronomical Institute, Academy of Sciences, Bo\vcni II, CZ-14131 Prague, Czech Republic (4) Instituto de Astrofisica de Andalucia (IAA)-CSIC, Camino Bajo de Huetor 50, 18008 Granada, Spain (5) Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan Paper : A&A, 2009, in press EPrint : 0911.4659 Abstract: We report on the results of new simulations of near-infrared (NIR) observations of the Sagittarius A* (Sgr A*) counterpart associated with the super-massive black hole at the Galactic Center. Our goal is to investigate and understand the physical processes behind the variability associated with the NIR flaring emission from Sgr A*. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and CIAO NIR camera on the Subaru telescope (13 June 2004, 30 July 2005, 1 June 2006, 15 May 2007, 17 May 2007 and 28 May 2008). We used a model of synchrotron emission from relativistic electrons in the inner parts of an accretion disk. The relativistic simulations have been carried out using the Karas-Yaqoob (KY) ray-tracing code. We probe the existence of a correlation between the modulations of the observed flux density light curves and changes in polarimetric data. Furthermore, we confirm that the same correlation is also predicted by the hot spot model. Correlations between intensity and polarimetric parameters of the observed light curves as well as a comparison of predicted and observed light curve features through a pattern recognition algorithm result in the detection of a signature of orbiting matter under the influence of strong gravity. This pattern is detected statistically significant against randomly polarized red noise. Expected results from future observations of VLT interferometry like GRAVITY experiment are also discussed. The observed correlations between flux modulations and changes in linear polarization degree and angle can be a sign that the NIR flares have properties that are not expected from purely random red-noise. We find that the geometric shape of the emission region plays a major role in the predictions of the model. From fully relativistic simulations of a spiral shape emitting region, we conclude that the observed swings of the polarization angle during NIR flares support the idea of compact orbiting spots instead of extended patterns. The effects of gravitational shearing, fast synchrotron cooling of the components and confusion from a variable accretion disk have been taken into account. Simulated centroids of NIR images lead us to the conclusion that a clear observation of the position wander of the center of NIR images with future infrared interferometers will prove the existence of orbiting hot spots in the vicinity of our Galactic super-massive black hole. ------------------------------------------------------------------------ (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 ========================================================================