======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 23, No. 5 Feb 2, 2006 Recently submitted papers: -------------------------- 1) A Chandra Catalog of X-ray Sources in the Central 150 pc of the Galaxy (Muno et al., ApJ) 2) Isolated, Massive Supergiants near the Galactic Center (Muno et al., ApJ) 3) QPOs in microquasars and Sgr A^*: measuring the black hole spin (Toeroek, Astron.Nachr.) 4) A Successful Targeted Search for Hypervelocity Stars (Brown et al., ApJL) ------------------------------------------------------------------------ Email : mmuno@astro.UCLA.EDU Title : A Chandra Catalog of X-ray Sources in the Central 150 pc of the Galaxy Author(s): M. P. Muno, F. E. Bauer, R. M. Bandyopadhyay, Q. D. Wang Paper : ApJ submitted EPrint : astro-ph/0601627 Abstract: We present the catalog of X-ray sources detected in a shallow Chandra survey of the inner 2 DEGree*0.8 DEGree of the Galaxy, and in two deeper observations of the Radio Arches and Sgr B2. The catalog contains 1352 objects that are highly-absorbed (N_H> 4*10^22 cm^-2) and are therefore likely to lie near the Galactic center (D 8 kpc), and 549 less-absorbed sources that lie within < 6 kc of Earth. Based on the inferred luminosities of the X-ray sources and the expected numbers of various classes of objects, we suggest that the sources with L_X < 10^33 \ergs that comprise 90% of the catalog are cataclysmic variables, and that the 100 brighter objects are accreting neutron stars and black holes, young isolated pulsars, and Wolf-Rayet and O stars in colliding-wind binaries. We find that the spatial distribution of X-ray sources matches that of the old stellar population observed in the infrared, which supports our suggestion that most of the X-ray sources are old cataclysmic variables. However, we find that there is an apparent excess of 10 bright sources in the Radio Arches region. That region is already known to be the site of recent star formation, so we suggest that the bright sources in this region are young high-mass X-ray binaries, pulsars, or WR/O star binaries. We briefly discuss some astrophysical questions that this catalog can be used to address. ------------------------------------------------------------------------ Email : mmuno@ucla.edu Title : Isolated, Massive Supergiants near the Galactic Center Author(s): M. P. Muno, G. C. Bower, A. J. Burgasser, F. K. Baganoff, M. R. Morris, W. N. Brandt Paper : ApJ, 638, 183, 2006 in press EPrint : astro-ph/0509617 Abstract: We have carried out a pilot project to assess the feasibility of using radio, infrared, and X-ray emission to identify young, massive stars located between 1 and 25 pc from the Galactic center. We first compared catalogs compiled from the Very Large Array, the Chandra X-ray Observatory, and 2MASS. We identified two massive, young stars: the previously-identified star that is associated with the radio HII region H2, and a newly-identified star that we refer to as CXOGC J174516.1-290315. The infrared spectra of both stars exhibit very strong Brackett- gamma and He I lines, and resemble those of massive supergiants that have evolved off of the main sequence, but not yet reached the Wolf-Rayet phase. We estimate that each star has a bolometric luminosity > 10^6L_o. These two stars are also associated with bright mid-infrared sources from the Midcourse Space Experiment survey, although the origin of this emission is uncertain. Likewise, the detection of these two sources in X-rays is surprising, because stars at similar evolutionary states are not uniformly bright X-ray sources. Therefore, we suggest that both stars are in binary systems that contain either OB stars whose winds collide with those of the luminous supergiants, or compact objects that are accreting from the winds of the supergiants. We also identify X-ray emission from a nitrogen-type Wolf-Rayet star and place upper limits on the X-ray luminosities of three more evolved, massive stars that previously have been identified between 1 and 25 pc from Sgr A*. Finally, we briefly discuss the implications that future searches for young stars will have for our understanding of the recent history of star formation near the Galactic center. ------------------------------------------------------------------------ Email : terek@volny.cz Title : QPOs in microquasars and Sgr A^*: measuring the black hole spin Author(s): G. Toeroek Institute: Institute of Physics, Faculty of Philosophy and Science, Silesian University in Opava, Bezru\v covo nam. 13, 746 01 Opava, Czech Republic and Nordita, Blegdamsvej 17, 2100 Copenhagen, Denmark Paper : Astron.Nachr., 9 (2005) 856-860 EPrint : astro-ph/0510669 Web : http://xxx.lanl.gov/ps/astro-ph/0510669 Abstract: In all four microquasars which show double peak kHz QPOs, the ratio of the two frequencies is 3:2. This strongly supports the suggestion that twin peak kHz QPOs are due to a resonance between some modes of accretion disk oscillations. Here, we stress that fits to observations of the hypothetical resonances between vertical and radial epicyclic frequencies (particularly of the parametric resonance) give an accurate estimate of the spin for the three microquasars with known mass. Measurement of double peak QPOs frequencies in the Galaxy centre seems also to be consistent with the 3:2 ratio established by previous observations in microquasars, however the Sgr A* data are rather difficult for the same exact analysis. If confirmed, the 3:2 ratio of double peak QPOs in Sgr A^* would be of a fundamental importance for the black hole accretion theory and the precise measurement could help to solve the question of QPOs nature. ------------------------------------------------------------------------ Email : wbrown@cfa.harvard.edu Title : A Successful Targeted Search for Hypervelocity Stars Author(s): Warren R. Brown, Margaret J. Geller, Scott J. Kenyon, Michael J. Kurtz Institute: Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 Paper : ApJ Letters, Jan 2006, submitted EPrint : astro-ph/0601580 Abstract: Hypervelocity stars (HVSs) travel with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Following the discovery of the first HVS by Brown and collaborators, we have undertaken a dedicated survey for more HVSs in the Galactic halo and present here the resulting discovery of two new HVSs: SDSS J091301.0+305120 and SDSS J091759.5+672238, traveling with Galactic rest-frame velocities at least +558+/-12 and +638+/-12 km s^-1, respectively. Assuming the HVSs are B8 main sequence stars, they are at distances 75 and 55 kpc, respectively, and have travel times from the Galactic Center consistent with their lifetimes. The existence of two B8 HVSs in our 1900 deg^2 survey, combined with the Yu \& Tremaine HVS rate estimates, is consistent with HVSs drawn from a standard initial mass function but inconsistent with HVS drawn from a truncated mass function like the one in the top-heavy Arches cluster. The travel times of the five currently known HVSs provide no evidence for a burst of HVSs from a major in-fall event at the Galactic Center in the last 160 Myr. ------------------------------------------------------------------------ (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 ========================================================================