======================================================================== 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. 18 Feb 26, 2010 Recently submitted papers: -------------------------- 1) Locating the VHE source in the Galactic Centre with milli-arcsecond accuracy (Abramowski et al., MNRAS) 2) Nuclear interaction gamma-ray lines from the Galactic center region (Dogiel et al., A&A) 3) Expanded VLA Detection of 36.2 GHz Class I Methanol Masers in Sagittarius A (Sjouwerman et al., ApJLett) 4) Discovery of a Luminous Blue Variable with an Ejection Nebula Near the Quintuplet Cluster (Mauerhan et al., ApJL) ------------------------------------------------------------------------ Email : attila.abramowski@googlemail.com Title : Locating the VHE source in the Galactic Centre with milli-arcsecond accuracy Author(s): A. Abramowski(1), S. Gillessen(2), D. Horns(1), H.-S. Zechlin(1) Institute: (1) Department of Physics, University of Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany (2) MPI for Extraterrestrial Physics, Giessenbachstrasse, D-85748 Garching, Germany Paper : MNRAS, Feb.2010, 2010MNRAS.402.1342A Abstract: Very high-energy gamma -rays (VHE; E>100 GeV) have been detected from the direction of the Galactic Centre up to energies E>10 TeV. Up to now, the origin of this emission is unknown due to the limited positional accuracy of the observing instruments. One of the counterpart candidates is the super-massive black hole (SMBH) Sgr A^*. If the VHE emission is produced within 10^15 cm 1000 r_G (r_G=G M/c^2 is the Schwarzschild radius) of the SMBH, a decrease of the VHE photon flux in the energy range 100-300 GeV is expected whenever an early type or giant star approaches the line of sight within milli-arcseconds (mas). The dimming of the flux is due to absorption by pair-production of the VHE photons in the soft photon field of the star, an effect we refer to as pair-production eclipse (PPE). Based upon the currently known orbits of stars in the inner arcsecond of the Galaxy we find that PPEs lead to a systematic dimming in the 100-300 GeV band at the level of a few per cent and lasts for several weeks. Since the PPE affects only a narrow energy band and is well correlated with the passage of the star, it can be clearly discriminated against other systematic or even source-intrinsic effects. While the effect is too small to be observable with the current generation of VHE detectors, upcoming high count-rate experiments like the Cherenkov telescope array (CTA) will be sufficiently sensitive. Measuring the temporal signature of the PPE bears the potential to locate the position and size of the VHE emitting region within the inner 1000 r_G or in the case of a non-detection exclude the immediate environment of the SMBH as the site of gamma -ray production altogether. ------------------------------------------------------------------------ Email : cmko@gm.astro.ncu.edu.tw Title : Nuclear interaction gamma-ray lines from the Galactic center region Author(s): V. A. Dogiel(1,2), V. Tatischeff(3), K. S. Cheng(4), D. O. Chernyshov(1,2,4,5), C. M. Ko(2), W. H. Ip(2) Institute: (1) I.E.Tamm Theoretical Physics Division of P.N.Lebedev Institute, Leninskii pr, 53, 119991 Moscow, Russia (2) Institute of Astronomy, National Central University, Jhongli 320, Taiwan (3) Centre de Spectrometrie Nucleaire et de Spectrometrie de Masse, IN2P3-CNRS and Univ Paris-Sud, F-91405 Orsay Campus, France (4) Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, China (5) Moscow Institute of Physics and Technology, Institutskii lane, 141700 Moscow Region, Dolgoprudnii, Russia Paper : A&A 508, 1 (2009) EPrint : 0909.2110 Abstract: The accretion of stars onto the central supermassive black hole at the center of the Milky Way is predicted to generate large fluxes of subrelativistic ions in the Galactic center region. We analyze the intensity, shape, and spatial distribution of de-excitation gamma-ray lines produced by nuclear interactions of these energetic particles with the ambient medium. We first estimated the amount and mean kinetic energy of particles released from the central black hole during star disruption. We then calculated the energy and spatial distributions of these particles in the Galactic center region from a kinetic equation. These particle distributions we are then used to derive the characteristics of the main nuclear interaction gamma-ray lines. Because the time period of star capture by the supermassive black hole is expected to be shorter than the lifetime of the ejected fast particles against Coulomb losses, the gamma-ray emission is predicted to be stationary. We find that the nuclear de-excitation lines should be emitted from a region with a maximum 5^o angular radius. The total gamma-ray line flux below 8 MeV is calculated to be 10^-4 photons cm^-2 s^-1. The most promising lines for detection are those at 4.44 and 6.2 MeV, with a predicted flux in each line of 10^-5 photons cm^-2 s^-1. Unfortunately, it is unlikely that this emission can be detected with the INTEGRAL observatory. But the predicted line intensities appear to be within reach of future gamma-ray space instruments. A future detection of de-excitation gamma-ray lines from the Galactic center region would provide unique information on the high-energy processes induced by the central supermassive black hole and the physical conditions of the emitting region. ------------------------------------------------------------------------ Email : lsjouwer@nrao.edu Title : Expanded VLA Detection of 36.2 GHz Class I Methanol Masers in Sagittarius A Author(s): Lorant O. Sjouwerman(1), Ylva M. Pihlstroem(2), Vincent L. Fish(3) Institute: (1) National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801 (2) Department of Physics and Astronomy, University of New Mexico, 800 Yale Boulevard NE, Albuquerque NM 87131 (3) Massachusetts Institute of Technology, Haystack Observatory, Route 40, Westford, MA 01886 Paper : ApJLett, January 2010 Vol 710 L111 Abstract: We report on the interferometric detection of 36.2 GHz Class I methanol emission with the new 27-40 GHz Ka band receivers available on the Expanded Very Large Array (EVLA). The brightness temperatures of the interferometric 36 GHz detections unambiguously indicate for the first time that the emission is maser emission. The 36 GHz methanol masers are not co-spatial with 1720 MHz OH masers, indicating that the two species trace different shocks. The 36 GHz and 44 GHz methanol masers, which both are collisionally pumped, do not necessarily co-exist and may trace different methanol gas. The methanol masers seem correlated with NH_3(3,3) density peaks. We favor an explanation in which the 36 GHz Class I methanol masers outline regions of cloud-cloud collisions, perhaps just before the onset of the formation of individual massive stars. The transition of the Very Large Array (VLA) to the EVLA is well under way, and these detections demonstrate the bright future of this completely renewed instrument. ------------------------------------------------------------------------ Email : mauerhan@ipac.caltech.edu Title : Discovery of a Luminous Blue Variable with an Ejection Nebula Near the Quintuplet Cluster Author(s): J.C. Mauerhan(1), M.R. Morris(2), A. Cotera(3), H. Dong(4), Q.D. Wang(4), S.R. Stolovy(1), C. Lang(5), I.S. Glass(6) Institute: (1) Spitzer Science Center, California Institute of Technology, Mail Code 220-6, 1200 East California Blvd., Pasadena, CA 91125, USA; mauerhan@ipac.caltech.edu (2) Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA (3) SETI Institute, 515 North Whisman Road, Mountain View, CA 94043, USA (4) Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA (5) Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52245, USA (6) South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa Paper : ApJL, February 2010, accepted EPrint : 1002.3379 Abstract: We report the discovery of a Luminous Blue Variable (LBV) lying 7 pc in projection from the Quintuplet cluster. This source, which we call LBV G0.120-0.048, was selected for spectroscopy owing to its detection as a strong source of Paschen-alpha excess in a recent narrow-band imaging survey of the Galactic center region with HST/NICMOS. The K-band spectrum is similar to that of the Pistol Star and other known LBVs. The new LBV was previously cataloged as a photometric variable star, exhibiting brightness fluctuations of up to 1 magnitude between 1994 and 1997, with significant variability also occurring on month-to-month time scales. The luminosity of LBV G0.120-0.048, as derived from 2MASS photometry, is approximately equivalent to that of the Pistol Star. However, the time-averaged brightness of LBV G0.120-0.048 between 1994 and 1997 exceeded that of the Pistol Star; LBV G0.120-0.048 also suffers more extinction, which suggests that it was intrinsically more luminous in the infrared than the Pistol Star between 1994 and 1997. Paschen-alpha images reveal a thin circular nebula centered on LBV G0.120-0.048 with a physical radius of 0.8 pc. We suggest that this nebula is a shell of ejected material launched from a discrete eruption that occurred between 5000 and 10,000 years ago. Because of the very short amount of time that evolved massive stars spend in the LBV phase, and the close proximity of LBV G0.120-0.048 to the Quintuplet cluster, we suggest that this object might be coeval with the cluster, and may have once resided within it. ------------------------------------------------------------------------ (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 ========================================================================