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F Yusefzadeh - One of the best experts on this subject based on the ideXlab platform.
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turbulent origin of the Galactic Center magnetic field nonthermal radio filaments
The Astrophysical Journal, 2006Co-Authors: Stanislav Boldyrev, F YusefzadehAbstract:A great deal of study has been conducted over the last 20 years on the origin of the magnetic activity in the Galactic Center. One of the most popular hypotheses assumes a milligauss magnetic field with poloidal geometry, pervading the inner few hundred parsecs of the Galactic Center region. However, there is growing observational evidence for the large-scale distribution of a much weaker field of B 10 μG in this region. Here we propose that the Galactic Center magnetic field originates from turbulent activity, which is known to be extreme in the central hundred parsecs. In this picture, the spatial distribution of the magnetic field energy is highly intermittent, and the regions of strong field have filamentary structure. We propose that the observed nonthermal radio filaments appear in (or, possibly, may be identified with) such strongly magnetized regions. At the same time, the large-scale diffuse magnetic field is weak. Both results of our model can explain the magnetic field measurements of the Galactic Center region. In addition, we discuss the role of ionized outflow from stellar clusters in producing the long magnetized filaments perpendicular to the Galactic plane.
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the origin of the Galactic Center nonthermal radio filaments young stellar clusters
The Astrophysical Journal, 2003Co-Authors: F YusefzadehAbstract:The unusual class of magnetized nonthermal radio filaments, threads, and streaks, with their unique physical characteristics, is found only within the inner couple of degrees of the Galactic Center. Also, a number of young, mass-losing, and rare stellar clusters are recognized as lying in the Galactic Center region. The latter characteristic of the Galactic Center region is used to explain the origin of the nonthermal radio filaments. We consider a mechanism in which the collective winds of massive W-R and OB stars within a dense stellar environment produce shock waves that can accelerate particles to relativistic energies. This mechanism is an extension of a model originally proposed in 1996 by Rosner & Bodo, who suggested that energetic nonthermal particles are produced in a terminal shock of mass-losing stars. The large-scale distribution of the magnetic field in the context of this model is argued to be neither poloidal in geometry nor pervasive throughout the Galactic Center region.
James M. Cordes - One of the best experts on this subject based on the ideXlab platform.
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A VLA Survey for Compact Radio Sources in the Galactic Center
The Astrophysical Journal Supplement Series, 2008Co-Authors: T. Joseph W. Lazio, James M. CordesAbstract:The scattering diameters of Sgr A* and several nearby OH masers (1 -->'' at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic Center. This region of enhanced scattering implies that radio pulsars in or beyond the Galactic Center will suffer pulse broadening of hundreds of seconds at 1 GHz. We have used the VLA at 1.4 GHz to survey the inner 2° of the Galactic Center for radio pulsar candidates. Because of the large pulse broadening suffered by Galactic Center radio pulsars, our strategy has been to identify compact radio sources, without regard to whether they are pulsed radio sources. We describe our survey and present the catalog of 170 sources. Comparison with other radio surveys of the area shows that just over half of these sources have not been detected previously; this same comparison reveals 29 sources that are not in higher frequency surveys, suggesting that the sources are either variable or steep spectrum, and a comparable number of sources in other surveys that should have been detected in this survey but were not. Comparison with infrared surveys shows that 59 (35%) sources have infrared counterparts and are likely to be H II regions. On the basis of the number of sources as yet unidentified, we estimate that there could have been no more than roughly 104-105 supernovae in the past several million years in the Galactic Center. This estimate is consistent with the number required to produce the X-ray-emitting plasma in the Galactic Center and is marginally consistent with that required to explain the 1.8 MeV emission from -->26Al.
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Radio pulsars and transients in the Galactic Center
Journal of Physics: Conference Series, 2006Co-Authors: Joseph Lazio, James M. Cordes, J. S. Deneva, Geoffrey C. Bower, Scott D. Hyman, Donald C. Backer, R. Bhat, Shami Chatterjee, Paul Demorest, Scott M. RansomAbstract:Radio pulsars and transients provide powerful probes of the star formation history, interstellar medium, and gravitational potential of the Galactic Center. Historical radio observations of the Galactic Center have not emphasized the time domain aspect of observing this region. We summarize a series of recent searches for and observations of radio transients and pulsars that make use of two advances in technology. The first is the formation of large fields of view ( 1°) at relatively longer wavelengths (λ > 1 m), and the second is the construction of receivers and instruments capable of collecting data on microsecond time scales at relatively short wavelengths (≈ 3 cm).
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Finding Radio Pulsars in and Beyond the Galactic Center
The Astrophysical Journal, 1997Co-Authors: James M. Cordes, T. Joseph W. LazioAbstract:Radio-wave scattering is enhanced dramatically for Galactic Center sources in a region with radius >~ 15 arc min. Using scattering from Sgr A* and other sources, we show that pulse broadening for pulsars in the Galactic Center is {\em at least} 6.3 \nu^{-4} seconds (\nu = radio frequency in GHz) and is most likely 50--200 times larger because the relevant scattering screen appears to be within the Galactic Center region itself. Pulsars beyond---but viewed through---the Galactic Center suffer even greater pulse broadening and are angularly broadened by ~ 7 GHz and by testing for small numbers of harmonics in the power spectrum. The optimal frequency is $\nu ~ 7.3 GHz (\Delta_{0.1}P\sqrt{\alpha})^{-1/4}$ where \Delta_{0.1} is the distance of the scattering region from Sgr A* in units of 0.1 kpc, P is the period (seconds), and \alpha is the spectral index. A search for compact sources using aperture synthesis may be far more successful than searches for periodicities because the angular broadening is not so large as to desensitize the survey. We estimate that the number of {\em detectable} pulsars in the Galactic Center may range from
T. Joseph W. Lazio - One of the best experts on this subject based on the ideXlab platform.
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A VLA Survey for Compact Radio Sources in the Galactic Center
The Astrophysical Journal Supplement Series, 2008Co-Authors: T. Joseph W. Lazio, James M. CordesAbstract:The scattering diameters of Sgr A* and several nearby OH masers (1 -->'' at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic Center. This region of enhanced scattering implies that radio pulsars in or beyond the Galactic Center will suffer pulse broadening of hundreds of seconds at 1 GHz. We have used the VLA at 1.4 GHz to survey the inner 2° of the Galactic Center for radio pulsar candidates. Because of the large pulse broadening suffered by Galactic Center radio pulsars, our strategy has been to identify compact radio sources, without regard to whether they are pulsed radio sources. We describe our survey and present the catalog of 170 sources. Comparison with other radio surveys of the area shows that just over half of these sources have not been detected previously; this same comparison reveals 29 sources that are not in higher frequency surveys, suggesting that the sources are either variable or steep spectrum, and a comparable number of sources in other surveys that should have been detected in this survey but were not. Comparison with infrared surveys shows that 59 (35%) sources have infrared counterparts and are likely to be H II regions. On the basis of the number of sources as yet unidentified, we estimate that there could have been no more than roughly 104-105 supernovae in the past several million years in the Galactic Center. This estimate is consistent with the number required to produce the X-ray-emitting plasma in the Galactic Center and is marginally consistent with that required to explain the 1.8 MeV emission from -->26Al.
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Finding Radio Pulsars in and Beyond the Galactic Center
The Astrophysical Journal, 1997Co-Authors: James M. Cordes, T. Joseph W. LazioAbstract:Radio-wave scattering is enhanced dramatically for Galactic Center sources in a region with radius >~ 15 arc min. Using scattering from Sgr A* and other sources, we show that pulse broadening for pulsars in the Galactic Center is {\em at least} 6.3 \nu^{-4} seconds (\nu = radio frequency in GHz) and is most likely 50--200 times larger because the relevant scattering screen appears to be within the Galactic Center region itself. Pulsars beyond---but viewed through---the Galactic Center suffer even greater pulse broadening and are angularly broadened by ~ 7 GHz and by testing for small numbers of harmonics in the power spectrum. The optimal frequency is $\nu ~ 7.3 GHz (\Delta_{0.1}P\sqrt{\alpha})^{-1/4}$ where \Delta_{0.1} is the distance of the scattering region from Sgr A* in units of 0.1 kpc, P is the period (seconds), and \alpha is the spectral index. A search for compact sources using aperture synthesis may be far more successful than searches for periodicities because the angular broadening is not so large as to desensitize the survey. We estimate that the number of {\em detectable} pulsars in the Galactic Center may range from
R. Ramaty - One of the best experts on this subject based on the ideXlab platform.
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Positron annihilation radiation from the Galactic Center region
AIP Conference Proceedings, 2008Co-Authors: R. Ramaty, Richard E. LingenfelterAbstract:Observations show that there are two components of positron annihilation radiation from the region of the Galactic Center: a variable component resulting from one or just a few compact sources at or near the Galactic Center and a steady, diffuse component resulting from positron annihilation in the Galactic disk. We model the diffuse component using the observed longitude distributions of 70–150 MeV gamma rays, CO, and hot plasma revealed by Fe line emission. We review recent results on positron annihilation in the interstellar medium and discuss the implications of the annihilation processes on the fraction of positrons annihilating via positronium and on the shape of the 511 keV annihilation line. We also review the sources of diffuse Galactic positrons and discuss the nature of the compact source of annihilation radiation near the Galactic Center.
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The origin of the Galactic Center annihilation radiation
AIP Conference Proceedings, 2008Co-Authors: Richard E. Lingenfelter, R. RamatyAbstract:Observations of the e+−e− annihilation radiation from the Galactic Center suggest that something truly extraordinary is occurring there. We review the observations of this intense, time‐varying, 0.511 MeV emission and discuss the implications of these and other recent observations of the positron production process, the annihilation region and the fundamental nature of the Galactic Center source.
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TGRS Observation of the Galactic Center Annihilation Line
The Astrophysical Journal, 1996Co-Authors: Bonnard J. Teegarden, R. Ramaty, T. Cline, N. Gehrels, D. M. Palmer, H. Seifert, K. Hurley, D. A. Landis, Norman W. Madden, D. MaloneAbstract:The TGRS (Transient Gamma-Ray Spectrometer) experiment is a high-resolution germanium detector launched on the WIND satellite on Nov. 1, 1994. Although primarily intended to study gamma-ray bursts and solar flares, TGRS also has the capability of studying slower transients (e.g. x-ray novae) and certain steady sources. We present here results on the narrow 511 keV annihilation line from the general direction of the Galactic Center accumulated over the period Jan. 1995 through Oct. 1995. These results were obtained from the TGRS occultation mode, in which a lead absorber occults the Galactic Center region for 1/4 of each spacecraft rotation, thus chopping the 511 keV signal. The occulted region is a band in the sky of width 16 degrees that passes through the Galactic Center. We detect the narrow annihilation line from the Galactic Center with flux = $(1.64\pm0.09)\times10^{-3} {photons} {cm}^{-2} {s}^{-1}$. The data are consistent with a single point source at the Galactic Center, but a distributed source of extent up to ~30 degrees cannot be ruled out. No evidence for temporal variability on time scales longer than 1 month was found.
K. Ferrière - One of the best experts on this subject based on the ideXlab platform.
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The interstellar magnetic field near the Galactic Center
Proceedings of the International Astronomical Union, 2010Co-Authors: K. FerrièreAbstract:We review the observational knowledge that has built up over the past 25 years on the interstellar magnetic field within ~ 150 pc of the Galactic Center. We also provide a critical discussion of the main observational findings and comment on their possible theoretical interpretations. To conclude, we propose a coherent view of the interstellar magnetic field near the Galactic Center, which accounts at best for the vast body of observations.
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interstellar magnetic fields in the Galactic Center region
Astronomy and Astrophysics, 2009Co-Authors: K. FerrièreAbstract:Aims. We seek to obtain a picture of the interstellar magnetic field in the Galactic Center region that is as clear and complete as possible. Methods. We review the observational knowledge that has built up over the past 25 years on interstellar magnetic fields within ∼200 pc of the Galactic Center. We then critically discuss the various theoretical interpretations and scenarios proposed to explain the existing observations. We also study the possible connections with the general Galactic magnetic field and describe the observational situation in external galaxies. Results. We propose a coherent picture of the magnetic field near the Galactic Center, which reconciles some of the seemingly divergent views and which best accounts for the vast body of observations. Our main conclusions are the following. In the diffuse intercloud medium, the large-scale magnetic field is approximately poloidal and its value is generally close to equipartition with cosmic rays (∼10 μG), except in localized filaments where the field strength can reach ∼1 mG. In dense interstellar clouds, the magnetic field is approximately horizontal and its value is typically ∼ 1m G.
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Interstellar magnetic fields in the Galactic Center region
Astronomy & Astrophysics, 2009Co-Authors: K. FerrièreAbstract:We seek to obtain a picture of the interstellar magnetic field in the Galactic Center region that is as clear and complete as possible. To that end, we review the observational knowledge that has built up over the past 25 years on interstellar magnetic fields within ~ 200 pc of the Galactic Center. We then critically discuss the various theoretical interpretations and scenarios proposed to explain the existing observations. We also study the possible connections with the general Galactic magnetic field and describe the observational situation in external galaxies. We propose a coherent picture of the magnetic field near the Galactic Center, which reconciles some of the seemingly divergent views and which best accounts for the vast body of observations. Our main conclusions are the following. In the diffuse intercloud medium, the large-scale magnetic field is approximately poloidal and its value is generally close to equipartition with cosmic rays (~ 10 microG), except in localized filaments where the field strength can reach ~ 1 mG. In dense interstellar clouds, the magnetic field is approximately horizontal and its value is typically ~ 1 mG.
