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| SMA Research: Galactic Center | ||||
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The principal focus of the SMA for the study of the Galactic Center region is to investigate the structure, time variability and polarization of the Sgr A*, the compact radio source associated with the black hole in center of our galaxy. Precise astrometric observations at near infrared wavelengths over the past 15 years of stars orbiting Sgr A* show there is a central mass of 3.6 x 107 M⊙ within a volume of 10-7 pc3. Since the lifetime of any stellar cluster of this density is rather short, the central mass is most probably a black hole. Direct evidence of an accretion envelope around this black hole can be obtained by measuring the rotation of the plane of polarization of a linearly polarized wave traversing the envelope due to the effect of Faraday rotation. The rotation is proportional to the product of the longitudinal magnetic field strength, the electron density, the path length and the wavelength squared. The SMA was the first instrument to definitively measure the Faraday rotation of Sgr A* from observations at different wavelengths simultaneously. The so called rotation measure was found to be 5 x 105 rad m-2. Based on equipartition of energy arguments and other assumptions, the accretion rate corresponding to this rotation measure is about 10-8 M⊙ yr-1. This low accretion rate supports the idea that the black hole is in a quiescent period with highly sub-Eddington luminosity. The emission fluctuates at all wavelengths, and coordinated observations have been made with the SMA and other telescopes at longer radio, IR and X-Ray wavelengths. Some flare "events" have been observed, which provide limits on the expanding plasmoid model of synchrotron emission and the SSC radiation mechanism. The SMA shows that although the rotation measure of the accretion envelope stays relatively constant, the polarization of the Sgr A* radio source fluctuates on a sub-hourly basis. Some quasi-periodic events have been identified, which may be caused by transient emission regions in the inner accretion envelope near the last stable orbit or in a precessing jet.
The SMA and JCMT have been linked to form a VLBI station at 230 GHz in a network that includes telescopes on Mauna Kea, Mt Hopkins, Arizona, and Cedar Flat, CA. Observations in 2007 have definitively resolved the intrinsic emission region of Sgr A*. Its diameter is 37 μas (3.7 Rs, where Rs is the Schwarzschild radius of 1 x 1012 cm) based on a Gaussian source model. The data are also consistent with a thick annulus model with inner and outer diameters of 35 and 80 μas, respectively. If this model is correct, it provides the first indication of the existence of the predicted black hole "shadow." Future observations with higher sensitivity and additional stations, e.g. Mexico (LMT) or Chile (ASTE or APEX), will improve the VLBI imaging capabliity dramatically.
CfA:
ASIAA:
Ongoing collaborators:
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The polarization position angle as a function of wavelength squared measured by the SMA in 2005. The relatively constant slope in position angle between 220 and 230 GHz among the six measurements indicates that the accretion flow is fairly stable. However, the intrinsic position angle of the intrinsic radio emission from Sgr A* is quite variable.
The position angle of linear polarization for a 6 hour period on March 31, 2007. There appears to be a mostly monotonic progression in position angle of a variable component along with a constant polarized component with Q = -0.2 Jy and U = 0.1 Jy.
VLBI observation in March 2007 at 230 GHz with stations on Mauna Kea (JCMT/SMA), Mt. Hopkins (SMT), and Cedar Flat (CARMA). The visibility versus baseline length can be interpreted as a Gaussian source (solid line) of 43 μas diameter (37 after removal of effect of interstellar scattering), or as a thick annular source (dotted line) of 80 μas outer diameter. | ||
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